Balz
Frei
ABSTRACF We exposed human blood plasma and lowdensity lipoprotein (LDL) to many different oxidative challenges and followed the temporal consumption ofendogenous antioxidants in relation to the initiation of oxidative damage. Under all types of oxidizing conditions, ascorbic acid completely protects lipids in plasma and LDL against detectable peroxidative damage as assessed by a specific and highly sensitive assay for lipid peroxidation. Ascorbic acid proved to be superior to the other water-soluble plasma antioxidants bilirubin, uric acid, and protein thiols as well as to the lipoprotein-associated antioxidants a-tocopherol, ubiquinol-lO, lycopene, and fl-carotene. Although these antioxidants can lower the rate of detectable lipid peroxidation, they are not able to prevent its initiation. Only ascorbic acid is reactive enough to effectively intercept oxidants in the aqueous phase before they can attack and cause detectable oxidative damage to lipids. Am J Gun Nuir 199 1 ;54: 11l3S-l8S. KEY WORDS Ascorbic acid, vitamin C, antioxidants, human blood plasma, low-density lipoprotein, lipid peroxidation, neutrophils, cigarette smoke, cancer, atherosclerosis Infroduction
Prooxidant
states have been linked to certain types of cancer also play important roles in atherogenesis (2) and a number of other human diseases associated with aging (3). Antioxidants, by preventing or suppressing prooxidant states, can act as anticarcinogens ( 1 4) and probably as antiatherogens (2, 5). We have investigated the relative effectiveness of selected physiological antioxidants in preventing oxidative damage to lipids in human blood plasma under various types of oxidative stress. We found that ascorbic acid is the most effective lowmolecular-weight antioxidant in thisextracellularfluid.This artide summarizes our work on antioxidant defenses and oxidative damage in plasma (6-10) and discusses some of our more recent studies on the role of ascorbic acid in preventing cigarette smokeinduced oxidations (11) and oxidation of isolated human lowdensity lipoprotein (LDL) (12). (1) and
may
,
azobis(2-amidinopropane) hydrochloride (AAPH), human neutrophils activated with phorbol 12-myristate 13-acetate, and the gas phase of cigarette smoke. The radical-initiator AAPH, through thermal decomposition, generates water-soluble peroxyl radicals at a constant rate (14). Activated neutrophils release a wide variety of oxidants (superoxide anion, hydrogen peroxide, hypochiorous acid, chloramines, nitric oxide, etc) after a single, so-called respiratory burst (1 5, 16). Cigarette smoke contains many oxidants, including nitric oxide and its oxidation product nitrogen dioxide, carbon-centered radicals, alkoxyl radicals, peroxyl radicals, and semiquinone radicals capable of generating superoxide anion, hydrogen peroxide, and hydroxyl radicals (17). We have investigated antioxidant defenses in human blood plasma under these three types of oxidizing conditions by measuring the temporal consumption of endogenous ascorbic acid, uric acid, a-tocopherol, ubiquinol-lO, albumin-bound bilirubin, and protein thiols in relation to the appearance of oxidatively damaged plasma constituents. Oxidative damage was assessed by measuring lipid peroxidation with a sensitive and selective high-performance liquid chromatography assay coupled to a postcolumn chemiluminescence detection system (13, 18). This assay, which measures lipid hydroperoxides themselves rather than indirect indices of lipid peroxidation [such as diene conjugation or thiobarbituric acid-reactive substances (19)], can detect various classes of lipid hydroperoxides at plasma levels as low as 10 nmol/L (13). The same assay was used to investigate peroxidative damage to LDL exposed to AAPH or activated neutrophils (12). For a more detailed description of the experimental procedures for exposure ofplasma or LDL to oxidative stress and the quantitation of antioxidants and lipid hydroperoxides, the interested reader is referred to references 6, 7, 1 1, 12, and
18.
Results
and discussion
Human
blood
plasma
exposed
to aqueous
peroxyl
radicals
In plasma exposed to aqueous peroxyl radicals generated at a constant rate by decomposition of the chemical radical-initiator AAPH, the endogenous antioxidants are consumed progressively
Methods Fresh human blood plasma or LDL [free amounts of lipid hydroperoxides (12, 13)1 from molipidemic individuals was exposed to various dizing conditions, viz the water-soluble radical
Am J Clin Nuir
1991 ;54: 1113S-18S.
Printed
of detectable healthy, nortypes of oxiinitiator 2,2’-
in USA. © 1991 American
‘From the Department Boston, MA. 2Ad(fr
reprint
School of Public
Society
for Clinical
requests Health,
Nutrition
of Nutrition,
Harvard
School of Public Health,
to B Frei, Department 665 Huntington
Avenue,
of Nutrition, Boston,
Harvard
MA 02115.
lll3S
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Ascorbic acid protects lipids in human plasma and lowdensity lipoprotein against oxidative damage1’2
lll4S
C
U) U) U)
0. 01 U) -j
Ascorbate
(mmol/L)
FIG 1. Extension by ascorbic acid ofthe lag phase preceding detectable lipid peroxidation in plasma exposed to aqueous peroxyl radicals. Plasma supplemented with ascorbic acid to the total (endogenous plus added) concentrations indicated was incubated at 37 #{176}C with 50 mmol AAPH/ L and the lag phase during which no lipid hydroperoxides could be detected (concentration 10 nmol/L) was determined. From reference 7.
peroxyl radicals, lipid peroxidation is initiated immediately without lag in detectable lipid peroxidative damage (7). Adding back ascorbic acid, but none of the other low-molecular-weight compounds (eg, uric acid), restores the lag period before lipid peroxidation is detected; the added ascorbic acid is consumed completely during this period. Third, exogenous ascorbic acid was added to plasma after endogenous ascorbic acid had been consumed completely and lipid peroxidation had already been initiated (7). As shown in Figure 2, addition of ascorbic acid after 150 mm of incubation ofplasma with AAPH results in complete and transient standstill of lipid peroxidation as well as transient cessation of bilirubin, uric acid, and a-tocopherol consumption. Only the plasma proteins’ thiols are not spared by ascorbic acid, possibly because they are oxidized by peroxyl radical-induced autoxidation (7). After the added ascorbic acid has been consumed, lipid peroxidation and consumption of the other plasma antioxidants resumes (Fig 2). From these data we conclude that ascorbic acid is the only endogenous antioxidant in plasma that can completely protect lipids from detectable peroxidative damage induced by aqueous peroxyl radicals. Under this type of oxidative stress, ascorbic acid is a much more effective antioxidant than are albuminbound biirubin, uric acid, the plasma proteins’ thiols, ubiquinol10, and a-tocopherol. Ascorbic acid appears to trap the peroxyl radicals in the aqueous phase with a rate constant large enough to intercept virtually all these radicals before they can diffuse into the plasma lipids. Once ascorbic acid has been consumed completely, the remaining water-soluble antioxidants uric acid, albumin-bound bilirubin, and protein thiols provide only a partial trap for aqueous peroxyl radicals. The peroxyl radicals that escape the antioxidants in the aqueous phase diffuse into the lipids, where they initiate lipid peroxidation. The propagation of peroxidation is inhibited by the chain-breaking antioxidants associated with the lipoproteins, such as ubiquinol-lO and atocopherol. However, these lipid-soluble antioxidants can only
