Transport of coenzyme A in plant mitochondria

Plant mitochondria - past, present and future

The study of plant mitochondria started in earnest around 1950 with the first isolations of mitochondria from animal and plant tissues. The first 35 years were spent establishing the basic properties of plant mitochondria and plant respiration using biochemical and physiological approaches. A number of unique properties (compared to mammalian mitochondria) were observed: (i) the ability to oxidize malate, glycine and cytosolic NAD(P)H at high rates; (ii) the partial insensitivity to rotenone, which turned out to be due to the presence of a second NADH dehydrogenase on the inner surface of the inner mitochondrial membrane in addition to the classical Complex I NADH dehydrogenase; and (iii) the partial insensitivity to cyanide, which turned out to be due to an alternative oxidase, which is also located on the inner surface of the inner mitochondrial membrane, in addition to the classical Complex IV, cytochrome oxidase. With the appearance of molecular biology methods around 1985, followed by genomics, further unique properties were discovered: (iv) plant mitochondrial DNA (mtDNA) is 10-600 times larger than the mammalian mtDNA, yet it only contains approximately 50% more genes; (v) plant mtDNA has kept the standard genetic code, and it has a low divergence rate with respect to point mutations, but a high recombinatorial activity; (vi) mitochondrial mRNA maturation includes a uniquely complex set of activities for processing, splicing and editing (at hundreds of sites); (vii) recombination in mtDNA creates novel reading frames that can produce male sterility; and (viii) plant mitochondria have a large proteome with 2000-3000 different proteins containing many unique proteins such as 200-300 pentatricopeptide repeat proteins. We describe the present and fairly detailed picture of the structure and function of plant mitochondria and how the unique properties make their metabolism more flexible allowing them to be involved in many diverse processes in the plant cell, such as photosynthesis, photorespiration, CAM and C4 metabolism, heat production, temperature control, stress resistance mechanisms, programmed cell death and genomic evolution. However, it is still a challenge to understand how the regulation of metabolism and mtDNA expression works at the cellular level and how retrograde signaling from the mitochondria coordinates all those processes.

Seed Germination in Oil Palm (Elaeis guineensis Jacq.): A Review of Metabolic Pathways and Control Mechanisms

Oil palm is an oil-producing crop of major importance at the global scale. Oil palm mesocarp lipids are used for myriads industrial applications, and market demand has been growing for decades. In addition, oil palm seeds are oleaginous, and the oil extracted therefrom can be used for several purposes, from food to cosmetics. As such, there is a huge need in oil palm seeds to maintain the global cohort of more than 2 billion trees. However, oil palm seed germination is a rather difficult process, not only to break dormancy, but also because it is long and often reaches lower-than-expected germination rates. Surprisingly, despite the crucial importance of germination for oil palm plantation management, our knowledge is still rather limited, in particular about germinating oil palm seed metabolism. The present review incorporates different pieces of information that have been obtained in the past few years, in oil palm and in other palm species, in order to provide an overview of germination metabolism and its control. Further insights can also be gained from other oleaginous model plants, such as Arabidopsis or canola, however, palm seeds have peculiarities that must be accounted for, to gain a better understanding of germinating seed metabolism.

Biochemical characterization of a new mitochondrial transporter of dephosphocoenzyme A in Drosophila melanogaster

CoA is an essential cofactor that holds a central role in cell metabolism. Although its biosynthetic pathway is conserved across the three domains of life, the subcellular localization of the eukaryotic biosynthetic enzymes and the mechanism behind the cytosolic and mitochondrial CoA pools compartmentalization are still under debate. In humans, the transport of CoA across the inner mitochondrial membrane has been ascribed to two related genes, SLC25A16 and SLC25A42 whereas in D. melanogaster genome only one gene is present, CG4241, phylogenetically closer to SLC25A42. CG4241 encodes two alternatively spliced isoforms, dPCoAC-A and dPCoAC-B. Both isoforms were expressed in Escherichia coli, but only dPCoAC-A was successfully reconstituted into liposomes, where transported dPCoA and, to a lesser extent, ADP and dADP but not CoA, which was a powerful competitive inhibitor. The expression of both isoforms in a Saccharomyces cerevisiae strain lacking the endogenous putative mitochondrial CoA carrier restored the growth on respiratory carbon sources and the mitochondrial levels of CoA. The results reported here and the proposed subcellular localization of some of the enzymes of the fruit fly CoA biosynthetic pathway, suggest that dPCoA may be synthesized and phosphorylated to CoA in the matrix, but it can also be transported by dPCoAC to the cytosol, where it may be phosphorylated to CoA by the monofunctional dPCoA kinase. Thus, dPCoAC may connect the cytosolic and mitochondrial reactions of the CoA biosynthetic pathway without allowing the two CoA pools to get in contact.

