Metabolic interactions between peroxisomes and mitochondria with a special focus on acylcarnitine metabolism

Carnitine plays an essential role in mitochondrial fatty acid β-oxidation as a part of a cycle that transfers long-chain fatty acids across the mitochondrial membrane and involves two carnitine palmitoyltransferases (CPT1 and CPT2). Two distinct carnitine acyltransferases, carnitine octanoyltransferase (COT) and carnitine acetyltransferase (CAT), are peroxisomal enzymes, which indicates that carnitine is not only important for mitochondrial, but also for peroxisomal metabolism. It has been demonstrated that after peroxisomal metabolism, specific intermediates can be exported as acylcarnitines for subsequent and final mitochondrial metabolism. There is also evidence that peroxisomes are able to degrade fatty acids that are typically handled by mitochondria possibly after transport as acylcarnitines. Here we review the biochemistry and physiological functions of metabolite exchange between peroxisomes and mitochondria with a special focus on acylcarnitines.     

Molecular cloning and expression of human carnitine octanoyltransferase: evidence for its role in the peroxisomal beta-oxidation of branched-chain fatty acids.

To study the putative role of human carnitine octanoyltransferase (COT) in the beta-oxidation of branched-chain fatty acids, we identified and cloned the cDNA encoding human COT and expressed it in the yeast Saccharomyces cerevisiae. Enzyme activity measurements showed that COT efficiently converts one of the end products of the peroxisomal beta-oxidation of pristanic acid, 4, 8-dimethylnonanoyl-CoA, to its corresponding carnitine ester. Production of the carnitine ester of this branched/medium-chain acyl-CoA within the peroxisome is required for its transport to the mitochondrion where further beta-oxidation occurs. In contrast, 4, 8-dimethylnonanoyl-CoA is not a substrate for carnitine acetyltransferase, another acyltransferase localized in peroxisomes, which catalyzes the formation of carnitine esters of the other products of pristanic acid beta-oxidation, namely acetyl-CoA and propionyl-CoA. Our results shed new light on the function of COT in fatty acid metabolism and point to a crucial role of COT in the beta-oxidation of branched-chain fatty acids.     

Changes in carnitine octanoyltransferase activity induce alteration in fatty acid metabolism

The peroxisomal beta oxidation of very long chain fatty acids (VLCFA) leads to the formation of medium chain acyl-CoAs such as octanoyl-CoA. Today, it seems clear that the exit of shortened fatty acids produced by the peroxisomal beta oxidation requires their conversion into acyl-carnitine and the presence of the carnitine octanoyltransferase (CROT). Here, we describe the consequences of an overexpression and a knock down of the CROT gene in terms of mitochondrial and peroxisomal fatty acids metabolism in a model of hepatic cells. Our experiments showed that an increase in CROT activity induced a decrease in MCFA and VLCFA levels in the cell. These changes are accompanied by an increase in the level of mRNA encoding enzymes of the peroxisomal beta oxidation. In the same time, we did not observe any change in mitochondrial function. Conversely, a decrease in CROT activity had the opposite effect. These results suggest that CROT activity, by controlling the peroxisomal amount of medium chain acyls, may control the peroxisomal oxidative pathway.     

Effects of unsaturated fatty acids on the peroxisomal enzyme activities of Tetrahymena pyriformis

The effects of unsaturated fatty acids on the activities of peroxisomal enzymes of Tetrahymena pyriformis were investigated. When saturated fatty acids and the corresponding unsaturated fatty acids (C18) were added to the culture medium at 0.05%, the activities of peroxisomal enzymes [fatty acyl-CoA oxidase (FAO), carnitine acetyltransferase (CAT), isocitrate lyase (ICL), and malate synthase (MS)] were significantly increased. The order of effectiveness was linoleic acid greater than oleic acid greater than stearic acid. However, alpha-linolenic acid and gamma-linolenic acid at the same concentration were lethal to the cells. The inhibitory effect on growth disappeared upon addition of an antioxidant, alpha-tocopherol. Lipid peroxides derived from unsaturated fatty acids induced marked cell lysis. In the presence of a low concentration (0.005%) of linolenic acid the production of lipid peroxide was lower and no inhibitory effect on the growth was observed, while the activities of peroxisomal enzymes participating in lipid metabolism and that of catalase were significantly increased. These results indicate that the peroxisomal enzyme systems related to the beta-oxidations of fatty acids and the glyoxylate cycle are regulated by unsaturated long-chain fatty acids, including linolenic acid, at low concentrations, as well as by saturated fatty acid in the medium.     

