Dietary

effects

on cytochromes

P450,

xenobiotic

metabolism,

and toxicity CHUNG

S. YANG,’

JOHN

F. BRADY,2

AND

HONG

JUN-YAN

Laboratory

for Cancer Research, Department of Chemical Biology Rutgers University, Piscataway, New Jersey 08855-0789, USA

ABSTRACT The levels and activities of cytochrome P450 enzymes are influenced by a variety of factors, including the diet. In this article, the effects of selected nonnutritive dietary chemicals, macronutrients, micronutrients, and ethanol on cytochromes P450 and xenobiotic metabolism are reviewed in the light of our current understanding of the multiplicity and substrate specificity of cytochrome P450 enzymes. Although the mechanisms of action of several dietary chemicals on specific cytochrome P450 isozymes have been established, those for macroand micronutrients are largely unknown. It is known, however, that specific nutrients may have varied effects on different cytochrome P450 forms and thus may affect the metabolism of various drugs differently. Nutritional deficiencies generally cause lowered rates of xenobiotic metabolism. In certain cases, such as thiamin deficiency and mild riboflavin deficiency, however, enhanced rates of metabolism of xenobiotics were observed. The effects of dietary modulation of xenobiotic metabolism on chemical toxicity and carcinogenicity are discussed. -Yang, C. S.; Brady, J. F.; Hong, -Y. Dietary effects on cytochromes P450, xenobiotic metabolism, and toxicity. FASEBJ. 6: 737-744; 1992.

J.

Key Words: diet tion xenobiotics

.

nutrition cytochromes P450 . enzyme drug metabolism . toxicity . carcinogens

regula-

CLOSE RELATIONSHIP BETWEEN DIET and xenobiotic metabolism may be traced back to prehistoric days in “animalplant warfare” during evolution (1). Plants synthesized chemicals for self-protection and animals had to develop xenobiotic-metabolizing enzymes such as cytochrome P450 (P450)3 for the detoxication of these chemicals. The evolution of the large number of P450 2 genes 400 million years ago may correspond to the advance of animals onto land where they encountered new terrestrial plants and phytochemicals. The work of many investigators in the past 30 years has clearly established that various dietary factors have marked effects on the metabolism of drugs, environmental chemicals, and certain endogenous substrates. This topic has been reviewed extensively (2-9). However, only recently have we begun to understand some of these effects at the molecular level. Dietary influences on xenobiotic metabolism may alter the therapeutic effects of drugs and the toxicity or carcinogenicity of environmental chemicals. In this article, we review the mechanisms by which dietary chemicals and nutritional status affect the levels and activities of P450 enzymes, xenobiotic metabolism, as well as chemical toxicity and carcinogenicity. Because of space limitations, we have chosen to use selected examples to illustrate key concepts rather than to conduct an exhaustive review on this topic. Review articles are cited instead of original papers.

THE

and

MODULATION METABOLISM CHEMICALS

Pharmacognosy,

College

OF P450 LEVELS BY NONNUTRITIVE

of Pharmacy,

AND

XENOBIOTIC DIETARY

Dietary

chemicals may affect the levels and activities of P450 at different steps as shown in Fig. 1. Dietary chemicals may affect the levels of P450 species by altering the rates of: 1) the transcription of specific P450 genes, 2) the degradation of specific mRNA, 3) the translation process, and 4) P450 degradation through protein turnover or by suicide inhibition. Many dietary chemicals are substrates of the P450-dependent monooxygenase system. They or their metabolites may inhibit or enhance the activities of this system by binding to P450 species or to NADPH:P450 reductase, by affecting the interaction between these enzymes, or by affecting key steps in the catalytic cycle. isozymes

Induction

of P450-dependent

activities

The

effect of diet on P450-dependent monooxygenase activiwas clearly demonstrated in the pioneering work of Wattenberg (10), who discovered that rats on commercial rat chow had 68-fold higher intestinal benzo(a)pyrene (BP) hydroxylase activity than those on a purified diet. By adding dry vegetable powder to the purified diet, Brussels sprouts, cabbage, turnips, and other vegetables were found to be inducers of intestinal BP hydroxylase in rats. The effects of cruciferous vegetables and their components on drug metabolism have since been studied extensively. Three autolytic products of indolylmethyl glucosinolate (glucobrassicin): indole-3-carbinol, indole-3-acetonitrile, and indole-3-carboxyaldehyde, isolated from Brussels sprouts, were found to induce intestinal and hepatic BP hydroxylase in rats. The structures of some of the nonnutritive dietary compounds are illustrated in Fig. 2. Indole-3-carbinol is the most potent inducing agent among these indoles. Its induction of BP hydroxylase and ethoxyresorufin dealkylase activities is believed to be mainly due to the induction of P450 1A1 in the intestine and P450s IA1 and 1A2 in the liver (11). It was demonstrated that after an oral dose of indole-3carbinol to rats, P450 lAl mRNA was elevated severalfold ties