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in the sequence ascorbic acid = ubiquinol-lO = protein thiols > albumin-bound bilirubin > uric acid > a-tocopherol (6, 7). During the consumption ofascorbic acid, no lipid peroxidation can be detected (ie, concentration oflipid hydroperoxides 10 nmol/L). Immediately after the complete consumption of ascorbic acid, hydroperoxides of plasma phospholipids, cholesterol esters, and triglycerides begin to form in micromolar concentrations. Lipid peroxidation occurs despite the presence of physiological or near-physiological concentrations of protein thiols, albumin-bound bilirubin, uric acid, ubiquinol-lO, and atocopherol. During consumption of these latter antioxidants, the average chain length oflipid peroxidation is 0.36, indicating that 64% of the aqueous peroxyl radicals generated by AAPH are trapped whereas 36% escape the antioxidant defenses and cause peroxidative damage to lipids (assuming that there is no propagation of lipid peroxidation) (6). Thus, although protein thiols, uric acid, albumin-bound bilirubin, ubiquinol-lO, and atocopherol moderate the rate oflipid peroxidation, they do not prevent formation of micromolar concentrations of lipid hydroperoxides. In contrast, ascorbic acid appears to be capable of completely protecting plasma lipids against detectable peroxidative damage. To conclusively demonstrate the superior effectiveness of ascorbic acid against aqueous peroxyl radicals, we performed the following three types of experiments (7). First, we added increasing amounts of ascorbic acid to plasma before it was challenged with AAPH and then measured the period during which no lipid hydroperoxides could be detected. We found that with increasing ascorbic acid concentrations the lag phase preceding detectable lipid peroxidation also increases (Fig 1). The lag period increases sublinearly with ascorbic acid concentration, reflecting decreased radical-trapping efficiency of ascorbic acid as its concentration increases. Indeed, it was shown that the number of peroxyl radicals trapped by each molecule of ascorbic acid (its n value) is concentration dependent and decreases from 1.7 at 1 j.mol/L to -0.5 at 50 jmol/L (20, 21). This decrease in the n value most probably is due to loss of ascorbic acid by peroxyl-catalyzed autoxidation, which is of increasing importance at higher initial concentrations of ascorbic acid (20). Note that ascorbic acid provides strictly increased protection with every increase in its concentration (Fig 1), implying that the higher the ascorbic acid concentration, the better, or longer lasting, the protection against aqueous peroxyl radicals. This strict increase in antioxidant protection also demonstrates that in plasma ascorbic acid exerts only antioxidant and no prooxidant activity. It is known that ascorbic acid can switch from anti- to prooxidant activity, depending on its concentration and the presence of free transition metal ions (20,22). Thus, the fact that in plasma ascorbic acid preserves its antioxidant activity, even at very high concentrations, confirms that in this extracellular fluid transition metal ions are bound tightly and are not available for free radical reactions outside the metal-binding proteins(lO, 23). Second, we removed endogenous ascorbic acid from plasma by gel filtration using a rapid column-centrifugation technique that yields undiluted plasma devoid of low-molecular-weight compounds (7). Albumin-bound bilirubin, protein thiols, and lipoprotein-associated antioxidants (a-tocopherol, ubiquinol-lO, carotenoids, etc) are not removed from plasma by this procedure. When plasma treated by gel filtration is exposed to aqueous
FREI
PREVENTION
OF
OXIDATIVE
DAMAGE
TO
11 l5S
LIPIDS
0
60
120 Time
180 (mm)
240
300
0
60
120 180 Time (mm)
240
300
FIG 2. Interruption by ascorbic acid of ongoing lipid peroxidation (panels A and B) and antioxidant depletion (panels C and D) in plasma exposed to aqueous peroxyl radicals. Plasma containing 8 1 Mmol endogenous ascorbic acid/L was incubated at 37 #{176}C with 50 mmol AAPH/L, and after 150 mm of incubation 100 tmol ascorbic acid was added (open symbols, +). No ascorbic acid was added to the control incubation (closed symbol, -). Lipid-OOHs, lipid hydroperoxides; PL-OOH, phospholipid hydroperoxides; CE-OOH, cholesterol ester hydroperoxides; TG-OOH, triglyceride hydroperoxides. 100% = 100 mol ascorbic acid/L; 75 mol phospholipid hydroperoxides/L; 146 tmol cholesterol ester hydroperoxides or triglyceride hydroperoxides/L; 9.9 Mmol bilirubin/L; 334 tmo1 urate/L; 25.1 &mol a-tocopherol/L. From reference 7.
trap chain-carrying lipid peroxyl radicals and therefore can only lower the rate of detectable lipid peroxidation but not prevent its initiation. Only ascorbic acid can do so.
Human
blood
plasma
exposed
to activated
neutrophils
Activation ofleukocytes is an important endogenous source of oxidants in vivo, which has been implicated in the etiology of many human diseases, including cancer (1, 4), atherosclerosis (2, 24), and autoimmune diseases (3). We observed that challenging human blood plasma with oxidants released from activated neutrophils leads to sequential depletion ofascorbic acid = ubiquinol10 = protein thiols > uric acid, without significant oxidation of albumin-bound bilirubin and a-tocopherol (6, 8). Protein thiols and uric acid are depleted only partially during the first 30 mm of incubation. The cessation of their oxidation after this period most probably reflects cessation of oxidant production by the neutrophils. Ascorbic acid is consumed immediately and very rapidly upon neutrophil activation. As with aqueous peroxyl radicals, there is no detectable lipidperoxidation ( 10 nmol lipid hydroperoxides/L) as long as detectable concentrations(> 1 mol/ L) of ascorbic acid are present. Once ascorbic acid has been consumed completely, hydroperoxides of plasma phospholipids, cholesterol esters, and triglycerides are formed simultaneously (6).
Although similar in some respects, the oxidation processes in plasma triggered by aqueous peroxyl radicals and activated neutrophils differ in some important aspects (6). First, ascorbic acid is depleted more rapidly by activated neutrophils than by aqueous peroxyl radicals despite a drastically lower rate of subsequent lipid peroxidation. Second, a-tocopherol is not oxidized after neutrophil activation whereas it is after exposure of plasma to aqueous peroxyl radicals. Third, albumin-bound bilirubin is not oxidized by the oxidants released from neutrophils despite partial depletion of uric acid whereas albumin-bound biirubin is oxidized before uric acid by aqueous peroxyl radicals. These ohservations suggest that the effects of activated neutrophils on plasma antioxidants and lipids are not caused by aqueous peroxyl radicals. More likely candidates are 1) hypochlorous acid, known to be produced by the enzyme myeloperoxidase released from neutrophil granules (15) and to be scavenged by ascorbic acid (20) and uric acid (25) but not albumin-bound bilirubin (26), and 2) nitric oxide, known to be generated by neutrophils (16), and its oxidation product nitrogen dioxide. In summary, under the (patho)physiologically relevant type of oxidative stress exerted by activated human neutrophils, ascorbic acid is highly effective, and the only antioxidant in plasma capable of completely preventing detectable lipid peroxidative damage. All the other water-soluble antioxidants in
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‘-I
11 l6S
FREI
plasma
and the lipid-soluble
lipoproteins
antioxidants
are considerably
associated
with plasma
less effective. 0.6
Human smoke
blood
plasma
exposed
to the gas phase
of cigarette I
smoke,
which
to cause
cancer
biological
effects
ofcigarette
contains ( 1 , 27).
a large The
number
smoke
appear
Cl)
of oxidants,
mechanisms
ofthe
to include
0.4
adverse oxidative
damage to essential biological constituents (1, 17). When we exposed fresh human blood plasma to the gas phase of cigarette smoke (1 1), ascorbic acid was the first antioxidant to be consumed (Fig 3). Protein thiols and albumin-bound bilirubin are also
oxidized
at significant
rates
whereas
oxidations
of uric
which
subsequently
results
of Figure
phase
ofcigarette
peroxidation
oxidize
3 demonstrate
smoke in plasma
is exposed
cigarette
ascorbic
rate (30% spectively). this
control
(17).
oxidants
present
in the gas
and
are formed.
