Testicular steroid sulfatase: Substrate specificity and inhibition

Abstract

Kinetic studies of the cleavage of dehydroepiandrosterone-sulfate (1) and androstenediol-3-sulfate by a particulate enzyme preparation from a rat testicular microsomal fraction gave Km values of 2.04 x 10-6M for DHA-S and 0.935 x 10-6M for androstenediol-3-sulfate with identical Vmax values. Inhibition studies with equimolar concentrations of substrate and inhibitor demonstrated that 5[alpha]-androstane-3[beta],17[beta]-diol was the most potent inhibitor among fifteen C-19 and C-18 unconjugated steroids investigated. Substitution of: 1) a [Delta]4 or [Delta]5 bond or phenolic A ring for a saturated A ring, 2) 17[alpha]-hydroxyl group for a 17[beta]-hydroxyl group, or 3) a 3[alpha]-hydroxyl group for a 3[beta]-hydroxyl group, markedly decreased the inhibitory effect of the steroid. Ki values of 1.7 x 10-6 M, 3.3 x 10-6M and 11.8 x 10-6M were found with 5[alpha]-androstane-3[beta], 17[beta]-diol, 5[alpha]-androstane-3[alpha], 17[beta]-diol and testosterone, respectively. The kinetic data related to inhibition are consistent with partial competitive inhibition.