Mixed function oxidases in sterol metabolism. Source of reducing equivalents.

Either NADH or NADPH can serve as a cofactor for oxidative demethylation of [30,31-14C]4,4-dimethyl-5 alpha-cholest-7-en-3 beta-ol and oxidative deformylation of 4-hydroxy[14C]methylene-5 alpha-cholest-7-en-3-one. This report suggests that the cofactors interact with these two oxidase systems differently depending upon whether the reduced cofactor arises intra- or extramicrosomally. Marked differences in oxidative activity are observed depending on whether NADPH is generated in the microsomes or is added as an exogenous cofactor. Thus, the concentration of added NADPH required to yield maximal rates of sterol oxidation is 500 muM or greater. Nearly equivalent rates of sterol oxidation are obtained from NADPH generated in the microsomes where the NADPH concentration is no greater than 0.454 muM. Similar results are observed with NADH. In this case, NADH is generated in the microsomes from added NAD+ by microsomal reactions. The rate of sterol oxidation when NADH is generated from added NAD+ is nearly the same as that obtained from added NADH; although the concentration of NADH generated from NAD+ is 0.403 muM, the concentration of added NADH is 100 muM, and Km for added NADH is 1.7 muM.