An enzyme-bound intermediate in the biosynthesis of 3-hydroxy-3-methylglutaryl-coenzyme A

1. Purified 3-hydroxy-3-methylglutaryl-CoA synthase from baker's yeast (free from acetoacetyl-CoA thiolase activity) catalysed an exchange of acetyl moiety between 3'-dephospho-CoA and CoA. The exchange rate was comparable with the overall velocity of synthesis of 3-hydroxy-3-methylglutaryl-CoA. 2. Acetyl-CoA reacted with the synthase, giving a rapid ;burst' release of CoA proportional in amount to the quantity of enzyme present. The ;burst' of CoA was released from acetyl-CoA, propionyl-CoA and succinyl-CoA (3-carboxypropionyl-CoA) but not from acetoacetyl-CoA, hexanoyl-CoA, dl-3-hydroxy-3-methylglutaryl-CoA, or other derivatives of glutaryl-CoA. 3. Incubation of 3-hydroxy-3-methylglutaryl-CoA synthase with [1-(14)C]acetyl-CoA yielded protein-bound acetyl groups. The K(eq.) for the acetylation was 1.2 at pH7.0 and 4 degrees C. Acetyl-labelled synthase was isolated free from [1-(14)C]acetyl-CoA by rapid gel filtration at pH6.1. The [1-(14)C]acetyl group was removed from the protein by treatment with hydroxylamine, CoA or acetoacetyl-CoA but not by acid. When CoA or acetoacetyl-CoA was present the radioactive product was [1-(14)C]acetyl-CoA or 3-hydroxy-3-methyl-[(14)C]glutaryl-CoA respectively. 4. The isolated [1-(14)C]acetyl-enzyme was slowly hydrolysed at pH6.1 and 4 degrees C with a first-order rate constant of 0.005min(-1). This rate could be stimulated either by raising the pH to 7.0 or by the addition of desulpho-CoA. 5. These properties are interpreted in terms of a mechanism in which 3-hydroxy-3-methyl-glutaryl-CoA synthase is acetylated by acetyl-CoA to give a stable acetyl-enzyme, which then condenses with acetoacetyl-CoA yielding a covalent derivative between 3-hydroxy-3-methylglutaryl-CoA and the enzyme which is then rapidly hydrolysed to free enzyme and product.     

3-Hydroxy-3-methylglutaryl-coenzyme A synthase from ox liver. Purification, molecular and catalytic properties.

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (EC 4.1.3.5) was purified to homogeneity from ox liver and obtained essentially free from acetoacetyl-CoA thiolase activity. The purification procedure included substrate elution from cellulose phosphate and chromatofocusing. The relative molecular mas was about 100 000 and S20,w0 was 6.36S. The enzyme appears to be a dimer of identical subunits (Mr 47 900). The Km for acetoacetyl-CoA is extremely low (less than 0.5 microM), and acetoacetyl-CoA (Acac-CoA) gives marked substrate inhibition (KiAcac-CoA = 3.5 microM) that is competitive with respect to acetyl-CoA. Both CoA and DL-3-hydroxy-3-methylglutaryl-CoA give mixed product inhibition with respect to acetyl-CoA, which is compatible with a Ping Pong mechanism in which both products can form kinetically significant complexes with two forms of the enzyme. The two forms are most likely to be free enzyme and an acetyl-enzyme intermediate.     

Some properties of 3-hydroxy-3-methylglutaryl-coenzyme A synthase from ox liver.

Mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (EC 4.1.3.5) was purified from ox liver, and obtained essentially free from 3-oxoacyl-CoA thiolases. The kinetic behaviour, like that of the synthases from chicken liver and yeast, is compatible with a reaction pathway involving condensation of an acetyl-enzyme with acetoacetyl-CoA. The Km for acetoacetyl-CoA, less than 1 micronM at pH 7.8, may possibly be low enough to permit rapid ketogenesis under physiological conditions without the need for a binary complex between the synthase and oxoacyl-CoA thiolase.     

Intracellular localization of the 3-hydroxy-3-methylglutaryl coenzme A cycle enzymes in liver. Separate cytoplasmic and mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A generating systems for cholesterogenesis and ketogenesis.

