Rat liver 3-hydroxy-3-methylglutaryl-CoA reductase. Catalysis of the reverse reaction and two half-reactions.

Rat liver 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase catalyzes, in addition to its normal biosynthetic or forward reaction (HMG-CoA + 2 NADPH + 2H+—-mevalonate + 2 NAD+ + CoASH), the reverse reaction (mevalonate + CoASH + 2 NADP+—-HMG-CoA + 2 NADPH + 2H+) and two “half-reactions” that involve the presumed intermediate mevaldate (mevaldate + CoASH + NADP+—-HMG-CoA + NADPH + H+ and mevaldate + NADPH + H+—-mevalonate + NADP+). These reactions were studied using both enzyme solubilized by the traditional freeze-thaw method and enzyme solubilized with a nonionic detergent in the presence of inhibitors of proteolysis. All four reactions were inhibited by mevinolin, a known inhibitor of the forward (biosynthetic) reaction catalyzed by HMG-CoA reductase. When the enzyme was inactivated by ATP and a cytosolic, ADP-dependent HMG-CoA reductase kinase, the rates of both the forward reaction and the half-reactions decreased to comparable extents. Although coenzyme A is not a stoichiometric participant in the second half-reaction (mevaldate + NADPH + H+—-mevalonate + NADP+), it was required as an activator of this reaction. This observation implies that coenzyme A may remain bound to the enzyme throughout the normal catalytic cycle of HMG-CoA reductase.