Carnitine-dependent transport of acetyl coenzyme A in Candida albicans is essential for growth on nonfermentable carbon sources and contributes to biofilm formation

In eukaryotes, acetyl coenzyme A (acetyl-CoA) produced during peroxisomal fatty acid beta-oxidation needs to be transported to mitochondria for further metabolism. Two parallel pathways for acetyl-CoA transport have been identified in Saccharomyces cerevisiae; one is dependent on peroxisomal citrate synthase (Cit), while the other requires peroxisomal and mitochondrial carnitine acetyltransferase (Cat) activities. Here we show that the human fungal pathogen Candida albicans lacks peroxisomal Cit, relying exclusively on Cat activity for transport of acetyl units. Deletion of the CAT2 gene encoding the major Cat enzyme in C. albicans resulted in a strain that had lost both peroxisomal and mitochondrion-associated Cat activities, could not grow on fatty acids or C(2) carbon sources (acetate or ethanol), accumulated intracellular acetyl-CoA, and showed greatly reduced fatty acid beta-oxidation activity. The cat2 null mutant was, however, not attenuated in virulence in a mouse model of systemic candidiasis. These observations support our previous results showing that peroxisomal fatty acid beta-oxidation activity is not essential for C. albicans virulence. Biofilm formation by the cat2 mutant on glucose was slightly reduced compared to that by the wild type, although both strains grew at the same rate on this carbon source. Our data show that C. albicans has diverged considerably from S. cerevisiae with respect to the mechanism of intracellular acetyl-CoA transport and imply that carnitine dependence may be an important trait of this human fungal pathogen.     

Fatty acid cellular metabolism and lactone production by the yeast Pichia guilliermondii

Methyl ricinoleate conversion into γ-decalactone by fungi is already widely used by the aromatic industry. It offers an interesting alternative to chemical synthesis by permitting acquisition of a natural label. Peroxisomal β-oxidation has been described as the probable transformation mechanism. This paper provides information about this metabolism and shows the importance of the step catalysed by carnitine octanoyltransferase. After culture of the yeast Pichia guilliermondii on a medium containing methyl ricinoleate as sole carbon source, we confirmed that mitochondrial β-oxidation could not be responsible for the biotransformation. We also observed the effect of chlorpromazine, an inhibitor of carnitine octanoyltransferase, on peroxisomal β-oxidation and therefore on lactone production, and on lipid accumulation by the yeasts. The presence of chlorpromazine caused a reduction in aromatic specific production yield. This reduction was inversely proportional to the amount of chlorpromazine present in the medium. A considerable accumulation of methyl ricinoleate derivatives was also observed. We therefore concluded that the metabolism responsible for the bioconversion was peroxisomal β-oxidation. The effects of chlorpromazine suggested that the entry of fatty acids into the peroxisomes took place in a carnitine-dependent manner. This step might be a limiting step in the metabolism. Received: 26 June 1995/Received revision: 16 November 1995/Accepted: 4 December 1995     

Peroxisomes contribute to the acylcarnitine production when the carnitine shuttle is deficient