acid
to air instead acid
experiment
ofinducing
ascorbic
and
no detectable
When
detectable lipid has been depleted
at a slow
after three antioxidants
and
amounts
However,
of
after
peroxides/L
lipid
amounts
endogenous
hydroperoxides
six exposures
can
be detected,
effectively
scavenges
the
after
indicating
that
suggest
oxidants
acid
or supplemented
fore cigarette smoke because of treatment
that
ascorbic
of gas-phase
is initiated
ascorbic
cigarette
significantly
whereas
endogenous plasma
antioxidant lipids against
induced
by gas-phase
oxidants augmented
to cigarette
in plasma
initiation ofdetectable lipid (I 1). These data demonstrate
the only protecting
the argument
exogenous
that
in plasma
acid acid
has very
smoke,
To further inof endogenous ascorbic
acid
be-
exposure. In plasma devoid ofascorbic acid with ascorbate oxidase, lipid peroxidation
immediately, acid
with
exposures.
ester hydro-
ascorbic
preventing them from attacking plasma lipids. vestigate this question, plasma was either depleted ascorbic
oxidase/ of detect-
the lipids
in air once
smokers
in plasma detectable
containing
C
0.2
0.0 0
FIG 3. to the gas put into a to 0.2 kPa cigarette Cambridge
2
LDL
exposed
added
peroxidation is delayed that ascorbic acid is capable of completely peroxidative damage
of cigarette smoke; they strengthen ascorbic acid intake is of benefit
(29).
to oxidative
6
8
Number
of Puffs
10
12
Antioxidant defenses and lipid peroxidation in plasma exposed phase ofcigarette smoke. Forty milliliters offresh plasma was 500-mL filter flask. For each exposure, the flask was evacuated and then smoke from a University ofKentucky 2Rl standard (equivalent to two puffs by a smoker) was pulled through a
filter (rated to remove
99.9% of all particles
>
0.01
zm in
diameter) into the flask and the flask was sealed. After incubation for 20 mm at 37 #{176}C with vigorous shaking, samples were removed from the plasma fordetermination ofantioxidantsand lipid hydroperoxides. Levels ofantioxidants are given in percent oftheir initial concentrations, which
were as follows (per liter): ascorbic imol;
albumin-bound tocopherol, 38 imoL
acid, 45 zmol; protein
thiols, 421
bilirubin, 13.9 mol; uric acid, 373 tmol; and aPL-OOH, TG-OOH, and CE-OOH: hydroperoxides
ofplasma phospholipids, From reference 1 1.
thglycerides,
lycopene,
and
and cholesterol
esters, respectively.
f3-carotene,
lipid
peroxidative
completely
stress
Using isolated human LDL exposed to AAPH or activated neutrophils, we recently were able to confirm our results obtained with plasma (12). LDL contains a number of lipid-soluble, chainbreaking antioxidants, including a-tocopherol, ubiquinol-lO,
other
carotenoids
(10,
12, 13) but
no
water-soluble ascorbic acid. In LDL exposed to a mild flux of aqueous peroxyl radicals, ubiquinol-lO is consumed first, followed by slower partial depletion of lycopene, fl-carotene, and a-tocopherol. Detectable lipid peroxidation is initiated immediately, indicating that none ofthe LDL-associated antioxidants can completely protect the lipids against oxidative damage. The rate of radical-mediated formation of lipid hydroperoxides in LDL is low as long as ubiquinol-lO is present, but it increases rapidly after ubiquinol-lO is consumed even though > 80% of a-tocopherol, a-carotene, and lycopene are still present ( 12). When exogenous ascorbic acid is added to the incubation of LDL with AAPH, there is a lag phase preceding detectable lipid peroxidation because ofthe antioxidant activity of ascorbic acid, in agreement with the data discussed above for plasma exposed to AAPH. Similar observations were made when LDL was incubated with activated neutrophils in the presence and absence of added ascorbic acid. Exposure ofLDL at 0.5 g protein/L without added ascorbic acid for 2 h to activated neutrophils results in formation of 3.70 mol lipid hydroperoxides/L without lag in detectable
consumed Human
4
hy-
acid is first
three
cholesterol
rein
oflipid
ascorbic
0.02 imol
undergo very slow autoxidation been oxidized completely. The data in Figure 3 strongly
of
steady
six exposures, are not oxidized
oxidized by treatment ofplasma with 0.5 kU ascorbate L, subsequent air exposures do not lead to formation able
The
of the gas phase
is oxidized
and 55% oxidized All other plasma
droperoxides
radicals
that
once
completely. When plasma smoke,
to peroxyl
are capable
2
acid
and a-tocopherol are very slow. After the complete consumption of ascorbic acid, lipid peroxidation is initiated (Fig 3). These sequences of antioxidant consumption and lipid peroxidation are reminiscent of the oxidations seen in plasma exposed to aqueous peroxyl radicals. Indeed, it is known that nitrogen dioxide, formed by slow oxidation of nitric oxide [present at 300500 ppm in the gas phase of cigarette smoke (28)], can react with organic smoke constituents to form carbon-centered radicals,
I
0
incubation). 0.20
zmol
2 h incubation
damage.
whereas
slowly In the cholesterol and
Ubiquinol-lO
a-tocopherol,
and only presence ester
and
rapidly
and
/3-carotene
are
partially
(5-15% oxidized after 2 h of 50 mol ascorbic acid/L, only
hydroperoxides/L
no peroxidative
phospholipids can be detected that physiological concentrations
is oxidized
lycopene,
damage
are
formed
to triglycerides
after and
Other investigators showed of ascorbic acid also effectively
(12).
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Cigarette is known
PREVENTION
OF
OXIDATIVE
thus
suggest
that
ascorbic
acid
may
act as an antiath-
that of the lipidefficient that a-toLDL against per-
Conclusions Our findings summarized that ascorbic acid is the only completely
protect
lipids
in this article show conclusively physiological antioxidant that can
in plasma
and
LDL
against
detectable
peroxidative damage induced by many different types of oxidants. Although aqueous peroxyl radicals chemically generated by AAPH
are a convenient
the relative
model
for in vitro
article
is the
of antioxidants
investigations
of
they are pathologically oflittle interest. However, other types ofoxidative stress used in our studies, such as activated neutrophils and cigarette smoke, are highly relevant to human pathology and have been implicated in carcinogenesis and atherogenesis (1-4, 17, 24, 27). Another type of oxidative stress we have used but not discussed in this
effectiveness
(14),
xanthine-xanthine
oxidase
system,
possibly
important in myocardial reoxygenation injury (3). Studies used Cu2, macrophages, and endothelial cells as sources of oxidants (30-33). Under all these types of oxidative stress ascorbic acid successfully gesting
that
prevents
detectable
vitamin
C should
ofdegenerative and other a causative or exacerbating Note
ascorbic ical)
that
we
acid
in plasma,
could
concentrations
isolated
LDL
damage, helpful
in which
diseases
strongly
in the
sug-
prevention
oxidative
stress
plays
role. not
(7), in the
oxidative prove
observe
any
even
at extremely
and
when
absence
prooxidant
activity
of
high (nonphysiolog-
ascorbic
acid
of transition
metal
was ions
added
to
(12).
In
fact, even in the presence ofmicromolar concentrations of Cu2, ascorbic acid strongly protected against oxidative damage (3033), suggesting that also under these harsh conditions the antioxidant,
protective
potential
prooxidant,
properties damaging
of ascorbic effects.
acid In light
prevail of the
over
its
lack
of
any significant adverse health effects ofvitamin C to humans in general (34) and the potent antioxidant activity ofascorbic acid discussed in this article, it seems reasonable to consider vitamin C tissue saturation in humans to be desirable. 11 I am very grateful to Bruce N Ames for his support and enthusiasm over the years, and I thank my collaborators Roland Stocker and Carroll E Cross, with whom I did the LDL and cigarette smoke experiments, respectively. References 1. Cerutti PA. Prooxidant states and tumor promotion. Science l985;227:375-8 1. 2. Steinberg D, Parthasarathy 5, Carew TE, Khoo JC, Witztum JL. Beyond cholesterol: modifications of low-density lipoprotein that increase its atherogenicity. N EngI J Med l989;320:9l5-24. 3. Halliwell B, Gutteridge JMC. Free radicals in biology and medicine.