‘To whom correspondence should be addressed, at: Laboratory for Cancer Research, Department of Chemical Biology and Pharmacognosy, College of Pharmacy, Rutgers University, Piscataway, NJ 08855-0789, USA. 2Present address: Chemistry

Department, University of California, Davis, CA95616, USA. 3Abbreviations: P450(s), cytochromes P450; BP, benzo(a)pyrene; Ah receptor, aromatic hydrocarbon receptor; RXM, acid reaction products of indole-3-carbinol; NDMA, N-nitrosodimethylamine; NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone; BHA, butylated hydroxyanisole; B HT, butylated hydroxytoluene.

non, o/o,/nnne. n7l7Icnl en Th ded from www.fasebj.org by Edinburgh University (129.215.17.190) on November 23, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()}, pp

II

III

I

III

P450

gene

transcription P450 mRNA

degradation

translation

degradation

.(

P450 Inactivation

Reductase

Substrates

Inhibiton

Enhancement

Figure 1. Possible sites for dietary effects on P450 enzymes. The P450 2E1 gene is used in the illustration. In addition to affecting the rates of transcription and translation, dietary chemicals may affect the rates of degradation of P450 mRNA and protein. Dietary chemicals may serve as substrates. They may also interact with P450 enzymes and NADPH:P450 reductase, causing inactivation of P450, or inhibition or enhancement of the monooxygenase activities.

in the liver and colon, and P450 1A2 mRNA was also elevated in the liver (11). The induction of P450 IAI involves the binding of the inducer to the Ah receptor. Nevertheless, indole-3-carbinol has only low binding affinity to the Ah receptor, whereas indolo[3,2-b]carbazole has a much higher affinity (12). It is suggested that under the acidic conditions in the stomach, indole-3-carbinol can be converted to indolo[3,2-b]carbazole or other acid reaction products (RXM) (13) that bind to the Ah receptor and thereby increase the transcription of the P450 1A1 gene. This suggestion is supported by the observation that the inductive effect was found when indole-3-carbinol was given to rats orally but not when given intraperitoneally (13). Acid-treated indole-3carbinol was much more effective than untreated indole-3carbinol in inducing ethoxyresorufin activity in primary cultures of rat hepatocytes (14). Certain flavones may also induce P450 IA1 by binding to the Ah receptor and subsequently activating the P450 1A1 gene. A second example of transcriptional regulation is the induction of P450 2B1 by diallyl sulfide. Diallyl sulfide, a component of garlic oil, has been shown to induce rat hepatic P450 2B1 based on the increase of immunodetectable protein and pentoxyresorufin dealkylase activity (15). Recent studies in our laboratory indicate that this induction is accompanied by the elevation of 2B1 mRNA levels. Nuclear run-on experiments indicate that after administration of diallyl sulfide, the transcriptional rate of the P450 2B1 gene was increased markedly, and was observable at 4 h after treatment (16). In primary culture of rat hepatocytes, P450 2Bl was not induced by diallyl sulfide but by its metabolite diallyl sulfone (unpublished results). Posttranscriptional mechanisms may also be involved in the induction of P450s by dietary factors. In the induction of P450 2E1 by fasting, elevation of P450 2E1 mRNA was observed (17) but transcriptional activation could not be convincingly demonstrated by nuclear run-on experiments (unpublished results). Stabilization of the mRNA may play a role in this induction. In the induction of P450 2E1 by ethanol and acetone, elevation of the mRNA was not observed (18, 19). Protein stabilization or increased translation efficiency may be involved in this induction.