Acetoacetyl-CoA thiolase and 3-hydroxy-3-methylglutaryl coenzyme synthase which comprise the 3-hydroxy-3-methylglutaryl-CoA-generating system(s) for hepatic cholesterogenesis and ketogenesis exhibit dual mitochondrial and cytoplasmic localization. Twenty to forty per cent of the thiolase and synthase of avian and rat liver are localized in the cytoplasmic compartment, the remainder residing in the mitochondria. In contrast, 3-hydroxy-3 methylglutaryl-CoA lyase, an enzyme unique to the "3-hydroxy-3-methylglutaryl-CoA cycle" of ketogenesis, appears to be localized in the mitochondrion. The small proportion, 4 to 8 percent, of this enzyme found in the cytoplasmic fraction appears to arise via leakage from the mitochondria during cell fractionation in that its properties, pI and stability, are identical to those of the mitochondrial lyase. These results are consistent with the view that ketogenesis which involves all three enzymes, acetoacetyl-CoA thiolase, 3-hydroxy-3-methylglutaryl-CoA synthase and 3-hydroxy-3-methylglutaryl-CoA lyase, occurs exclusively in the mitochondrion, whereas cholesterogenesis, a pathway which involves only the 3-hydroxy-3-methylglutaryl-CoA synthesizing enzymes, is restricted to the cytoplasm. Further fractionation of isolated mitochondria from chicken and rat liver showed that all three of the 3-hydroxy-3-methylglutaryl-CoA cycle enzymes are soluble and are localized within the matrix compartment of the mitochondrion. Likewise, cytoplasmic acetoacetyl-CoA thiolase and 3-hydroxy-3-methylglutaryl-CoA synthase are soluble cytosolic enzymes, no thiolase or synthase activity being detectable in the microsomal fraction. Chicken liver mitochondrial 3-hydroxy-3methylglutaryl-CoA synthase activity consists of a single enzymic species with a pI of 7.2, whereas the cytoplasmic activity is composed of at least two species with pI values of 4.8 and 6.7. Thus it is evident that the mitochondrial and cytoplasmic species are molecularly distinct as has been shown to be the case for the mitochondrial and cytoplasmic acetoacetyl-CoA thiolases from avian liver (Clinkenbeard, K. D., Sugiyama, T., Moss, J., Reed, W. D., and Lane, M. D. (1973) J. Biol. Chem. 248, 2275). Substantial mitochondrial 3-hydroxy-3-methylglutaryl-CoA lyase activity is present in all tissues surveyed, while only liver and kidney possess significant mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase activity. Therefore, it is proposed that tissues other than liver and kidney are unable to generate acetoacetate because they lack the mitochondrial synthase.     

A survey of enzymes which generate or use acetoacetyl thioesters in rat liver

Reactions that generate and remove acetoacetyl-CoA and acetoacetate were measured in mitochondria and cytosol of rat liver. The activities surveyed include acetoacetyl-CoA hydrolase, acetoacetyl-glutathione hydrolase, acetoacetyl-CoA:glutathione acyl transferase, 3-ketothiolases I and II, 3-hydroxy-3-methylglutaryl-CoA lyase and synthase, and acetoacetyl-CoA synthetase. Phosphocellulose chromatography shows that cytosol contains at least four acetoacetyl-CoA hydrolase activities, two of which do not coincide with 3-ketothiolases or 3-hydroxy-3-methylglutaryl-CoA lyase, while mitochondria contain at least three acetoacetyl-CoA hydrolase activities that overlap partially or completely with 3-ketothiolases and 3-hydroxy-3-methyl-glutaryl-CoA lyase. Two of the mitochondrial acetoacetyl-CoA hydrolase activities are not found in cytosol. Cytosol contains at least two and mitochondrial extracts at least six acetoacetyl-glutathione hydrolase activities. Mitochondria and cytosol both contain two isozymes of 3-ketoacyl-CoA thiolase (thiolases Ia and Ib). Chain length specificities show that the mitochondrial and cytosolic forms of thiolase Ia differ from each other. We report a new isozyme of 3-ketoacyl-CoA thiolase (thiolase I) in rat liver cytosol.     

