Mixed function oxidases in sterol metabolism. Separate routes for electron transfer from NADH and NADPH.

Oxidative deformylation of 4-hydroxy[14C]methylene-5alpha-cholest-7-en-3-one and oxidative demethylation of [30,31-14C]4,4-dimethyl-5alpha-cholest-7-en-3beta-ol by rat liver microsomes have been compared with regard to the manner in which electrons are introduced from both NADH and NADPH. Evidence suggests that NADH and NADPH support oxidation of both substrates via separate routes of electron transfer. Thus, 10 micron cytochrome c will inhibit NADPH-supported oxidation to 40 to 50% of control activity leaving NADH-supported oxidation unaffected. Also, treatment of microsomes with subtilisin diminishes NADPH-supported oxidation to 10 to 30% of control activity for either substrate to 70 to 90% of control activity while NADH-supported oxidative activity is virtually unaffected. Studies on the oxidase activities and NADPH-cytochrome c reductase as well as NADH-ferricyanide reductase have shown marked differences in activity in the presence of inhibitors. Thus, 9 mM 2'-AMP inhibits NADPH-cytochrome c reductase to 10 to 20% of control activity while NADPH-supported oxidative demethyl ation and deformylation are essentially unchanged. Mersalyl at 15 to 25 nmol/mg of microsomal protein inhibits both reductases to 20 to 40% of control activity; oxidative demethylation is unaffected and oxidative deformylation stimulated slightly when NADPH is used. Finally, antibody to NADPH-cytochrome c reductase inhibits oxidase activity for either substrate to 70 to 90% of control activity while reductase activity is inhibited to 10 to 30% of control activity.