Effects of a supernatant protein activator on microsomal squalene-2,3-oxide-lanosterol cyclase.

A soluble protein termed "supernatant protein factor" (SPF) that stimulates microsomal squalene epoxidase has been isolated in this laboratory (Ferguson, J.B., and Bloch, K. (1977) J. Biol. Chem. 252, 5381-5385). We now show that the purified protein also stimulates microsomal squalene-2,3-oxide leads to lanosterol cyclase but has no effect on the subsequent conversion of lanosterol to cholesterol. Phospholipid, specifically phosphatidylglycerol or phosphatidylethanolamine, is required for maximal stimulation of the cyclase by purified SPF. The response of microsomal squalene epoxide-lanosterol cyclase to SPF was abolished by pretreatment of the membranes with phospholipase A2 or by low concentrations of deoxycholate, indicating that an intact membrane system is required. Digestion of intact microsomes with trypsin had no effect on the SPF-stimulated cyclase activity. However, in the presence of 0.4% deoxycholate, trypsin completely inhibited microsomal squalene epoxide-lanosterol cyclase. We conclude that the cyclase is located on the luminal side of the microsomal membrane. SPF also significantly enhances the formation of lanosterol from squalene-2,3-oxide already bound to microsomes. This finding is constant with the proposal that SPF influences intramembrane events.