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).     

Characteristics of rat liver microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

A procedure for the preparation of rat liver microsomal fractions essentially devoid of contaminating lysosomes is described. When this preparation was examined by immunoblotting with a rabbit antiserum to rat 3-hydroxy-3-methylglutaryl-CoA reductase, a single band corresponding to an Mr of 100000 was observed. No evidence was found for glycosylation of rat liver-3-hydroxy-3-methylglutaryl-CoA reductase. Native rat liver microsomal 3-hydroxy-3-methylglutaryl-CoA reductase differs from the purified proteolytically modified species in that it displays allosteric kinetics towards NADPH.     

Coordinate regulation of 3-hydroxy-3-methylglutaryl-coenzyme A synthase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, and prenyltransferase synthesis but not degradation in HepG2 cells

Human hepatoma HepG2 cells were used to demonstrate coordinate regulation of three enzymes of cholesterol synthesis under a variety of conditions. Addition of either delipidized serum and mevinolin or low density lipoprotein, 25-hydroxycholesterol, or mevalonic acid to HepG2 cells resulted in rapid changes both in the levels of the mRNAs and in the rates of synthesis of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, HMG-CoA reductase, and farnesyl pyrophosphate synthetase (prenyltranferase). In all cases, the changes in mRNA levels were paralleled by changes in the rates of specific protein synthesis. Pulse-chase techniques were used to determine the half-lives of all three proteins. Addition of low density lipoprotein to the media during the chase increased the rate of degradation of HMG-CoA reductase 4.6-fold but had no affect on the half-lives of HMG-CoA synthase or prenyltransferase. Therefore, we conclude that the coordinate regulation of these three enzymes under a variety of conditions occurs at the level of enzyme synthesis and not at the level of protein stability.     

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.     

Some properties of purified 3-hydroxy-3-methylglutaryl coenzyme A reductase phosphatases from rat liver

Incubation of four purified rat liver 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase phosphatases (G. Gil, M. Sitges, and F. G. Hegardt, (1981) Biochim. Biophys. Acta663, 211–221) with HMG-CoA, CoA, NADPH, or citrate caused a concentration-dependent inactivation of the enzyme activities. HMG-CoA and CoA showed similar patterns of inactivation and at 0.5 mm of both compounds, the four reductase phosphatases were fully inhibited. Half-maximal inactivation was comprised between 0.02 and 0.1 mm of HMG-CoA and CoA. NADPH at concentration ranging between 5 and 10 mm produced complete inactivation of reductase phosphatases. Citrate at 5 mm produced full inactivation, and half-maximal inhibition ranged from 0.1 to 0.4 mm for the different phosphatases. The behavior of fluoride varied with respect to the four phosphatases: Low molecular forms were inactivated in a similar manner as described for other protein phosphatases. However, high molecular forms were slightly inactivated, and phosphatase IIa at 100 mm showed a level of activity similar to the control. The effect of KCl on the four reductase phosphatases could explain this behavior since at high concentrations, KCl (and NaCl) produced activation in both high and low molecular forms, this effect being more enhanced in high M_r reductase phosphatases. The insensitivity to fluoride of high M_r reductase phosphatases could explain the discrepancies in percentage of the active form of HMG-CoA reductase described previously in literature.     

Subcellular localization of Arabidopsis 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Plants produce diverse isoprenoids, which are synthesized in plastids, mitochondria, endoplasmic reticulum (ER), and the nonorganellar cytoplasm. 3-Hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) catalyzes the synthesis of mevalonate, a rate-limiting step in the cytoplasmic pathway. Several branches of the pathway lead to the synthesis of structurally and functionally varied, yet essential, isoprenoids. Several HMGR isoforms have been identified in all plants examined. Studies based on gene expression and on fractionation of enzyme activity suggested that subcellular compartmentalization of HMGR is an important intracellular channeling mechanism for the production of the specific classes of isoprenoids. Plant HMGR has been shown previously to insert in vitro into the membrane of microsomal vesicles, but the final in vivo subcellular localization(s) remains controversial. To address the latter in Arabidopsis (Arabidopsis thaliana) cells, we conducted a multipronged microscopy and cell fractionation approach that included imaging of chimeric HMGR green fluorescent protein localizations in transiently transformed cell leaves, immunofluorescence confocal microscopy in wild-type and stably transformed seedlings, immunogold electron microscopy examinations of endogenous HMGR in seedling cotyledons, and sucrose density gradient analyses of HMGR-containing organelles. Taken together, the results reveal that endogenous Arabidopsis HMGR is localized at steady state within ER as expected, but surprisingly also predominantly within spherical, vesicular structures that range from 0.2- to 0.6-microm diameter, located in the cytoplasm and within the central vacuole in differentiated cotyledon cells. The N-terminal region, including the transmembrane domain of HMGR, was found to be necessary and sufficient for directing HMGR to ER and the spherical structures. It is believed, although not directly demonstrated, that these vesicle-like structures are derived from segments of HMGR-ER. Nevertheless, they represent a previously undescribed subcellular compartment likely capable of synthesizing mevalonate, which provides new evidence for multiorganelle compartmentalization of the isoprenoid biosynthetic pathways in plants.     

Associative properties of butyryl-coenzyme A synthetase from ox liver mitochondria.

Butyryl-coenzyme A synthetase (butyrate:CoA ligase (AMP-forming), EC 6.2.1.2) from an acetone-dried powder of ox liver mitochondria was found to have a molecular weight of approx. 40 000. The sedimentation equilibrium analysis suggested the presence in solution of higher molecular weight forms of the enzyme and these could also be obtained by extracting the enzyme from the mitochondrial powder in non-reducing conditions. The enzyme was inhibited by sulphydryl reagents, and was found to have at least one available thiol group/molecule. The relationship between enzymic activity and concentration was non-linear, and suggested that an inactive monomer-active dimer equilibrium was present. The 5--6-fold activation by bovine serum albumin required the presence of free thiol groups in the albumin and involved association of albumin with the enzyme.     

