Yeast peroxisomes

Requirements for Carnitine Shuttle-Mediated Translocation of Mitochondrial Acetyl Moieties to the Yeast Cytosol

In many eukaryotes, the carnitine shuttle plays a key role in intracellular transport of acyl moieties. Fatty acid-grown Saccharomyces cerevisiae cells employ this shuttle to translocate acetyl units into their mitochondria. Mechanistically, the carnitine shuttle should be reversible, but previous studies indicate that carnitine shuttle-mediated export of mitochondrial acetyl units to the yeast cytosol does not occur in vivo. This apparent unidirectionality was investigated by constitutively expressing genes encoding carnitine shuttle-related proteins in an engineered S. cerevisiae strain, in which cytosolic acetyl coenzyme A (acetyl-CoA) synthesis could be switched off by omitting lipoic acid from growth media. Laboratory evolution of this strain yielded mutants whose growth on glucose, in the absence of lipoic acid, was l-carnitine dependent, indicating that in vivo export of mitochondrial acetyl units to the cytosol occurred via the carnitine shuttle. The mitochondrial pyruvate dehydrogenase complex was identified as the predominant source of acetyl-CoA in the evolved strains. Whole-genome sequencing revealed mutations in genes involved in mitochondrial fatty acid synthesis (MCT1), nuclear-mitochondrial communication (RTG2), and encoding a carnitine acetyltransferase (YAT2). Introduction of these mutations into the nonevolved parental strain enabled l-carnitine-dependent growth on glucose. This study indicates intramitochondrial acetyl-CoA concentration and constitutive expression of carnitine shuttle genes as key factors in enabling in vivo export of mitochondrial acetyl units via the carnitine shuttle.

This study demonstrates, for the first time, that Saccharomyces cerevisiae can be engineered to employ the carnitine shuttle for export of acetyl moieties from the mitochondria and, thereby, to act as the sole source of cytosolic acetyl-CoA. Further optimization of this ATP-independent mechanism for cytosolic acetyl-CoA provision can contribute to efficient, yeast-based production of industrially relevant compounds derived from this precursor. The strains constructed in this study, whose growth on glucose depends on a functional carnitine shuttle, provide valuable models for further functional analysis and engineering of this shuttle in yeast and other eukaryotes.

Transcriptomic analyses during the transition from biomass production to lipid accumulation in the oleaginous yeast Yarrowia lipolytica

We previously developed a fermentation protocol for lipid accumulation in the oleaginous yeast Y. lipolytica. This process was used to perform transcriptomic time-course analyses to explore gene expression in Y. lipolytica during the transition from biomass production to lipid accumulation. In this experiment, a biomass concentration of 54.6 g(CDW)/l, with 0.18 g/g(CDW) lipid was obtained in ca. 32 h, with low citric acid production. A transcriptomic profiling was performed on 11 samples throughout the fermentation. Through statistical analyses, 569 genes were highlighted as differentially expressed at one point during the time course of the experiment. These genes were classified into 9 clusters, according to their expression profiles. The combination of macroscopic and transcriptomic profiles highlighted 4 major steps in the culture: (i) a growth phase, (ii) a transition phase, (iii) an early lipid accumulation phase, characterized by an increase in nitrogen metabolism, together with strong repression of protein production and activity; (iv) a late lipid accumulation phase, characterized by the rerouting of carbon fluxes within cells. This study explores the potential of Y. lipolytica as an alternative oil producer, by identifying, at the transcriptomic level, the genes potentially involved in the metabolism of oleaginous species.

Characterization and carbon metabolism in fungi pathogenic to Triatoma infestans, a chagas disease vector

