Cloning, expression, and purification of glyoxysomal 3-oxoacyl-CoA thiolase from sunflower cotyledons

The glyoxysomal beta-oxidation system in sunflower (Helianthus annuus L.) cotyledons is distinguished by the coexistence of two different thiolase isoforms, thiolase I and II. So far, this phenomenon has only been described for glyoxysomes from sunflower cotyledons. Thiolase I (acetoacetyl-CoA thiolase, EC 2.3.1.9) recognizes acetoacetyl-CoA only, while thiolase II (3-oxoacyl-CoA thiolase, EC 2.3.1.16) exhibits a more broad substrate specificity towards 3-oxoacyl-CoA esters of different chain length. Here, we report on the cloning of thiolase II from sunflower cotyledons. The known DNA sequence of Cucumis sativus 3-oxoacyl-CoA thiolase was used to generate primers for cloning the corresponding thiolase from sunflower cotyledons. RT-PCR was then used to generate an internal fragment of the sunflower thiolase gene and the termini were isolated using 5'- and 3'-RACE. Full-length cDNA was generated using RT-PCR with sunflower thiolase-specific primers flanking the coding region. The resultant gene encodes a thiolase sharing at least 80% identity with other plant thiolases at the amino acid level. The recombinant sunflower thiolase II was expressed in a bacterial system in an active form and purified to apparent homogeneity in a single step using Ni-NTA agarose chromatography. The enzyme was purified 53.4-fold and had a specific activity of 235 nkat/mg protein. Pooled fractions from the Ni-NTA column resulted in an 83% yield of active enzyme to be used for further characterization.

Elevated basal expression of liver peroxisomal beta-oxidation enzymes and CYP4A microsomal fatty acid omega-hydroxylase in STAT5b(-/-) mice: cross-talk in vivo between peroxisome proliferator-activated receptor and signal transducer and activator of transcription signaling pathways

Long-term treatment of rodents with peroxisome proliferator chemicals, a group of structurally diverse nongenotoxic carcinogens, leads to liver cancer in a process dependent on the nuclear receptor peroxisome proliferator-activated receptor-alpha (PPARalpha). Previous in vitro studies have shown that growth hormone (GH) can inhibit PPARalpha-dependent gene expression by down-regulation of PPARalpha expression and by a novel inhibitory cross-talk involving the GH-activated transcription factor STAT5b. Presently, we evaluate the role of STAT5b in mediating these inhibitory actions of GH on PPAR function using a STATb-deficient mouse model. Protein levels of three PPARalpha-responsive peroxisomal beta-oxidation pathway enzymes (fatty acyl-CoA oxidase, 3-ketoacyl-CoA thiolase, and L-bifunctional enzyme) were increased up to two- to threefold in STAT5b(-/-) relative to wild-type control mouse liver, as was the basal expression of two PPARalpha-regulated cytochrome P450 4A proteins. In contrast, protein levels of two PPARalpha-unresponsive peroxisomal enzymes, catalase and urate oxidase, were not affected by the loss of STAT5b. A corresponding increase in expression of fatty acyl-CoA oxidase and L-bifunctional enzyme mRNA, as well as PPARalpha mRNA, was observed in the STAT5b-deficient mice, suggesting a transcriptional mechanism for the observed increases. Although basal liver expression of PPARalpha and its target genes was thus elevated in STAT5b(-/-) mice, the clofibrate-induced level of enzyme expression was unaffected, suggesting that the inhibitory effects of STAT5b are overcome at high concentrations of PPARalpha activators. These findings support the hypothesis that GH and potentially other endogenous activators of STAT5b help to maintain liver PPARalpha function at a low basal level and may thereby moderate PPARalpha-dependent hepatocarcinogenesis and other responses stimulated by exposure to low levels of environmental chemicals of the peroxisome proliferator class.