Fatty acid β-oxidation may occur in both mitochondria and peroxisomes. While peroxisomes oxidize specific carboxylic acids such as very long-chain fatty acids, branched-chain fatty acids, bile acids, and fatty dicarboxylic acids, mitochondria oxidize long-, medium-, and short-chain fatty acids. Oxidation of long-chain substrates requires the carnitine shuttle for mitochondrial access but medium-chain fatty acid oxidation is generally considered carnitine-independent. Using control and carnitine palmitoyltransferase 2 (CPT2)- and carnitine/acylcarnitine translocase (CACT)-deficient human fibroblasts, we investigated the oxidation of lauric acid (C12:0). Measurement of the acylcarnitine profile in the extracellular medium revealed significantly elevated levels of extracellular C10- and C12-carnitine in CPT2- and CACT-deficient fibroblasts. The accumulation of C12-carnitine indicates that lauric acid also uses the carnitine shuttle to access mitochondria. Moreover, the accumulation of extracellular C10-carnitine in CPT2- and CACT-deficient cells suggests an extramitochondrial pathway for the oxidation of lauric acid. Indeed, in the absence of peroxisomes C10-carnitine is not produced, proving that this intermediate is a product of peroxisomal β-oxidation. In conclusion, when the carnitine shuttle is impaired lauric acid is partly oxidized in peroxisomes. This peroxisomal oxidation could be a compensatory mechanism to metabolize straight medium- and long-chain fatty acids, especially in cases of mitochondrial fatty acid β-oxidation deficiency or overload.     

OCTN3 is a mammalian peroxisomal membrane carnitine transporter

Carnitine is a zwitterion essential for the beta-oxidation of fatty acids. The role of the carnitine system is to maintain homeostasis in the acyl-CoA pools of the cell, keeping the acyl-CoA/CoA pool constant even under conditions of very high acyl-CoA turnover, thereby providing cells with a critical source of free CoA. Carnitine derivatives can be moved across intracellular barriers providing a shuttle mechanism between mitochondria, peroxisomes, and microsomes. We now demonstrate expression and colocalization of mOctn3, the intermediate-affinity carnitine transporter (Km 20 microM), and catalase in murine liver peroxisomes by TEM using immunogold labelled anti-mOctn3 and anti-catalase antibodies. We further demonstrate expression of hOCTN3 in control human cultured skin fibroblasts both by Western blotting and immunostaining analysis using our specific anti-mOctn3 antibody. In contrast with two peroxisomal biogenesis disorders, we show reduced expression of hOCTN3 in human PEX 1 deficient Zellweger fibroblasts in which the uptake of peroxisomal matrix enzymes is impaired but the biosynthesis of peroxisomal membrane proteins is normal, versus a complete absence of hOCTN3 in human PEX 19 deficient Zellweger fibroblasts in which both the uptake of peroxisomal matrix enzymes as well as peroxisomal membranes are deficient. This supports the localization of hOCTN3 to the peroxisomal membrane. Given the impermeability of the peroxisomal membrane and the key role of carnitine in the transport of different chain-shortened products out of peroxisomes, there appears to be a critical need for the intermediate-affinity carnitine/organic cation transporter, OCTN3, on peroxisomal membranes now shown to be expressed in both human and murine peroxisomes. This Octn3 localization is in keeping with the essential role of carnitine in peroxisomal lipid metabolism.     

Purification and properties of peroxisomal carnitine palmitoyltransferase in chick embryo liver