Inactivation

of P450

enzymes

When diallyl sulfide was given orally to rats, microsomal Nnitrosodimethylamine (NDMA) demethylase (indicative of P450 2E1 activity) decreased markedly in several hours. This was followed by a lowering of the immunodetectable P450 2El protein level in microsomes (15). The inactivation of P450 2E1 was also demonstrated in vitro when diallyl sulfone, an oxidative metabolite of diallyl sulfide, was incubated with microsomes in the presence of an NADPH-generating system (15). The inactivation was time, concentration, and NADPH-dependent, following a typical suicide inhibition pattern. It is believed that diallyl sulfone is converted by P450 2E1 to a reactive intermediate that modifies the heme moiety of P450 2E1 (20). Phenethyl isothiocyanate, occurring as a glucusinolate in a variety of cruciferous vegetables, also inactivates P450 2E1 by a suicide mechanism (21). Psoralens, which are found in edible plants such as figs, celery, parsley, and parsnip, were shown to decrease 7-ethoxycoumarin and BP hydroxylase activities when added to human liver microsomal incubations in the presence of NADPH (22). It was suggested that methoxsalen (8-methoxypsoralen), bergapten (5-methoxypsoralen), and psoralen are suicide inhibitors of P450 enzymes; P450 lAl or 1A2 may be involved. Enhancement

of monooxygenase

activities

Flavone, tangeretin, and nobiletin were found to increase BP hydroxylase activity and aflatoxin B, activation in human liver microsomes (4). The metabolism of antipyrine and zoxazolamine was also stimulated, but that of hexobarbital, coumarin, and 7-ethoxycoumarin was not. The stimulatory effect was P450 isozyme-specific. With purified P450 isozymes in a reconstituted system, the effect was observed with rabbit P450s 3A6 and 1A2 but not with P450s 2B4, 2C3, or 1AI. It was proposed that these flavonoids stimulate the monooxygenase activity by enhancing the interactions be-

cxc

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CH,CH.N #{149} C - S

DM oISd

Phneth1

Sot’SocyaSai. OCH.

c)OO\ cX0

cOOk

PS

Flavone

Quercebn

Figure

2. Structures

Tange,otin

Nob,I.bn

Kaenpfeoi

of some dietary

or related

compounds.

The FASEBJournal 738 Vol. 6 lanuarv 1992 YANC FT At ded from www.fasebj.org by Edinburgh University (129.215.17.190) on November 23, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()}, pp

tween P450 and NADPH:P450 reductase and facilitating the flow of electrons to P450. Administration of flavone to neonatal rats also increased the rate of zoxazolamine metabolism several-fold (4). Inhibition

of monooxygenase

activities

Dietary compounds can bind to the active sites of P450 enzymes, serving as substrates or competitive inhibitors. For example, diallyl sulfide and phenethyl isothiocyanate are competitive inhibitors of P450 2El-catalyzed reactions (20, 21). In addition, phenethyl isothiocyanate also competitively inhibits the metabolism of 4-(methylnitrosamino)-1-(3-pyridyl)-1butanone (NNK), a potent tobacco carcinogen, in lung microsomes (23). In this case, a purely competitive mode of inhibition was not observed, possibly due to the inactivation of the enzyme during the incubation, and P450 2El was not involved. Many dietary flavonoids have been shown to inhibit monooxygenase activities. For example, quercetin, kaempferol, morin, and chrysin inhibited BP hydroxylase activity in human liver microsomes (4). Quercetin, kaempferol, and naringenin inhibited nifedipen and filodipen oxidation catalyzed by P450 3A4 in human microsomes (24). It appears that flavones having free hydroxy groups on the A ring are inhibitors, whereas flavonoids containing no hydroxy groups (flavone, tangeretin, and nobiletin) are activators (stimulators) of selected monooxygenase activities.

EFFECTS OF MACRONUTRIENTS METABOLISM

ON

XENOBIOTIC

Compared with the effects of nonnutritive dietary chemicals, the effects of macronutrients on drug metabolism are more difficult to understand. In the former case, the actions of a compound or its metabolites can be studied, but in the latter case we have to deal with a nutritional state or status. Therefore, despite extensive investigations, our understanding of the latter subject is mostly at the descriptive level. Only recently have researchers begun to look at specific changes on selected enzymes and the molecular mechanisms involved. Many previously seemingly contradictory results may be interpretable based on our new knowledge on P450 isozymes. A dietary manipulation of nutritional status may increase the levels of a certain group of P450s but decrease those of others. Therefore, depending on the substrates used in the study, opposite effects on drug metabolism may be observed. Protein Diets with low protein content or lower quality of protein usually result in lower rates of xenobiotic metabolism than a normal diet. This effect was observed in human volunteers concerning the metabolism of antipyrine and theophylline (2, 4) and in animals with a variety of xenobiotics (25). The P450 enzymes may be affected because protein synthesis is retarded under protein deficiency conditions. However, the effect on xenobiotic metabolism may be observed even without common signs of nutritional protein deficiency. It is possible that dietary protein level influences the physiological state, such as hormone levels, which affect the level of P450 enzymes.