2nd ed. Oxford,
UK: Clarendon
Press, 1989.
4. Ames BN. Dietary carcinogens and anticarcinogens: and degenerative diseases. Science 1983;22l:1256-64.
LIPIDS
11 17S
oxygen radicals
antioxidants in vivo can selectively inhibit low density lipoprotein degradation in macrophage-rich fatty streaks and slow the progression of atherosclerosis in the Watanabe heritable hyperlipidemic rabbit. Proc Natl Acad Sci USA l987;84:7725-9. 6. Frei B, Stocker R, Ames BN. Antioxidant defenses and lipid peroxidation in human blood plasma. Proc Nail Acad Sci USA l988;85: 9748-52. 7. Frei B, England L, Ames BN. Ascorbate is an outstanding antioxidant in human blood plasma. Proc Natl Acad 5th USA 1989;86:637781. 8. Cross CE, Forte T, Stocker R, et al. Oxidative stress and abnormal cholesterol metabolism in patients with adult respiratory distress syndrome. J Lab Gin Med 1990;l 15:396-404. 9. Frei B, Stocker R, England L, Ames BN. Ascorbate: the most effective antioxidant in human blood plasma. In: Emerit I, Packer L, Auclair C, eds. Antioxidants in therapy and preventive medicine. New York:
Plenum
Press, 1990:155-63.
10. Stocker R, Frei B. Endogenous antioxidant defenses in human blood plasma. In: Sies H, ed. Oxidative stress: oxidants and antioxidants. London: Academic Press, 1991:213-43. 1 1. Frei B, Forte T, Ames BN, Cross CE. Gas-phase oxidants of cigarette smoke induce lipid peroxidation and changes in lipoprotein properties in human blood plasma: protective effects of ascorbic acid. Biochem J l991;277:l33-8. 12. Stocker R, Bowry VW, Frei B. Ubiquinol-lO protects human low density lipoprotein more efficiently against lipid peroxidation than does a-tocopherol. Proc Natl Acad Sci USA 199 1;88: 1646-50. 13. Frei B, Yamamoto Y, Niclas D, Ames BN. Evaluation of an isoluminol chemiluminescence assay for the detection of hydroperoxides in human blood plasma. Anal Biochem 1988;1 75:120-30. 14. Barclay LRC, Locke SJ, MacNeil JM, Van Kessel J, Burton GW, Ingold KU. Autooxidation of micelles and model membranes: quantitative kinetic measurements can be made by using either watersoluble or lipid-soluble initiators with water-soluble or lipid-soluble chain-breaking antioxidants. J Am Chem Soc 1984;l06:2479-81. 15. Hamers MN, Roos D. Oxidative stress in human neutrophilic granulocytes: host defence and self-defence. In: Sies H, ed. Oxidative stress. London: Academic Press, 1985:351-81. 16. McCall TB, Boughton-Smith NK, Palmer RM, Whittle BJ, Moncada S. Synthesis of nitric oxide from L-arglnine by neutrophils: release and interaction with superoxide anion. Biochem J l989;26l: 293-6. 17. Church DF, Pryor WA. Free-radical chemistry of cigarette smoke and its toxicological implications. Environ Health Persp l985;64: 1 1 1-26. 18. Yamamoto Y, Frei B, Ames BN. Assay oflipid hydroperoxides using high-performance liquid chromatography with isoluminol chemiluminescence detection. Methods Enzymol l990;l86:371-80. 19. Kappus H. Lipid peroxidation: mechanisms, analysis, enzymology and biological relevance. In: Sies H, ed. Oxidative stress. London: Academic Press, 1985:273-3 10. 20. Bendich A, Machim LI, Scandurra 0, Burton GW, Wayner DDM. The antioxidant role ofvitamin C. Adv Free Radic Biol Med 1986;2: 419-44.
21.
Wayner DDM, Burton GW, Ingold KU. The antioxidant efficiency of vitamin C is concentration-dependent. Biochim Biophys Acts 1986;884:1 19-23. 22. Sadrzadeh SMH, Eaton JW. Hemoglobin-mediated oxidant damage to the central nervous system requires endogenous ascorbate. J Clin Invest 1988;82: 1510-5. 23. Halliwell B, Gutteridge JMC. The antioxidants of human extracellular fluids. Arch Biochem Biophys 190280:1-8.
Downloaded from https://academic.oup.com/ajcn/article-abstract/54/6/1113S/4715083 by Washington University, Law School Library user on 21 September 2018
and
5)
erogen. In addition, our results demonstrate soluble antioxidants, ubiquinol-lO is more copherol and the carotenoids in protecting oxidative damage.
TO
5. Carew TE, Schwenke DC, Steinberg D. Antiatherogenic effect of probucol unrelated to its hypocholesterolemic effect: evidence that
protect isolated LDL against Cu2-, macrophage-, or endothelial cell-induced oxidative damage (30-33). The data are particularly interesting because lipid peroxidation in LDL has been implicated as causative factor in atherosclerosis (2,
DAMAGE
1 1 18S
29.
30.
31.
32.
33. 34.
Murata A, Shiraishi I, Fukuzaki K, Kitahara 1, Harada Y. Lower levels of vitamin C in plasma and urine of Japanese male smokers. Int J Vitam Nutr Res l989;59:l84-9. Esterbauer H, Striegl G, Puhl H, et al. The role of vitamin E and carotenoids in preventing oxidation oflow density lipoprotein. Ann NY Acad Sci 1989;570:254-67. Jialal I, Lena Vega G, Grundy SM. Physiologic levels of ascorbate inhibit the oxidative modification of low density lipoprotein. Atherosclerosis l990;82: 185-91. Jialal I, Grundy SM. Preservation of the endogenous antioxidants in low density lipoprotein by ascorbate but not probucol during oxidative modification. J Gin Invest 199 l;87:597-60l. Steinbrecher UP. Role of superoxide in endothelial-cell modification of low density lipoproteins. Biochim Biophys Acts l988;959:20-30. Sestili MA. Possible adverse health effects of vitamin C and ascorbic acid. Semin Oncol 1983;lO:299-304.
Downloaded from https://academic.oup.com/ajcn/article-abstract/54/6/1113S/4715083 by Washington University, Law School Library user on 21 September 2018
24. Cathcart MK. Morel DW, Chisolm GW. Monocytes and neutrophils oxidize low density lipoprotein making it cytotoxic. J Leukoc Biol l985;38:34 1-50. 25. Grootveld M, Halliwell B, Moorhouse CP. Action of uric acid, allopurinol and oxypurinol on the myeloperoxidase-derived oxidant hypochlorous acid. Free Radic Res Commun 1987;4:69-76. 26. Stocker R, Lai A, Peterhans E, Ames BN. Antioxidant properties of bilirubin and biliverdin. In: Hayaishi 0, Niki E, Kondo M, Yoshikawa T, eds. Medical, biochemical and chemical aspects of free radicals. Amsterdam: Elsevier, 1988:465-8. 27. US Public Health Service. Smoking and health: the report of the Surgeon General. Washington, DC: US Department of Health, Education and Welfare, 1979. 28. Guerin MR. Chemical composition of cigarette smoke. In: Gori GB, Bock FG, eds. Banbury Report: a safe cigarette? Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, l980;19l-204.