The pathogenicity of Metarhizium anisopliae (Ma) and Beauveria bassiana (Bb) isolates against Triatoma infestans, the major vector of Chagas disease in Argentina is reported. A 100% mortality was achieved with mean lethal times varying form 5.8 (Ma6) to 7.7 (Bb5) or 11.1 days (Bb10). The fatty acid, hydrocarbon, and total lipid patterns were compared for glucose-grown and alkane-grown Bb10 cultures. The alkane-grown cells showed a lipid pattern different from that of glucose-grown cells, with triacylglyercol as the major lipid fraction, whereas sterols prevailed in the glucose-grown cells. A significant reduction in the relative amounts of linoleic acid diminished the unsaturated/saturated fatty acid ratio for alkane-grown cells; in addition, large amounts of heptacosanoic and eicosanoic acids were detected in the saturated fraction. The hydrocarbon profile of Bb10 showed a saturated chain length distribution,with a marked prevalence for straight chains, ranging from n-C18 to n-C37 in the carbon skeleton, with n-C22 as the major component. Alkane-grown cells showed no qualitative changes in their hydrocarbon fraction, but a similar ratio for odd/even carbon chains. After 48-h incubation assays,[1-(14)C]acetate uptake was largely diminished following a period of alkane growth induction. Glucose-grown cells readily incorporated 19% of the labelinto phospholipids, hydrocarbons, triacylglycerols, and free fatty acids. In contrast, incorporation was reduced to 5.3% for alkane-grown cells, accounting only for phospholipid synthesis.

The facC gene of Aspergillus nidulans encodes an acetate-inducible carnitine acetyltransferase

Mutations in the facC gene of Aspergillus nidulans result in an inability to use acetate as a sole carbon source. This gene has been cloned by complementation. The proposed translation product of the facC gene has significant similarity to carnitine acetyltransferases (CAT) from other organisms. Total CAT activity was found to be inducible by acetate and fatty acids and repressed by glucose. Acetate-inducible activity was found to be absent in facC mutants, while fatty acid-inducible activity was absent in an acuJ mutant. Acetate induction of facC expression was dependent on the facB regulatory gene, and an expressed FacB fusion protein was demonstrated to bind to 5' facC sequences. Carbon catabolite repression of facC expression was affected by mutations in the creA gene and a CreA fusion protein bound to 5' facC sequences. Mutations in the acuJ gene led to increased acetate induction of facC expression and also of an amdS-lacZ reporter gene, and it is proposed that this results from accumulation of acetate, as well as increased expression of facB. A model is presented in which facC encodes a cytosolic CAT enzyme, while a different CAT enzyme, which is acuJ dependent, is present in peroxisomes and mitochondria, and these activities are required for the movement of acetyl groups between intracellular compartments.

Peroxisomal and mitochondrial carnitine acetyltransferases of the n-alkane-assimilating yeast Candida Tropicalis. Analysis of gene structure and translation products

A genomic DNA clone encoding carnitine acetyltransferases (EC 2.3.1.7), localized in two subcellular organelles, peroxisomes and mitochondria of an n-alkane-assimilating yeast Candida tropicalis, was isolated from the yeast lambda EMBL library using a carnitine acetyltransferase CDNA probe. Nucleotide sequence analysis disclosed that the open reading frame was 1881 bp, corresponding to 627 amino acids with a molecular mass of 70760 Da. Comparison of the predicted amino acid sequence of the C. tropicalis enzyme with that of Saccharomyces cerevisiae mitochondrial matrix carnitine acetyltransferase revealed 46.3% identity. It was noticeable that the C. tropicalis enzymes had amino acid sequences similar to both proposed mitochondrial and peroxisomal targeting signals. When the C. tropicalis gene was expressed in S. cerevisiae using its own 5'-upstream region, a 12-fold increase in activity was observed. Western blot analysis revealed the presence of two major proteins whose sizes corresponded to the peroxisomal and mitochondrial proteins detected in C. tropicalis. This suggested that peroxisomal and mitochondrial carnitine acetyltransferases were encoded by one gene, as suggested for the S. cerevisiae enzyme. Furthermore, we have separated and purified these enzymes from peroxisomes and mitochondria of C. tropicalis, and analyzed the amino-terminal amino acid sequences of each. The amino-terminal sequence of the mitochondrial enzyme suggested that a signal sequence had been cleaved during translocation into mitochondria. Concerning the peroxisomal enzyme, the evidence obtained indicated that in vivo the translation was initiated at the second methionine of the open reading frame.

