Vol. 182, No. 2, 1992 January 31, 1992
BIOCHEMICAL
P?ZRWATE/NALATE
Anna AtlanteO, OCentro
ANTIPORTER
Salvatore
Studio
RESEARCH COMMUNICATIONS Pages 931-938
IN RAT LIVER
Passarella*
HITOCRONDRIA
and Ernest0
Quagliariello+
Mitocondri e Metabolism0 Energetic0 - CNR, Bari, Italy *Dipartimento di Scienze Animali, Vegetali e dell'Ambiente, Universith de1 Molise, Campobasso, Italy +Dipartimento di Biochimica e Biologia Molecolare, Universita di Bari, Bari, Italy Recei.ved
di
AND BIOPHYSICAL
December
20,
sui
1991
To gain some insight into the process by which both acetylCoA and NADPH, needed for fatty acid synthesis, are obtained, in the cytosol, from the effluxed intramitochondrial via citrate lyase and malate dehydrogenase plus malic citrate, enzyme respectively, the capability of externally added pyruvate to cause efflux of malate from rat liver mitochondria was tested. The occurrence of a pyruvate/malate translocator is here shown: pyruvate/malate exchange shows saturation features (Km and Vmax values, measured at 20°C and at pH 7.20, were found to be about 0.25 mM and 2.7 nmoles/min x mg mitochondrial protein, respectively) and is inhibited by certain impermeable compounds. This carrier, together the previously with reported tricarboxylate and oxodicarboxylate translocators proved to allow for citrate and oxaloacetate efflux due to externally added pyruvate. Q 1992 Academic Press, Inc. Summary.
The
pathway
cytoso1 pyruvate de
novo
matrix
citrate
most
described
to
account
for
the
translocation
of acetyl groups generated during the oxidation (taken up by mitochondria via its own carrier) for
to of the
synthesis of fatty acids, starts in the mitochondrial with citrate synthesis from acetylCoA and OAA. The is then transported into cytosol where acetylCoA and OAA
Abbreviations: ARS, arsenite; BTA, benzentricarboxylate; CCN, (Y -cyanocinnamate; CITR, citrate; CoA, coenzyme A; LDH, lactate dehydrogenase; MAL, malate; MDS, malate detecting system; OAA, oxaloacetate; PDH, pyruvate dehydrogenase; PYR, pyruvate; RIM, tricarboxylate carrier; TxlOO, rat liver mitochondria; TRIC, Triton-x-100. 0006-291x/92
931
$1.50
Copyright 0 1992 by Academic Press. Inc. All rights of reproduction in any form reserved.
Vol.
182,
are
No.
2,
BIOCHEMICAL
1992
regenerated
via
a mechanism, cycle.
ATP-citrate
OAA must
This
could
AND BIOPHYSICAL
liase
be taken
RESEARCH COMMUNICATIONS
(1).
To substantiate
up by mitochondria
be accomplished
via
a cytosolic
to
such
respect
the
OAA reduction
to malate and consequent malate/citrate antiport mediated by the tricarboxylate carrier (2); in the matrix malate could give further OAA via MDH. However, in this case no malate would be available in the cytosol where malic enzyme is located, the activity necessary
of
phosphate
which is for fatty pathway
responsible
for
strictly acid
has
required synthesis.
been
to In
reported
to
(3).
Thus
NADPH production
as to the metabolic fate of the extramitochondrial phase from citrate malate dehydrogenase.
malate
via
produce the NADPH fact the pentose be only partially the question arises generated in the citrate liase and
To solve this puzzle, i.e. the impossibility of cytosolic malate being both the counteranion for exported citrate and the reducing equivalent donor via the malic enzyme, a catalytic role for malate in exporting citrate in exchange with incoming pyruvate Citrate
could efflux
result (2)
which
of and in
the of
be proposed. caused by externally combined work the pyruvate/malate
RLM is
here
shown
of
(see
OUT
added
pyruvate
the citrate/malate translocator the Scheme 1).
M.I.M.
IN
GLucYsE
FATTY A,c” SYNTHESIS 1 CoA
SCHEME
932
1
is
shown
translocator existence
as a of
Vol.