Type-II 3-oxoacyl-CoA thiolase of the nematode Caenorhabditis elegans is located in peroxisomes, highly expressed during larval stages and induced by clofibrate

We examined the expression and localization of type-II 3-oxoacyl-CoA thiolase in the nematode Caenorhabditis elegans. Type-II thiolase acts on 3-oxoacyl-CoA esters with a methyl group at the alpha carbon, whereas conventional thiolases do not. Mammalian type-II thiolase, which is also termed sterol carrier protein x (SCPx) or SCP2/3-oxoacyl-CoA thiolase, is located in the peroxisomes and involved in phytanic acid degradation and most probably in bile acid synthesis. The nematode enzyme lacks the SCP2 domain, which carries the peroxisomal-targeting signal, but produces bile acids in a cell-free system. Northern and Western blot analyses demonstrated that C. elegans expressed type-II thiolase throughout its life cycle, especially during the larval stages, and that the expression was significantly enhanced by the addition of clofibrate at 5 mM or more to the culture medium. Whole-mount in situ hybridization and immunostaining of L4 larvae revealed that the enzyme was mainly expressed in intestinal cells, which are multifunctional like many of the cell types in C. elegans. Subcellular fractionation and indirect immunoelectron microscopy of the nematode detected the enzyme in the matrix of peroxisomes. These results indicate the fundamental homology between mammalian SCPx and the nematode enzyme regardless of whether the SCP2 part is fused, suggesting their common physiological roles.

Improved production of poly(4-hydroxybutyrate) by Comamonas acidovorans and its freeze-fracture morphology

Production of poly(4-hydroxybutyrate) [P(4HB)] by Comamonas acidovorans JCM10181 was studied by introducing additional copies of its PHA synthase gene and the beta-ketothiolase gene. A multi-copy-number broad-host-range plasmid vector, pJRD215, was modified to contain the strong hybrid trc promoter in order to express these genes in the wild-type C. acidovorans. Increased copy-number of genes resulted in significant increase in the activities of corresponding enzymes, which could further be increased by inducing with isopropyl-beta-D-thiogalactopyranoside (IPTG), indicating that the expression is under the transcriptional control of the trc promoter. P(4HB) biosynthesis in the recombinant C. acidovorans increased 2-fold to constitute more than 60 wt% of the dry cell weight. No significant decrease in the number-average molecular weights of P(4HB) in the recombinant strain was observed when compared with that of the wild-type. Freeze-fracture electron microscopy of intracellular P(4HB) granules revealed almost similar fracture morphology to the well-known mushroom-type deformation shown by polyhydroxyalkanoates with medium-chain-length monomers.

The role of the peroxisome proliferator-activated receptor alpha (PPAR alpha) in the control of cardiac lipid metabolism

The postnatal mammalian heart uses mitochondrial fatty acid oxidation (FAO) as the chief source of energy to meet the high energy demands necessary for pump function. Flux through the cardiac FAO pathway is tightly controlled in accordance with energy demands dictated by diverse physiologic and dietary conditions. In this report, we demonstrate that the lipid-activated nuclear receptor, peroxisome proliferator-activated receptor alpha (PPARalpha), regulates the expression of several key enzymes involved in cardiac mitochondrial FAO. In response to the metabolic stress imposed by pharmacologic inhibition of mitochondrial long-chain fatty acid import with etomoxir, PPARa serves as a molecular 'lipostat' factor by inducing the expression of target genes involved in fatty acid utilization including enzymes involved in mitochondrial and peroxisomal beta-oxidation pathways. In mice lacking PPARalpha (PPARalpha-/- mice), etomoxir precipitates a cardiac phenotype characterized by myocyte lipid accumulation. Surprisingly, this metabolic regulatory response is influenced by gender as demonstrated by the observation that male PPARalpha-/- mice are more susceptible to the metabolic stress compared to female animals. These results identify an important role for PPARalpha in the control of cardiac lipid metabolism.

Comparison of the stability and substrate specificity of purified peroxisomal 3-oxoacyl-CoA thiolases A and B from rat liver

The specific activities and substrate specificities of 3-oxoacyl-CoA thiolase A (thiolase A) purified from normal rat liver peroxisomes and 3-oxoacyl-CoA thiolase B (thiolase B) isolated from livers of rats treated with the peroxisome proliferator clofibrate were virtually identical. The enzymes could be distinguished by their N-terminal amino acid sequences, their isoelectric points and their stability, the latter being higher for thiolase A. Contrary to thiolase B, which showed a marked cold lability in the presence of KCl by dissociating into monomers with poor activity, thiolase A retained its full activity and its homodimeric structure under these conditions.