Expression of an endogenous and a heterologous gene in Candida maltosa by using a promoter of a newly-isolated phosphoglycerate kinase (PGK) gene

A gene encoding phosphoglycerate kinase (PGK) was isolated from the genomic library of C. maltosa to construct an expression vector for this yeast. The PGK gene had an open reading frame of 1,251 base pairs encoding approximately 47-kDa polypeptide of 417 amino-acid residues. Expression of this gene assayed by Northern-blot analysis was significantly induced in cells grown on glucose but not in cells grown on n-tetradecane, n-tetradecanol, or oleic acid. By using the promoter region of this gene, an expression vector (termed pMEA1) for C. maltosa was constructed and expression of an endogenous gene (P450alk1 encoding one of cytochrome P450s for n-alkane hydroxylation in C. maltosa) and a heterologous gene (LAC4 encoding Kluyveromyces lactis beta-galactosidase) was tested. Expression of P450alk1 gene was confirmed at both mRNA and protein levels. LAC4 gene expression was confirmed by determining beta-galactosidase activity. The activity in cells grown on various carbon sources correlated very well with the expression levels of PGK mRNA in these cells.

Inhibitory action of palmitic acid on the growth of Saccharomyces cerevisiae

High concentrations of long-chain fatty acids have been found to be harmful to mammalian cells and prokaryotic organisms. This effect was investigated in Saccharomyces cerevisiae. Addition of 3 mmol/L palmitate to a yeast extract-peptone medium caused a significant inhibition of cell growth during the first 2 d of incubation, followed by renewed growth and palmitate utilization. Inhibition was also observed with palmitate concentrations down to 0.1 mmol/L. As inferred from catalase activity determinations, this effect was found to correlate with the absence of peroxisome proliferation. Finally, no inhibition was observed in exponential-phase cultures or in the presence of 0.1 g/L glucose, this suggesting that the physiological state of the cell may determine whether its growth will be inhibited by fatty acids.

Study on fatty acid binding by proteins in yeast. Dissimilar results in Saccharomyces cerevisiae and Yarrowia lipolytica

1. The presence of soluble proteins with fatty acid binding activity was investigated in cell-free extracts from Saccharomyces cerevisiae and Yarrowia lipolytica cultures. 2. No significant fatty acid binding by proteins was detected in S. cerevisiae, even when grown on a fatty acid-rich medium, thus indicating that such proteins are not essential to fatty acid metabolism. 3. An inducible fatty acid binding protein (K0.5 = 3-4 microM) was found in Y. lipolytica which had grown on a minimal medium with palmitate as the sole source of carbon and energy. 4. The relative molecular mass of this protein was 100,000 as inferred from Sephacryl S-200 gel filtration.

Sequences of two tandem genes regulated by carbon sources, one being essential for n-alkane assimilation in Candida maltosa

Several n-alkane-inducible clones were isolated from the genomic library of an n-alkane-assimilation yeast, Candida maltosa, by the differential hybridization method. Among these, one of the most predominantly expressed clones was analyzed. The nucleotide sequence of the cloned DNA fragment showed that it contained two open reading frames, one encoding a protein of 127 amino acids (aa) and the other a protein of 276 aa. The former was named POX18Cm, because the sequence was highly homologous to that of the Candida tropicalis gene, POX18, which already had been identified as encoding a small oleate-inducible peroxisomal protein. The latter, named ALI1, had no homologous sequences in the EMBL database (1990 release). Northern-blot hybridization indicated that the expression of these two genes was regulated by carbon sources in the media. From gene-disruption experiments, it was concluded that ALI1 was essential for assimilation of n-alkane by C. maltosa.

Proliferation and metabolic significance of peroxisomes in Candida boidinii during growth on D-alanine or oleic acid as the sole carbon source

We have studied the induction of peroxisomes in the methylotrophic yeast Candida boidinii by D-alanine and oleic acid. The organism was able to utilize each of these compounds as the sole carbon source and grew with growth rates of mu = 0.20 h-1 (on D-alanine) or mu = 0.43 h-1 (on oleic acid). Growth was associated with the development of many peroxisomes in the cells. On D-alanine a cluster of tightly interwoven organelles was observed which made up 6.3% of the cytoplasmic volume and were characterized by the presence of D-amino acid oxidase and catalase. On oleic acid rounded to elongated peroxisomes were dominant which were scattered throughout the cytoplasm. These organelles contained increased levels of beta-oxidation enzymes; their relative volume fraction amounted 12.8% of the cytoplasmic volume.