The kinetic mechanism of 3-hydroxy-3-methylglutaryl-coenzyme A synthase from baker''s yeast

1. The effect of independent variation of both acetyl-CoA and acetoacetyl-CoA on the initial velocity at pH8.0 and pH8.9 gives results compatible with a sequential mechanism involving a modified enzyme tentatively identified as an acetyl-enzyme, resulting from the reaction with acetyl-CoA in the first step of a Ping Pong (Cleland, 1963a) reaction. 2. Acetoacetyl-CoA gives marked substrate inhibition that is competitive with acetyl-CoA. This suggests formation of a dead-end complex with the unacetylated enzyme and is in accord with the inhibition pattern given by 3-oxohexanoyl-CoA, an inactive analogue of acetoacetyl-CoA. 3. The inhibition pattern given by products of the reaction is compatible with the above mechanism. CoA gives mixed inhibition with respect to both substrates, whereas dl-3-hydroxy-3-methylglutaryl-CoA competes with acetyl-CoA but gives uncompetitive inhibition with respect to acetoacetyl-CoA. 4. 3-Hydroxy-3-methylglutaryl-CoA analogues lacking the 3-hydroxyl group are found to compete, like 3-hydroxy-3-methylglutaryl-CoA, with acetyl-CoA but have K(i) values ninefold higher, indicating the importance of the 3-hydroxyl group in the interaction. 5. A comparison of inhibition by CoA and desulpho-CoA at pH8.0 and pH8.9 shows that at the higher pH value a kinetically significant reversal of the formation of acetyl-enzyme can occur. 6. Acetyl-CoA homologues do not act as substrates and compete only with acetyl-CoA. A study of the variation of K(i) with acyl-chain length suggests the presence near the active centre of a hydrophobic region. 7. These results are discussed in terms of a kinetic mechanism in which there is only one CoA-binding site the specificity of which is altered by acetylation of the enzyme. 8. The rate of 3-hydroxy-3-methylglutaryl-CoA synthesis in yeast is calculated from the kinetic constants determined for purified 3-hydroxy-3-methylglutaryl-CoA synthase and from estimates of the physiological substrate concentrations. The rate of synthesis of 12nmol of 3-hydroxy-3-methylglutaryl-CoA/min per g wet wt. of yeast is still greater than the rate of utilization in spite of the extremely low (calculated) acetoacetyl-CoA concentration (1.8nm).     

Inhibition of sterol synthesis by citrinin in a cell-free system from rat liver and yeast.

Citrinin, a fungal metabolite known as an antibiotic, strongly inhibited the labeled acetate incorporation into nonsaponifiable lipids by a cell-free system from rat liver but not the labeled mevalonate incorporation. Of the enzymes involved in cholesterol synthesis, two enzymes, acetoacetyl-CoA thiolase (EC 2.3.1.9) and 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34), were specifically inhibited by the antibiotic. The concentration required for 50% inhibition was 0.2 mM for the former enzyme and 0.5 mM for the latter. Essentially the same results were obtained with a cell-free system from yeast although higher concentrations of the antibiotic were required for inhibition.     

Acetoacetate metabolism in rat brain. Development of acetoacetyl-coenzyme A deacylase and 3-hydroxy-3-methylglutaryl-coenzyme A synthase.

1. Data are provided that indicate that the rat brain acetoacetyl-CoA deacylase is almost exclusively mitochondrial. Developmental studies show that this enzyme more than doubles its activity during suckling (0--21 days) and then maintains this activity in adults (approx. 1.1 units/g wet wt.). 2. Kinetic studies (on the acetoacetyl-CoA deacylase) in a purified brain mitochondrial preparation give a Vmax. of 47 nmol/min per mg of protein, and a Km for acetoacetyl-CoA of 5.2 micron and are compatible with substrate inhibition by acetoacetyl-CoA above concentrations of 47 micron. 3. The total brain 3-hydroxy-3-methyl-glutaryl-CoA synthase remains constant in the developing and adult rat brain (approx. 1.2 units/g wet wt.). This enzyme is located in both the mitochondrial and cytosolic fractions. During suckling (0--21 days) the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase represents approx. one-third of the total, but this increases markedly to about 60% of the total in the adult. The cytosolic enzyme correspondingly falls to approx. 40% of the total. 4. The role of the acetoacetyl-CoA deacylase in providing cytosolic acetoacetate for biosynthetic activities in the developing brain is discussed.     