Genetic evaluation of physiological functions of thiolase isoenzymes in the n-alkalane-assimilating yeast Candida tropicalis

The n-alkane-assimilating diploid yeast Candida tropicalis possesses three thiolase isozymes encoded by two pairs of alleles: cytosolic and peroxisomal acetoacetyl-coenzyme A (CoA) thiolases, encoded by CT-T1A and CT-T1B, and peroxisomal 3-ketoacyl-CoA thiolase, encoded by CT-T3A and CT-T3B. The physiological functions of these thiolases have been examined by gene disruption. The homozygous ct-t1a delta/t1bdelta null mutation abolished the activity of acetoacetyl-CoA thiolase and resulted in mevalonate auxotrophy. The homozygous ct-t3a delta/t3b delta null mutation abolished the activity of 3-ketoacyl-CoA thiolase and resulted in growth deficiency on n-alkanes (C10 to C13). All thiolase activities in this yeast disappeared with the ct-t1a delta/t1bdelta and ct-t3a delta/t3bdelta null mutations. To further clarify the function of peroxisomal acetoacetyl-CoA thiolases, the site-directed mutation leading acetoacetyl-CoA thiolase without a putative C-terminal peroxisomal targeting signal was introduced on the CT-T1A locus in the ct-t1bdelta null mutant. The truncated acetoacetyl-CoA thiolase was solely present in cytoplasm, and the absence of acetoacetyl-CoA thiolase in peroxisomes had no effect on growth on all carbon sources employed. Growth on butyrate was not affected by a lack of peroxisomal acetoacetyl-CoA thiolase, while a retardation of growth by a lack of peroxisomal 3-ketoacyl-CoA thiolase was observed. A defect of both peroxisomal isozymes completely inhibited growth on butyrate. These results demonstrated that cytosolic acetoacetyl-CoA thiolase was indispensable for the mevalonate pathway and that both peroxisomal acetoacetyl-CoA thiolase and 3-ketoacyl-CoA thiolase could participate in peroxisomal beta-oxidation. In addition to its essential contribution to the beta-oxidation of longer-chain fatty acids, 3-ketoacyl-CoA thiolase contributed greatly even to the beta-oxidation of a C4 substrate butyrate.

Newly identified Chinese hamster ovary cell mutants defective in peroxisome biogenesis represent two novel complementation groups in mammals

We isolated peroxisome biogenesis mutants from Chinese hamster ovary (CHO) cells, using the 9-(1'-pyrene)nonanol/ultraviolet (P9OH/ UV) method and wild-type CHO-K1 cells that had been stably transfected with cDNA encoding Pex2p (formerly peroxisome assembly factor-1, PAF-1). Three mutant cell clones, ZP110, ZP111, and ZP114, showed cytosolic localization of catalase, thereby indicating a defect in peroxisome biogenesis, whereas ZP112 and ZP113 contained fewer but larger catalase-positive particles. Mutant ZP115 displayed an aberrant, tubular structure immunoreactive to anti-catalase antibody. Mutants lacking morphologically recognizable peroxisomes also showed the typical peroxisome assembly-defective phenotype such as severe loss of catalase latency and resistance to 12-(1'-pyrene)dodecanoic acid (P12)/UV treatment. ZP110 and ZP111, and ZP114 were found to belong to two novel complementation groups, respectively, by complementation group analysis with cDNA transfection and cell fusion. Cell fusion with fibroblasts from patients with peroxisome biogenesis disorders such as Zellweger syndrome revealed that ZP110 and ZP114 could not be classified to any of human complementation groups. Thus, ZP110/ZP111 and ZP114 are the first, two peroxisome-deficient cell mutants of newly identified complementation groups distinct from the ten mammalian groups previously characterized.

