Allosteric Activation of Rat Liver Microsomal

NADH and NAD+ are neither substrates nor inhibitors of 3-hydroxy3-methylglutaryl coenzyme A (HMGCoA) reductase in concentrations up to 1 mM. Addition of either NADH or NAD+ enhanced the activity of rat liver microsomal reductase, yet NADH failed to affect the activity of the freeze-thaw solubilized enzyme. The degree of enhancement of enzyme activity by NADH decreased as GSH concentration in the assay increased. Addition of 500 p~ NADH to the asssay converted the sigmoidal (Hill coefficient = 2.0) NADPH-dependent kinetic curve of the microsomal reductase into Michaelis-Menten kinetics (Hill coefficient = 1.1). Furthermore, the kinetic curves were shifted to the left, resulting in an up to 35% decrease in the concentration of NADH required to obtain half-maximal velocity (So.a) in the presence of 500 PM NADH. Again, this effect of NADH was diminished as GSH concentrations increased. These results demonstrate that NAD(H) is an allosteric activator of HMG-CoA reductase. These results also indicate that HMG-CoA reductase has NAD(H) binding site(s) distinct from the catalytic NADPH site(s).