Peroxisomal carnitine palmitoyltransferase was purified by solubilization using Tween 20 and KCl from the large granule fraction of the liver of clofibrate-treated chick embryo, DEAE-Sephacel and blue Sepharose CL-6B column chromatography. The peroxisomal carnitine palmitoyltransferase was an Mr 64,000 polypeptide; the mitochondrial carnitine palmitoyltransferase had a subunit molecular weight of 69,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The carnitine acetyltransferase was an Mr 64,000 polypeptide. Antibody against purified peroxisomal carnitine palmitoyltransferase reacted only with peroxisomal carnitine palmitoyltransferase, but not with mitochondrial carnitine palmitoyltransferase or carnitine acetyltransferase. In addition, anti-peroxisomal carnitine palmitoyltransferase reacted only with the protein in peroxisomes purified from chick embryo liver by sucrose density gradient centrifugation. Thus, it was confirmed that purified peroxisomal carnitine palmitoyltransferase was a peroxisomal protein. Compared with mitochondrial carnitine palmitoyltransferase, peroxisomal carnitine palmitoyltransferase was extremely resistant to inactivation by trypsin. The pH optimum of peroxisomal carnitine palmitoyltransferase was 8.5, differing from that of mitochondrial carnitine palmitoyltransferase. The Km value of peroxisomal carnitine palmitoyltransferase for palmitoyl-CoA (32 microM) was similar to that of the mitochondrial one, whereas those values for L-carnitine (140 microM), palmitoyl-L-carnitine (43 microM) and CoA (9 microM) were lower than those of mitochondrial carnitine palmitoyltransferase. Peroxisomal carnitine palmitoyltransferase exhibited similar substrate specificities in both the forward and reverse reactions, with the highest activity toward lauroyl derivatives. Furthermore, this enzyme showed relatively high affinities for long-chain acyl derivatives (C10-C16) and similar Km values (30-50 microM) for acyl-CoAs, acylcarnitine and CoA, and a constant Km value (approximately 150 microM) for carnitine. These results indicate that peroxisomal carnitine palmitoyltransferase played a role in the modulation of the intracellular CoA/long-chain acyl-CoA ratio at the hatching stage of chicken when long-chain fatty acids are actively oxidized in peroxisomes.     

Identification of peroxisomal acyl-CoA thioesterases in yeast and humans

A computer-based screen of the Saccharomyces cerevisiae genome identified YJR019C as a candidate oleate-induced gene. YJR019C mRNA levels were increased significantly during growth on fatty acids, suggesting that it may play a role in fatty acid metabolism. The YJR019C product is highly similar to tesB, a bacterial acyl-CoA thioesterase, and carries a tripeptide sequence, alanine-lysine-phenylalanineCOOH, that closely resembles the consensus sequence for type-1 peroxisomal targeting signals. YJR019C directed green fluorescence protein to peroxisomes, and biochemical studies revealed that YJR019C is an abundant component of purified yeast peroxisomes. Disruption of the YJR019C gene caused a significant decrease in total cellular thioesterase activity, and recombinant YJR019C was found to exhibit intrinsic acyl-CoA thioesterase activity of 6 units/mg. YJR019C also shared significant sequence similarity with hTE, a human thioesterase that was previously identified because of its interaction with human immunodeficiency virus-Nef in the yeast two-hybrid assay. We report here that hTE is also a peroxisomal protein, demonstrating that thioesterase activity is a conserved feature of peroxisomes. We propose that YJR019C and hTE be renamed as yeast and human PTE1 to reflect the fact that they encode peroxisomal thioesterases. The physical segregation of yeast and human PTE1 from the cytosolic fatty acid synthase suggests that these enzymes are unlikely to play a role in formation of fatty acids. Instead, the observation that PTE1 contributes to growth on fatty acids implicates this thioesterase in fatty acid oxidation.     

Chlorpromazine and carnitine-dependency of rat liver peroxisomal beta-oxidation of long-chain fatty acids.

The enzyme targets for chlorpromazine inhibition of rat liver peroxisomal and mitochondrial oxidations of fatty acids were studied. Effects of chlorpromazine on total fatty acyl-CoA synthetase activity, on both the first and the third steps of peroxisomal beta-oxidation, on the entry of fatty acyl-CoA esters into the peroxisome and on catalase activity, which allows breakdown of the H2O2 generated during the acyl-CoA oxidase step, were analysed. On all these metabolic processes, chlorpromazine was found to have no inhibitory action. Conversely, peroxisomal carnitine octanoyltransferase activity was depressed by 0.2-1 mM-chlorpromazine, which also inhibits mitochondrial carnitine palmitoyltransferase activity in all conditions in which these enzyme reactions are assayed. Different patterns of inhibition by the drug were, however, demonstrated for both these enzyme activities. Inhibitory effects of chlorpromazine on mitochondrial cytochrome c oxidase activity were also described. Inhibitions of both cytochrome c oxidase and carnitine palmitoyltransferase are proposed to explain the decreased mitochondrial fatty acid oxidation with 0.4-1.0 mM-chlorpromazine reported by Leighton, Persico & Necochea [(1984) Biochem. Biophys. Res. Commun. 120, 505-511], whereas depression by the drug of carnitine octanoyltransferase activity is presented as the factor responsible for the decreased peroxisomal beta-oxidizing activity described by the above workers.     

