Structural alterations of human immortal astrocytes after inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase by lovastatin

This investigation was designed to study the specific structural alterations of astroglial cells following the inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, which is a key enzyme in cholesterol synthesis. In our previous investigations, it has been demonstrated that lovastatin, which is a specific competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, effectively inhibited cholesterol synthesis in human primary and immortal astrocytes in serum and lipid-free media, and we also showed that the effects on both astrocytes of primary cultures and immortal astrocytes (ASCh-7) were very similar. In the present study we therefore examined effects of lovastatin at a concentration of 100 ng/ml on human immortal astrocytes (ASCh-7) using electron microscopic analysis. We have found that lovastatin significantly affects human immortal astrocytes, resulting in degenerative ultrastructural changes including accumulation of a large number of phagosomes, oedema of the cytoplasm and destruction of gliofilaments. The results obtained suggest that the human immortal astrocyte cell line (ASCh-7) can be not only very useful for screening drugs in pharmacological research but can also provide a useful model for examining fine structural alterations of cells following selective disturbances of metabolism.

N-[2-[N'-pentyl-(6,6-dimethyl-2,4-heptadiynyl)amino]ethyl]- (2-methyl-1-naphthylthio)acetamide (FY-087). A new acyl coenzyme a:cholesterol acyltransferase (ACAT) inhibitor of diet-induced atherosclerosis formation in mice

FY-087 (N-[2-[N'-pentyl-(6,6-dimethyl-2,4-heptadiynyl)amino]ethyl]- (2-methyl-1-naphthylthio)acetamide) was found to be a competitive inhibitor of human microsomal acyl coenzyme A:cholesterol acyltransferase (ACAT) with an IC50 value of 0.11 microM. Under our assay conditions, other ACAT inhibitors tested, specifically YM-750, E-5324, and melinamide, all of which are now in phase I clinical trials or in clinical use in Japan, inhibited this enzyme with IC50 values of 0.18, 0.14, and 3.2 microM, respectively. FY-087 also inhibited ACAT in acetyl-low density lipoprotein loaded human macrophages (THP-1 cells) with an IC50 of 0.17 microM. Following the oral administration of FY-087 (30 mg/kg) to rats, the plasma concentration of FY-087 reached 0.42 microgram/mL after 2 hr. This concentration of FY-087 was enough to inhibit blood vessel ACAT activity. Cholesterol-lowering and anti-atherogenic effects of FY-087 were examined using C57BL/6J mice fed an atherogenic diet. In this mouse model, treatment with FY-087 (28 mg/kg) inhibited the increase in plasma cholesterol levels by about 20% and decreased the hepatic accumulation of free and esterified cholesterol by 61 and 67%, respectively. FY-087 also significantly inhibited the atherogenic diet-induced increase in the fatty-streak lesion area of the proximal aorta by 57% in C57BL/6J mice. These results indicate that FY-087 is not only a therapeutically bioavailable ACAT inhibitor that lowers plasma cholesterol levels, but also an effective anti-atherogenic agent in mice fed an atherogenic diet.

Decreases in serum ubiquinone concentrations do not result in reduced levels in muscle tissue during short-term simvastatin treatment in humans

Statins, which are commonly used drugs for hypercholesterolemia, inhibit 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme in cholesterol synthesis. Important nonsterol compounds, such as ubiquinone, are also derived from the same synthetic pathway. Therefore it has been hypothesized that statin treatment causes ubiquinone deficiency in muscle cells, which could interfere with cellular respiration causing severe adverse effects. In this study we observed decreased serum levels but an enhancement in muscle tissue ubiquinone levels in patients with hypercholesterolemia after 4 weeks of simvastatin treatment. These results indicate that ubiquinone supply is not reduced during short-term statin treatment in the muscle tissue of subjects in whom myopathy did not develop.

