Evidence that estradiol-2/4-hydroxylase activities in rat hypothalamus and hippocampus differ qualitatively and involve multiple forms of P-450: ontogenetic and inhibition studies.

Estradiol-2/4-hydroxylase activity was studied in hypothalamic and hippocampal microsomes of male and female Wistar rats between 22 and 75 days after birth. The activity exhibited substrate saturation (50-100 microM estradiol) required NADPH and was inhibited by carbon monoxide, alpha-naphthoflavone and metyrapone. With 30 microM estradiol, 50% inhibition required 50-70 microM alpha-naphthoflavone compared to 200 microM metyrapone. Metyrapone exhibited biphasic inhibition curves which did not differ significantly between hypothalamus and hippocampus, whereas alpha-naphthoflavone was a more potent inhibitor of the hippocampal enzyme than of the hypothalamic enzyme. The Km and Vmax of the hippocampal estradiol-2/4-hydroxylase were significantly greater than that of the hypothalamus in both sexes at all ages studied. In female rats the Km of the hypothalamic enzyme changed from 14 microM at 23 days of age, to 47 microM at 71 days, but remained constant at about 29 microM in males. The Km of the hippocampal enzyme showed no significant change with age in either sex. The present findings indicate that catechol estrogen formation in the brain is catalyzed by multiple forms of microsomal P-450. They also suggest that these enzyme activities in the rat hypothalamus and hippocampus differ qualitatively. Ontogenetic changes in the Km of estradiol-2/4-hydroxylases appeared to be limited to the female hypothalamus. This might reflect a specific biological requirement of the female hypothalamus during critical stages of sexual differentiation of the brain. The relatively high hippocampal activity might reflect the catalytic versatility of different P-450 isozymes and does not necessarily imply a physiologically meaningful role with respect to catechol estrogen biosynthesis in this particular brain area.