182, No. 2, 1992
BIOCHEMICAL
AND BIOPHYSICAL
RESEARCH COMMUNICATIONS
MATERIALS AND METHODS. All reagents used were from SIGMA (St. Louis, U.S.A.). RLM were isolated according to (3) with mitochondrial protein determined as in (4). Malate and OAA effluxes from mitochondria caused by externally added anions were monitored as previously reported (5,6) by adding 0.2 e.u. malic enzyme plus 0.25 mM NADP+ (in the case of malpte) , and 2 e.u. MDH plus 0.2 mM NADH (in the case of OAA) outside the mitochondria and following NADP+ reduction and NADH oxidation, respectively, as a function of time. Citrate efflux caused by externally added anions was monitored by photometrically following NADH (0.2 mM) oxidation in the presence of RLM added with 2 e.u. MDH, 1 mM coenzyme A and 2.5 e.u. citrate lyase. In particular, the rate of citrate appearance was measured by automatically substracting the rate of NADH oxidation measured in the absence of citrate lyase, due to the reduction of effluxed oxaloacetate (reference cuvette), from the rate of NADH oxidation measured in the presence of the complete citrate detecting system. Changes in the redox state of NAD(P) were followed photometrically using a Perkin-Elmer Lambda 5 spectrophotometer. The rates of change of absorbance were obtained as tangent to the initial part of the experimental curve and expressed as nmoles NADP+ reduced (or NADH oxidized)/min x mg mitochondrial protein. added The capability of externally AND DISCUSSION. to cause efflux of intramitochondrial malate from RLM in Fig. 1. The extramitochondrial concentration of is shown in fact no change in the absorbance due to malate is negligible, of malic enzyme and NADP+. NADPH is observed in the presence a fast However following the addition of 125 YM pyruvate,
RESULTS pyruvate
increase malate efflux
in in was
absorbance is observed which shows the appearance of the extramitochondrial phase. The rate of malate found to depend on the rate of transport across the
mitochondrial membrane as revealed by a Triton experiment (5) Externally added 2 mM sodium arsenite, (see the inset). was found consistently to inhibitor of pyruvate dehydrogenase, 0.1 mM a-cyanocinnamate prevent malate efflux. Interestingly, was found to have no effect on the exchange reaction. This excludes via
the
pyruvate
possibility carrier
that
pyruvatefmalate
exchange
occurs
(7).
A possible pyruvate
explanation for this finding is shown in the scheme: enters mitochondria in exchange with endogenous malate inside the a putative pyruvate/malate antiporter; once via of pyruvate metabolism via pyruvate matrix, as a result carboxylase (E.C. dehydrogenase (E-C. 1.2.7.1), pyruvate malate is 6.4.1.1.) and malate dehydrogenase (E.C. 1.1.1.37), formed
which
in
turn
exchanges
for 933
further
pyruvate.
Vol.
182,
No.
2,
BIOCHEMICAL
1992
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
t
MDS I PYR ACETYL CoA OHLw6 PYR(-OAA MAL/ FIG. 1. APPEARANCE OF NALATE IN THE INCUBATION MEDIUM INDUCED BY THE ADDITION OF PYRWATE TO RAT LIVER MITOCBONDRIA. RLM (1.5 mg protein) were incubated at 20°C in 2.0 ml of standard medium, consisting of 0.2 M sucrose, 10 mM KCl, 1 mM MgCl2, 20 mM Hepes-Tris pH 7.2, added with 2 pg rotenone in the presence of 1 lus 1 mM ATP (in A). Where indicated additions were as F!x:~~3 p : malate detecting system (MDS), 125 PM pyruvate (PYR), 2 mM sodium arsenite (CCN), 0.5% tARSI t 100 FM c-cyanocinnamate Triton-X-100 (TXlOO). NADP+ reduction change was followed photometrically and the rate measured as the tangent to the initial part of the progress curve expressed in nmoles NADP' reduced/min x mg mitochondrial protein. The increase in the rate of NADP+ reduction, after Triton-X-100 addition to mitochondria, shows that the transport step is the rate limiting step of the measured process.