Sterol-independent regulation of 3-hydroxy-3-methylglutaryl-CoA reductase by mevalonate in Chinese hamster ovary cells. Magnitude and specificity

In this paper, we assess the relative degree of regulation of the rate-limiting enzyme of isoprenoid biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, by sterol and nonsterol products of mevalonate by utilizing cultured Chinese hamster ovary cells blocked in sterol synthesis. We also examine the two other enzymes of mevalonate biosynthesis, acetoacetyl-CoA thiolase and HMG-CoA synthase, for regulation by mevalonate supplements. These studies indicate that in proliferating fibroblasts, treatment with mevalonic acid can produce a suppression of HMG-CoA reductase activity similar to magnitude to that caused by oxygenated sterols. In contrast, HMG-CoA synthase and acetoacetyl-CoA thiolase are only weakly regulated by mevalonate when compared with 25-hydroxycholesterol. Furthermore, neither HMG-CoA synthase nor acetoacetyl-CoA thiolase exhibits the multivalent control response by sterol and mevalonate supplements in the absence of endogenous mevalonate synthesis which is characteristic of nonsterol regulation of HMG-CoA reductase. These observations suggest that nonsterol regulation of HMG-CoA reductase is specific to that enzyme in contrast to the pleiotropic regulation of enzymes of sterol biosynthesis observed with oxygenated sterols. In Chinese hamster ovary cells supplemented with mevalonate at concentrations that are inhibitory to reductase activity, at least 80% of the inhibition appears to be mediated by nonsterol products of mevalonate. In addition, feed-back regulation of HMG-CoA reductase by endogenously synthesized nonsterol isoprenoids in the absence of exogenous sterol or mevalonate supplements also produces a 70% inhibition of the enzyme activity.     

The pathway of ketogenesis in rumen epithelium of the sheep.

A method for the fractionation of sheep rumen epithelium with limited mitochondrial breakage is described. The distributions of the enzymes of the 3-hydroxy-3-methylglutaryl-CoA pathway of ketogenesis indicate that this process is exclusively mitochondrial. Enzyme activities are sufficient to account for the ketogenic rates found in vivo. The failure of (-)-hydroxycitrate to block ketogenic flux supports this view. 3-Hydroxybutyrate dehydrogenase activity is largely associated with particulate material in the mitochondrial fraction. ATP citrate lyase activity was found, with appreciable acetoacetyl-CoA thiolase and 3-hydroxy-3-methylglutaryl-CoA synthase in the cytoplasmic fraction.     

Enzymes involved in the metabolism of 3-hydroxy-3-methylglutaryl-coenzyme A in Catharanthus roseus

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) is an important intermediate in various metabolic pathways, e.g. sterol biosynthesis, ketogenesis and leucine catabolism. The reactions and enzymes involved in the metabolism of HMG-CoA are briefly reviewed. These enzymes have been studied in Catharanthus roseus, a model system for studies on the regulation of secondary metabolic pathways, particularly those leading to terpenoidindole alkaloids. By using HPLC, three HMG-CoA catabolizing enzyme activities have been detected in protein extracts from suspension cultured C. roseus cells: HMG-CoA lyase, 3-nucleotidase and (tentatively identified) 3-methylglutaconyl-CoA hydratase (HMG-CoA hydrolyase). The enzymes have been partially purified. HMG-CoA is formed from three molecules of acetyl-CoA, via reactions which are catalyzed by two (as in yeast and animal cells, via intermediacy of acetoacetyl-CoA) or by just one enzyme (as in e.g. radish). It is yet not clear which process occurs in C. roseus.Abbreviations AACT acetoacetyl-CoA thiolase - AACT/HMGS acetoacetyl-COA thiolase/HMG-CoA synthase - CoASH coenzyme A (reduced form) - HMG-CoA 3-hydroxy-3-methylglutaryl-CoA - MG-CoA 3-methylglutaconyl-CoA     

Regulation of early enzymes of ergosterol biosynthesis in Saccharomyces cerevisiae.

In order to determine the regulation mechanisms of ergosterol biosynthesis in yeast, we developed growth conditions leading to high or limiting ergosterol levels in wild type and sterol-auxotrophic mutant strains. An excess of sterol is obtained in anaerobic sterol-supplemented cultures of mutant and wild type strains. A low sterol level is obtained in aerobic growth conditions in mutant strains cultured with optimal sterol supplementation and in wild type strain deprived of pantothenic acid, as well as in anaerobic cultures without sterol supplementation. Measurements of the specific activities of acetoacetyl-CoA thiolase, HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) synthase and HMG-CoA reductase (the first three enzymes of the pathway), show that in cells deprived of ergosterol, acetoacetyl-CoA thiolase and HMG-CoA synthase are generally increased. In an excess of ergosterol, in anaerobiosis, the same enzymes are strongly decreased. A 5-10-fold decrease is observed for acetoacetyl-CoA thiolase and HMG-CoA synthase. In contrast, HMG-CoA reductase is only slightly affected by these conditions. These results show that ergosterol could regulate its own synthesis, at least partially, by repression of the first two enzymes of the pathway. Our results also show that exogenous sterols, even if strongly incorporated by auxotrophic mutant cells, cannot suppress enzyme activities in aerobic growth conditions. Measurement of specific enzyme activities in mutant cells also revealed that farnesyl pyrophosphate thwarts the enhancement of the activities of the two first enzymes.     