Lipid

and

carbohydrate

In comparison to a fat-free diet, feeding a 3-10% corn oil diet to rats caused an increase in the microsomal metabolism of a variety of substrates, including aminopyrine, ethyl-

fllrrAPv

crrrrc

rmi

vrMr,o,cyr,r

.ACTAPCI

ICkA

morphine, hexobarbital, heptachlor, BP, NDMA, and aniline (8, 26, 27). Generally, fat levels are important in affecting P450 levels, and those rich in polyunsaturated fatty acids are more effective than those rich in saturated and monounsaturated fatty acids. Dietary lipids are also essential in producing an optimal induction of P450 enzymes by inducers such as phenobarbital, and the effect is observable at the mRNA level (27). Although most studies suggested that the factors responsible for the increased drug metabolism were lipids, the lipid-to-carbohydrate ratio may be important. Other components or contaminants in oils such as vitamin E, cholesterol, and lipid peroxides may have also been contributing factors in certain experiments (26). Recent results from our laboratory indicate that, in comparison to a fat-free diet, a 20% corn oil diet produced a twofold higher constitutive level of P450 2E1 but did not affect the maximal acetone-inducible level of this enzyme (27). When diets containing different amounts of corn oil were fed to rats, those on the higher fat diet had higher P450 2E1 levels and higher blood acetone levels (27), consistent with the hypothesis that ketone bodies and ketosis are key factors for the regulation of P450 2E1 (28). Menhaden oil and corn oil were more effective than olive oil and lard in maintaining high levels of P450 2E1, suggesting that factors other than ketosis were involved. The total concentration of microsomal P450 was higher in rats on a diet with higher lipid content; specifically, P450s 3A and 2A1 were higher, whereas 2B1 and 2C11 were not affected (29). Carbohydrates are usually used as variable caloric sources for isocaloric diets in studying the effects of dietary proteins or lipids on xenobiotic metabolism. The higher P450 levels observed with higher protein or lipid diets have usually been attributed to the protein or lipids. However, metabolic glucose deprivation, seen in cases of fasting and diabetes, caused induction of P450 2E1, possibly due to the ketotic conditions produced. Sucrose, glucose, or fructose, when given in drinking water to mice maintained on a rodent chow, were reported to decrease drug metabolism rates in vivo and in vitro (30). The high sugar intake from the drinking water may have reduced the dietary intake of protein (or other nutrients) that affected monooxygenase activities. Alternatively, a high dietary or blood glucose level may inhibit the synthesis of P450s due to inhibition of y-aminolevulinic acid synthesis (31).

Fasting

and

dietary

restriction

During fasting, microsomal aminopyrine N-demethylase and hexobarbital hydroxylase activities were decreased, but aniline p-hydroxylase and p-nitroanisole O-demethylase activities were increased in male rats (32). The induction of P450 2E1 by fasting (17) can account for the increased aniline hydroxylase and NDMA demethylase activities. Fasting for 2 or 3 days caused a 50% decrease in the level of the male-specific P450 2C11 (33), and this may account for the previously observed decrease in aminopyrine demethylase activity. During fasting, the mRNA for P450 2E1 was significantly elevated, a situation similar to diabetes (34), but such elevation was not observed during the induction of P450 2E1 by acetone pretreatment (19). The results Suggest that there are differences in the mechanisms of P450 2E1 induction under these conditions. In nutritional studies with experimental animals, pair feeding is frequently used to allow the animals in the control group eating a quantity of diet equivalent to that consumed by animals in the experimental group. If the pair-fed animals consume this limited amount of food in 2 h, usually early in

739

ded from www.fasebj.org by Edinburgh University (129.215.17.190) on November 23, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()}, pp

the

morning,

this

will

which increases in P450 served (35). The practice periment may also cause

create 2E1

a fasting and its

of overnight an increase

situation activities

of 22 h in can be ob-

fasting before an exin the P450 2E1 level.

