Abstract
The ability of NADH to function as an alternative cofactor for the support of estrogen biosynthesis was validated. NADH supported rates of aromatization of up to 80% of those obtained with NADPH, with an apparent K m of 0.70 mM, and stimulated the NADPH-supported reaction only when supplies of the normal cofactor were limiting, both additive and synergistic effects being observed. NADH-supported aromatization was inhibited competitively by NADP + and 2′-AMP with K i values of 5 μM and 22 μM, respectively. Support by both cofactors was lost in parallel with the selective removal of NADPH-cytochrome c reductase from microsomes by graded subtilisin treatment. NADH-supported aromatization was differentiated from NADPH-supported aromatization by its sensitivity to inhibition by NAD + and its response to changes in ionic strength. NADH appears to function, at high concentrations, as a surrogate for NADPH at the reduced nucleotide-binding site of NADPH-cytochrome c reductase but additional roles for NADH are also suggested both when acting alone and as a supplement to NADPH. A common oxidase (cytochrome P-450) appears to catalyze both NADH- and NADPH-supported aromatization.
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