Identification of mitochondrial coenzyme a transporters from maize and Arabidopsis

Plants make coenzyme A (CoA) in the cytoplasm but use it for reactions in mitochondria, chloroplasts, and peroxisomes, implying that these organelles have CoA transporters. A plant peroxisomal CoA transporter is already known, but plant mitochondrial or chloroplastic CoA transporters are not. Mitochondrial CoA transporters belonging to the mitochondrial carrier family, however, have been identified in yeast (Saccharomyces cerevisiae; Leu-5p) and mammals (SLC25A42). Comparative genomic analysis indicated that angiosperms have two distinct homologs of these mitochondrial CoA transporters, whereas nonflowering plants have only one. The homologs from maize (Zea mays; GRMZM2G161299 and GRMZM2G420119) and Arabidopsis (Arabidopsis thaliana; At1g14560 and At4g26180) all complemented the growth defect of the yeast leu5Δ mitochondrial CoA carrier mutant and substantially restored its mitochondrial CoA level, confirming that these proteins have CoA transport activity. Dual-import assays with purified pea (Pisum sativum) mitochondria and chloroplasts, and subcellular localization of green fluorescent protein fusions in transiently transformed tobacco (Nicotiana tabacum) Bright Yellow-2 cells, showed that the maize and Arabidopsis proteins are targeted to mitochondria. Consistent with the ubiquitous importance of CoA, the maize and Arabidopsis mitochondrial CoA transporter genes are expressed at similar levels throughout the plant. These data show that representatives of both monocotyledons and eudicotyledons have twin, mitochondrially located mitochondrial carrier family carriers for CoA. The highly conserved nature of these carriers makes possible their reliable annotation in other angiosperm genomes.

A novel member of solute carrier family 25 (SLC25A42) is a transporter of coenzyme A and adenosine 3',5'-diphosphate in human mitochondria

Mitochondrial carriers are a family of proteins that transport metabolites, nucleotides, and cofactors across the inner mitochondrial membrane thereby connecting cytosolic and matrix functions. The essential cofactor coenzyme A (CoA) is synthesized outside the mitochondrial matrix and therefore must be transported into mitochondria where it is required for a number of fundamental processes. In this work we have functionally identified and characterized SLC25A42, a novel human member of the mitochondrial carrier family. The SLC25A42 gene (Haitina, T., Lindblom, J., Renström, T., and Fredriksson, R., 2006, Genomics 88, 779-790) was overexpressed in Escherichia coli, purified, and reconstituted into phospholipid vesicles. Its transport properties, kinetic parameters, and targeting to mitochondria demonstrate that SLC25A42 protein is a mitochondrial transporter for CoA and adenosine 3',5'-diphosphate. SLC25A42 catalyzed only a counter-exchange transport, exhibited a high transport affinity for CoA, dephospho-CoA, ADP, and adenosine 3',5'-diphosphate, was saturable and inhibited by bongkrekic acid and other inhibitors of mitochondrial carriers to various degrees. The main physiological role of SLC25A42 is to import CoA into mitochondria in exchange for intramitochondrial (deoxy)adenine nucleotides and adenosine 3',5'-diphosphate. This is the first time that a mitochondrial carrier for CoA and adenosine 3',5'-diphosphate has been characterized biochemically.

Pantothenate synthetase is essential but not limiting for pantothenate biosynthesis in Arabidopsis