Independent regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and chylomicron remnant receptor activities in rat liver.

Treatment of rats with pharmacological doses of oestrogen resulted in a 3-fold decrease in the activity of hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) and a 4-fold increase in saturable binding of 125I-labelled chylomicron remnants to liver membranes in vitro. Intragastric administration of mevalonolactone to rats did not affect the capacity of the liver membranes to bind to labelled chylomicron remnants even though there was a substantial decrease in the activity of HMG-CoA reductase. Similar results were obtained after cholesterol feeding. Simultaneous treatment of rats with cholestyramine and compactin increased hepatic HMG-CoA reductase activity 6-fold. However, liver membranes derived from these animals showed no change in their capacity to bind to labelled chylomicron remnants in vitro. Administration of mevalonolactone to the cholestyramine/compactin-treated animals also failed to produce a change in remnant-binding capacity. Although administration of mevalonolactone alone produced a significant 3-fold decrease in the activity of hepatic HMG-CoA reductase it was unable to suppress significantly the increase in enzyme activity caused by treatment with cholestyramine and compactin.     

Slow binding inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase

The mechanism of slow binding inhibition of 3-hydroxy-3-methylglutaryl- coenzyme A reductase by lovastatin, fluindostatin, and related compounds was studied. Several of these compounds, including lovastatin, were found to be slow binding, while other less potent inhibitors were not. From a comparison of kinetic parameters obtained by steady-state measurements and progress curve analysis, it was concluded that the slow binding inhibitors bind by a mechanism which is more accurately described by biphasic binding than by single-step binding. The overall association rates of the slow binding inhibitors range from 1 x 10(6) to 4 x 10(-7) M-1 s-1, and the dissociation rates are in the range of 10(-3) s-1. The structures of slow binding and reversible inhibitors were compared by using molecular modeling methods. From these comparisons, it was proposed that the slow binding and very potent inhibition of, for instance, lovastatin, is not simply a result of binding of a transition state or reaction intermediate analogue. The various lipophilic groups of the inhibitors that do not seem to be related to structural features of the substrate may also play a crucial role in determining the mechanism of binding of HMGR inhibitors.     

Solubilization of the 97-kDa native form of liver microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase

Liver microsomal 3-hydroxy-3-methylglutaryl-CoA reductase was partially purified from cholestyramine-fed rats by sequential extraction of the membrane with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) and polyethylene glycol nonylphenyl ether (Triton N-101) and solubilized by incorporation of the resulting insoluble protein preparation into a detergent mixture of Triton N-101 and sodium N-lauroylsarcosinate (Sarkosyl) in the presence of high salt. The purification procedure resulted in approximately a 3-4-fold increase in specific activity compared with the microsomal fraction, and the enzyme was recovered with yields as high as 63%. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and a blotting experiment using antiserum to the purified 53,000-dalton reductase fragment showed that the major immunoreactive polypeptide had a Mr of 97,000, that expected for the native intact form of the enzyme (Chin, D. J., Gil, G., Russell, D. W., Liscum, L., Luskey, K. L., Basu, S. K., Okayama, H., Berg, P., Goldstein, J. L., and Brown, M. S. (1984) Nature 308, 613-617). In addition, the effect of various detergents on the activity and stability of the membrane-bound and the partially purified enzyme was determined, and a method for protection of the reductase from inactivation caused by the addition of anionic detergents to the assay mixture is described.     

Regulated degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase in permeabilized cells.

We have studied the regulated degradation of the enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase within the endoplasmic reticulum in cells permeabilized with digitonin. Using Chinese hamster ovary cells transfected with a plasmid encoding HMGal, a chimeric protein containing the membrane domain of HMG-CoA reductase coupled to beta-galactosidase, we have demonstrated mevalonate and sterol-stimulated loss of beta-galactosidase activity. In pulse-chase experiments we have demonstrated mevalonate-stimulated degradation of both HMGal and HMG-CoA reductase. The rate of mevalonate-stimulated degradation observed in permeabilized cells tends to be slightly slower than that observed in intact cells treated with mevalonate and is dependent upon incubation of cells with mevalonate prior to permeabilization. The degradation process measured in this report extends a previous report of HMG-CoA reductase degradation in digitonin-permeabilized cells (Leonard, D. A., and Chen, H. W. (1987) J. Biol. Chem. 262, 7914-7919) by mimicking key physiological features of the in vivo process, including: stimulation by regulatory molecules, specifically mevalonate and sterols; inhibition by cycloheximide; and inhibition by an inhibitor of neutral cysteine proteases.     

Development of monoclonal antibodies to 3-hydroxy-3-methylglutaryl-coenzyme A reductase

This report describes the development of a series of monoclonal antibodies to rat liver 3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR). Sera from hybridoma tumor-bearing mice were used to remove and characterize HMGR activity from a mixture of rat liver proteins. Two IgG₂ monoclonal antibodies removed separately greater than 80% HMGR activity while non-immune mouse or negative hybridoma-derived sera were ineffective. Radiolabeled immunoprecipitates of enzyme preparations resolved in one- and two-dimensional SDS-PAGE showed two predominant subunits at M_r 52,000 and 54,000. Our results indicate that in these preparations of rat liver proteins HMGR exists as a heteropolymer with at least two distinct subunits of different molecular weights.