Study of the coinduction by fatty acids of catalase A and acyl-CoA oxidase in standard and mutant Saccharomyces cerevisiae strains

Evidence is presented that Saccharomyces cerevisiae can metabolize fatty acids via the inducible peroxisomal beta-oxidation pathway even when these acids are not the sole carbon source. The fatty acids of chain length of C10-C18 induce acyl-CoA oxidase simultaneously with catalase A but have no effect on catalase T and acyl-CoA dehydrogenase. The coinduction of both acyl-CoA oxidase and catalase A is recorded in strains with both active catalase A and T or displaying only catalase A activity. In mutants lacking catalase A, the induction of acyl-CoA oxidase is observed without a concomitant increase in catalase activity. After centrifugation in a linear Ficoll gradient of the particulate fraction from the cells grown on ethanol and oleate the activity of acyl-CoA oxidase cosediments with catalase A. The relationship of catalase A to acyl-CoA oxidase is discussed.

Fatty acid dependent hydrogen peroxide production in Lactobacillus

Lactobacillus leichmanii growing in complex medium supplemented with decanoic acid accumulated high concentrations of hydrogen peroxide in the culture. The H2O2-generating system was specifically induced by one of the saturated fatty acids from 4:0 to 16:0 or oleic acid. The induction of this system was associated with the presence of a fatty acyl-CoA-dependent H2O2-generating activity in the cell-free extracts. This activity is shown for the first time in a procaryote organism.

Construction of a host-vector system in Candida maltosa by using an ARS site isolated from its genome

To construct a host-vector system in an n-alkane-assimilating yeast, Candida maltosa, the isolation of an ARS site from its genome which replicates autonomously in C. maltosa was attempted. Leu- mutants of C. maltosa were transformed with a gene library prepared by using YEp13 (LEU2+) as a vector, and Leu+ transformants were obtained at a high frequency. A plasmid named pCS1 was isolated from the recipient cells. pCS1 contained a 6.3-kilobase (kb) fragment of the C. maltosa genome, and a 3.8-kb fragment with ARS activity was subcloned and designated the TRA (transformation ability) region. Vectors (pTRA1 and pTRA11) for C. maltosa J288 were constructed that contained this 3.8-kb fragment, pBR322, and the LEU2 gene of Saccharomyces cerevisiae. Transformation of C. maltosa J288 with these plasmids was successful by both spheroplast and lithium acetate methods. Southern blot analysis suggested that the copy number of pTRA1 in C. maltosa was between 10 and 20, and it was stably maintained during growth without selective pressure in the medium. It was also found that these vectors could transform S. cerevisiae leu2- to LEU2+, suggesting that the TRA region contained an ARS site(s) that was specific not only for C. maltosa but also for S. cerevisiae.

Purification of peroxisomal malate synthase from alkane-grown Candida tropicalis and some properties of the purified enzyme

Malate synthase, one of the key enzymes in the glyoxylate cycle, was purified from peroxisomes of alkane-grown yeast, Candida tropicalis. The enzyme was mainly localized in the matrix of peroxisomes, judging from subcellular fractionation followed by exposure of the organelles to hypotonic conditions. The molecular mass of this peroxisomal malate synthase was determined to be 250,000 daltons by gel filtration on a Sepharose 6B column as well as by ultracentrifugation. On sodium dodecylsulfate/polyacrylamide slab-gel electrophoresis, the molecular mass of the subunit of the enzyme was demonstrated to be 61,000 daltons. These results revealed that the native form of this enzyme was homo-tetrameric. Peroxisomal malate synthase showed the optimal activity pH at 8.0 and absolutely required Mg2+ for enzymatic activity. The Km values for Mg2+, acetyl-CoA and glyoxylate were 4.7 mM, 80 microM and 1.0 mM, respectively.