Nonetheless,
as
RLM contain
both
pyruvate/OAA
and
malate/OAA
antiporters (6), the possibility was considered that malate efflux due to externally added pyruvate could also occur as a result of the pyruvate metabolism and the activities of these translocators (see scheme Fig. 2). Thus, to gain further insight into the mechanism of malate efflux, comparison was made in the same experiment of the pyruvate/malate, pyruvate/OAA and OAA/malate exchange kinetics (Fig. 2). The double reciprocal plots showed the occurrence of saturation characteristics with different Vmax and Km values. Vmax values, which reflect the 934
Vol.
182, No. 2, 1992
BIOCHEMICAL
l/[SUBSTRATE]
FIG. 2. KINETIC ANALYSIS AND PYRWATEfOXALOACETATE PLOT.
AND BIOPHYSICAL
RESEARCH COMMUNICATIONS
ImId’)
OF PYRWATEIMALATE, EXCHANGES
USING
THE
OXALOACETATE/MALATE DOUBLE RECIPROCAL
RLM (1.5 mg protein) were incubated at 20°C in 2.0 ml of standard medium added with 2 pg rotenone in the presence of either the malate detecting system (in the case of both the pyruvatejmalate (A) and oxaloacetate/malate ( X ) exchanges) or the oxaloacetate detecting system (in the case of the pyruvatefoxaloacetate (e) exchange) (for details see the Methods). The exchange reactions were started by adding either oxaloacetate or pyruvate at the indicated concentrations. The rate V is measured as described in the Methods is expressed as NADP' reduced (or NADH oxidized) nmoles/min x mg mitochondrial protein.
rate
of
transport
across
the
mitochondrial
membrane
(see
Fig.
1
and ref.6), were 2.6, 6.0 and 12.6 nmolesjmin x mg protein and Km values, i.e. the substrate concentration which gives half maximum rate of uptake, were equal to 0.25, 0.12 and 0.17 mM in the reported pyruvate/OAA
experiment for pyruvate/malate, exchanges respectively.
The reported findings proposed pyruvate/malate Consistently, 14C-malate
in loaded
strongly antiporter.
suggest
another experiment mitochondria, the
OAA/malate the
in which capability
existence use
and of
the
was made of externally
added pyruvate to cause efflux of malate from the mitochondrial matrix was confirmed (not shown). In agreement with the existence of a pyruvate/malate translocator, externally added malate proved to cause efflux 935
of
Vol.
182,
No.
2, 1992
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
PYAKITR
MALKITR
I
0
5
10
15
l/[SUBSTRATE]
FIG. 3. NALATEICITRATE
Pyruvate
(
KINETIC ANALYSIS OF THE PYRWATE/CITRATE EXCHANGES USING THE DOUBLE RECIPROCAL PLOT. l
concentrations. The experiment The rate V mitochondrial Mitochondrial
from
mitochondria
following
the
(E.C. 1.1.1.27) concentration
20
(mM-‘)
)
or
( 0 )
malate
were
added
at
the
AND
indicated
was carried out as described in the Methods. is expressed as nmoles NADP+ reduced/min protein. protein was 1.5 mg.
of decrease
pyruvate, of
measured
NADH absorbance
(not shown). was increased
In by
this case incubating
essentially in
the
x
as
in
presence
mitochondrial RLM with
mg
(6), of
LDH
pyruvate alanine and
oxoglutarate whose uptake into mitochondria (8,9) provides the substrate pair for intramitochondrial glutamate-pyruvate transaminase (E.C. 2.6.1.2.). Arsenite and a-cyanocinnamate were also present to inhibit PDH and pyruvate transport via its own carrier, respectively. Pyruvate/malate found to be
exchange (measured by using 0.5 mM pyruvate) inhibited by phenylsuccinate and benzylmalonate
was (5
mM each) (85 and 50% inhibition respectively). However, use of dicarboxylate carrier in pyruvate/malate exchange was excluded since no inhibition by pyruvate of both the oxidation of succinate (10) and the uptake of 14C-dicarboxylates occur (11). In the light of the existence of a specific pyruvate/malate translocator, a catalytic role could be proposed for malate movement in the efflux of citrate caused by uptake and 936
Vol.