FREI
Frei
ABSTRACF We exposed human blood plasma and lowdensity lipoprotein (LDL) to many different oxidative challenges and followed the temporal consumption ofendogenous antioxidants in relation to the initiation of oxidative damage. Under all types of oxidizing conditions, ascorbic acid completely protects lipids in plasma and LDL against detectable peroxidative damage as assessed by a specific and highly sensitive assay for lipid peroxidation. Ascorbic acid proved to be superior to the other water-soluble plasma antioxidants bilirubin, uric acid, and protein thiols as well as to the lipoprotein-associated antioxidants a-tocopherol, ubiquinol-lO, lycopene, and fl-carotene. Although these antioxidants can lower the rate of detectable lipid peroxidation, they are not able to prevent its initiation. Only ascorbic acid is reactive enough to effectively intercept oxidants in the aqueous phase before they can attack and cause detectable oxidative damage to lipids. Am J Gun Nuir 199 1 ;54: 11l3S-l8S. KEY WORDS Ascorbic acid, vitamin C, antioxidants, human blood plasma, low-density lipoprotein, lipid peroxidation, neutrophils, cigarette smoke, cancer, atherosclerosis Infroduction
Prooxidant
states have been linked to certain types of cancer also play important roles in atherogenesis (2) and a number of other human diseases associated with aging (3). Antioxidants, by preventing or suppressing prooxidant states, can act as anticarcinogens ( 1 4) and probably as antiatherogens (2, 5). We have investigated the relative effectiveness of selected physiological antioxidants in preventing oxidative damage to lipids in human blood plasma under various types of oxidative stress. We found that ascorbic acid is the most effective lowmolecular-weight antioxidant in thisextracellularfluid.This artide summarizes our work on antioxidant defenses and oxidative damage in plasma (6-10) and discusses some of our more recent studies on the role of ascorbic acid in preventing cigarette smokeinduced oxidations (11) and oxidation of isolated human lowdensity lipoprotein (LDL) (12). (1) and
may
,
azobis(2-amidinopropane) hydrochloride (AAPH), human neutrophils activated with phorbol 12-myristate 13-acetate, and the gas phase of cigarette smoke. The radical-initiator AAPH, through thermal decomposition, generates water-soluble peroxyl radicals at a constant rate (14). Activated neutrophils release a wide variety of oxidants (superoxide anion, hydrogen peroxide, hypochiorous acid, chloramines, nitric oxide, etc) after a single, so-called respiratory burst (1 5, 16). Cigarette smoke contains many oxidants, including nitric oxide and its oxidation product nitrogen dioxide, carbon-centered radicals, alkoxyl radicals, peroxyl radicals, and semiquinone radicals capable of generating superoxide anion, hydrogen peroxide, and hydroxyl radicals (17). We have investigated antioxidant defenses in human blood plasma under these three types of oxidizing conditions by measuring the temporal consumption of endogenous ascorbic acid, uric acid, a-tocopherol, ubiquinol-lO, albumin-bound bilirubin, and protein thiols in relation to the appearance of oxidatively damaged plasma constituents. Oxidative damage was assessed by measuring lipid peroxidation with a sensitive and selective high-performance liquid chromatography assay coupled to a postcolumn chemiluminescence detection system (13, 18). This assay, which measures lipid hydroperoxides themselves rather than indirect indices of lipid peroxidation [such as diene conjugation or thiobarbituric acid-reactive substances (19)], can detect various classes of lipid hydroperoxides at plasma levels as low as 10 nmol/L (13). The same assay was used to investigate peroxidative damage to LDL exposed to AAPH or activated neutrophils (12). For a more detailed description of the experimental procedures for exposure ofplasma or LDL to oxidative stress and the quantitation of antioxidants and lipid hydroperoxides, the interested reader is referred to references 6, 7, 1 1, 12, and
18.
Results
and discussion
Human
blood
plasma
exposed
to aqueous
peroxyl
radicals
In plasma exposed to aqueous peroxyl radicals generated at a constant rate by decomposition of the chemical radical-initiator AAPH, the endogenous antioxidants are consumed progressively
Methods Fresh human blood plasma or LDL [free amounts of lipid hydroperoxides (12, 13)1 from molipidemic individuals was exposed to various dizing conditions, viz the water-soluble radical
Am J Clin Nuir
1991 ;54: 1113S-18S.
Printed
of detectable healthy, nortypes of oxiinitiator 2,2’-
in USA. © 1991 American
‘From the Department Boston, MA. 2Ad(fr
reprint
School of Public
Society
for Clinical
requests Health,
Nutrition
of Nutrition,
Harvard
School of Public Health,
to B Frei, Department 665 Huntington
Avenue,
of Nutrition, Boston,
Harvard
MA 02115.
lll3S
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Ascorbic acid protects lipids in human plasma and lowdensity lipoprotein against oxidative damage1’2
lll4S
C
U) U) U)
0. 01 U) -j
Ascorbate
(mmol/L)
FIG 1. Extension by ascorbic acid ofthe lag phase preceding detectable lipid peroxidation in plasma exposed to aqueous peroxyl radicals. Plasma supplemented with ascorbic acid to the total (endogenous plus added) concentrations indicated was incubated at 37 #{176}C with 50 mmol AAPH/ L and the lag phase during which no lipid hydroperoxides could be detected (concentration 10 nmol/L) was determined. From reference 7.
peroxyl radicals, lipid peroxidation is initiated immediately without lag in detectable lipid peroxidative damage (7). Adding back ascorbic acid, but none of the other low-molecular-weight compounds (eg, uric acid), restores the lag period before lipid peroxidation is detected; the added ascorbic acid is consumed completely during this period. Third, exogenous ascorbic acid was added to plasma after endogenous ascorbic acid had been consumed completely and lipid peroxidation had already been initiated (7). As shown in Figure 2, addition of ascorbic acid after 150 mm of incubation ofplasma with AAPH results in complete and transient standstill of lipid peroxidation as well as transient cessation of bilirubin, uric acid, and a-tocopherol consumption. Only the plasma proteins’ thiols are not spared by ascorbic acid, possibly because they are oxidized by peroxyl radical-induced autoxidation (7). After the added ascorbic acid has been consumed, lipid peroxidation and consumption of the other plasma antioxidants resumes (Fig 2). From these data we conclude that ascorbic acid is the only endogenous antioxidant in plasma that can completely protect lipids from detectable peroxidative damage induced by aqueous peroxyl radicals. Under this type of oxidative stress, ascorbic acid is a much more effective antioxidant than are albuminbound biirubin, uric acid, the plasma proteins’ thiols, ubiquinol10, and a-tocopherol. Ascorbic acid appears to trap the peroxyl radicals in the aqueous phase with a rate constant large enough to intercept virtually all these radicals before they can diffuse into the plasma lipids. Once ascorbic acid has been consumed completely, the remaining water-soluble antioxidants uric acid, albumin-bound bilirubin, and protein thiols provide only a partial trap for aqueous peroxyl radicals. The peroxyl radicals that escape the antioxidants in the aqueous phase diffuse into the lipids, where they initiate lipid peroxidation. The propagation of peroxidation is inhibited by the chain-breaking antioxidants associated with the lipoproteins, such as ubiquinol-lO and atocopherol. However, these lipid-soluble antioxidants can only