Peroxisomal beta-oxidation system of Candida tropicalis. Purification of a multifunctional protein possessing enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase and 3-hydroxyacyl-CoA epimerase activities

A multifunctional protein from oleate-grown cells of Candida tropicalis has been purified and partially characterized. A simple two-step purification has been developed involving ion-exchange chromatography followed by dye-ligand chromatography on blue Sepharose CL-6B. Homogeneous enzyme with a subunit Mr of 102 000 is obtained in 60% yield. The native relative molecular mass, determined by three different methods, yielded values which suggest that the enzyme is dimeric. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the purified protein revealed a single polypeptide band and reverse-phase high-performance liquid chromatography indicated a single component suggesting that this protein may consist either of two identical or very similar subunits. Three beta-oxidation activities, enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase and 3-hydroxyacyl-CoA epimerase, co-purified with this protein. The ratio of the three beta-oxidation enzyme activities remained constant during purification and was unchanged by additional chromatographic methods (adsorption and affinity chromatography), thus indicating the multifunctional nature of this protein. Enzymatic staining of the purified protein for 3-hydroxyacyl-CoA dehydrogenase and epimerase, following electrophoresis in a polyacrylamide density gradient, further supported the multifunctionality of this protein. After isopycnic centrifugation of a particulate fraction from oleate-grown cells in a linear sucrose gradient the activities of all individual beta-oxidation enzymes cosedimented with catalase and with the glyoxylate bypass enzymes. This result demonstrated the peroxisomal localization of the multifunctional enzyme. The relationship of this multifunctional protein to the two bifunctional beta-oxidation enzymes isolated from peroxisomes of rat liver and from glyoxysomes of cucumber seeds is discussed.

Increase of translatable mRNA for major microsomal proteins in n-alkane-grown Candida maltosa

In an n-alkane-assimilating Candida sp., transfer from glucose- to n-alkane-containing medium induced changes in the microsomal proteins, and several distinctive polypeptides were demonstrated in the solubilized microsomal fraction derived from n-alkane-grown cells. Long-term-labeling and pulse-labeling experiments in vivo demonstrated the synthesis of the specific microsomal polypeptides. The polypeptides were synthesized as in vitro translation products directed by polyadenylated RNA extracted from n-alkane-grown cells. Two major polypeptides were partially purified from the microsomal fraction from n-alkane-grown cells, and antiserum was prepared in a rabbit. Immunoprecipitation of these two polypeptides was accompanied by an increase in the amount of translatable mRNA. The molecular weights of the polypeptides derived from long-term-labeling, pulse-labeling and in vitro translation experiments appeared to be identical.

Synthesis in vitro of precursor-type carnitine acetyltransferase with messenger RNA from Candida tropicalis

Carnitine acetyltransferase was synthesized in vitro in the mRNA-dependent reticulocyte system with mRNA from alkane-grown or propionate-grown cells of Candida tropicalis. The protein synthesized in vitro was isolated by immunoprecipitation with antibody against peroxisomal or mitochondrial carnitine acetyltransferase and was compared with peroxisomal carnitine acetyltransferase (Mr of subunits, 64 000 and 57 000) and the mitochondrial enzyme (Mr of subunits, 64 000 and 52 000) of C. tropicalis by electrophoresis in the presence of sodium dodecyl sulfate. Nascent carnitine acetyltransferase prepared in vitro showed a hetero-oligomeric property, like the peroxisomal and mitochondrial enzymes isolated from C. tropicalis. The molecular weights of the subunits of nascent carnitine acetyltransferase were estimated to be 71 000 and 57 000, indicating the existence of the precursor form of the enzyme. By sucrose density gradient centrifugation of total mRNA, these two subunit proteins were shown to be synthesized with respective mRNAs of different sizes. The same precursor-type of carnitine acetyltransferase was obtained with the mRNAs from the alkane-grown cells and the propionate-grown cells. The results obtained suggest that a common precursor will be post-translationally modified to form the peroxisomal and mitochondrial enzymes.