Effects of an HMG-CoA reductase inhibitor, pravastatin, and bile sequestering resin, cholestyramine, on plasma plant sterol levels in hypercholesterolemic subjects

To study exogenous sterol metabolism during the suppression or stimulation of cholesterol biosynthesis induced by treatments for hyperlipidemia, we determined plasma plant sterol concentrations before and after administration of an HMG-CoA reductase inhibitor, pravastatin, and compared these with changes in these plasma sterol levels by the bile-sequestrating resin, cholestyramine. The effects of the drugs were also studied in a sitosterolemic patient who has had increased plasma levels of plant sterols. Plasma cholesterol levels determined by the HPLC method were decreased significantly after administration of pravastatin. Plasma plant sterol (sitosterol and campesterol) as well as cholestanol concentrations were also significantly reduced. Cholestyramine administration decreased plasma levels of cholesterol, but did not change those of plant sterols in the hypercholesterolemic subjects. Pravastatin had little effect in a sitosterolemic patient on plasma levels of sterols, where cholestyramine decreased the plasma levels of both cholesterol and cholestanol. These results indicate that treatment with the HMG-CoA reductase inhibitor decreases plasma plant sterol concentrations, and suggest that the increased plasma plant sterol levels in sitosterolemia might not be due to the decreased cholesterol biosynthesis in vivo.

Species difference in cholesteryl ester cycle and HDL-induced cholesterol efflux from macrophage foam cells

The species difference in the turnover rates of the cholesteryl ester (CE) cycle in macrophage foam cells (MFC) was examined in mice and rats. MFC were induced by acetyl-LDL and pulsed with [3H]oleate, followed by a chase with [14C]oleate. The replacement of the initial amount of cholesteryl [3H]oleate by cholesteryl [14C]oleate within 24 hours was 63% in mouse MFC, whereas it was 33% in rat MFC. The corresponding replacement in rabbit MFC was < 10%. In addition, HDL removed 41% of the CE mass from mouse MFC but only 22% from rat MFC. HDL-induced CE reduction from mouse MFC was enhanced by 40% by the inhibitor for acyl-coenzyme A:cholesterol acyltransferase (58-035), whereas the enhancing effect was not observed with rat MFC. These results indicate that the rate of CE turnover may serve as a critical factor to determine the capacity of MFC to respond to HDL-induced CE reduction, suggesting the possibility that the species difference in the turnover rates of the CE cycle in MFC might explain, in part, the species difference in susceptibility to experimental atherosclerosis.

Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in the rat ileum: effects of bile acids and lovastatin

We investigated the effects of intestinal bile acid flux, orientation of the 7-hydroxy group, and administration of lovastatin on the regulation of intestinal 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity in the rat ileum. HMG-CoA reductase activities in villous and crypt cells from the ileal mucosa were similar, and the study was performed on whole mucosa that contained both cell types. Taurocholate feeding decreased ileal reductase activity 48%, whereas tauroursocholate, the 7 beta-hydroxy epimer of taurocholate, had no effect. Feeding lovastatin (inhibitor of HMG-CoA reductase) stimulated total ileal HMG-CoA reductase activity threefold in washed microsomes, which were dissociated from the inhibitor. However, the proportion of active enzyme in the ileum of lovastatin-fed rats was 50% lower than in controls, whereas there was no change in the percentage of expressed enzyme with bile acid treatments. Interruption of the enterohepatic circulation (bile fistula) increased HMG-CoA reductase activity in the ileum 73%. Duodenal infusion of taurocholate to bile-fistula rats significantly decreased microsomal HMG-CoA reductase activity in the ileal mucosa. In contrast, infusion of the 7 beta-hydroxy epimer tauroursocholate failed to inhibit the derepressed HMG-CoA reductase activity in the ileum of bile-fistula rats. The inhibition of intestinal HMG-CoA reductase activity by taurocholate occurred without accumulation of mucosal cholesterol. Furthermore, the stimulation of total ileal HMG-CoA reductase activity by lovastatin treatment was observed without a decrease in mucosal cholesterol. In summary, the regulation of ileal HMG-CoA reductase activity by the intestinal luminal flux of bile acids is dependent on the orientation of the hydroxyl groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of two acylcoenzyme A: cholesterol acyltransferase (ACAT) inhibitors, cyclandelate and a non-hydrolysable ether analogue, benzyl3,3,5-trimethylcyclohexanol on low density lipoprotein metabolism in macrophages and hepatocytes