chrome

c reductase

microsomal ethoxycoumarin hydroxylase

sumption ALCOHOL CONSUMPTION METABOLISM

gus

ON

postulated (36). been demonstrated enzyme catalyzes

drug

of P450 2E1 has subsequently and humans (38, 39). This of small molecules such as

ethanol, other alcohols, NDMA, acetone, aniline, enflurane, ether, acetaminophen, benzene, chloroform, carbon tetrachloride, dihaloethanes, and other small halogenated hydrocarbons (Fig. 3) (40, 41). It is understandable, therefore, that chronic consumption of ethanol will increase the rate of the metabolism and the toxicity of the aforementioned compounds. On the other hand, the acute effect of alcohol consumption is an inhibition of P450 2E1 function due to the competitive inhibition by ethanol. As reviewed by Lieber (36), chronic administration of ethanol increases the rate of the metabolic clearance of a variety of aminopyrine,

drugs such tolbutamide,

as

meprobamate, propranolol,

pentobarbital, rifampicin,

and

testosterone. As a result of acute ethanol consumption, however, impairment of the metabolism of drugs has been observed with meprobamate, benzodiazepines, phenothiazine, barbiturate, morphine, methadone, warfarin, xylene, and other compounds. The induction and inhibition of P450 2E1 may account for only a small portion of these inhibitory actions. The induction of other P450s such as P450 2B1 and possible inhibition of the metabolism of these drugs by ethanol remain to be studied. Chronic ethanol administration to rats caused a proliferation of the endoplasmic reticulum of the upper small intestine along with increases in P450 content and NADPH:cyto-

The

extent

The

Substrates Acetone,Alcohols Aniline, Pyridine Benzene, Phenol, Styrene

Alkanes CHCI3, Cd4 Vinyl chloride Dihaloethanes Trichloroethylene N-Nitrosodimethylamine

N-Nitrosodiethylamine Azoxymethane t-Butylhydroperoxide Ethers Enflurane, Halothane Chlorzoxazone Acetaminophen Figure 3. Dietary effectorsand substrates for P450 2E1.

Ianuarv

1992

Th

cAcIR

7-

and

aniline

ethanol concontent and the

P450

of N-nitrosopyrrolidine these

intestinal

demethylase,

in

effects

are

due

rat

esophato the

in-

to be determined. lung has also been

(19, 43).

EFFECTS OF MICRONUTRIENTS METABOLISM effects

of vitamins,

metabolism 9). Seemingly

ON

especially

vitamin

XENOBIOTIC

deficiencies,

have been investigated extensively conflicting results in older literature

on

(2,

5, 8,

may be more understandable with some new insights on this topic. 1) Whereas a general effect of all severe vitamin deficiencies is the decreased metabolic functions and lowered levels of P450-dependent metabolic activities, mild deficiency in a certain nutrient may enhance P450-dependent activities. Therefore, depending on the severity of the deficiency, opposite effects on xenobiotic metabolism may be observed. 2) Deficiency in a single nutrient may produce varied effects on the metabolism of different xenobiotics due to the different effects on specific P450 isozymes. These points are illustrated

in the

During a gradual possibly

cases

of riboflavin

and

thiamin

deficiencies.

the development of riboflavin deficiency, decrease in the activity of NADPH:P450 due

to the

lowered

levels

of cellular

FAD

there was reductase, and

FMN.

On the other hand, during early stages of riboflavin deficiency, the levels of some P450s may be increased, possibly to compensate for the decreased NADPH:P450 reductase levels; some of the monooxygenase activities such as NDMA demethylase, aniline hydroxylase, and aminopyrine demethylase activities were elevated. When the deficiency was prolonged, the level of P450 enzymes decreased, resulting in depressed levels of all P450-dependent activities using substrateS such as BP, aminopyrine, ethylmorphine, N-methylaniline, aniline, and acetanilide. Therefore, mild deficiency and severe deficiency had opposite effects on the metabolism of certain drugs and carcinogens (8).

reductase

Vol. 6

to increase

to which

Rats fed a thiamin-deficient tions of P450, cytochrome

740

increased

duction of P450 2E1 or its activity remains The induction of P450 2E1 in kidney and

observed

induction in animals the metabolism

In addition, benzphetamine

deethylase, BP hydroxylase, have been observed (36). Chronic activation

(42).