Effects of ubiquinone and mevalonic acid on hepatic peroxisomal enzymes induced by dehydroepiandrosterone

The adrenal steroid, dehydroepiandrosterone (DHEA) has been identified as a peroxisome proliferator. We examined the effects of the cellular antioxidant ubiquinone and its precursor mevalonic acid on the induction of enzymes associated with DHEA-mediated peroxisome proliferation in male F-344 rats. Upon treatment with DHEA (300 mg/kg orally for 14 days), there was a significant increase in hepatic activities of peroxisomal beta-oxidation (3 fold), 3-ketoacyl-CoA thiolase (4 fold) and catalase (1.8 fold). Co-administration of either mevalonic acid (100 mg/kg intraperitoneally) or ubiquinone (50 mg/kg orally) with DHEA significantly attenuated the DHEA-mediated induction of these enzymes. However, neither ubiquinone nor mevalonic acid alone significantly altered peroxisomal enzyme activities in rat liver. These data suggest that exogenous administration of ubiquinone or mevalonic acid can modulate the induction of the enzymes involved in peroxisome proliferation.

The requirement of heat shock cognate 70 protein for mitochondrial import varies among precursor proteins and depends on precursor length

The cytosolic heat shock cognate 70-kDa protein (hsc70) is required for efficient import of ornithine transcarbamylase precursor (pOTC) into rat liver mitochondria (K. Terada, K. Ohtsuka, N. Imamoto, Y. Yoneda, and M. Mori, Mol. Cell. Biol. 15:3708-3713, 1995). The requirement of hsc70 for mitochondrial import of various precursor proteins and truncated pOTCs was studied by using an in vitro translation import system in which hsc70 was completely depleted. hsc70-dependent import of pOTC was about 60% of the total import, while import of the aspartate aminotransferase precursor, the serine:pyruvate aminotransferase precursor, and 3-oxoacyl coenzyme A thiolase was about 50, 30, and 0%, respectively. The subunit sizes of these four precursor proteins were 40 to 47 kDa. When pOTC was serially truncated from the COOH terminal, the hsc70 requirement decreased gradually and was not evident for the shortest truncated pOTCs of 90 and 72 residues. These truncated pOTCs were imported and proteolytically processed rapidly in 0.5 to 2 min at 25 degrees C, and the processed mature portions and the presequence portion were rapidly degraded. Sucrose gradient centrifugation analysis followed by import assay showed that pOTC synthesized in rabbit reticulocyte lysate forms an import-competent complex of about 11S in an hsc70-dependent manner. S values of import-competent forms of aspartate aminotransferase precursor, serine:pyruvate aminotransferase precursor, and 3-oxoacyl coenzyme A thiolase were 9S, 9S, and 4S, respectively. Thus, the S value decreased as the hsc70 dependency decreased. Precursor proteins were coimmunoprecipitated from the reticulocyte lysate containing the newly synthesized precursor proteins with an hsc70 antibody. The amount of coimmunoprecipitated proteins was much larger in the absence of ATP than in its presence. Among the four precursor proteins, the amount of coimmunoprecipitated protein decreased as the hsc70 dependency decreased.

cDNA cloning and expression of a gene for 3-ketoacyl-CoA thiolase in pumpkin cotyledons

A cDNA clone for 3-ketoacyl-CoA thiolase (EC 2.3.1.16) was isolated from a lambda gt11 cDNA library constructed from the poly(A)+ RNA of etiolated pumpkin cotyledons. The cDNA insert contained 1682 nucleotides and encoded 461 amino acid residues. A study of the expression in vitro of the cDNA and analysis of the amino-terminal sequence of the protein indicated that pumpkin thiolase is synthesized as a precursor which has a cleavable amino-terminal presequence of 33 amino acids. The amino-terminal presequence was highly homologous to typical amino-terminal signals that target proteins to microbodies. Immunoblot analysis showed that the amount of thiolase increased markedly during germination but decreased dramatically during the light-inducible transition of microbodies from glyoxysomes to leaf peroxisomes. By contrast, the amount of mRNA increased temporarily during the early stage of germination. In senescing cotyledons, the levels of the thiolase mRNA and protein increased again with the reverse transition of microbodies from leaf peroxisomes to glyoxysomes, but the pattern of accumulation of the protein was slightly different from that of malate synthase. These results indicate that expression of the thiolase is regulated in a similar manner to that of other glyoxysomal enzymes, such as malate synthase and citrate synthase, during seed germination and post-germination growth. By contrast, during senescence, expression of the thiolase is regulated in a different manner from that of other glyoxysomal enzymes.

