Activation of purified 3-hydroxy-3-methylglutaryl-CoA reductase by phospholipids

3-Hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase), the enzyme that catalyzes the rate-limiting step in cholesterol biosynthesis, has been purified by two previously reported procedures. Enzyme purified by the method of Heller, R. and Shrewsbury, M. (1976) J. Biol. Chem. 251, 3815-3822) shows up to 3-fold enhancement of activity by various types of lipid dispersions while the enzyme purified by the procedure of Tormanen et al. ((1976) Biochem. Biophys. Res. Commun. 68, 754-762) shows no activation. These results suggest that interaction with microsomal membrane lipids may be important in determining the activity of this enzyme. Analysis of bound lipid showed that enzyme prepared by the procedure of Tormanen contained at last 50 times as much phospholipid on a weight basis as enzyme prepared by Heller and Shrewsbury. Analysis of both preparations by gel-electrophoresis indicates that enzyme activities of the two comigrate, but in neither case does activity coincide with the major protein species.