6. Steroid metabolism in neuroendocrine structures. Specific and nonspecific components in the oxidative metabolism of estradiol by the male rat brain in vitro.

The oxidative transformation of estradiol at the C-2 and C-4 positions in the male rat brain was studied by the transfer of 3H into water from [2-3H]- and [4-3H]-estradiol. The nonenzymatic transformation at either site was linear with substrate concentration. The addition of pyridine nucleotide cofactors had no effect on the transformation at the C-4 site. The C-2 reaction was greatly enhanced by NADPH or NADH and exhibited the greatest velocity in the microsomal formation in the presence of NADH. The C-2 reaction exhibited saturation with respect to substrate and NADH with apparent KM of 1.82 microM and 0.34 mM respectively and Vmax of 81.3-76.3 pmol/mg protein/60 min. The reaction at C-4 failed to exhibit substrate saturation up to 30 microM. These data indicate that male rat brain estrogen hydroxylase is specific for the C-2 position, is NADH dependent and thus different from the corresponding liver enzyme. The reaction at C-4 in the brain tissues fails to exhibit enzymatic characteristics and the formation of 4-hydroxyestrogens in that tissue may be artifactual.