J. Biol. Chem., Vol. 255, Issue 22, 10624-10629, Nov, 1980
Mixed function oxidases in sterol metabolism. Source of reducing equivalents
DR Brady, RD Crowder and WJ Hayes
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.