Comparative studies of the aromatization of testosterone and epitestosterone by human placental aromatase

The aromatization of epitestosterone (17 alpha-hydroxy-4-androsten-3-one) and testosterone by lyophilized human placental microsomes was studied. Upon incubation of epitestosterone, 12% was converted to 17 alpha-estradiol, 15% to 19-keto-epitestosterone (17 alpha-hydroxy-4-oxo-4-androsten-19-al), 10% to 19-hydroxyepitestosterone (17 alpha, 19-dihydroxy-4-androsten-3-one), and about 10% to several unidentified products. A similar incubation with testosterone resulted in 60% conversion to 17 beta-estradiol; 30% was unchanged. At increasing substrate concentrations (0.1-50 microM), the aromatization rate of epitestosterone increased gradually and did not reach a plateau, whereas aromatization rate of testosterone plateaued at about 3 microM. The presence of either testosterone or 17 beta-estradiol in concentrations 0.1-10 times the concentration of epitestosterone inhibited the aromatization of epitestosterone by about 70%, while the aromatization of testosterone was not inhibited by either epitestosterone or 17 alpha-estradiol. Lyophilization of fresh microsomes or storage of the lyophilized microsomes at -20 C greatly reduced the aromatizing activity upon epitestosterone but not upon testosterone. These results suggest that the aromatizing system for epitestosterone is different from that for testosterone.