Pantothenate (vitamin B5) is the universal precursor for coenzyme A (CoA), an essential cofactor that is required in the metabolism of carbohydrates and fatty acids. The final step of bacterial and eukaryotic pantothenate biosynthesis is catalyzed by pantothenate synthetase (PTS), which is encoded by a single gene in Arabidopsis thaliana (AtPTS). There was debate whether PTS represents the only mode of pantothenate production because previous biochemical evidence pointed to an additional pantothenate pathway in plants. Here we show that insertional mutant alleles of AtPTS confer a recessive embryo-lethal phenotype with mutant embryos arrested at the preglobular stage. Exogenous pantothenate was required for normal seed development and germination and also facilitated the remaining life cycle. Complementation of the mutant phenotype was likewise achieved by heterologous expression of E. coli PTS (panC). The panC transgene increased the total PTS activity in leaves by up to 500-fold but did not affect the steady-state level of pantothenate, indicating that PTS is essential but not limiting for pantothenate production. The auxotrophic AtPTS knockout phenotype suggests that the embryo and possibly all other tissues are autonomous for the biosynthesis of pantothenate. This view is consistent with the near-ubiquitous expression of AtPTS as judged by promoter:beta-glucuronidase analysis. Given the high demand for CoA during storage oil accumulation, we analyzed transcript and metabolite patterns of CoA biosynthesis in seeds. The data indicate that the pantothenate and CoA contents follow distinct developmental programs and that both transcriptional and posttranslational control mechanisms are important for CoA homeostasis.

Plant coenzyme A biosynthesis: characterization of two pantothenate kinases from Arabidopsis

In bacterial and animal coenzyme A (CoA) biosynthesis, pantothenate kinase (PANK) activity is critical in regulating intracellular CoA levels. Less is known about the role of PANK in plants, although a single plant isozyme from Arabidopsis, AtPANK1, was previously cloned and analyzed in vitro. We report here the characterization of a second pantothenate kinase of Arabidopsis, AtPANK2, as well as characterization of the physiological roles of both plant enzymes. The activity of the second pantothenate kinase, AtPANK2, was confirmed by its ability to complement the temperature-sensitive mutation of the bacterial pantothenate kinase in E. coli strain ts9. Knock-out mutation of either AtPANK1 or AtPANK2 did not inhibit plant growth, whereas pank1-1/pank2-1 double knockout mutations were embryo lethal. The phenotypes of the mutant plants demonstrated that only one of the AtPANK enzymes is necessary and sufficient for producing adequate CoA levels, and that no other enzyme can compensate for the loss of both isoforms. Real-time PCR measurements of AtPANK1 and AtPANK2 transcripts indicated that both enzymes are expressed with similar patterns in all tissues examined, further suggesting that AtPANK1 and AtPANK2 have complementary roles. The two enzymes have homologous pantothenate kinase domains, but AtPANK2 also carries a large C-terminal protein domain. Sequence comparisons indicate that this type of "bifunctional" pantothenate kinase is conserved in other higher eukaryotes as well. Although the function of the C-terminal domain is not known, homology structure modeling suggests it contains a highly conserved cluster of charged residues that likely constitute a metal-binding site.

An Arabidopsis mutant impaired in coenzyme A biosynthesis is sugar dependent for seedling establishment

Once the plant coenzyme A (CoA) biosynthetic pathway has been elucidated by comparative genomics, it is feasible to analyze the physiological relevance of CoA biosynthesis in plant life. To this end, we have identified and characterized Arabidopsis (Arabidopsis thaliana) T-DNA knockout mutants of two CoA biosynthetic genes, HAL3A and HAL3B. The HAL3A gene encodes a 4'-phosphopantothenoyl-cysteine decarboxilase that generates 4'-phosphopantetheine. A second gene, HAL3B, whose gene product is 86% identical to that of HAL3A, is present in the Arabidopsis genome. HAL3A appears to have a predominant role over HAL3B according to their respective mRNA expression levels. The hal3a-1, hal3a-2, and hal3b mutants were viable and showed a similar growth rate as that in wild-type plants; in contrast, a hal3a-1 hal3b double mutant was embryo lethal. Unexpectedly, seedlings that were null for HAL3A and heterozygous for HAL3B (aaBb genotype) displayed a sucrose (Suc)-dependent phenotype for seedling establishment, which is in common with mutants defective in beta-oxidation. This phenotype was genetically complemented in aaBB siblings of the progeny and chemically complemented by pantethine. In contrast, seedling establishment of Aabb plants was not Suc dependent, proving a predominant role of HAL3A over HAL3B at this stage. Total fatty acid and acyl-CoA measurements of 5-d-old aaBb seedlings in medium lacking Suc revealed stalled storage lipid catabolism and impaired CoA biosynthesis; in particular, acetyl-CoA levels were reduced by approximately 80%. Taken together, these results provide in vivo evidence for the function of HAL3A and HAL3B, and they point out the critical role of CoA biosynthesis during early postgerminative growth.