Characterization of peroxisomal and mitochondrial carnitine acetyltransferases purified from alkane-grown Candida tropicalis

Properties of peroxisomal and mitochondrial carnitine acetyltransferases purified from an alkane-grown yeast, Candida tropicalis, were compared each other. The molecular weight of both enzymes was estimated to be about 420 000 by analytical ultracentrifugation and gel filtration chromatography with Sepharose 6B. However, each enzyme gave two subunits on the polyacrylamide slab gel electrophoresis in the presence of sodium dodecyl sulfate: the peroxisomal enzyme (64 000 and 57 000) and the mitochondrial enzyme (64 000 and 52 000). The subcellularly distinct enzymes gave a similar amino acid composition except for the contents of some amino acids: glycine, valine, glutamic acid and aspartic acid. Their isoelectric point was somewhat different: 5.11 for the peroxisomal enzyme and 5.22 for the mitochondrial enzyme. Both enzymes had the same amino-terminal residue (glutamic acid or glutamine) and the heat stability, and was indistinguishable immunochemically. These results suggest that peroxisomal and mitochondrial carnitine acetyltransferases of C. tropicalis cells may be products of the same nuclear gene. Differences in the molecular weight of the subunits of the enzymes would result from modification or processing of the common protein in the step of distribution to the respective organelles, that is, so-called post-translational modification.

Induction and subcellular localization of enzymes participating in propionate metabolism in Candida tropicalis

Candida tropicalis, a representative alkane- and higher fatty acid-utilizing yeast, can grow on propionate used as sole carbon and energy source. Initial pH of the medium markedly affected the growth of the yeast on propionate. In propionate-grown cells, several enzymes associated with peroxisomes and/or participating in propionate metabolism were induced in connection with the appearance of the characteristic peroxisomes. Acetate-grown cells of this yeast had only few peroxisomes, while alkane-grown cells contained conspicuous numbers of the organelles. As compared with alkane-grown cells, some specific features were observed in peroxisomes and enzymes associated with the organelles of propionate-grown cells: The shape of peroxisomes was large but the number was small; unlike localization of catalase in peroxisomes of alkane-grown cells, the enzyme of propionate-grown cells was mainly localized in cytoplasm; as for carnitine acetyltransferase localized almost equally in peroxisomes and mitochondria in alkane-grown cells, propionate-grown cells contained mainly the mitochondrial type enzyme. A propionate-activating enzyme, which was different from acetyl-CoA synthetase, was also induced in cytoplasm of propionate-grown cells. The role of carnitine acetyltransferase and the propionate-activating enzyme in propionate metabolism is discussed in comparison with the role of carnitine acetyltransferase and acetyl-CoA synthetase in acetate metabolism.

Cell-free translation and regulation of Candida tropicalis catalase messenger RNA

To gain information on metabolic control and peroxisome biogenesis in Candida tropicalis growing on n-alkanes, cell-free translation of catalase (H2O2:H2O2 oxidoreductase, EC 1.11.1.6), a general marker enzyme of peroxisomes, was performed. The level of catalase activity in alkane-grown cells was approximately 9-fold and 27-fold higher than that in ethanol-grown and glucose-grown cells, respectively. Immunochemical titration experiments with rabbit antiserum against the purified peroxisomal catalase from alkane-grown C. tropicalis indicated that the remarkable variation in the enzyme activity level on different carbon sources was ascribable to a corresponding change in the amount of the enzyme protein. When cell-free translation was carried out with the mRNA-dependent reticulocyte lysate system, total RNA prepared from alkane-grown cells was shown to direct the synthesis of catalase subunit in vitro. The identity of the cell-free translation product was ascertained by the following evidence: (a) the translation product was immuno-reactive with specific antibody to catalase and competed effectively with the authentic enzyme for immunoprecipitation; (b) it possessed a molecular weight indistinguishable from that of authentic catalase subunit (Mr 54000); (c) its peptide fragments formed by partial digestion with Staphylococcus aureus V8 protease were identical with those from the authentic enzyme. With the use of the cell-free translation system, it was indicated that the significant change in the amount of catalase protein on different carbon sources nearly paralleled that in the activity of the mRNA encoding the enzyme.