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in the sequence ascorbic acid = ubiquinol-lO = protein thiols > albumin-bound bilirubin > uric acid > a-tocopherol (6, 7). During the consumption ofascorbic acid, no lipid peroxidation can be detected (ie, concentration oflipid hydroperoxides 10 nmol/L). Immediately after the complete consumption of ascorbic acid, hydroperoxides of plasma phospholipids, cholesterol esters, and triglycerides begin to form in micromolar concentrations. Lipid peroxidation occurs despite the presence of physiological or near-physiological concentrations of protein thiols, albumin-bound bilirubin, uric acid, ubiquinol-lO, and atocopherol. During consumption of these latter antioxidants, the average chain length oflipid peroxidation is 0.36, indicating that 64% of the aqueous peroxyl radicals generated by AAPH are trapped whereas 36% escape the antioxidant defenses and cause peroxidative damage to lipids (assuming that there is no propagation of lipid peroxidation) (6). Thus, although protein thiols, uric acid, albumin-bound bilirubin, ubiquinol-lO, and atocopherol moderate the rate oflipid peroxidation, they do not prevent formation of micromolar concentrations of lipid hydroperoxides. In contrast, ascorbic acid appears to be capable of completely protecting plasma lipids against detectable peroxidative damage. To conclusively demonstrate the superior effectiveness of ascorbic acid against aqueous peroxyl radicals, we performed the following three types of experiments (7). First, we added increasing amounts of ascorbic acid to plasma before it was challenged with AAPH and then measured the period during which no lipid hydroperoxides could be detected. We found that with increasing ascorbic acid concentrations the lag phase preceding detectable lipid peroxidation also increases (Fig 1). The lag period increases sublinearly with ascorbic acid concentration, reflecting decreased radical-trapping efficiency of ascorbic acid as its concentration increases. Indeed, it was shown that the number of peroxyl radicals trapped by each molecule of ascorbic acid (its n value) is concentration dependent and decreases from 1.7 at 1 j.mol/L to -0.5 at 50 jmol/L (20, 21). This decrease in the n value most probably is due to loss of ascorbic acid by peroxyl-catalyzed autoxidation, which is of increasing importance at higher initial concentrations of ascorbic acid (20). Note that ascorbic acid provides strictly increased protection with every increase in its concentration (Fig 1), implying that the higher the ascorbic acid concentration, the better, or longer lasting, the protection against aqueous peroxyl radicals. This strict increase in antioxidant protection also demonstrates that in plasma ascorbic acid exerts only antioxidant and no prooxidant activity. It is known that ascorbic acid can switch from anti- to prooxidant activity, depending on its concentration and the presence of free transition metal ions (20,22). Thus, the fact that in plasma ascorbic acid preserves its antioxidant activity, even at very high concentrations, confirms that in this extracellular fluid transition metal ions are bound tightly and are not available for free radical reactions outside the metal-binding proteins(lO, 23). Second, we removed endogenous ascorbic acid from plasma by gel filtration using a rapid column-centrifugation technique that yields undiluted plasma devoid of low-molecular-weight compounds (7). Albumin-bound bilirubin, protein thiols, and lipoprotein-associated antioxidants (a-tocopherol, ubiquinol-lO, carotenoids, etc) are not removed from plasma by this procedure. When plasma treated by gel filtration is exposed to aqueous
FREI
PREVENTION
OF
OXIDATIVE
DAMAGE
TO
11 l5S
LIPIDS
0
60
120 Time
180 (mm)
240
300
0
60
120 180 Time (mm)
240
300
FIG 2. Interruption by ascorbic acid of ongoing lipid peroxidation (panels A and B) and antioxidant depletion (panels C and D) in plasma exposed to aqueous peroxyl radicals. Plasma containing 8 1 Mmol endogenous ascorbic acid/L was incubated at 37 #{176}C with 50 mmol AAPH/L, and after 150 mm of incubation 100 tmol ascorbic acid was added (open symbols, +). No ascorbic acid was added to the control incubation (closed symbol, -). Lipid-OOHs, lipid hydroperoxides; PL-OOH, phospholipid hydroperoxides; CE-OOH, cholesterol ester hydroperoxides; TG-OOH, triglyceride hydroperoxides. 100% = 100 mol ascorbic acid/L; 75 mol phospholipid hydroperoxides/L; 146 tmol cholesterol ester hydroperoxides or triglyceride hydroperoxides/L; 9.9 Mmol bilirubin/L; 334 tmo1 urate/L; 25.1 &mol a-tocopherol/L. From reference 7.
trap chain-carrying lipid peroxyl radicals and therefore can only lower the rate of detectable lipid peroxidation but not prevent its initiation. Only ascorbic acid can do so.
Human
blood
plasma
exposed
to activated
neutrophils
Activation ofleukocytes is an important endogenous source of oxidants in vivo, which has been implicated in the etiology of many human diseases, including cancer (1, 4), atherosclerosis (2, 24), and autoimmune diseases (3). We observed that challenging human blood plasma with oxidants released from activated neutrophils leads to sequential depletion ofascorbic acid = ubiquinol10 = protein thiols > uric acid, without significant oxidation of albumin-bound bilirubin and a-tocopherol (6, 8). Protein thiols and uric acid are depleted only partially during the first 30 mm of incubation. The cessation of their oxidation after this period most probably reflects cessation of oxidant production by the neutrophils. Ascorbic acid is consumed immediately and very rapidly upon neutrophil activation. As with aqueous peroxyl radicals, there is no detectable lipidperoxidation ( 10 nmol lipid hydroperoxides/L) as long as detectable concentrations(> 1 mol/ L) of ascorbic acid are present. Once ascorbic acid has been consumed completely, hydroperoxides of plasma phospholipids, cholesterol esters, and triglycerides are formed simultaneously (6).
Although similar in some respects, the oxidation processes in plasma triggered by aqueous peroxyl radicals and activated neutrophils differ in some important aspects (6). First, ascorbic acid is depleted more rapidly by activated neutrophils than by aqueous peroxyl radicals despite a drastically lower rate of subsequent lipid peroxidation. Second, a-tocopherol is not oxidized after neutrophil activation whereas it is after exposure of plasma to aqueous peroxyl radicals. Third, albumin-bound bilirubin is not oxidized by the oxidants released from neutrophils despite partial depletion of uric acid whereas albumin-bound biirubin is oxidized before uric acid by aqueous peroxyl radicals. These ohservations suggest that the effects of activated neutrophils on plasma antioxidants and lipids are not caused by aqueous peroxyl radicals. More likely candidates are 1) hypochlorous acid, known to be produced by the enzyme myeloperoxidase released from neutrophil granules (15) and to be scavenged by ascorbic acid (20) and uric acid (25) but not albumin-bound bilirubin (26), and 2) nitric oxide, known to be generated by neutrophils (16), and its oxidation product nitrogen dioxide. In summary, under the (patho)physiologically relevant type of oxidative stress exerted by activated human neutrophils, ascorbic acid is highly effective, and the only antioxidant in plasma capable of completely preventing detectable lipid peroxidative damage. All the other water-soluble antioxidants in
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‘-I
11 l6S
FREI
plasma
and the lipid-soluble
lipoproteins
antioxidants
are considerably
associated
with plasma
less effective. 0.6
Human smoke
blood
plasma
exposed
to the gas phase
of cigarette I
smoke,
which
to cause
cancer
biological
effects
ofcigarette
contains ( 1 , 27).
a large The
number
smoke
appear
Cl)
of oxidants,
mechanisms
ofthe
to include
0.4
adverse oxidative
damage to essential biological constituents (1, 17). When we exposed fresh human blood plasma to the gas phase of cigarette smoke (1 1), ascorbic acid was the first antioxidant to be consumed (Fig 3). Protein thiols and albumin-bound bilirubin are also
oxidized
at significant
rates
whereas
oxidations
of uric
which
subsequently
results
of Figure
phase
ofcigarette
peroxidation
oxidize
3 demonstrate
smoke in plasma
is exposed
cigarette
ascorbic
rate (30% spectively). this
control
(17).
oxidants
present
in the gas
and
are formed.
