Immunotitration of 3-hydroxy-3-methylglutaryl-coenzyme A reductase in various physiological states

The immunotitration of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase [mevalonate:NADP+ oxidoreductase [CoA-acylating), EC 1.1.1.34], the major regulatory enzyme in cholesterol biosynthesis, by HMG-CoA reductase antiserum is described. This technique provides the advantage that one can rapidly measure relative changes in both enzyme concentration and enzyme activity in different physiological states by using enzyme samples all the way from crude liver microsomes to purified enzyme. Regarding the diurnal rhythm of HMG-CoA reductase, the major difference noted between animals killed near the middle of the dark period (D 4.5) compared to animals killed near the middle of the light period (L 4.5) was in the concentrations of HMG-CoA reductase present, rather than in the activity of the enzyme, with substantially more enzyme found near mid-dark. Cholestyramine treatment resulted in both an increased concentration of HMG-CoA reductase and a catalytically more active enzyme. On the other hand, cholesterol feeding resulted in both a decreased concentration of HMG-CoA reductase and a catalytically less active enzyme.