Subcellular localization of the methylcitric-acid-cycle enzymes in propionate metabolism of Yarrowia lipolytica

The subcellular localization of the four characteristic enzymes of the methylcitric acid cycle was studied with glucose-grown as well as n-alkane-grown cells of Yarrowia lipolytica. Microsomes and peroxisomes showed no cycle enzyme activities. The four cycle enzymes were constitutively localized in mitochondria, with the exception of the dual localization of the fourth enzyme, 2-methylisocitrate lyase, in mitochondria and cytoplasm, where the lyase may function to supply pyruvate (the end-product of the catabolism of the propionate residue) to various reactions.

Properties of catalase purified from whole cells and peroxisomes of n-alkane-grown Candida tropicalis

Peroxisomes appear profusely, in harmony with a marked enhancement of catalase activity level, in yeast cells growing on n-alkanes or higher fatty acids as the sole carbon source. Catalase (H2O2:H2O2 oxidoreductase, EC 1.11.1.6) was purified to homogeneity from the crude extract and from the peroxisome-containing particulate fraction of alkane-grown Candida tropicalis cells. The purified enzyme from each source was a similar protein of molecular weight 210000 composed of four identical subunits of molecular weight 54000, namely a kind of homotetramer. The enzyme contained one molecule of heme per subunit, giving the absorption spectrum characteristic of hemoprotein. Beta-(3,4-Dihydroxyphenyl)-L-alanine served as a substrate for the peroxidatic reaction by the enzyme. Ouchterlony double-diffusion analysis and immunochemical titration with rabbit antiserum against peroxisomal catalase of n-alkane-grown C. tropicalis have indicated that cytoplasmic catalase of the yeast is immunologically indistinguishable with peroxisomal catalase.

Peroxisomal and mitochondrial carnitine acetyltransferases in alkane-grown yeast Candida tropicalis

Two types of carnitine acetyltransferases (EC 2.3.1.7) were first isolated from a microorganism, alkane-grown yeast Candida tropicalis. Carnitine acetyltransferase activity was induced in the alkane-grown cells, reaching about twenty times higher than that in the glucose-grown cells. Localization of the enzyme activity was demonstrated, at least, in peroxisomes (microbodies), profusely occurred in the alkane-grown cells, and in mitochondria. Peroxisomal and mitochondrial carnitine acetyltransferases could be separated using the method of DEAE-Sephacel column chromatography and both types were found to exist in the alkane-grown cells of C. tropicalis. Each carnitine acetyltransferase was purified using Sephadex G-200, Sepharose 6B, DEAE-Sephacel and Blue-Sepharose CL-6B. In DEAE-Sephacel chromatography, peroxisomal carnitine acetyltransferase was eluted below 0.15 M KCl concentration and mitochondrial carnitine acetyltransferase above 0.15 M KCl concentration. Except for the localization, little difference was observed in their kinetic properties, substrate specificity and so on. These two carnitine acetyltransferase preparations were only specific to acetyl and propionyl groups, the substrate specificity not being so broad as that of carnitine acetyltransferase obtained from mammalian tissues. Roles of these carnitine acetyltransferases in alkane metabolism in yeast are also discussed.

Subcellular localization of long-chain alcohol dehydrogenase and aldehyde dehydrogenase in n-alkane-grown Candida tropicalis

Long-chain alcohol dehydrogenase and long-chain aldehyde dehydrogenase were induced in the cells of Candida tropicalis grown on n-alkanes. Subcellular localization of these dehydrogenases, together with that of acyl-CoA synthetase and glycerol-3-phosphate acyltransferase, was studied in terms of the metabolism of fatty acids derived from n-alkane substrates. Both long-chain alcohol and aldehyde dehydrogenases distributed in the fractions of microsomes, mitochondria and peroxisomes obtained from the alkane-grown cells of C. tropicalis. Acyl-CoA synthetase was also located in these three fractions. Glycerol-3-phosphate acyltransferase was found in microsomes and mitochondria, in contrast to fatty acid beta-oxidation system localized exclusively in peroxisomes. Similar results of the enzyme localization were also obtained with C. lipolytica grown on n-alkanes. These results suggest strongly that microsomal and mitochondrial dehydrogenases provide long-chain fatty acids to be utilized for lipid synthesis, whereas those in peroxisomes supply fatty acids to be degraded via beta-oxidation to yield energy and cell constituents.