Human placental steroid-sulfatase. Kinetics of the in-vitro hydrolysis of dehydroepiandrosterone 3-sulfate and of 16 alpha-hydroxydehydroepiandrosterone 3-sulfate.

35S-labeled sulfate esters of dehydroepiandrosterone and 16 alpha-hydroxydehydroepiandrosterone were synthesized and used as substrates for the in vitro kinetic assay of human placental steroid-sulfatase. Both steroid sulfates were hydrolysed by placenta homogenates and microsomal fractions with V values comparable to each other. The Km value of the 16 alpha-hydroxy compound, however, was found to be about tenfold higher than that of dehydroepiandrosterone sulfate. Both sulfate esters competitively inhibited each other's hydrolysis. The results suggest that dehydroepiandrosterone sulfate, as compared to its 16 alpha-hydroxy derivative, is the preferred substrate of the sulfatase. As far as conclusions can be drawn from experiments in vitro, this finding excludes the possibility that the preponderance of placental estriol production over that of estradiol and estrone in human late pregnancy is due to a preferential binding and cleavage of the estriol precursor 16 alpha-hydroxydehydroepiandrosterone sulfate by the placental steroid-sulfatase.