182,
No.
2,
metabolism
1992
of
malate
BIOCHEMICAL
pyruvate
effluxed
from
could exchange a pyruvatelcitrate capability of synthesized In the pyruvate
in
added the rate
citrate
due
membrane made and
kinetics
is
the
found.
and
that efflux
neither
externally
rate
pyruvate
malatejcitrate
14C-citrate added
However due to
from
(not
shown),
pyruvate
the
in
no difference in x mg mitochondrial proposal (see scheme added pyruvate tricarboxylate
the
(Fig. 3). and 0.07 mM
respectively,
efflux
of
between
exchanges were 0.3
an the
clearly (not across
step
experiment
Km values
externally via the
for the newly
added decrease
liase, which mitochondria the transport limiting
in the same malatelcitrate
were
be noted citrate
(see also 6). is observed
ATP-dependent citrate of citrate outside was used to show that
pyruvatelcitrate
to
with
1):
was tested.
NADH absorbance of NADH oxidation
CoA and the appearance Triton-X-100
Consistently
exchange
Scheme
malate dehydrogenase, externally of OAA from RLM, as shown by the
of
should cause
(see
intramitochondrial citrate thus allowing exchange. To substantiate this conclusion externally added pyruvate to cause efflux of
experiment, whereas, as expected, values was found (1.6 nmoles/min thus confirming the above reported It to
COMMUNICATIONS
with
the mitochondrial process. Comparison was pyruvatelcitrate for
RESEARCH
mitochondria
mitochondria
presence of causes efflux
Saturation
BIOPHYSICAL
energized
intramitochondrial
in externally increase in addition shows shown).
in
AND
the Vmax protein), Fig. 3).
proved to carrier
citrate nor
this
loaded inhibition
fail (2) *
RLM of
pyruvate/malate exchange in the presence of BTA were found. The reported experimental findings show that pyruvate and citrate do not share the same carrier and that citrate efflux due to pyruvate depends on the combined pyruvate/malate antiporter and the tricarboxylate This paper shows the existence in RLM of antiporter, i.e. the pyruvate/malate
the
synthesis remains to this carrier answers further significant
confirming role
in
pyruvate/malate translocator
in
be established, the question that mitochondrial several metabolic 937
work a
of carrier. so far
translocator. the in vivo however raised in
the (3)
the
new
unknown
The role of fatty acid presence as well
transport can pathways located
play both
of as a in
Vol.
182,
No.
2, 1992
cytosol
and
research
in
BIOCHEMICAL
in this
the
AND
organelles,
BIOPHYSICAL
RESEARCH
thus
necessitating
COMMUNICATIONS
further
field.
REFERENCES. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Spencer, A.F., and Lowensyein J.M. (1962) J. Biol. Chem. 237, 3640-3648. Palmieri, F., Stipani, I., and Quagliariello E. (1972) Eur. J. Biochem. 26, 587-594. Klingenberg, M., and Slenczka, N. (1959) Biochem. 2. 331, 486-517. Waddel, W.J., and Hill, C. (1956) J. Lab. Clin. Med. 48, 311-314. Atlante, A., Passarella, S., Giannattasio, S., and Quagliariello, E. (1985) Biochem. Biophys. Res. Common. 132, 8-18. Passarella, S., Atlante, A., and Quagliariello, E. (1985) Biochem. Biophys. Res. Commun. 129, l-10. Halestrap, A.P., Scott, R.D., and Thomas A.P. (1980) Int. J. 11, 97-105. Biochem. Passarella, S., and Quagliariello, E. (1982) Abstr. lo Conv. Naz. Biol. Cell. p. 69. Brosnan, J.T., Redmond, W., Morgan, D., and Whalen, P. (1980) Int. J. Biochem. 12, 131-133. Quagliariello, E., and Palmieri, F. (1968) Eur. J. Biochem. 4, 20-27. Palmieri, F., Prezioso, G., Quagliariello, E., and Klingenberg, M. (1971) Eur. J. Biochem. 22, 66-74.
