Evidence of Direct Conversion of Testosterone Sulfate to Estradiol 17-Sulfate by Human Placental Microsomes

Ring A aromatization of testosterone sulfate (TS), using human term placental microsomes as a enzyme source, was examined under a reduced nicotinamide adenine dinucleotide phosphate (NADPH)-generating system. The aromatization to estradiol 17-sulfate (ES) without removal of the C-17 conjugate group was confirmed. Judging from the results of various experiments, including an inhibition experiment, the conversion was considered to occur by placental aromatase. Apparent Km and Vmax values for this aromatization were determined to be 37.4 μM and 330 pmol/mg protein/min respectively. The physiological role of direct conversion of TS by placenta to ES is discussed in connection with pregnancy.