Hormonal Regulation of the mRNA Encoding the Ketogenic Enzyme Mitochondrial 3-Hydroxy-3-Methylglutaryl-CoA Synthase in Neonatal Primary Cultures of Cortical Astrocytes and Meningeal Fibroblasts

Abstract: We have previously identified cerebellum to contain significantly higher levels, compared with other brain regions, of the mRNA encoding the key ketogenic enzyme mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHS). In this report, we extend these observations, using primary cultures of cerebellar astrocytes and cerebellar granule neurons, and show that mHS mRNA was not readily detected in these cell types, suggesting that other cerebellar cell types account for mHS mRNA abundances observed in cerebellum. In contrast, we report, for the first time, the ready detection of mHS mRNA together with the mRNAs encoding the remaining enzymes of the 3-hydroxy-3-methylglutaryl-CoA cycle, namely, mitochondrial acetoacetyl-CoA thiolase and 3-hydroxy-3-methylglutaryl-CoA lyase, in primary cultures of neonatal meningeal fibroblasts. Based on observations of the effects of fetal calf serum in the culture medium and the documented effects of various hormones on mHS mRNA levels in liver, we show that the glucocorticoid hydrocortisone effects a selective fourfold increase in mHS mRNA abundances in both neonatal meningeal fibroblasts and neonatal cortical astrocytes cultured in a serum-free/hormone-free medium.     

3-Hydroxy-3-methylglutaryl-coenzyme A synthase from ox liver. Properties of its acetyl derivative.

Ox liver mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (EC 4.1.3.5) reacts with acetyl-CoA to form a complex in which the acetyl group is covalently bound to the enzyme. This acetyl group can be removed by addition of acetoacetyl-CoA or CoA. The extent of acetylation and release of CoA were found to be highly temperature-dependent. At temperatures above 20 degrees C, a maximum value of 0.85 mol of acetyl group bound/mol of enzyme dimer was observed. Below this temperature the extent of rapid acetylation was significantly lowered. Binding stoichiometries close to 1 mol/mol of enzyme dimer were also observed when the 3-hydroxy-3-methylglutaryl-CoA synthase activity was titrated with methyl methanethiosulphonate or bromoacetyl-CoA. This is taken as evidence for a 'half-of-the-sites' reaction mechanism for the formation of 3-hydroxy-3-methylglutaryl-CoA by 3-hydroxy-3-methylglutaryl-CoA synthase. The Keq. for the acetylation was about 10. Isolated acetyl-enzyme is stable for many hours at 0 degrees C and pH 7, but is hydrolysed at 30 degrees C with a half-life of 7 min. This hydrolysis is stimulated by acetyl-CoA and slightly by succinyl-CoA, but not by desulpho-CoA. The site of acetylation has been identified as the thiol group of a reactive cysteine residue by affinity-labelling with the substrate analogue bromo[1-14C]acetyl-CoA.     

3-Hydroxy-3-methylglutaryl coenzyme A synthase. Evidence for an acetyl-S-enzyme intermediate and identification of a cysteinyl sulfhydryl as the site of acetylation.