Organisation of the pantothenate (vitamin B5) biosynthesis pathway in higher plants

Pantothenate (vitamin B5) is the precursor for the biosynthesis of the phosphopantetheine moiety of coenzyme A and acyl carrier protein, and is synthesised in Escherichia coli by four enzymic reactions. Ketopantoate hydroxymethyltransferase (KPHMT) and pantothenate synthetase (PtS) catalyse the first and last steps, respectively. Two genes encoding KPHMT and one for PtS were identified in the Arabidopsis thaliana genome, and cDNAs for all three genes were amplified by PCR. The cDNAs were able to complement their respective E. coli auxotrophs, demonstrating that they encoded functional enzymes. Subcellular localisation of the proteins was investigated using green fluorescent protein (GFP) fusions and confocal microscopy. The two KPHMT-GFP fusion proteins were targeted exclusively to mitochondria, whereas PtS-GFP was found in the cytosol. This implies that there must be transporters for pathway intermediates. KPHMT enzyme activity could be measured in purified mitochondria from both pea leaves and Arabidopsis suspension cultures. We investigated whether Arabidopsis encoded homologues of the remaining two pantothenate biosynthesis enzymes from E. coli, l-aspartate-alpha-decarboxylase (ADC) and ketopantoate reductase (KPR). No homologue of ADC could be identified using either conventional blast or searches with the program fugue in which the structure of the E. coli ADC was compared to all the annotated proteins in Arabidopsis. ADC also appears to be absent from the genome of the yeast, Saccharomyces cerevisiae, by the same criteria. In contrast, a putative Arabidopsis oxidoreductase with some similarity to KPR was identified with fugue.

PPARalpha controls the intracellular coenzyme A concentration via regulation of PANK1alpha gene expression

Pantothenate kinase (PanK) is thought to catalyze the first rate-limiting step in CoA biosynthesis. The full-length cDNA encoding the human PanK1alpha protein was isolated, and the complete human PANK1 gene structure was determined. Bezafibrate (BF), a hypolipidemic drug and a peroxisome proliferator activator receptor-alpha (PPARalpha) agonist, specifically increased hPANK1alpha mRNA expression in human hepatoblastoma (HepG2) cells as a function of time and dose of the drug, compared with hPANK1beta, hPANK2, and hPANK3, which did not significantly increase. Four putative PPARalpha response elements were identified in the PANKIalpha promoter, and BF stimulated hPANK1alpha promoter activity but did not alter the mRNA half-life. Increased hPANK1alpha mRNA resulted in higher hPanK1 protein, localized in the cytoplasm, and elevated PanK enzyme activity. The enhanced hPANK1alpha gene expression translated into increased activity of the CoA biosynthetic pathway and established a higher steady-state CoA level in HepG2 cells. These data are consistent with a key role for PanK1alpha in the control of cellular CoA content and point to the PPARalpha transcription factor as a major factor governing hepatic CoA levels by specific modulation of PANK1alpha gene expression.

Identification and reconstitution of the yeast mitochondrial transporter for thiamine pyrophosphate

The genome of Saccharomyces cerevisiae contains 35 members of a family of transport proteins that, with a single exception, are found in the inner membranes of mitochondria. The transport functions of the 15 biochemically identified mitochondrial carriers are concerned with shuttling substrates, biosynthetic intermediates and cofactors across the inner membrane. Here the identification of the mitochondrial carrier for the essential cofactor thiamine pyrophosphate (ThPP) is described. The protein has been overexpressed in bacteria, reconstituted into phospholipid vesicles and identified by its transport properties. In confirmation of its identity, cells lacking the gene for this carrier had reduced levels of ThPP in their mitochondria, and decreased activity of acetolactate synthase, a ThPP-requiring enzyme found in the organellar matrix. They also required thiamine for growth on fermentative carbon sources.