938
BIOCHEMICAL
P?ZRWATE/NALATE
Anna AtlanteO, OCentro
ANTIPORTER
Salvatore
Studio
RESEARCH COMMUNICATIONS Pages 931-938
IN RAT LIVER
Passarella*
HITOCRONDRIA
and Ernest0
Quagliariello+
Mitocondri e Metabolism0 Energetic0 - CNR, Bari, Italy *Dipartimento di Scienze Animali, Vegetali e dell'Ambiente, Universith de1 Molise, Campobasso, Italy +Dipartimento di Biochimica e Biologia Molecolare, Universita di Bari, Bari, Italy Recei.ved
di
AND BIOPHYSICAL
December
20,
sui
1991
To gain some insight into the process by which both acetylCoA and NADPH, needed for fatty acid synthesis, are obtained, in the cytosol, from the effluxed intramitochondrial via citrate lyase and malate dehydrogenase plus malic citrate, enzyme respectively, the capability of externally added pyruvate to cause efflux of malate from rat liver mitochondria was tested. The occurrence of a pyruvate/malate translocator is here shown: pyruvate/malate exchange shows saturation features (Km and Vmax values, measured at 20°C and at pH 7.20, were found to be about 0.25 mM and 2.7 nmoles/min x mg mitochondrial protein, respectively) and is inhibited by certain impermeable compounds. This carrier, together the previously with reported tricarboxylate and oxodicarboxylate translocators proved to allow for citrate and oxaloacetate efflux due to externally added pyruvate. Q 1992 Academic Press, Inc. Summary.
The
pathway
cytoso1 pyruvate de
novo
matrix
citrate
most
described
to
account
for
the
translocation
of acetyl groups generated during the oxidation (taken up by mitochondria via its own carrier) for
to of the
synthesis of fatty acids, starts in the mitochondrial with citrate synthesis from acetylCoA and OAA. The is then transported into cytosol where acetylCoA and OAA
Abbreviations: ARS, arsenite; BTA, benzentricarboxylate; CCN, (Y -cyanocinnamate; CITR, citrate; CoA, coenzyme A; LDH, lactate dehydrogenase; MAL, malate; MDS, malate detecting system; OAA, oxaloacetate; PDH, pyruvate dehydrogenase; PYR, pyruvate; RIM, tricarboxylate carrier; TxlOO, rat liver mitochondria; TRIC, Triton-x-100. 0006-291x/92
931
$1.50
Copyright 0 1992 by Academic Press. Inc. All rights of reproduction in any form reserved.
Vol.
182,
are
No.
2,
BIOCHEMICAL
1992
regenerated
via
a mechanism, cycle.
ATP-citrate
OAA must
This
could
AND BIOPHYSICAL
liase
be taken
RESEARCH COMMUNICATIONS
(1).
To substantiate
up by mitochondria
be accomplished
via
a cytosolic
to
such
respect
the
OAA reduction
to malate and consequent malate/citrate antiport mediated by the tricarboxylate carrier (2); in the matrix malate could give further OAA via MDH. However, in this case no malate would be available in the cytosol where malic enzyme is located, the activity necessary
of
phosphate
which is for fatty pathway
responsible
for
strictly acid
has
required synthesis.
been
to In
reported
to
(3).
Thus
NADPH production
as to the metabolic fate of the extramitochondrial phase from citrate malate dehydrogenase.
malate
via
produce the NADPH fact the pentose be only partially the question arises generated in the citrate liase and
To solve this puzzle, i.e. the impossibility of cytosolic malate being both the counteranion for exported citrate and the reducing equivalent donor via the malic enzyme, a catalytic role for malate in exporting citrate in exchange with incoming pyruvate Citrate
could efflux
result (2)
which
of and in
the of
be proposed. caused by externally combined work the pyruvate/malate
RLM is
here
shown
of
(see
OUT
added
pyruvate
the citrate/malate translocator the Scheme 1).
M.I.M.
IN
GLucYsE
FATTY A,c” SYNTHESIS 1 CoA
SCHEME
932
1
is
shown
translocator existence
as a of
Vol.