Cyclandelate (3,3,5-trimethylcyclohexanylmandelate) caused a dose-dependent decrease in the metabolism of radioiodinated low density lipoprotein [125I-LDL] by J774 mouse macrophages. This was probably an indirect effect due to the inhibition of cholesterol esterification by the cells rather than a direct one on the interaction of LDL with its receptor, since no inhibition was seen in cells which had been cholesterol-depleted by prior incubation with lipoprotein-depleted serum for 48 hr. Cyclandelate also inhibited immediately de novo synthesis of cholesterol from [1-14C]acetate in J774 cells, suggesting a direct action of the drug on an enzyme of the cholesterol biosynthetic pathway. The drug was an efficient inhibitor of hamster and rat intestinal acylcoenzyme A: cholesterol acyltransferase (ACAT) activity in vitro with an IC50 of 20 microM. Addition of cyclandelate to the diet of meal-fed rats caused a marked inhibition of the rate of appearance of dietary [4-14C]cholesterol in the plasma. A nonhydrolysable ether analogue of cyclandelate, benzyl3,3,5-trimethylcyclohexanol, was prepared to compare hepatic and extrahepatic actions of the two molecules. The analogue inhibited cholesterol esterification in J774 cells, transformed human macrophages U937 and human umbilical vein endothelial cells with an IC50 of 20 microM and had effects similar to those of cyclandelate on 125I-LDL metabolism in J774 cells. Differences between the analogue and cyclandelate were seen in hepatocytes and hepatic microsomal fractions, where preincubation with the analogue inhibited cholesterol esterification in both systems while cyclandelate had no inhibitory action in either. Consequently, preincubation of rat hepatocytes with benzyl3,3,5-trimethylcyclohexanol for 17 hr caused a marked decrease in the binding of 125I-LDL to the cells, whereas binding to cells preincubated with cyclandelate was the same as to control cells.

Inhibition of cholesterol biosynthesis by allicin and ajoene in rat hepatocytes and HepG2 cells

Exposure of primary rat hepatocytes and human HepG2 cells to allicin and ajoene resulted in the concentration-dependent inhibition of cholesterol biosynthesis at different steps of this metabolic pathway. At low concentrations of ajoene sterol biosynthesis from [14C]acetate in rat hepatocytes was decreased by 18% with an IC50-value of 15 microM, while allicin was almost uneffective. In HepG2 cells, both compounds significantly inhibited sterol biosynthesis by 14% and 19% with IC50-values of 7 and 9 microM for allicin and ajoene, respectively. This inhibition was exerted at the level of HMG-CoA-reductase as revealed by the absence of inhibition, if [14C]acetate was replaced by [14C]mevalonate as a precursor, and by direct determination of enzyme activity. At somewhat higher concentrations inhibition of cholesterol biosynthesis by both, allicin and ajoene, was also observed at late steps resulting in the accumulation of the precursor lanosterol. Alliin instead was completely inactive. In the case of allicin, small amounts of dihydrolanosterol and 7-dehydrocholesterol were formed at intermediate concentrations of 5-10 microM. From these results it is concluded that a major point of inhibition at the late steps occurs at the level of lanosterol 14 alpha-demethylase.

Skeletal muscle mitochondrial beta-oxidation of dicarboxylates

(1) The oxidation of [U-14C]hexadecanedionoyl-mono-CoA by rat skeletal muscle mitochondrial fractions is carnitine dependent and is inhibited by cyanide. (2) [U-14C]hexadecanedionoyl-mono-CoA was oxidised at a rate 8% of that of [U-14C]hexadecanoyl-CoA. (3) Oxidations were saturable and no substrate inhibition was observed. (4) We demonstrate the formation of dicarboxylyl-mono-CoA esters and the corresponding carnitine derivatives. (5) We conclude that, although skeletal muscle mitochondria are capable of the beta-oxidation of dicarboxylic acids, this is unlikely to be of great physiological significance.

The inhibition of long-chain fatty acyl-CoA synthetase by enoximone in rat heart mitochondria

The mechanism by which enoximone, a reported phosphodiesterase inhibitor, inhibits the oxidation of long-chain fatty acids was studied in isolated rat heart mitochondria using a series of 14C-labeled substrates. Enoximone decreased palmitate oxidation in a time- and concentration-dependent manner. Fifty percent inhibition of palmitate oxidation was achieved with 250 microM of enoximone. In contrast to its effect on palmitate, enoximone (250 microM) increased octanoate oxidation by 30%, whereas pyruvate oxidation was unaffected by enoximone. At that dose there was no effect on the oxidation of palmitoyl-CoA and palmitoyl carnitine. The degree of palmitate oxidation inhibited by enoximone was parallel to the inhibition of acyl-CoA synthetase in both rat heart mitochondria and microsomes. These results suggest that enoximone is a reversible inhibitor of long-chain fatty acyl-CoA synthetase. Moreover, the reaction, which is catalyzed by this enzyme, is a rate-limiting step in the pathway of fatty acid oxidation in rat heart mitochondria.