Contributions of Carnitine Acetyltransferases to Intracellular Acetyl Unit Transport in Candida albicans

Transport of acetyl-CoA between intracellular compartments is mediated by carnitine acetyltransferases (Cats) that reversibly link acetyl units to the carrier molecule carnitine. The genome of the opportunistic pathogenic yeast Candida albicans encodes several (putative) Cats: the peroxisomal and mitochondrial Cat2 isoenzymes encoded by a single gene and the carnitine acetyltransferase homologs Yat1 and Yat2. To determine the contributions of the individual Cats, various carnitine acetyltransferase mutant strains were constructed and subjected to phenotypic and biochemical analyses on different carbon sources. We show that mitochondrial Cat2 is required for the intramitochondrial conversion of acetylcarnitine to acetyl-CoA, which is essential for a functional tricarboxylic acid cycle during growth on oleate, acetate, ethanol, and citrate. Yat1 is cytosolic and contributes to acetyl-CoA transport from the cytosol during growth on ethanol or acetate, but its activity is not required for growth on oleate. Yat2 is also cytosolic, but we were unable to attribute any function to this enzyme. Surprisingly, peroxisomal Cat2 is essential neither for export of acetyl units during growth on oleate nor for the import of acetyl units during growth on acetate or ethanol. Oxidation of fatty acids still takes place in the absence of peroxisomal Cat2, but biomass formation is absent, and the strain displays a growth delay on acetate and ethanol that can be partially rescued by the addition of carnitine. Based on our results, we present a model for the intracellular flow of acetyl units under various growth conditions and the roles of each of the Cats in this process.     

Molecular characterization of carnitine-dependent transport of acetyl-CoA from peroxisomes to mitochondria in Saccharomyces cerevisiae and identification of a plasma membrane carnitine transporter, Agp2p.

In Saccharomyces cerevisiae, beta-oxidation of fatty acids is confined to peroxisomes. The acetyl-CoA produced has to be transported from the peroxisomes via the cytoplasm to the mitochondrial matrix in order to be degraded to CO(2) and H(2)O. Two pathways for the transport of acetyl-CoA to the mitochondria have been proposed. The first involves peroxisomal conversion of acetyl-CoA into glyoxylate cycle intermediates followed by transport of these intermediates to the mitochondria. The second pathway involves peroxisomal conversion of acetyl-CoA into acetylcarnitine, which is subsequently transported to the mitochondria. Using a selective screen, we have isolated several mutants that are specifically affected in the second pathway, the carnitine-dependent acetyl-CoA transport from the peroxisomes to the mitochondria, and assigned these CDAT mutants to three different complementation groups. The corresponding genes were identified using functional complementation of the mutants with a genomic DNA library. In addition to the previously reported carnitine acetyl-CoA transferase (CAT2), we identified the genes for the yeast orthologue of the human mitochondrial carnitine acylcarnitine translocase (YOR100C or CAC) and for a transport protein (AGP2) required for carnitine transport across the plasma membrane.     

A phytol-enriched diet induces changes in fatty acid metabolism in mice both via PPARalpha-dependent and -independent pathways