Homogeneous liver 3-hydroxy-3-methylglutaryl coenzyme A synthase, which catalyzes the condensation of acetyl-CoA with acetoacetyl-CoA to form 3-hydroxy-3-methylglutaryl-CoA, also carries out: (a) a rapid transacetylation from acetyl-CoA to 31-dephospho-CoA and (b) a slow hydrolysis of acetyl-CoA to acetate and CoA. Transacetylation and hydrolysis occur at 50 and 1 percent, respectively, the rate of the synthasecatalyzed condensation reaction. It appears that an acetyl-enzyme intermediate is involved in the transacetylase and hydrolase reactions of 3-hydroxy-3-methylglutaryl-CoA synthase, as well as in the over-all condensation process. Covalent binding to the enzyme of a [14C]acetyl group contributed by [1(-14)C]acetyl-CoA is indicated by migration of the [14C]acetyl group with the dissociated synthase upon electrophoresis in dodecyl sulfate-urea and by precipitation of [14C]acetyl-enzyme with trichloroacetic acid. At 0 degrees and a saturating level of acetyl-CoA, the synthase is rapidly (less than 20 s) acetylated yielding 0.6 acetyl group/enzyme dimer. Performic acid oxidation completely deacetylates the enzyme, suggesting the site of acetylation to be a cysteinyl sulfhydryl group. Proteolytic digestion of [14C]acetyl-S-enzyme under conditions favorable for intramolecular S to N acetyl group transfer quantitatively liberates a labeled derivative with a [14C]acetyl group stable to performic acid oxidation. The labeled oxidation product is identified as N-[14C]acetylcysteic acid, thus demonstrating a cysteinyl sulfhydryl group as the original site of acetylation. The ability of the acetylated enzyme, upon addition of acetoacetyl-CoA, to form 3-hydroxy-3-methylglutaryl-CoA indicates that the acetylated cysteine residue is at the catalytic site.     

Regulation of thiolases from pig heart. Control of fatty acid oxidation in heart

The effects of various mitochondrial coenzymes and metabolities on the activities of 3-oxoacyl-CoA thiolase (EC 2.3.1.16) and acetoacetyl-CoA thiolase (EC 2.3.1.9) from pig heart were investigated with the aim of elucidating the possible regulation of these two enzymes. Of the compounds tested, acetyl-CoA was the most effective inhibitor of both thiolases. However, 3-oxoacyl-CoA thiolase was more severly inhibited by acetyl-CoA than was acetoacetyl-CoA thiolase. 3-Oxoacyl-CoA thiolase was also significantly inhibited by decanoyl-CoA while acetoacetyl-CoA thiolase was inhibited by 3-hydroxybutyryl-CoA as strongly as it was by acetyl-CoA. All other compounds either did not affect the thiolase activities or only at unphysiologically high concentrations. The inhibition of acetoacetyl-CoA thiolase by acetyl-CoA was linear and apparently noncompetitive with respect to CoASH (Ki = 125 microM) whereas that of 3-oxoacyl-CoA thiolase was nonlinear. However at low concentrations of acetyl-CoA the inhibition of 3-oxoacyl-CoA thiolase was linear competitive with respect to CoASH (Ki = 3.9 microM). It is concluded that 3-oxoacyl-CoA thiolase, but not acetoacetyl-CoA thiolase, will be completely inhibited by acetyl-CoA at concentrations of CoASH and acetyl-CoA which are assumed to exist intramitochondrially at state-4 respiration. It is suggested that fatty acid oxidation in heart muscle at sufficiently high concentrations of plasma free fatty acids is controlled via the regulation of 3-oxoacyl-CoA thiolase by the acetyl-CoA/CoASH ratio which is determined by the rate of the citric acid cycle and consequently by the energy demand of the tissue.     

Changes in sterol biosynthesis accompanying cessation of glial cell growth in serum-free medium

C-6 glioma cells, grown in medium supplemented with 5% delipidated foetal calf serum, were induced to enter a quiescent state by removing serum from the medium. Within 24h there was a 75–80% decline in the rate of incorporation of [¹⁴C]acetate or ³H₂O into digitonin-precipitable sterols. Experiments with [³H]mevalonolactone as a labelled sterol precursor suggested that the decline in sterol synthesis was regulated primarily at a point in the pathway before the formation of mevalonate. The specific activities of 3-hydroxy-3-methylglutaryl-CoA synthase and 3-hydroxy-3-methylglutaryl-CoA reductase decreased sharply in conjunction with the decline in sterol synthesis in the serum-free cultures; however, the activity of acetoacetyl-CoA thiolase was altered only slightly. The magnitude of the initial decline in reductase activity was not affected when 50-mm-NaF was included in the preincubation and assay buffers to prevent activation of physiologically inactive enzyme. However, after 6h of serum deprivation the decline in 3-hydroxy-3-methylglutaryl-CoA reductase activity was due to a decrease in the amount of latent activity. The sterol concentration in C-6 cells was unchanged after 24h in serum-free medium, although a 20% decrease in the sterol/fatty acid molar ratio occurred as a result of a small increase in the fatty-acid concentration. Incorporation of ³H₂O into fatty acids was inhibited in the serum-deprived glial cells; however, this inhibition developed more slowly and was not as pronounced as the diminution in sterol synthesis. The results suggest that in C-6 glia, which resemble the glial stem cells of the developing brain, the decreased demand for membrane sterols in the quiescent state results in a decline in sterol synthesis, mediated primarily through co-ordinate changes in the activities of 3-hydroxy-3-methylglutaryl-CoA synthase and 3-hydroxy-3-methylglutaryl-CoA reductase.     

