Regulation of rat liver hydroxymethylglutaryl coenzyme A reductase by a new class of noncompetitive inhibitors. Effects of dichloroacetate and related carboxylic acids on enzyme activity.

Abstract

A B S T R A C T Dichloroacetate (DCA) markedly reduces circulating cholesterol levels in animals and in patients with combined hyperlipoproteinemia or homozygous familial hypercholesterolemia (FH). To investigate the mechanism of its cholesterol-lowering action, we studied the effects of DCA and its hepatic metabolites, glyoxylate and oxalate, on the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) obtained from livers of healthy, reverse light-cycled rats. Oral administration of DCA for 4 d decreased HMG CoA reductase activity 46% at a dose of 50 mg/kg per d, and 82% at a dose of 100 mg/kg per d. A 24% decrease in reductase activity was observed as early as 1 h after a single dose of 50 mg/ kg DCA. The inhibitory effect of the drug was due to a fall in both expressed enzyme activity and the total number of reductase molecules present. DCA also decreased reductase activity when added to suspensions of isolated hepatocytes. With chronic administration, DCA inhibited 3H20 incorporation into cholesterol by 38% and into triglycerides by 52%. When liver microsomes were incubated with DCA, the pattern of inhibition of reductase activity was noncompetitive for both HMG CoA (inhibition constant [Ki] 11.8 mM) and NADPH (K; 11.6 mM). Inhibition by glyoxylate was also noncompetitive for both HMG CoA (K1 1.2 mM) and NADPH (K1 2.7 mM). Oxalate inhibited enzyme activity only at nonsaturating concentrations of NADPH (K, 5.6 mM Department of Medicine, College ylene glycol, all of which can form glyoxylate, also inhibited reductase activity. Using solubilized and 60-fold purified HMG CoA reductase, we found that the inhibitory effect of glyoxylate was reversible. Furthermore, the inhibition by glyoxylate was an effect exerted on the reductase itself, rather than on its regulatory enzymes, reductase kinase and reductase phosphatase. We conclude that the cholesterol-lowering effect of DCA is mediated, at least in part, by inhibition of endogenous cholesterol synthesis. The probable mechanisms are by inhibition of expressed reductase activity by DCA per se and by conversion of DCA to an active metabolite, glyoxylate, which noncompetitively inhibits HMG CoA reductase. These studies thus identify a new class of pharmacological agents that may prove useful in regulating cholesterol synthesis and circulating cholesterol levels in man.

In vivo regulation of human mononuclear leukocyte 3-hydroxy-3-methylglutaryl coenzyme A reductase. Studies in normal subjects.

“…Our studies show that the cholesterol-lowering drug, DCA, at a single dose lower than that reported to decrease cholesterol levels in patients with diabetes (34) or homozygous familial hypercholesterolemia (35), rapidly but transiently inhibited enzyme activity. These results are consistent with those obtained in rat liver (39) and cultured human lymphoid (IM-9) cells (32).…”

“…Dichloroacetate reduces cholesterol levels in man (34,35), inhibits hepatic cholesterol biosynthesis in rats (39) and is a noncompetitive inhibitor of HMG CoA reductase obtained from rat liver (39). Previous studies have also shown that dichloroacetate inhibits human leukocyte HMG CoA reductase activity in vitro (32).…”

Effects of Diisopropylamine Dichloroacetate on Proliferation and Differentiation of Normal Human Keratinocytes in vitro

“…As described in figure 2 and figure 3, the active component of DADA on keratinocytes is DCA, at least in vitro, which coincides with the cases previously reported on increasing vasodilation [26], lowering blood glucose in hyperglycemia [5] and suppressing lipid syn- Kitamura/Ota/Mimura thesis [27]. DCA is known to activate the pyruvate dehydrogenase complex by phosphorylating and inhibiting pyruvate dehydrogenase kinase [28], a regulatory protein of pyruvate dehydrogenase.…”

Pharmacogenetic Considerations with Dichloroacetate Dosing

“…DCA's inhibitory effect is observed in both rodent liver [5] and human leukocytes [6]; it likely accounts for the drugassociated reduction in total and lowdensity lipo protein (LDL) cholesterol in patients with LDL receptornegative homozygous familial hyper cholesterolemia [7] and for its desig nation as the first orphan product for this rare disease. Second, DCA inhibits de novo hepatic triglyceride synthesis in nondiabetic rodents [5] and decreases circulating triglyc eride and very low density lipoprotein levels in patients with Type 2 diabetes mellitus [4]. It also decreases blood ketone bodies in rats with experimentally induced diabetic ketoacidosis [8,9].…”