Cholesterol biosynthesis and metabolism

Cholesterol plays an essential role in cell membrane synthesis and in cell growth and differentiation. In mammalian cells, cholesterol can be synthesized from acetate precursors or taken up from dietary or exogenous sources. The major catabolic route for disposal of cholesterol involves conversion into excretable bile acids. The maintenance of cholesterol homeostasis is influenced and carefully controlled by multiple feedback mechanisms. The key regulatory targets of these feedback mechanisms are 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase in cholesterol biosynthesis, the low-density lipoprotein (LDL) receptor in cholesterol uptake, and cholesterol 7 alpha-hydroxylase in cholesterol catabolism. The elucidation of regulatory mechanisms in cholesterol metabolism has been greatly facilitated by the discovery of a new class of lipid-lowering drugs, the HMG-CoA reductase inhibitors. In addition to proving therapeutically useful in the treatment of hypercholesterolemia, these drugs have revealed novel regulatory steps in cholesterol metabolism and several new targets for future drug development. This manuscript reviews recent developments in the cholesterol biosynthetic pathway and the regulatory mechanisms that maintain cholesterol homeostasis.

Once-daily pravastatin in patients with primary hypercholesterolemia: a dose-response study

This multicenter, double-blind, placebo-controlled study was conducted to evaluate dose-response effects and safety of once-daily administration of pravastatin, a new inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. Pravastatin 5, 10, 20, 40 mg or placebo was administered at bedtime to 150 patients with primary hypercholesterolemia inadequately controlled on a low-fat, low-cholesterol (AHA Phase I) diet. After 8 weeks of treatment, pravastatin produced dose-dependent reductions in low-density lipoprotein (LDL) cholesterol of 19.2 to 34.1% (p less than or equal to .001 vs. baseline and placebo) and reductions in total cholesterol of 14.3 to 25.1% (p less than or equal to .01 to p less than or equal to .001 vs. placebo and p less than or equal to .001 vs. baseline). The relationship between the loge dose of pravastatin and decrease in LDL cholesterol was linear (p less than 0.002). High-density-lipoprotein cholesterol increased up to 11.7% and triglycerides decreased by as much as 23.9%. Pravastatin was well tolerated; no patient withdrew from the study as a consequence of treatment-related adverse events. Despite its relatively short serum half-life of approximately 2 h, once-daily administration of pravastatin provides a safe and effective means of reducing elevated LDL and total cholesterol.

Regulation of bile acid synthesis. V. Inhibition of conversion of 7-dehydrocholesterol to cholesterol is associated with down-regulation of cholesterol 7 alpha-hydroxylase activity and inhibition of bile acid synthesis

In the chronic bile fistula rat, the administration of a bolus dose of mevinolinic acid, an inhibitor of HMG-CoA reductase, was followed by rapid down-regulation of cholesterol 7 alpha-hydroxylase activity and a decrease in bile acid synthesis. These observations suggested that either newly synthesized cholesterol or some other metabolite of mevalonate may be involved in the regulation of bile acid synthesis. In order to distinguish between these two alternatives, we carried out experiments in which cholesterol synthesis was blocked by AY9944, a compound that inhibits the conversion of 7-dehydrocholesterol to cholesterol, a last step in the cholesterol biosynthesis pathway. Rats underwent biliary diversion for 72 h at which time they were given intravenously either a bolus dose of AY9944 (1 mg/kg) or control vehicle. At 0 (pre-treatment control), 0.5, 1.5, and 3 h post bolus, livers were harvested and specific activities of cholesterol 7 alpha-hydroxylase were determined. At 1.5, 3, and 6 h post bolus, AY9944 inhibited bile acid synthesis by 19 +/- 6%, 40 +/- 4%, and 41 +/- 6%, respectively, as compared to pretreatment baseline. Cholesterol 7 alpha-hydroxylase activity determined at 0.5, 1.5, and 3 h was decreased by 44 +/- 6%, 44 +/- 2%, and 36 +/- 2%, respectively, as compared to the control value. In in vitro experiments using microsomes from livers of control bile fistula rats, the addition of AY9944 (up to 100 microM) failed to inhibit cholesterol 7 alpha-hydroxylase activity. The results of this study demonstrate that, in the chronic bile fistula rat, acute inhibition of cholesterol synthesis at either early or late steps leads to a rapid down-regulation of cholesterol 7 alpha-hydroxylase activity and decrease in bile acid synthesis.