Consumption of alcoholic beverages is widespread and can account for a significant portion of caloric intake of certain individuals. Ethanol can affect the absorption, plasma protein binding, blood flow, and distribution of xenobiotics as well as have profound influence on phase I and phase II metabolism of xenobiotics (36, 37). The induction of P450 enzymes by ethanol has long been The

of

was also shown

metabolic

EFFECT OF XENOBIOTIC

activity.

activities

activity

in liver

diet

b5, and microsomes

had

higher

concentra-

NADPH:cytochrome than

those

fed

c a diet

sufficient in thiamin. The deficient rats also had increased rates in the metabolism of acetaminophen, NDMA, aminopyrine, ethylmorphine, zoxazolamine, heptachlor, aniline, N-methylaniline, acetanilide, and BP, but not in the metabolism of hexobarbital (8). Recent studies from our laboratory indicated that thiamin deficiency increased the hepatic microsomal P450 2E1 level (two- to fivefold) but not the P450 2Cll level (44). This observation provides an enzymatic basis for the enhanced rate of in vivo metabolism of aniline, NDMA, and acetaminophen, all of which are substrates for P450 2El. Thiamin deficiency was shown to increase cytosolic glutathione S-transferase activity moderately but not steroid isomerase activity (44). The mechanisms of these effects on drugmetabolizing enzymes remain to be elucidated. Mechanistic information on the effects of other micronutrients on xenobiotic metabolism is lacking. These effects, together with the aforementioned effects on xenobiotic metabolism by riboflavin and thiamin nutrition, are summarized in Table 1.

In,

,n.I

VAklt

CT

Al

ded from www.fasebj.org by Edinburgh University (129.215.17.190) on November 23, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()}, pp

TABLE

1. Effects of micronutrients

on xenobiotic

metabolism Xcnobiotic

Nutritional status

Vitamin

A deficiency

Vitamin

A-high

of aminopyrine,

I Metabolism

dose

of aniline,

Metabolism

Niacin deficiency

ethylmorphine,

aniline, BP, 7-ethoxycoumarin

7-ethoxycoumarin

of anesthetics

deficiency Reductase

Mild

I Metabolism

Severe

of NDMA,

Metabolism

of BP,

deficiency

Vitamin

C deficiency

P450, reductase Monooxygenase

activities

Vitamin

C

Monooxygenase

activities

Vitamin

E deficiency

Metabolism

Folic acid deficiency Aluminum Cadmium,

cobalt,

heavy

metals-high

Calcium

deficiency

Magnesium

Induction

of codeine,

ethylmorphine,

aniline, zoxazolamine,

acetanilide BP

BP

of P450 2B1 by barbiturates

1 P450 and related

dose

I Monooxygenase

ethylmorphine

activities activities

I Monooxygenase activities I Metabolism of aniline, hexobarbital

deficiency

I Metabolism

of BP

Iron deficiency

I Metabolism I Metabolism

of hexobarbital, of aniline

Iron-large

dose

I NADPH-dependent

Selenium

deficiency

I Induction

Zinc deficiency

DIETARY TOXICITY

N-methylaniline,

and other

deficiency

Abbreviations found in reviews

b5 aminopyrine,

ethylmorphine,

I Hepatic P450 I Metabolism of p-nitrophenole,

high dose

-

aminopyrine ethylmorphine,

1 P450 2E1, reductase, cytochrome I NDMA, acetaminophen, aniline,

high dose

-

aniline,

aminopyrine,

Thiamin

Copper

and enzymes

P450 Metabolism

Riboflavin

metabolism

I Metabolism

of pentobarbital,

I, decrease;

I, increase;

P450

2E1 AND

CHEMICAL

ON

lipid peroxidation

of P450 by phenobarbital

and signs used: (2, 3, 5, 8, 9).

EFFECTS

reductase,

In this section, P450 2E1 is used as an example to illustrate the dietary modulation of P450 enzymes and the effect of this modulation on toxicity and carcinogenicity of chemicals. As illustrated in Fig. 3, diet may provide inducers, suppressors, inhibitors, and substrates for P450 2El. In addition to affecting the hepatic microsomal P450 2E1 level measured in vitro, a low fat/high carbohydrate diet also resulted in a lower rate of enflurane metabolism in rats than a high fat/low carbohydrate diet (29). Based on these results, it may be suggested that individuals on a low fat diet may have, on average, lower P450 2E1 levels than those on a high fat diet. Frequent consumption of ethanol is known to elevate the level of P450 2E1 in humans, and is believed to be a key mechanism for the enhanced toxicity of acetaminophen, CCI,, and other chemicals. Fasting can be an important factor in raising P450 2E1 levels in individuals undergoing weight reduction by severe fasting or a low carbohydrate diet. The induction of P450 2E1 by fasting probably also contributes to the enhanced toxicity of acetaminophen, although other factors such as decreased glutathione levels are also important. The activity of P450 2E1 is also modulated by dietary inhibitors and suppressors of this enzyme. The consequence of this inhibition and suppression of P450 2E1 is the inhibition of metabolism and toxicity of certain xenobiotics. The inhi-

DIETARY EFFECTS ON XENORIOTIC

MFTAROI

1cM

aminopyrine

aminopyrine NADPH:

P450 reductase

activity.