182, No. 2, 1992
BIOCHEMICAL
AND BIOPHYSICAL
RESEARCH COMMUNICATIONS
MATERIALS AND METHODS. All reagents used were from SIGMA (St. Louis, U.S.A.). RLM were isolated according to (3) with mitochondrial protein determined as in (4). Malate and OAA effluxes from mitochondria caused by externally added anions were monitored as previously reported (5,6) by adding 0.2 e.u. malic enzyme plus 0.25 mM NADP+ (in the case of malpte) , and 2 e.u. MDH plus 0.2 mM NADH (in the case of OAA) outside the mitochondria and following NADP+ reduction and NADH oxidation, respectively, as a function of time. Citrate efflux caused by externally added anions was monitored by photometrically following NADH (0.2 mM) oxidation in the presence of RLM added with 2 e.u. MDH, 1 mM coenzyme A and 2.5 e.u. citrate lyase. In particular, the rate of citrate appearance was measured by automatically substracting the rate of NADH oxidation measured in the absence of citrate lyase, due to the reduction of effluxed oxaloacetate (reference cuvette), from the rate of NADH oxidation measured in the presence of the complete citrate detecting system. Changes in the redox state of NAD(P) were followed photometrically using a Perkin-Elmer Lambda 5 spectrophotometer. The rates of change of absorbance were obtained as tangent to the initial part of the experimental curve and expressed as nmoles NADP+ reduced (or NADH oxidized)/min x mg mitochondrial protein. added The capability of externally AND DISCUSSION. to cause efflux of intramitochondrial malate from RLM in Fig. 1. The extramitochondrial concentration of is shown in fact no change in the absorbance due to malate is negligible, of malic enzyme and NADP+. NADPH is observed in the presence a fast However following the addition of 125 YM pyruvate,
RESULTS pyruvate
increase malate efflux
in in was
absorbance is observed which shows the appearance of the extramitochondrial phase. The rate of malate found to depend on the rate of transport across the
mitochondrial membrane as revealed by a Triton experiment (5) Externally added 2 mM sodium arsenite, (see the inset). was found consistently to inhibitor of pyruvate dehydrogenase, 0.1 mM a-cyanocinnamate prevent malate efflux. Interestingly, was found to have no effect on the exchange reaction. This excludes via
the
pyruvate
possibility carrier
that
pyruvatefmalate
exchange
occurs
(7).
A possible pyruvate
explanation for this finding is shown in the scheme: enters mitochondria in exchange with endogenous malate inside the a putative pyruvate/malate antiporter; once via of pyruvate metabolism via pyruvate matrix, as a result carboxylase (E.C. dehydrogenase (E-C. 1.2.7.1), pyruvate malate is 6.4.1.1.) and malate dehydrogenase (E.C. 1.1.1.37), formed
which
in
turn
exchanges
for 933
further
pyruvate.
Vol.
182,
No.
2,
BIOCHEMICAL
1992
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
t
MDS I PYR ACETYL CoA OHLw6 PYR(-OAA MAL/ FIG. 1. APPEARANCE OF NALATE IN THE INCUBATION MEDIUM INDUCED BY THE ADDITION OF PYRWATE TO RAT LIVER MITOCBONDRIA. RLM (1.5 mg protein) were incubated at 20°C in 2.0 ml of standard medium, consisting of 0.2 M sucrose, 10 mM KCl, 1 mM MgCl2, 20 mM Hepes-Tris pH 7.2, added with 2 pg rotenone in the presence of 1 lus 1 mM ATP (in A). Where indicated additions were as F!x:~~3 p : malate detecting system (MDS), 125 PM pyruvate (PYR), 2 mM sodium arsenite (CCN), 0.5% tARSI t 100 FM c-cyanocinnamate Triton-X-100 (TXlOO). NADP+ reduction change was followed photometrically and the rate measured as the tangent to the initial part of the progress curve expressed in nmoles NADP' reduced/min x mg mitochondrial protein. The increase in the rate of NADP+ reduction, after Triton-X-100 addition to mitochondria, shows that the transport step is the rate limiting step of the measured process.