Branched-chain fatty acids (such as phytanic and pristanic acid) are ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha) in vitro. To investigate the effects of these physiological compounds in vivo, wild-type and PPARalpha-deficient (PPARalpha-/-) mice were fed a phytol-enriched diet. This resulted in increased plasma and liver levels of the phytol metabolites phytanic and pristanic acid. In wild-type mice, plasma fatty acid levels decreased after phytol feeding, whereas in PPARalpha-/- mice, the already elevated fatty acid levels increased. In addition, PPARalpha-/- mice were found to be carnitine deficient in both plasma and liver. Dietary phytol increased liver free carnitine in wild-type animals but not in PPARalpha-/- mice. Investigation of carnitine biosynthesis revealed that PPARalpha is likely involved in the regulation of carnitine homeostasis. Furthermore, phytol feeding resulted in a PPARalpha-dependent induction of various peroxisomal and mitochondrial beta-oxidation enzymes. In addition, a PPARalpha-independent induction of catalase, phytanoyl-CoA hydroxylase, carnitine octanoyltransferase, peroxisomal 3-ketoacyl-CoA thiolase, and straight-chain acyl-CoA oxidase was observed. In conclusion, branched-chain fatty acids are physiologically relevant ligands of PPARalpha in mice. These findings are especially relevant for disorders in which branched-chain fatty acids accumulate, such as Refsum disease and peroxisome biogenesis disorders.     

The involvement of carnitine intermediates in peroxisomal fatty acid oxidation: a study with 2-bromofatty acids

Metabolism-dependent inactivators of 3-ketothiolase I and carnitine acyltransferase I (CAT I) have been used to study the oxidation of fatty acids in intact hepatocytes. 2-Bromooctanoate inactivates mitochondrial and peroxisomal 3-ketothiolases I in a time-dependent manner. During the first 5 min of incubation, inactivation of 3-ketothiolase in mitochondria is five times faster than its inactivation in peroxisomes. Almost complete inactivation of 3-ketothiolase I in both types of organelle is achieved after incubation with 1 mM 2-bromooctanoate for 40 min. The inactivation is not affected by preincubating hepatocytes with 20 microM tetradecylglycidate (TDGA), an inactivator of CAT I, under conditions which cause greater than 95% inactivation of CAT I. 2-Bromododecanoate (1 mM) causes 60% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min. These inactivations are greatly reduced by preincubating hepatocytes with 20 microM TDGA, demonstrating that 2-bromododecanoate enters both mitochondria and peroxisomes via its carnitine ester. 2-Bromopalmitate (1 mM) causes less than 5% inactivation of mitochondrial and peroxisomal 3-ketothiolases I in 40 min, but causes 95% inactivation of CAT I during this time. Incubation of hepatocytes with 10-200 microM 2-bromopalmitoyl-L-carnitine causes inactivation of mitochondrial and peroxisomal 3-ketothiolases I at similar rates. This inactivation is decreased by palmitoyl-D-carnitine during the first 5 min of incubation. Pretreating hepatocytes with 20 microM TDGA does not affect the inactivation of mitochondrial or peroxisomal 3-ketothiolase I by 2-bromopalmitoyl-L-carnitine. These results demonstrate that in intact hepatocytes, peroxisomes oxidize fatty acids of medium-chain length by a carnitine-independent mechanism, whereas they oxidize long-chain fatty acids by a carnitine-dependent mechanism.     

Developmental changes in the activities of peroxisomal and mitochondrial beta-oxidation in chicken liver

The activities of peroxisomal and mitochondrial beta-oxidation and carnitine acyltransferases changed during the process of development from embryo to adult chicken, and the highest activities of peroxisomal beta-oxidation, palmitoyl-CoA oxidase, and carnitine acetyltransferase were found at the hatching stage of the embryo. The profiles of these alterations were in agreement with those of the contents of triglycerides and free fatty acids in the liver. The highest activities of mitochondrial beta-oxidation and palmitoyl-CoA dehydrogenase were observed at the earlier stages of the embryo; then the activities decreased gradually from embryo to adult chicken. The ratio of activities of carnitine acetyltransferase in peroxisomes and mitochondria (peroxisomes/mitochondria) increased from 0.54 to 0.82 during the development from embryo to adult chicken. The ratio of activities of carnitine palmitoyltransferase decreased from 0.82 to 0.25 during the development. The affinity of fatty acyl-CoA dehydrogenase toward the medium-chain acyl-CoAs (C6 and C8) was high in the embryo and decreased with development, whereas the substrate specificity of fatty acyl-CoA oxidase did not change. The substrate specificity of mitochondrial carnitine acyltransferases did not change with development. The affinity of peroxisomal carnitine acyltransferases toward the long-chain acyl-CoAs (C10 to C16) was high in the embryo, but low in adult chicken.     