Original

references

can be

bition of enflurane metabolism by diallyl sulfide and phenethyl isothiocyanate, which are competitive and suicide inhibitors of P450 2E1, has been demonstrated in rats (unpublished results). The inhibition of carcinogen metabolism in the liver can increase the carcinogen exposure to nonhepatic organs and thus may enhance nonhepatic carcinogenesis. This effect has been demonstrated with ethanol, which inhibited hepatocarcinogenesis of NDMA but enhanced tumorigenesis in the nasal cavity (45). The dual effects of ethanol, i.e., an acute effect of inhibition and a chronic effect of enhancement of NDMA-induced carcinogenesis, have also been observed (46). Knowing that many dietary factors can modulate P450 2E1, it may be questioned whether a higher or lower level of this enzyme is more beneficial to health. A possible physiological function of P450 2E1 is in the initial step for the conversion of acetone to glucose (47), but the rate of this metabolic pathway is rather low and its physiological importance remains to be established. Lowering P450 2E1 levels may thus decrease the susceptibility to many toxic chemicals, provided the parent compounds are not toxic and can be disposed of by other metabolic pathways or by exhalation. One exception is the dihaloethanes, which are activated by a glutathionedependent pathway and detoxified by P450 2E1-dependent metabolism (41). A second aspect of this question is that P450 2E1 is known to be effective in causing lipid peroxidation (48). It remains to be determined whether higher levels

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of P450 2El can contribute to oxidative stress in vivo. The application of dietary-derived P450 2E1 inhibitors and suppressors for the prevention of acetaminophen toxicity is being explored in our laboratory. Of particular importance are applications with alcoholics or patients taking isoniazid whose P450 2El levels are known to be elevated.

DIETARY

EFFECTS

TOXICITY,

AND

ON

DRUG

METABOLISM,

CARCINOGENESIS

Food and dietary components may affect the fate of a drug or toxicant by the following mechanisms: 1) altering the rates of its absorption and uptake, 2) reacting or tightly binding with the drug, 3) competing with the drug for binding to plasma proteins, and 4) affecting phase I and phase II metabolism. Interfering with P450-dependent metabolism appears to be the most selective mechanism by which dietary components exert their effects on drug metabolism and carcinogenesis. For example, the observation that the drinking of 200 ml of grapefruit juice markedly inhibited the oxidation of nifedipine and felodipine in human volunteers (49) may be interpreted on the basis that grapefruit is rich in quercetin and naringenin, which are effective inhibitors of P450 3A4, the isozyme responsible for the metabolism of these dihydropyridine drugs (24). P450 3A4 is also important in the activation of aflatoxin B, in humans (50). It may be speculated that grapefruit consumption would inhibit the bioactivation of aflatoxin B, and could inhibit hepatocarcinogenesis in populations exposed to rather high concentrations of this carcinogen. Indoles and isothiocyanates are two major classes of compounds that occur as glucosinolates in cruciferous vegetables. The actions of indole-3-carbinol and phenethyl isothiocyanate may account for some of the reported inhibitory actions

of cruciferous

vegetables

against

chemically

in the

trout,

the

inhibitory

action

of the

acid

reaction products of indole-3-carbinol (RXM) on the metabolic activation of aflatoxin B, may be a major mechanism for the inhibition of carcinogenesis by indole-3-carbinol (52). On the other hand, when indole-3-carbinol was given after the aflatoxin B, treatment period, it enhanced carcinogenesis in the trout (53); the mechanisms are not known. It has also been demonstrated that indole-3-carbinol, when incorporated into the diet, increased the estradiol 2-hydroxylase activity in rats and humans (54), possibly due to the induction of P450s lAl and 1A2. Because 2-hydroxylation converts estradiol to nonuterotropic and antiestrogenic metabolites, the increase in this metabolic activity was suggested to reduce the incidence of estrogen-related cancers. Phenethyl isothiocyanate was a very potent inhibitor of NNK-induced lung tumorigenesis (55) and N-nitrosomethylbenzylamine-induced esophageal carcinogenesis (56). The action can be attributed largely to its inhibition of carcinogen activation (23). Probably via a similar mechanism, diallyl sulfide was also an effective inhibitor in both carcinogenesis models (unpublished results; ref 57). Diallyl sulfide and related organosulfur compounds may be partially responsible for the negative association between the consumption of allium vegetables and incidence of gastric cancer (58).