Nonetheless,
as
RLM contain
both
pyruvate/OAA
and
malate/OAA
antiporters (6), the possibility was considered that malate efflux due to externally added pyruvate could also occur as a result of the pyruvate metabolism and the activities of these translocators (see scheme Fig. 2). Thus, to gain further insight into the mechanism of malate efflux, comparison was made in the same experiment of the pyruvate/malate, pyruvate/OAA and OAA/malate exchange kinetics (Fig. 2). The double reciprocal plots showed the occurrence of saturation characteristics with different Vmax and Km values. Vmax values, which reflect the 934
Vol.
182, No. 2, 1992
BIOCHEMICAL
l/[SUBSTRATE]
FIG. 2. KINETIC ANALYSIS AND PYRWATEfOXALOACETATE PLOT.
AND BIOPHYSICAL
RESEARCH COMMUNICATIONS
ImId’)
OF PYRWATEIMALATE, EXCHANGES
USING
THE
OXALOACETATE/MALATE DOUBLE RECIPROCAL
RLM (1.5 mg protein) were incubated at 20°C in 2.0 ml of standard medium added with 2 pg rotenone in the presence of either the malate detecting system (in the case of both the pyruvatejmalate (A) and oxaloacetate/malate ( X ) exchanges) or the oxaloacetate detecting system (in the case of the pyruvatefoxaloacetate (e) exchange) (for details see the Methods). The exchange reactions were started by adding either oxaloacetate or pyruvate at the indicated concentrations. The rate V is measured as described in the Methods is expressed as NADP' reduced (or NADH oxidized) nmoles/min x mg mitochondrial protein.
rate
of
transport
across
the
mitochondrial
membrane
(see
Fig.
1
and ref.6), were 2.6, 6.0 and 12.6 nmolesjmin x mg protein and Km values, i.e. the substrate concentration which gives half maximum rate of uptake, were equal to 0.25, 0.12 and 0.17 mM in the reported pyruvate/OAA
experiment for pyruvate/malate, exchanges respectively.
The reported findings proposed pyruvate/malate Consistently, 14C-malate
in loaded
strongly antiporter.
suggest
another experiment mitochondria, the
OAA/malate the
in which capability
existence use
and of
the
was made of externally
added pyruvate to cause efflux of malate from the mitochondrial matrix was confirmed (not shown). In agreement with the existence of a pyruvate/malate translocator, externally added malate proved to cause efflux 935
of
Vol.
182,
No.
2, 1992
BIOCHEMICAL
AND
BIOPHYSICAL
RESEARCH
COMMUNICATIONS
PYAKITR
MALKITR
I
0
5
10
15
l/[SUBSTRATE]
FIG. 3. NALATEICITRATE
Pyruvate
(
KINETIC ANALYSIS OF THE PYRWATE/CITRATE EXCHANGES USING THE DOUBLE RECIPROCAL PLOT. l
concentrations. The experiment The rate V mitochondrial Mitochondrial
from
mitochondria
following
the
(E.C. 1.1.1.27) concentration
20
(mM-‘)
)
or
( 0 )
malate
were
added
at
the
AND
indicated
was carried out as described in the Methods. is expressed as nmoles NADP+ reduced/min protein. protein was 1.5 mg.
of decrease
pyruvate, of
measured
NADH absorbance
(not shown). was increased
In by
this case incubating
essentially in
the
x
as
in
presence
mitochondrial RLM with
mg
(6), of
LDH
pyruvate alanine and
oxoglutarate whose uptake into mitochondria (8,9) provides the substrate pair for intramitochondrial glutamate-pyruvate transaminase (E.C. 2.6.1.2.). Arsenite and a-cyanocinnamate were also present to inhibit PDH and pyruvate transport via its own carrier, respectively. Pyruvate/malate found to be
exchange (measured by using 0.5 mM pyruvate) inhibited by phenylsuccinate and benzylmalonate
was (5
mM each) (85 and 50% inhibition respectively). However, use of dicarboxylate carrier in pyruvate/malate exchange was excluded since no inhibition by pyruvate of both the oxidation of succinate (10) and the uptake of 14C-dicarboxylates occur (11). In the light of the existence of a specific pyruvate/malate translocator, a catalytic role could be proposed for malate movement in the efflux of citrate caused by uptake and 936
Vol.