Acyl carrier protein (ACP) import into chloroplasts. Covalent modification by a stromal holoACP synthase is stimulated by exogenously added CoA and inhibited by adenosine 3',5'-bisphosphate

During the import of the precursor for the acyl carrier protein (ACP) into chloroplasts, apoACP is converted to holoACP by the attachment of a phosphopantetheine group transferred from coenzyme A (CoA) by a chloroplast holoACP synthase [Fernandez, M. and Lamppa, G. (1990) Acyl carrier protein import into chloroplasts does not require the phosphopantetheine: evidence for a chloroplast holoACP synthase, Plant Cell 2, 195-206]. Here it is shown that exogenous addition of CoA to intact chloroplasts in the import assay stimulates the conversion of apoACP to holoACP. If adenosine 3',5'-bisphosphate [Ado(3',5')P2], the byproduct of the transfer reaction, was also included the extent of conversion was greatly reduced. CoA has its effect after ACP precursor (pre-ACP) import and proteolytic removal of the transit peptide, thus indicating that the chloroplast holoACP synthase resides in the stroma where fatty acid synthase is found. When Ado(3',5')P2 was added alone to the import assay, it inhibited the synthesis of holoACP. Inhibition of the conversion of apo- to holoACP with Ado(3',5')P2 made it possible to examine whether the holoform of preACP could be imported into chloroplasts. Pre-apoACP was synthesized in Escherichia coli and shown to be competent for import in an ATP- and temperature-dependent manner. A partially purified chloroplast holoACP synthase converted 60-90% of the pre-apoACP to pre-holoACP. Pre-holoACP incubated with chloroplasts in the presence of Ado(3',5')P2 yielded > 60% holoACP, whereas the control reaction with pre-apoACP gave primarily apoACP. Hence the phosphopantetheine prosthetic group of ACP does not block precursor movement through the translocation apparatus of the chloroplast envelope.

Thiamine pyrophosphate uptake into isolated rat liver mitochondria

The fact that thiamine pyrophosphate is synthesized in cytosol necessitates its uptake into mitochondria. The ability of mitochondria to take up externally added thiamine pyrophosphate was investigated by measuring the intramitochondrial thiamine pyrophosphate content using an enzymatic method. Thiamine pyrophosphate uptake by isolated rat liver mitochondria was found to occur in a time- and temperature-dependent manner. Uptake shows saturation characteristics with Km and Vmax values equal to about 20 microM and 700 pmol/min x mg protein, respectively, and is inhibited by certain nonpenetrating compounds. The inhibition of thiamine uptake by thiamine pyrophosphate and the efflux of endogenous thiamine pyrophosphate, caused by externally added thiamine, suggest the existence of a thiamine pyrophosphate/thiamine antiporter which could play an active role in the turnover of intramitochondrial thiamine pyrophosphate linked enzymes.

2-Oxoglutarate dehydrogenase and pyruvate dehydrogenase activities in plant mitochondria: interaction via a common coenzyme a pool

2-Oxoglutarate (2-OG)-dependent O2 uptake by washed or purified turnip (Brassica rapa L.) and pea (Pisum sativum L. cv. Massey Gem) leaf mitochondria, in the presence of malonate, was inhibited between 65 and 90% by micromolar levels of pyruvate. The inhibition was not observed in the absence of malonate and was reversed by alpha-cyano-4-hydroxycinnamic acid. The inhibition was also reversed by oxaloacetate or by malate, but not by any other tricarboxylic acid cycle intermediates. The stimulation of O2 uptake by oxaloacetate was half maximal at 8-9 microM and was transient, indicating its action was not mediated through the complete metabolic removal of pyruvate. Pyruvate had not effect on 2-OG oxidation under conditions in which pyruvate dehydrogenase was not active, indicating that pyruvate metabolism, rather than pyruvate itself, was responsible for producing the inhibition of 2-OG oxidation. Similar results were obtained with detergent-treated mitochondrial extracts with the exception that the inhibition of 2-OG oxidation by pyruvate could also be reversed by coenzyme A. The results suggest that pyruvate inhibits 2-oxoglutarate oxidation, in intact plant mitochondria, by sequestering intramitochondrial CoA as acetyl-CoA and, in the absence of citrate synthase activity, reduces the amount of free coenzyme A available for 2-oxoglutarate dehydrogenase. These results indicate that pyruvate dehydrogenase and 2-oxoglutarate dehydrogenase share a common CoA pool within plant mitochondria and that the turnover of the acyl-CoA product of one enzyme will dramatically influence the activity of the other.