742

Vol. 6

January

1992

genesis

assays

in vitro,

may

be

of particular

importance.

It

is also important to consider the doses of these chemicals required to produce the possible harmful effects. Depending on the dose, a compound can have either beneficial or harmful effects, and sometimes the effects depend on the experimental model used. This point can be illustrated with the commonly used food additives, butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT). These antioxidants

may

be present

at a total

concentration

of up to 0.02%

in certain food items. When added to the diet at a concentration of 0.5%, BHA and BHT inhibited carcinogenesis in several animal models (59). Induction of phase II enzymes and inhibition of carcinogen activation have been proposed as the mechanisms of inhibition (8, 59). However, with 1 or 2% of BHT or BHA in the diet, respectively, tumor promotion activity has been demonstrated in a two-stage urinary bladder carcinogenesis model (60).

induced

carcinogenesis in animals and for the association between the frequent consumption of these vegetables with lower cancer incidences at different organ sites (51). The induction of P450 1A1 in the intestine may help to metabolize and eliminate dietary polyaromatic hydrocarbons in the intestine, and thus may reduce the exposure of internal organs to such carcinogens. In studies of aflatoxin B1-induced hepatocarcinogenesis

The amount of diallyl sulfide derived from garlic is only in quantities of 3-100 ftg/g. The estimated human intake of glucosinolates through consumption of cooked vegetables is about 30 mg per day. It may be questioned whether such small quantities of dietary inhibitors can have a significant effect in inhibiting carcinogenesis. Two aspects may be pertinent to this question: 1) Many dietary compounds may be competitive inhibitors of P450 enzymes; even when present at low concentrations, they could effectively inhibit the metabolism of low concentrations of carcinogens. 2) Many dietary chemicals can inhibit carcinogen activation. Although most of them are present only in small concentrations, in combination their actions can be significant. The human diet is also known to contain mutagens, carcinogens, and tumor promoters. The effects of these compounds on health have to be considered in light of the capability of the body to detoxify these dietary chemicals. Phase II metabolism, which is usually not considered in most muta-

CONCLUDING Studies

of the multiplicity

isozymes

effects

REMARKS

have

and

contributed

animals

the

level

to humans.

metabolism.

specificity

understanding

of certain

With

A dietary

of P450 of dietary

may inlevel of others; thus the rates of the metabolism of certain drugs may be enhanced and those of others lowered. The distinction between an inhibitory effect after an acute dose and an induction effect after a treatment also helps to explain the divergent effects of dietary chemicals on drug metabolism. In addition, a nutritional deficiency may have different effects on the metabolism of a certain drug; the rate may be enhanced in mild deficiency but decreased in severe deficiency. Most of the studies reviewed herein were carried out using liver microsomes from rats and mice. These results can provide us with some basic understanding of the mechanisms by which a dietary factor may affect drug metabolism. Caution must be applied when extrapolating the information obtained from hepatic tissues to nonhepatic tissues and from crease

on xenobiotic

substrate

to our P450s

the

and

understanding

chemical

decrease

the

of human

xeno-

biotic metabolism as a goal, researchers are faced with the following challenges: 1) to further elucidate the detailed mechanisms by which diet affects xenobiotic-metabolizing enzymes, 2) to understand the basis for the tissue and species specificities of xenobiotic-metabolizing enzymes, 3) to further characterize the catalytic properties of human xenobioticmetabolizing enzymes, and 4) to pursue well-planned human studies concerning the nutritional impact on drug metabolism and toxicity.

The FASEB lournal

YANC

FT Al

ded from www.fasebj.org by Edinburgh University (129.215.17.190) on November 23, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()}, pp

This work was supported by National Institutes of Health grants ES03938, CA46535, and CA37037, a grant from the American Institute for Cancer Research, and NIEHS Center grant ES05022. The authors wish to thank Ms. Dorothy Wong for her excellent secretarial pare

the

assistance

and

Ms.

Marie

Leithauser

for helping

to pre-

manuscript.

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