182,
No.
2,
metabolism
1992
of
malate
BIOCHEMICAL
pyruvate
effluxed
from
could exchange a pyruvatelcitrate capability of synthesized In the pyruvate
in
added the rate
citrate
due
membrane made and
kinetics
is
the
found.
and
that efflux
neither
externally
rate
pyruvate
malatejcitrate
14C-citrate added
However due to
from
(not
shown),
pyruvate
the
in
no difference in x mg mitochondrial proposal (see scheme added pyruvate tricarboxylate
the
(Fig. 3). and 0.07 mM
respectively,
efflux
of
between
exchanges were 0.3
an the
clearly (not across
step
experiment
Km values
externally via the
for the newly
added decrease
liase, which mitochondria the transport limiting
in the same malatelcitrate
were
be noted citrate
(see also 6). is observed
ATP-dependent citrate of citrate outside was used to show that
pyruvatelcitrate
to
with
1):
was tested.
NADH absorbance of NADH oxidation
CoA and the appearance Triton-X-100
Consistently
exchange
Scheme
malate dehydrogenase, externally of OAA from RLM, as shown by the
of
should cause
(see
intramitochondrial citrate thus allowing exchange. To substantiate this conclusion externally added pyruvate to cause efflux of
experiment, whereas, as expected, values was found (1.6 nmoles/min thus confirming the above reported It to
COMMUNICATIONS
with
the mitochondrial process. Comparison was pyruvatelcitrate for
RESEARCH
mitochondria
mitochondria
presence of causes efflux
Saturation
BIOPHYSICAL
energized
intramitochondrial
in externally increase in addition shows shown).
in
AND
the Vmax protein), Fig. 3).
proved to carrier
citrate nor
this
loaded inhibition
fail (2) *
RLM of
pyruvate/malate exchange in the presence of BTA were found. The reported experimental findings show that pyruvate and citrate do not share the same carrier and that citrate efflux due to pyruvate depends on the combined pyruvate/malate antiporter and the tricarboxylate This paper shows the existence in RLM of antiporter, i.e. the pyruvate/malate
the
synthesis remains to this carrier answers further significant
confirming role
in
pyruvate/malate translocator
in
be established, the question that mitochondrial several metabolic 937
work a
of carrier. so far
translocator. the in vivo however raised in
the (3)
the
new
unknown
The role of fatty acid presence as well
transport can pathways located
play both
of as a in
Vol.
182,
No.
2, 1992
cytosol
and
research
in
BIOCHEMICAL
in this
the
AND
organelles,
BIOPHYSICAL
RESEARCH
thus
necessitating
COMMUNICATIONS
further
field.
REFERENCES. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Spencer, A.F., and Lowensyein J.M. (1962) J. Biol. Chem. 237, 3640-3648. Palmieri, F., Stipani, I., and Quagliariello E. (1972) Eur. J. Biochem. 26, 587-594. Klingenberg, M., and Slenczka, N. (1959) Biochem. 2. 331, 486-517. Waddel, W.J., and Hill, C. (1956) J. Lab. Clin. Med. 48, 311-314. Atlante, A., Passarella, S., Giannattasio, S., and Quagliariello, E. (1985) Biochem. Biophys. Res. Common. 132, 8-18. Passarella, S., Atlante, A., and Quagliariello, E. (1985) Biochem. Biophys. Res. Commun. 129, l-10. Halestrap, A.P., Scott, R.D., and Thomas A.P. (1980) Int. J. 11, 97-105. Biochem. Passarella, S., and Quagliariello, E. (1982) Abstr. lo Conv. Naz. Biol. Cell. p. 69. Brosnan, J.T., Redmond, W., Morgan, D., and Whalen, P. (1980) Int. J. Biochem. 12, 131-133. Quagliariello, E., and Palmieri, F. (1968) Eur. J. Biochem. 4, 20-27. Palmieri, F., Prezioso, G., Quagliariello, E., and Klingenberg, M. (1971) Eur. J. Biochem. 22, 66-74.
938