Conversion of a nonprocessed mitochondrial precursor protein into one that is processed by the mitochondrial processing peptidase

Mitochondrial processing peptidase (MPP) cleaves the signal sequence from a variety of mitochondrial precursor proteins. A subset of mitochondrial proteins, including rhodanese and 3-oxoacyl-CoA thiolase, are imported into the matrix space, yet are not processed. Rhodanese signal peptide and translated protein were recognized by MPP, as both were inhibitors of processing. The signal peptide of precursor aldehyde dehydrogenase consists of a helix-linker-helix motif but when the RGP linker is removed, processing no longer occurs (Thornton, K., Wang, Y., Weiner, H., and Gorenstein, D. G. (1993) J. Biol. Chem. 268, 19906-19914). Disruption of the helical signal sequence of rhodanese by the addition of the RGP linker did not allow cleavage to occur. However, addition of a putative cleavage site allowed the protein to be processed. The same cleavage site was added to 3-oxoacyl-CoA thiolase, but this protein was still not processed. Thiolase and linker-deleted aldehyde dehydrogenase signal peptides were poor inhibitors of MPP. It can be concluded that both a processing site and the structure surrounding this site are important for MPP recognition.

Phosphate concentration regulates transcription of the Acinetobacter polyhydroxyalkanoic acid biosynthetic genes

The polyhydroxyalkanoic acid (PHA) biosynthetic gene locus was cloned and characterized from an Acinetobacter sp. isolated from activated sludge. Nucleotide sequence analysis identified three clustered genes, phaAAc (encoding a beta-ketothiolase), phaBAc (encoding an acetoacetyl coenzyme A reductase), and phaCAc (encoding a PHA synthase). In addition, an open reading frame (ORF1) with potential to encode a 13-kDa protein was identified within this locus. The sequence of the putative translational product of ORF1 does not show significant similarity to any sequences in the database. A plasmid containing the Acinetobacter pha locus conferred the ability to accumulate poly-beta-hydroxybutyrate on its Escherichia coli host. These genes appear to lie in an operon transcribed by two promoters upstream of phaBAc, an apparent constitutive promoter, and a second promoter induced by phosphate starvation and under pho regulon control. These as well as a number of additional potential transcription start points were identified by a combination of primer extension and promoter-chloramphenicol acetyltransferase gene fusion studies carried out in Acinetobacter or E. coli transformants.

Characterization of a transcriptional control element involved in proliferation of peroxisomes in yeast in response to oleate

Oleate induces the transcription of genes involved in peroxisome biogenesis and stimulates the proliferation of these organelles in Saccharomyces cerevisiae. Previously, we have reported the identification of a region containing a positive regulatory element in the 5' flanking region of the FOX3 gene encoding the peroxisomal enzyme 3-oxoacyl-CoA thiolase. This region contains a 23-bp imperfect inverted-repeat sequence. Full induction, in response to oleate, is mediated by the intact dyad. However, one half-site of the inverted repeat is also able to mediate induction of transcription in response to oleate, albeit to a small extent. Furthermore, the weak binding of protein to each part of the inverted repeat proved to be correlated with the weak activation of transcription, in support of oleate. A DNase-I footprint covered the entire dyad and DNA band-shift experiments indicated that one or more trans-acting factors bind to the imperfect palindrome. The binding of protein to this element seems to be correlated with transcriptional activation, since mutations in both halves of the inverted dyad affected both transcriptional activation and protein binding in vitro. Similar oleate-responsive elements are commonly found in the 5' flanking regions of genes encoding proteins involved in peroxisome biogenesis and the factor(s) binding to oleate-responsive element(s) could therefore be involved in coordination of the expression of oleate-inducible genes and the proliferation of peroxisomes.