Effects of mevinolin, an inhibitor of cholesterol synthesis, on the morphology and function of differentiating and differentiated rat adrenocortical cells in primary culture

Mevinolin, an inhibitor of cholesterol synthesis, was used to study the effect of endogenous cholesterol synthesis on the morphology and function of differentiating and differentiated fetal rat adrenocortical cells grown in primary culture. Upon adrenocorticotrophic hormone (ACTH) stimulation under conditions in which endogenous cholesterol synthesis was inhibited but exogenous (lipoprotein) cholesterol was available, the cells differentiated normally from glomerulosa-like to fasciculata-like cells; the steroid hormone secretion was maximally induced. Under conditions in which cholesterol synthesis was maximally inhibited by mevinolin and the cells had no access to exogenous cholesterol, the cells did not differentiate into fasciculata-like cells; the ACTH-induced steroid response was highly suppressed under these conditions. The addition of either human low-density lipoprotein (LDL) or high-density lipoprotein (HDL3) to the culture medium restored the ACTH-induced differentiation and steroid secretion. Thus, in the absence of exogenous cholesterol, endogenous cholesterol synthesis was a prerequisite for differentiation. In cultures grown in the presence of exogenous cholesterol and ACTH with mevinolin-inhibited cholesterol synthesis and high steroid output, an increase in cytoplasmic lipids was evident, suggesting upregulation of LDL and HDL receptors. The results also demonstrated that induction of phenotypic differentiation from glomerulosa-like into fasciculata-like cells can proceed in the presence of a cholesterol synthesis inhibitor like mevinolin; this differentiation in the absence of endogenous cholesterol synthesis is accompanied by the appearance of cytoplasmic cholesterol ester droplets, typical of fasciculata cells.

Eicosapentaenoic acid reduces hepatic synthesis and secretion of triacylglycerol by decreasing the activity of acyl-coenzyme A:1,2-diacylglycerol acyltransferase

The mechanism for the reduced hepatic production of triacylglycerol in the presence of eicosapentaenoic acid was explored in short-term experiments using cultured parenchymal cells and microsomes from rat liver. Oleic, palmitic, stearic, and linoleic acids were the most potent stimulators of triacyl[3H]glycerol synthesis and secretion by hepatocytes, whereas erucic, alpha-linolenic, gamma-linolenic, arachidonic, docosahexaenoic, and eicosapentaenoic acids (in decreasing order) were less stimulatory. There was a linear correlation (r = 0.85, P less than 0.01) between synthesis and secretion of triacyl[3H]glycerol for the fatty acids examined. The extreme and opposite effects of eicosapentaenoic and oleic acids on triacylglycerol metabolism were studied in more detail. With increasing number of free fatty acid molecules bound per molecule of albumin, the rate of synthesis and secretion of triacyl[3H]glycerol increased, most markedly for oleic acid. Cellular uptake of the two fatty acids was similar, but more free eicosapentaenoic acid accumulated intracellularly. Eicosapentaenoic acid caused higher incorporation of [3H]water into phospholipid and lower incorporation into triacylglycerol and cholesteryl ester as compared to oleic acid. No difference was observed between the fatty acids on incorporation into cellular free fatty acids, monoacylglycerol and diacylglycerol. The amount of some 16- and 18-carbon fatty acids in triacylglycerol was significantly higher in the presence of oleic acid compared with eicosapentaenoic acid. Rat liver microsomes in the presence of added 1,2-dioleoyl-glycerol incorporated eicosapentaenoic acid and eicosapentaenoyl-CoA into triacylglycerol to a lesser extent than oleic acid and its CoA derivative. Decreased formation of triacylglycerol was also observed when eicosapentaenoyl-CoA was given together with oleoyl-CoA, whereas palmitoyl-CoA, stearoyl-CoA, linoleoyl-CoA, linolenoyl-CoA, and arachi-donoyl-CoA had no inhibitory effect. In conclusion, inhibition of acyl-CoA:1,2-diacylglycerol O-acyltransferase (EC 2.3.1.20) by eicosapentaenoic acid may be important for reduced synthesis and secretion of triacylglycerol from the liver.