acid
to air instead acid
experiment
ofinducing
ascorbic
and
no detectable
When
detectable lipid has been depleted
at a slow
after three antioxidants
and
amounts
However,
of
after
peroxides/L
lipid
amounts
endogenous
hydroperoxides
six exposures
can
be detected,
effectively
scavenges
the
after
indicating
that
suggest
oxidants
acid
or supplemented
fore cigarette smoke because of treatment
that
ascorbic
of gas-phase
is initiated
ascorbic
cigarette
significantly
whereas
endogenous plasma
antioxidant lipids against
induced
by gas-phase
oxidants augmented
to cigarette
in plasma
initiation ofdetectable lipid (I 1). These data demonstrate
the only protecting
the argument
exogenous
that
in plasma
acid acid
has very
smoke,
To further inof endogenous ascorbic
acid
be-
exposure. In plasma devoid ofascorbic acid with ascorbate oxidase, lipid peroxidation
immediately, acid
with
exposures.
ester hydro-
ascorbic
preventing them from attacking plasma lipids. vestigate this question, plasma was either depleted ascorbic
oxidase/ of detect-
the lipids
in air once
smokers
in plasma detectable
containing
C
0.2
0.0 0
FIG 3. to the gas put into a to 0.2 kPa cigarette Cambridge
2
LDL
exposed
added
peroxidation is delayed that ascorbic acid is capable of completely peroxidative damage
of cigarette smoke; they strengthen ascorbic acid intake is of benefit
(29).
to oxidative
6
8
Number
of Puffs
10
12
Antioxidant defenses and lipid peroxidation in plasma exposed phase ofcigarette smoke. Forty milliliters offresh plasma was 500-mL filter flask. For each exposure, the flask was evacuated and then smoke from a University ofKentucky 2Rl standard (equivalent to two puffs by a smoker) was pulled through a
filter (rated to remove
99.9% of all particles
>
0.01
zm in
diameter) into the flask and the flask was sealed. After incubation for 20 mm at 37 #{176}C with vigorous shaking, samples were removed from the plasma fordetermination ofantioxidantsand lipid hydroperoxides. Levels ofantioxidants are given in percent oftheir initial concentrations, which
were as follows (per liter): ascorbic imol;
albumin-bound tocopherol, 38 imoL
acid, 45 zmol; protein
thiols, 421
bilirubin, 13.9 mol; uric acid, 373 tmol; and aPL-OOH, TG-OOH, and CE-OOH: hydroperoxides
ofplasma phospholipids, From reference 1 1.
thglycerides,
lycopene,
and
and cholesterol
esters, respectively.
f3-carotene,
lipid
peroxidative
completely
stress
Using isolated human LDL exposed to AAPH or activated neutrophils, we recently were able to confirm our results obtained with plasma (12). LDL contains a number of lipid-soluble, chainbreaking antioxidants, including a-tocopherol, ubiquinol-lO,
other
carotenoids
(10,
12, 13) but
no
water-soluble ascorbic acid. In LDL exposed to a mild flux of aqueous peroxyl radicals, ubiquinol-lO is consumed first, followed by slower partial depletion of lycopene, fl-carotene, and a-tocopherol. Detectable lipid peroxidation is initiated immediately, indicating that none ofthe LDL-associated antioxidants can completely protect the lipids against oxidative damage. The rate of radical-mediated formation of lipid hydroperoxides in LDL is low as long as ubiquinol-lO is present, but it increases rapidly after ubiquinol-lO is consumed even though > 80% of a-tocopherol, a-carotene, and lycopene are still present ( 12). When exogenous ascorbic acid is added to the incubation of LDL with AAPH, there is a lag phase preceding detectable lipid peroxidation because ofthe antioxidant activity of ascorbic acid, in agreement with the data discussed above for plasma exposed to AAPH. Similar observations were made when LDL was incubated with activated neutrophils in the presence and absence of added ascorbic acid. Exposure ofLDL at 0.5 g protein/L without added ascorbic acid for 2 h to activated neutrophils results in formation of 3.70 mol lipid hydroperoxides/L without lag in detectable
consumed Human
4
hy-
acid is first
three
cholesterol
rein
oflipid
ascorbic
0.02 imol
undergo very slow autoxidation been oxidized completely. The data in Figure 3 strongly
of
steady
six exposures, are not oxidized
oxidized by treatment ofplasma with 0.5 kU ascorbate L, subsequent air exposures do not lead to formation able
The
of the gas phase
is oxidized
and 55% oxidized All other plasma
droperoxides
radicals
that
once
completely. When plasma smoke,
to peroxyl
are capable
2
acid
and a-tocopherol are very slow. After the complete consumption of ascorbic acid, lipid peroxidation is initiated (Fig 3). These sequences of antioxidant consumption and lipid peroxidation are reminiscent of the oxidations seen in plasma exposed to aqueous peroxyl radicals. Indeed, it is known that nitrogen dioxide, formed by slow oxidation of nitric oxide [present at 300500 ppm in the gas phase of cigarette smoke (28)], can react with organic smoke constituents to form carbon-centered radicals,
I
0
incubation). 0.20
zmol
2 h incubation
damage.
whereas
slowly In the cholesterol and
Ubiquinol-lO
a-tocopherol,
and only presence ester
and
rapidly
and
/3-carotene
are
partially
(5-15% oxidized after 2 h of 50 mol ascorbic acid/L, only
hydroperoxides/L
no peroxidative
phospholipids can be detected that physiological concentrations
is oxidized
lycopene,
damage
are
formed
to triglycerides
after and
Other investigators showed of ascorbic acid also effectively
(12).
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Cigarette is known
PREVENTION
OF
OXIDATIVE
thus
suggest
that
ascorbic
acid
may
act as an antiath-
that of the lipidefficient that a-toLDL against per-
Conclusions Our findings summarized that ascorbic acid is the only completely
protect
lipids
in this article show conclusively physiological antioxidant that can
in plasma
and
LDL
against
detectable
peroxidative damage induced by many different types of oxidants. Although aqueous peroxyl radicals chemically generated by AAPH
are a convenient
the relative
model
for in vitro
article
is the
of antioxidants
investigations
of
they are pathologically oflittle interest. However, other types ofoxidative stress used in our studies, such as activated neutrophils and cigarette smoke, are highly relevant to human pathology and have been implicated in carcinogenesis and atherogenesis (1-4, 17, 24, 27). Another type of oxidative stress we have used but not discussed in this
effectiveness
(14),
xanthine-xanthine
oxidase
system,
possibly
important in myocardial reoxygenation injury (3). Studies used Cu2, macrophages, and endothelial cells as sources of oxidants (30-33). Under all these types of oxidative stress ascorbic acid successfully gesting
that
prevents
detectable
vitamin
C should
ofdegenerative and other a causative or exacerbating Note
ascorbic ical)
that
we
acid
in plasma,
could
concentrations
isolated
LDL
damage, helpful
in which
diseases
strongly
in the
sug-
prevention
oxidative
stress
plays
role. not
(7), in the
oxidative prove
observe
any
even
at extremely
and
when
absence
prooxidant
activity
of
high (nonphysiolog-
ascorbic
acid
of transition
metal
was ions
added
to
(12).