Induction of enzymes involved in fatty acid beta-oxidation in Pseudomonas fragi B-0771 cells grown in media supplemented with fatty acid

Induction of the enzymes involved in fatty acid beta-oxidation in Pseudomonas fragi B-0771 cells grown in a medium containing straight chain saturated fatty acids was studied. The acyl-CoA dehydrogenase (ACDH) activity was induced during the exponential phase in cells grown in palmitic acid-supplemented medium, reached a maximum at the early stationary phase, and then gradually decreased thereafter. Changes in the overall activities of 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase, both existing on the multienzyme complex (HDT) involved in fatty acid beta-oxidation, were similar to that in ACDH activity. Straight chain saturated fatty acids having more than 6 carbon atoms could induce both the ACDH and HDT activities, and C13-C15 fatty acids caused the greatest induction of both activities. Changes in the overall activities of 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase correlated with that in the amount of the alpha-subunit of HDT during the entire culture period in the medium containing palmitic acid. Surprisingly, the stoichiometry of the alpha- and beta-subunit proteins of HDT was not maintained into the stationary phase culture, though the genes encoding the alpha- and beta-subunits are tandemly coded in bacterial genomic DNA.

Immunocytochemical localization of peroxisomal beta-oxidation enzymes in human fetal liver

In the majority of congenital peroxisomal disorders, beta-oxidation of very long chain fatty acids is deficient. We have investigated the appearance and localization of the three peroxisomal beta-oxidation enzymes in normal fetal liver (fertilization age between 5 and 18 weeks) with protein A-gold immunocytochemistry and silver enhancement for light microscopic visualization. With specificity-tested polyclonal antibodies, acyl-CoA-oxidase, bifunctional enzyme, and 3-oxoacyl-CoA thiolase were localized in the peroxisomes of the parenchymal cells, which appear as brown or black granules. In the youngest specimen, no immunopositive reaction was obtained. A weak reaction with anti-thiolase was obtained at the age of 6-7 weeks. At a fertilization age of 8 weeks, peroxisomes could be distinctly visualized after immunostaining for all three enzymes. From a staining series with anti-thiolase on simultaneously treated slides, it appears that the amount of antigen per peroxisome and the organelle size increase between the seventh and eighteenth weeks. These data should enable a more specific diagnosis in fetal liver biopsies from pregnancies at risk and after termination of pregnancy.

A noncleavable signal for mitochondrial import of 3-oxoacyl-CoA thiolase

Rat 3-oxoacyl-CoA thiolase, an enzyme of the fatty acid beta-oxidation cycle, is located in the mitochondrial matrix. Unlike most mitochondrial matrix proteins, the thiolase is synthesized with no transient presequence and possesses information for mitochondrial targeting and import in the mature protein of 397 amino acid residues. cDNA sequences encoding various portions of the thiolase were fused in frame to the cDNA encoding the mature portion of rat ornithine transcarbamylase (lacking its own presequence). The fusion genes were transfected into COS cells, and subcellular localization of the fusion proteins was analyzed by cell fractionation with digitonin. When the mature portion of ornithine transcarbamylase was expressed, it was recovered in the soluble fraction. On the other hand, the fusion proteins containing the NH2-terminal 392, 161, or 61 amino acid residues of the thiolase were recovered in the particulate fraction, whereas the fusion protein containing the COOH-terminal 331 residues (residues 62-392) was recovered in the soluble fraction. Enzyme immunocytochemical and immunoelectron microscopic analyses using an anti-ornithine transcarbamylase antibody showed mitochondrial localization of the fusion proteins containing the NH2-terminal portions of the thiolase. These results indicate that the NH2-terminal 61 amino acids of rat 3-oxoacyl-CoA thiolase function as a noncleavable signal for mitochondrial targeting and import of this enzyme protein. Pulse-chase experiments showed that the ornithine transcarbamylase precursor and the thiolase traveled from the cytosol to the mitochondria with half-lives of less than 5 min, whereas the three fusion proteins traveled with half-lives of 10-15 min. Interestingly, in the cells expressing the fusion proteins, the mitochondria showed abnormal shapes and were filled with immunogold-positive crystalloid structures.