Pyruvate metabolism in castor-bean mitochondria

We report the isolation of mitochondria from the endosperm of castor beans (Ricinus communis). These mitochondria oxidized succinate, external NADH, malate and pyruvate with respiratory-control and ADP/O ratios consistent with those found previously with mitochondria from other plant sources. The mitochondria exhibited considerable sensitivity to the electron-transport-chain inhibitors antimycin A and cyanide when oxidizing succinate and external NADH. Pyruvate-dependent O2 uptake was relatively insensitive to these inhibitors, although the residual O2 uptake could be inhibited by salicylhydroxamic acid. We conclude that a cyanide-insensitive alternative terminal oxidase is functional in these mitochondria. However, electrons from the succinate dehydrogenase or external NADH dehydrogenase seem to have no access to this pathway. There is little interconnection between the salicylhydroxamic acid-sensitive and cyanide-sensitive pathways of electron transport. alpha-Cyanocinnamate and its analogues, compound UK5099 [alpha-cyano-beta-(1-phenylindol-3-yl)acrylate] and alpha-cyano-4-hydroxycinnamate, were all found to be potent non-competitive inhibitors of pyruvate oxidation in castor-bean mitochondria. The accumulation of pyruvate by castor-bean mitochondria was determined by using a silicone-oil-centrifugation technique. The accumulation was shown to observe Michaelis-Menten kinetics, with a Km for pyruvate of 0.10 mM and a Vmax. of 0.95 nmol/min per mg of mitochondrial protein. However, the observed rates of pyruvate accumulation were insufficient to account for the pyruvate oxidation rates found in the oxygen-electrode studies. We were able to demonstrate that this is due to the immediate export of the accumulated radiolabel in the form of malate and citrate. Compound UK5099 inhibited the accumulation of [2-14C]pyruvate by castor-bean mitochondria at concentrations similar to those required to inhibit pyruvate oxidation.

The control of malate dehydrogenase activity by adenine nucleotides in purified potato tuber (Solanum tuberosum L.) mitochondria

The limiting factors of the involvement of malate dehydrogenase in mitochondrial malate oxidation were investigated by using Percoll-purified potato tuber mitochondria. The respective roles of reduced pyridine nucleotides, oxaloacetate, and adenine nucleotides were studied under conditions of high or low phosphorylation potential (Pi + ADP/ATP ratio). Under conditions of high phosphorylation potential, the limitation of malate dehydrogenase activity was caused by the accumulation of oxaloacetate in the medium. In the absence of ADP (phosphorylation potential close to zero), ATP was responsible for the inhibition of malate dehydrogenase activity rather than oxaloacetate or reduced pyridine nucleotides.

Inhibition of 2-oxoglutarate oxidation in plant mitochondria by pyruvate

Oxidation of 2-oxoglutarate (in the presence of malonate) by mitochondria isolated from turnip, pea leaf and cauliflower tissue was dramatically inhibited by micromolar concentrations of pyruvate. Pyruvate, however, had little or no effect on 2-OG oxidation when carried out in the absence of malonate. The inhibition was reversed by alpha-cyano-4-hydroxycinnamic acid, indicating pyruvate uptake into the matrix was required for the inhibitory effect. In contrast, pyruvate had no effect on 2-oxoglutarate oxidation by mitochondria isolated from rat heart. The possible significance of the effect in terms of the control of 2-oxoglutarate dehydrogenase activity during the operation of a malate/aspartate shuttle in plant mitochondria is discussed.

Activation of NAD-linked malic enzyme in intact plant mitochondria by exogenous coenzyme A

O2 uptake by potato and cauliflower bud mitochondria oxidizing malate was progressively inhibited as the pH of the external medium was increased, in response to accumulation of oxaloacetate. Adding 0.5 mM coenzyme A to the medium reversed this trend by stimulating intramitochondrial NAD-linked malic enzyme at alkaline pH. In intact potato mitochondria, coenzyme A stimulation of malic enzyme was not observed when the external pH was above 7.5; in cauliflower mitochondria, coenzyme A stimulated even at pH 8. This difference in the response of intact mitochondria was attributed to an inherent difference in the properties of malic enzyme from the two tissues. Malic enzyme solubilized from potato mitochondria was inactive at pH values above 7.8, while that from cauliflower mitochondria retained its activity at pH 8 in the presence of coenzyme A. In potato mitochondria, coenzyme A stimulation of O2 uptake at alkaline pH was only observed when NAD+ was also provided exogenously. The results show that coenzyme A can be taken up by intact mitochondria and that pH, NAD+, and coenzyme A levels in the matrix act together to regulate malate oxidation.