In
fact, even in the presence ofmicromolar concentrations of Cu2, ascorbic acid strongly protected against oxidative damage (3033), suggesting that also under these harsh conditions the antioxidant,
protective
potential
prooxidant,
properties damaging
of ascorbic effects.
acid In light
prevail of the
over
its
lack
of
any significant adverse health effects ofvitamin C to humans in general (34) and the potent antioxidant activity ofascorbic acid discussed in this article, it seems reasonable to consider vitamin C tissue saturation in humans to be desirable. 11 I am very grateful to Bruce N Ames for his support and enthusiasm over the years, and I thank my collaborators Roland Stocker and Carroll E Cross, with whom I did the LDL and cigarette smoke experiments, respectively. References 1. Cerutti PA. Prooxidant states and tumor promotion. Science l985;227:375-8 1. 2. Steinberg D, Parthasarathy 5, Carew TE, Khoo JC, Witztum JL. Beyond cholesterol: modifications of low-density lipoprotein that increase its atherogenicity. N EngI J Med l989;320:9l5-24. 3. Halliwell B, Gutteridge JMC. Free radicals in biology and medicine.
2nd ed. Oxford,
UK: Clarendon
Press, 1989.
4. Ames BN. Dietary carcinogens and anticarcinogens: and degenerative diseases. Science 1983;22l:1256-64.
LIPIDS
11 17S
oxygen radicals
antioxidants in vivo can selectively inhibit low density lipoprotein degradation in macrophage-rich fatty streaks and slow the progression of atherosclerosis in the Watanabe heritable hyperlipidemic rabbit. Proc Natl Acad Sci USA l987;84:7725-9. 6. Frei B, Stocker R, Ames BN. Antioxidant defenses and lipid peroxidation in human blood plasma. Proc Nail Acad Sci USA l988;85: 9748-52. 7. Frei B, England L, Ames BN. Ascorbate is an outstanding antioxidant in human blood plasma. Proc Natl Acad 5th USA 1989;86:637781. 8. Cross CE, Forte T, Stocker R, et al. Oxidative stress and abnormal cholesterol metabolism in patients with adult respiratory distress syndrome. J Lab Gin Med 1990;l 15:396-404. 9. Frei B, Stocker R, England L, Ames BN. Ascorbate: the most effective antioxidant in human blood plasma. In: Emerit I, Packer L, Auclair C, eds. Antioxidants in therapy and preventive medicine. New York:
Plenum
Press, 1990:155-63.
10. Stocker R, Frei B. Endogenous antioxidant defenses in human blood plasma. In: Sies H, ed. Oxidative stress: oxidants and antioxidants. London: Academic Press, 1991:213-43. 1 1. Frei B, Forte T, Ames BN, Cross CE. Gas-phase oxidants of cigarette smoke induce lipid peroxidation and changes in lipoprotein properties in human blood plasma: protective effects of ascorbic acid. Biochem J l991;277:l33-8. 12. Stocker R, Bowry VW, Frei B. Ubiquinol-lO protects human low density lipoprotein more efficiently against lipid peroxidation than does a-tocopherol. Proc Natl Acad Sci USA 199 1;88: 1646-50. 13. Frei B, Yamamoto Y, Niclas D, Ames BN. Evaluation of an isoluminol chemiluminescence assay for the detection of hydroperoxides in human blood plasma. Anal Biochem 1988;1 75:120-30. 14. Barclay LRC, Locke SJ, MacNeil JM, Van Kessel J, Burton GW, Ingold KU. Autooxidation of micelles and model membranes: quantitative kinetic measurements can be made by using either watersoluble or lipid-soluble initiators with water-soluble or lipid-soluble chain-breaking antioxidants. J Am Chem Soc 1984;l06:2479-81. 15. Hamers MN, Roos D. Oxidative stress in human neutrophilic granulocytes: host defence and self-defence. In: Sies H, ed. Oxidative stress. London: Academic Press, 1985:351-81. 16. McCall TB, Boughton-Smith NK, Palmer RM, Whittle BJ, Moncada S. Synthesis of nitric oxide from L-arglnine by neutrophils: release and interaction with superoxide anion. Biochem J l989;26l: 293-6. 17. Church DF, Pryor WA. Free-radical chemistry of cigarette smoke and its toxicological implications. Environ Health Persp l985;64: 1 1 1-26. 18. Yamamoto Y, Frei B, Ames BN. Assay oflipid hydroperoxides using high-performance liquid chromatography with isoluminol chemiluminescence detection. Methods Enzymol l990;l86:371-80. 19. Kappus H. Lipid peroxidation: mechanisms, analysis, enzymology and biological relevance. In: Sies H, ed. Oxidative stress. London: Academic Press, 1985:273-3 10. 20. Bendich A, Machim LI, Scandurra 0, Burton GW, Wayner DDM. The antioxidant role ofvitamin C. Adv Free Radic Biol Med 1986;2: 419-44.
21.
Wayner DDM, Burton GW, Ingold KU. The antioxidant efficiency of vitamin C is concentration-dependent. Biochim Biophys Acts 1986;884:1 19-23. 22. Sadrzadeh SMH, Eaton JW. Hemoglobin-mediated oxidant damage to the central nervous system requires endogenous ascorbate. J Clin Invest 1988;82: 1510-5. 23. Halliwell B, Gutteridge JMC. The antioxidants of human extracellular fluids. Arch Biochem Biophys 190280:1-8.
Downloaded from https://academic.oup.com/ajcn/article-abstract/54/6/1113S/4715083 by Washington University, Law School Library user on 21 September 2018
and
5)
erogen. In addition, our results demonstrate soluble antioxidants, ubiquinol-lO is more copherol and the carotenoids in protecting oxidative damage.
TO
5. Carew TE, Schwenke DC, Steinberg D. Antiatherogenic effect of probucol unrelated to its hypocholesterolemic effect: evidence that
protect isolated LDL against Cu2-, macrophage-, or endothelial cell-induced oxidative damage (30-33). The data are particularly interesting because lipid peroxidation in LDL has been implicated as causative factor in atherosclerosis (2,
DAMAGE
1 1 18S
29.
30.
31.
32.
33. 34.
Murata A, Shiraishi I, Fukuzaki K, Kitahara 1, Harada Y. Lower levels of vitamin C in plasma and urine of Japanese male smokers. Int J Vitam Nutr Res l989;59:l84-9. Esterbauer H, Striegl G, Puhl H, et al. The role of vitamin E and carotenoids in preventing oxidation oflow density lipoprotein. Ann NY Acad Sci 1989;570:254-67. Jialal I, Lena Vega G, Grundy SM. Physiologic levels of ascorbate inhibit the oxidative modification of low density lipoprotein. Atherosclerosis l990;82: 185-91. Jialal I, Grundy SM. Preservation of the endogenous antioxidants in low density lipoprotein by ascorbate but not probucol during oxidative modification. J Gin Invest 199 l;87:597-60l. Steinbrecher UP. Role of superoxide in endothelial-cell modification of low density lipoproteins. Biochim Biophys Acts l988;959:20-30. Sestili MA. Possible adverse health effects of vitamin C and ascorbic acid. Semin Oncol 1983;lO:299-304.
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24. Cathcart MK. Morel DW, Chisolm GW. Monocytes and neutrophils oxidize low density lipoprotein making it cytotoxic. J Leukoc Biol l985;38:34 1-50. 25. Grootveld M, Halliwell B, Moorhouse CP. Action of uric acid, allopurinol and oxypurinol on the myeloperoxidase-derived oxidant hypochlorous acid. Free Radic Res Commun 1987;4:69-76. 26. Stocker R, Lai A, Peterhans E, Ames BN. Antioxidant properties of bilirubin and biliverdin. In: Hayaishi 0, Niki E, Kondo M, Yoshikawa T, eds. Medical, biochemical and chemical aspects of free radicals. Amsterdam: Elsevier, 1988:465-8. 27. US Public Health Service. Smoking and health: the report of the Surgeon General. Washington, DC: US Department of Health, Education and Welfare, 1979. 28. Guerin MR. Chemical composition of cigarette smoke. In: Gori GB, Bock FG, eds. Banbury Report: a safe cigarette? Cold Spring Harbor, NY: Cold Spring Harbor Laboratory, l980;19l-204.
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