Transport of proteins into mitochondrial matrix. Evidence suggesting a common pathway for 3-ketoacyl-CoA thiolase and enzymes having presequences

Rat liver 3-ketoacyl-CoA thiolase, a mitochondrial matrix enzyme which catalyzes a step of fatty acid beta-oxidation, was synthesized in a rabbit reticulocyte lysate cell-free system. The in vitro product was apparently the same in molecular size and charge as the subunit of the mature enzyme. The enzyme synthesized in vitro was transported into isolated rat liver mitochondria in an energy-dependent manner. In pulse experiments with isolated rat hepatocytes at 37 degrees C, the radioactivity of the newly synthesized enzyme in the cytosolic fraction remained essentially unchanged during 5-20 min of incubation, whereas that of the enzyme in the particulate fraction increased with time during the incubation. The pulse-labeled enzyme disappeared with an apparent half-life of less than 3 min from the cytosolic fraction, in pulse-chase experiments. Purified 3-ketoacyl-CoA thiolase inhibited the mitochondrial uptake and processing of the precursors of the other matrix enzymes, ornithine carbamoyltransferase, medium-chain acyl-CoA dehydrogenase and acetoacetyl-CoA thiolase. These results indicate that 3-ketoacyl-CoA thiolase has an internal signal which is recognized by the mitochondria and suggest that this enzyme and the three others are transported into the mitochondria by a common pathway.

Synthesis of 3-ketoacyl-CoA thiolase of rat liver peroxisomes on free polyribosomes as a larger precursor. Induction of thiolase mRNA activity by clofibrate

The site of synthesis and induction by clofibrate of peroxisomal 3-ketoacyl-CoA thiolase (acetyl-CoA acyltransferase; EC 2.3.1.16) was investigated. Free and membrane-bound polyribosomal RNA species from the livers of normal rats and rats treated with clofibrate, a hypolipidaemic drug that causes marked proliferation of peroxisomes, were translated in a nuclease-treated rabbit reticulocyte-lysate cell-free protein-synthesizing system with [35S]methionine as label. The cell-free translation products were immunoprecipitated with monospecific X rabbit anti-thiolase serum and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and fluorography. Thiolase mRNA was found predominantly in free polyribosomes, in both normal and clofibrate-treated rats. Clofibrate treatment increased mRNA activity for thiolase approx. 20-fold. The translation product of clofibrate-induced thiolase mRNA migrated slightly faster in sodium dodecyl sulphate/polyacrylamide-gel electrophoresis than did the translation product of normal thiolase mRNA. Both the normal and the clofibrate-induced translation products were approx. 6000 Da larger than the 41000-Da subunit of the purified enzyme. Immunoblot analysis of liver homogenates, isolated peroxisomes and the purified enzyme indicated that the thiolase subunit was approx. 41000 Da in all samples, ruling out proteolysis during the purification of thiolase. Thiolase biogenesis thus differs from that of rat liver peroxisomal proteins studied previously in that it is synthesized as a larger precursor, implying post-translational import of thiolase into peroxisomes with proteolytic processing. Clofibrate apparently alters the size as well as the amount of the translation product.

Evidence for a complex of three beta-oxidation enzymes in Escherichia coli: induction and localization

The enzymes for beta-oxidation of fatty acids in inducible and constitutive strains of Escherichia coli were assayed in soluble and membrane fractions of disrupted cells by using fatty acid and acyl-coenzyme A (CoA) substrates containing either 4 or 16 carbon atoms in the acyl moieties. Cell fractionation was monitored, using succinic dehydrogenase as a membrane marker and glucose 6-phosphate dehydrogenase as a soluble marker. Acyl-CoA synthetase activity was detected exclusively in the membrane fraction, whereas acyl-CoA dehydrogenase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase activities that utilized both C4 and C16 acyl-CoA substrates were isolated from the soluble fraction. 3-Hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and 3-ketoacyl-CoA thiolase activities assayed with both C4 and C16 acyl-CoA substrates co-chromatographed on gel filtration and ion-exchange columns and cosedimented in glycerol gradients. The data show that these three enzyme activities of the fad regulon can be isolated as a multienzyme complex. This complex dissociates in very dilute preparations; however, in those preparations where the three activities are separated, the fractionated species retain activity with both C4 and C16 acyl-CoA substrates.