Peroxisomal beta-oxidation: insights from comparative biochemistry

The activities of fatty acyl-CoA oxidase (FAO) and carnitine palmitoyl transferase (CPT), indices of the capacities of peroxisomal beta-oxidation and mitochondrial beta-oxidation, respectively, were determined in livers of several vertebrate species notable for differences in dietary fatty acid composition. In suckling rats FAO activities were half that in adult rats and CPT/FAO ratios twice that of adult rats. As their milk diet is dominated by medium chain fatty acids, this observation is consistent with current ideas about the role of peroxisomal beta-oxidation in rat liver in oxidation of long chain unsaturated fatty acids. In nectar-feeding hummingbirds (fatty acids synthesized de novo) FAO activities were 50% greater than adult rats and CPT/FAO ratios one-third less than adult rats, suggesting that peroxisomal beta-oxidation is relatively more important in this species, despite a fatty-acid-poor diet. In marine fish (herring, dogfish shark, hagfish) FAO activities were all less than 15% that of rats and undetectable in hagfish. CPT/FAO ratios were greater in herring (8.1) and hagfish (greater than 30) than adult rats (3.1), suggesting that peroxisomal beta-oxidation is relatively less important in these species despite a natural diet containing high levels of long chain polyunsaturated fatty acids. These data are discussed in relation to current ideas about the role of peroxisomes in beta-oxidation of fatty acids.     

Intracellular acetyl unit transport in fungal carbon metabolism

Acetyl coenzyme A (acetyl-CoA) is a central metabolite in carbon and energy metabolism. Because of its amphiphilic nature and bulkiness, acetyl-CoA cannot readily traverse biological membranes. In fungi, two systems for acetyl unit transport have been identified: a shuttle dependent on the carrier carnitine and a (peroxisomal) citrate synthase-dependent pathway. In the carnitine-dependent pathway, carnitine acetyltransferases exchange the CoA group of acetyl-CoA for carnitine, thereby forming acetyl-carnitine, which can be transported between subcellular compartments. Citrate synthase catalyzes the condensation of oxaloacetate and acetyl-CoA to form citrate that can be transported over the membrane. Since essential metabolic pathways such as fatty acid β-oxidation, the tricarboxylic acid (TCA) cycle, and the glyoxylate cycle are physically separated into different organelles, shuttling of acetyl units is essential for growth of fungal species on various carbon sources such as fatty acids, ethanol, acetate, or citrate. In this review we summarize the current knowledge on the different systems of acetyl transport that are operational during alternative carbon metabolism, with special focus on two fungal species: Saccharomyces cerevisiae and Candida albicans.     

Preliminary characterization of Yor180Cp: identification of a novel peroxisomal protein of saccharomyces cerevisiae involved in fatty acid metabolism.

Here we report the preliminary characterization of Yor180Cp, a novel peroxisomal protein involved in fatty acid metabolism in the yeast Saccharomyces cerevisiae. A computer-based screen identified Yor180Cp as a putative peroxisomal protein, and Yor180Cp targeted GFP to peroxisomes in a PEX8-dependent manner. Yor180Cp was also detected by mass spectrometric analysis of an HPLC-separated extract of yeast peroxisomal matrix proteins. YOR180C is upregulated during growth on oleic acid, and deletion of YOR180C from the yeast genome resulted in a mild but significant growth defect on oleic acid, indicating a role for Yor180Cp in fatty acid metabolism. In addition, we observed that yor180cDelta cells fail to efficiently import the enzyme Delta3,Delta2-enoyl-CoA isomerase (Eci1p) to peroxisomes. This result suggested that Yor180Cp might associate with Eci1p in vivo, and a Yor180Cp-Eci1p interaction was detected using the yeast two-hybrid system. Potential roles for Yor180Cp in peroxisomal fatty acid metabolism are discussed.