17 beta-Oxidation of estradiol and testosterone in the human testis.

The oxidation of 17 beta-estradiol in the human testis was studied in order to characterize a potentially important regulatory step in the action of estradiol in the human testis. 17 beta-Estradiol was shown to be converted to estrone, a weaker estrogen, in a reaction which was not inhibited by testosterone to any greater extent. When compared with the corresponding 17 beta-oxidation of testosterone, differences in coenzyme preferences, hydrogen ion sensitivity and inhibitions by estradiol and testosterone were observed, suggesting to a presence of different 17 beta-hydroxysteroid dehydrogenases in the human testis. Thus the human testis may counteract some of the deleterious effects of 17 beta-estradiol on androgen biosynthesis by metabolizing it to a weaker estrogen, in a reaction which probably is not inhibited by high testicular testosterone concentrations.