Effect of phenylmethylsulfonyl fluoride on sterol biosynthesis in 10,000 x g supernatant fraction of rat liver homogenates

Phenylmethylsulfonyl fluoride (PMSF), a reagent commonly employed for the inhibition of serine proteases, has been found to cause significant inhibition of the incorporation of labeled acetate, but not mevalonate, into nonsaponifiable lipid and digitonin-precipitable sterols in the 10,000 X g supernatant fraction of rat liver homogenate preparations. In two experiments, the extent of inhibition of the synthesis of digitonin-precipitable sterols from acetate by PMSF at 1 mM was 81 and 65%. PMSF inhibited the synthesis of nonsaponifiable lipid from acetate at concentrations as low as 0.1 microM. Preincubation of the 10,000 X g supernatant fraction of rat liver homogenates with PMSF (1 mM) resulted in a significant reduction of the activities of acetate thiokinase and 3-hydroxy-3-methylglutaric acid (HMG)-CoA synthase, but did not affect the activities of acetoacetyl-CoA thiolase. Preincubation of rat liver microsomes with PMSF (1 mM) caused a 50% reduction in the level of HMG-CoA reductase activity. The combined results indicate that major sites of action of PMSF in the inhibition of sterol biosynthesis from labeled acetate appear to be on the activities of acetate thiokinase, HMG-CoA synthase, and HMG-CoA reductase. Another reagent used to inhibit serine proteases, diisopropylfluorophosphate, had (at a concentration of 1 mM) no effect on the activities of cytosolic acetoacetyl-CoA thiolase, HMG-CoA synthase, and HMG-CoA reductase.     

Acetoacetyl-CoA thiolase regulates the mevalonate pathway during abiotic stress adaptation

Acetoacetyl-CoA thiolase (EC 2.3.1.9), also called thiolase II, condenses two molecules of acetyl-CoA to give acetoacetyl-CoA. This is the first enzymatic step in the biosynthesis of isoprenoids via mevalonate (MVA). In this work, thiolase II from alfalfa (MsAACT1) was identified and cloned. The enzymatic activity was experimentally demonstrated in planta and in heterologous systems. The condensation reaction by MsAACT1 was proved to be inhibited by CoA suggesting a negative feedback regulation of isoprenoid production. Real-time RT-PCR analysis indicated that MsAACT1 expression is highly increased in roots and leaves under cold and salinity stress. Treatment with mevastatin, a specific inhibitor of the MVA pathway, resulted in a decrease in squalene production, antioxidant activity, and the survival of stressed plants. As expected, the presence of mevastatin did not change chlorophyll and carotenoid levels, isoprenoids synthesized via the plastidial MVA-independent pathway. The addition of vitamin C suppressed the sensitive phenotype of plants challenged with mevastatin, suggesting a critical function of the MVA pathway in abiotic stress-inducible antioxidant defence. MsAACT1 over-expressing transgenic plants showed salinity tolerance comparable with empty vector transformed plants and enhanced production of squalene without altering the 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) activity in salt-stress conditions. Thus, acetoacetyl-CoA thiolase is a regulatory enzyme in isoprenoid biosynthesis involved in abiotic stress adaptation.     

Occurrence and possible roles of acetoacetyl-CoA thiolase and 3-ketoacyl-CoA thiolase in peroxisomes of an n-alkane-grown yeast, Candida tropicalis

Two kinds of 3-ketoacyl-CoA thiolases were found in the peroxisomes of Candida tropicalis cells grown on n-alkanes (C10-C13). One was a typical acetoacetyl-CoA thiolase specific only to acetoacetyl-CoA, while another was 3-ketoacyl-CoA thiolase showing high activities on the longer chain substrates. A high level of the latter thiolase activity in alkane-grown cells was similar to that of other enzymes constituting the fatty acid beta-oxidation system in yeast peroxisomes. These facts suggest that the complete degradation of fatty acids to acetyl-CoA is carried out in yeast peroxisomes by the cooperative contribution of acetoacetyl-CoA thiolase and 3-ketoacyl-CoA thiolase.