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase: search for the enzyme's repressor derived from mevalonate

Three inhibitors of squalene 2,3-oxide-lanosterol cyclase (AMO 1618, 4,4,10 beta-trimethyl-trans-decal-3 beta-ol (TMD) and 2,3-iminosqualene (ISq] were used to study effects on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, and on sterol and polyprenyl synthesis from [14C]acetate and [14C]mevalonate in cultured rat hepatoma (H4) cells. After a 4 h exposure of cultures to AMO 1618 or TMD, followed by removal of the inhibitors, the utilization of [14C]acetate for synthesis of digitonin-precipitable sterols increased about twofold, an increase parallelled by the rise in HMG-CoA reductase. Mevalonate at 2.3 mM counteracted the effects of these inhibitors on the reductase. When (R)-[2-14C]mevalonate at 2.3 mM was included with the two inhibitors in the culture media, the cells were still able to synthesize cholesterol although in lesser amounts than the controls. In the presence of TMD the H4 cells also accumulated [14C]squalene 2,3-oxide and [14C]squalene 2,3-22,23-dioxide. ISq added to cells kept in full-growth medium (10 micrograms ml-1) caused an almost complete and irreversible inactivation of the squalene oxide-lanosterol cyclase but did not inhibit polyprenyl synthesis, as the amount of [14C]mevalonate converted into squalene, squalene 2,3-oxide, squalene 2,3-22,23-dioxide plus a little cholesterol was equal to the amount converted by control cells into cholesterol plus squalene. After a 24 h exposure of cells kept in full-growth medium to ISq (10 micrograms ml-1), the levels of HMG-CoA reductase rose about twofold. ISq completely abolished the suppressive effect of 2.3 mM (R)-mevalonate on the reductase. Chromatin isolated from cell nuclei contains cholesterol, which is renewed biosynthetically. It is argued that the suppressor of HMG-CoA reductase, derived from mevalonate, is a sterol and not a non-steroidal product of mevalonate metabolism.

Inhibition of protein kinase C from polymorphonuclear neutrophils by long chain acyl coenzyme A and counteraction by Mg-ATP

The Ca2+- and phospholipid-dependent protein kinase (protein kinase C) from bovine polymorphonuclear neutrophils was inhibited by micromolar amounts of long chain acyl-CoAs. The extent of inhibition at a given concentration of the acyl-CoAs depended on the length of the chain. A chain length of at least 12C was required for inhibition. Inhibition of protein kinase C activity was counteracted specifically by Mg-ATP.

The use of amphotericin B to detect inhibitors of cellular cholesterol biosynthesis

Pores formed in the membranes of animal cells by complexes of sterols and the polyene antibiotic amphotericin B can efficiently kill the cells. Thus, in the absence of exogenous sources of cholesterol, inhibitors of enzymes in the cholesterol biosynthetic pathway render cells resistant to amphotericin B. Preincubation of Chinese hamster ovary cells with compactin or 25-hydroxycholesterol, inhibitors of the synthesis of the key intermediate mevalonate, protected cells from amphotericin B killing and this protection was reversed by the addition of exogenous mevalonate. The ability of compactin to confer amphotericin B resistance on normal cells was abolished when cells were provided exogenous cholesterol by the receptor-mediated endocytosis of low density lipoprotein. Low density lipoprotein receptor-defective Chinese hamster ovary cells were not subject to this low density lipoprotein-dependent amphotericin B killing. Exogenous mevalonate did not prevent 4,4,10 beta-trimethyl-trans-decal-3 beta-ol, an inhibitor of mevalonate conversion to sterols, from protecting cells from amphotericin B. A simple two-step protocol in which cells are preincubated (15-24 h) with potential inhibitors and then treated (3-6 h) with amphotericin B was devised to provide a sensitive method for detecting direct (e.g., competitive) and regulatory inhibitors of cholesterol biosynthesis. This protocol may prove useful in detecting potential antihypercholesterolemia drugs and is currently being used to isolate mutants in receptor-mediated endocytosis.

The acylation of lysophosphatidylcholine in the rat testicular tissue: the combined activity of acyl-CoA synthetase and lysolecithin acyltransferase

The activity of lysophosphatidylcholine acyltransferase (EC 2.3.1.23) in combination with acyl-CoA synthetase (EC 6.2.1.3) has been determined in the homogenates and subcellular fractions of rat testis. The enzyme activity was found to be maximal at pH 7.4 ATP and CoASH were required for optimal incorporation of [1-14C] oleic acid into phosphatidylcholine. The sulfhydryl-binding reagents showed inhibitory effect on the acyltransferase activity. Dibutyryl cyclic AMP and beta-mercaptoethanol did not affect the enzyme activity. Subcellular distribution patterns of markers, marker enzymes and lysolecithin acyltransferase have shown that the acyltransferase activity was found to be predominantly localized in the microsomal fraction, though significant activity was also present in the mitochondrial fraction. These findings, together with our previous studies on testicular phospholipases A, suggest that the deacylation-reacylation cycle is operative in rat testicular tissue.

Acetoacetyl-CoA reductase activity of lactating bovine mammary fatty acid synthase

Fatty acid synthase, purified from lactating bovine mammary gland, utilizes coenzyme A esters of acetoacetic, 3-hydroxybutyric, and crotonic acids as substrates for its partial reactions at micromolar concentrations. The NADPH:acetoacetyl-CoA reductase had a Km of 5 microM acetoacetyl-CoA and a Vmax of about 4 mumol of NADPH oxidized min-1 mg-1. In contrast, the Km for the model compound, acetoacetyl pantetheine was 820 microM and that of S-acetoacetyl-N-acetylcysteamine was over 40 mM. The reduction of acetoacetyl-CoA was observed with the enzyme from rat tissues also but not with those from avian tissues or yeast. With the bovine mammary enzyme, the reaction was found to oxidize 2 mol of NADPH for every mol of acetoacetyl-CoA consumed. Butyrate was the major product of reduction. The reductase activity was susceptible to inhibition by several sulfhydryl reagents; it was lost when the synthase was dissociated into one-half molecular weight subunits or when the incubation mixture was depleted of CoA. It was competitively inhibited by acetyl-CoA, butyryl-CoA, methylmalonyl-CoA, and 2-methylcrotonyl-CoA. These results as well as its use as a primer in fatty acid synthesis by the enzyme suggest that the acetoacetyl group from acetoacetyl-CoA is transferred to the enzyme, presumably to its 4'-phosphopantheine prosthetic group. The acyl group is then expected to remain attached to the enzyme while it is reduced, dehydrated, and reduced again to form a butyryl group which can either undergo chain elongation, if malonyl-CoA is present, or be released from the enzyme by hydrolysis or transfer to free CoA.

Mechanism of action of glutaryl-CoA and butyryl-CoA dehydrogenases. Purification of glutaryl-CoA dehydrogenase

Glutaryl-CoA dehydrogenase, a flavoprotein, catalyzes the reaction -OOCCH3CH2--CH2COSR (FAD leads to FADH2) leads to CH3CH = CHCOSR + CO2 (SR = CoA or pantetheine). With the isolated enzyme, a dye serves as the final electron acceptor. The enzyme from Pseudomonas fluorescens (ATCC 11250) has been purified to homogeneity. It was established with appropriate isotopic substitutions that the proton which is added to the gamma position of the product, subsequent to decarboxylation, is not derived from the solvent but is derived from the alpha position of the substrate. Under conditions where no net conversion of substrate occurs, i.e., in the absence of electron acceptor, the enzyme catalyzes the exchange of the beta hydrogen of the substrate with solvent protons. Butyryl-CoA dehydrogenase (M. elsedenii), which catalyzes an analogous reaction, catalyzes the exchange of both the alpha and beta hydrogens with solvent protons in the absence of electron acceptor. Glutaryl-CoA dehydrogenase and butyryl-CoA dehydrogenase are irreversibly inactivated by the substrate analogues 3-butynoylpantetheine and 3-pentynoylpantetheine. These inactivators do not form an adduct with the flavin and probably react with a nucleophile at the active site. Upon inactivation, the spectrum of the enzyme-bound flavin is essentially unchanged, and the flavin can be reduced by Na2S2O4. We suggest that inactivation involves intermediate allene formation. We proposed that these results support an oxidation mechanism for glutaryl-CoA dehydrogenase and butyryl-CoA dehydrogenase which is initiated by proton abstraction. With glutaryl-CoA dehydrogenase, the base, which abstracts the substrate alpha proton, is shielded from the solvent and is then used to protonate the carbanion (CH2--CH--CHCOSCoA) formed after oxidation and decarboxylation.