The assay and partial characterization of 3β-hydroxysteroid sulfotransferase of the hamster epididymis

Abstract

Using a partially purified enzyme preparation obtained from hamster epididymis, a simple assay has been developed to measure the sulfurylation of dehydroisoandrosterone (DHA) and desmosterol in the presence of 3′-phosphoadenosine 5′-phospho[35S]sulfate ([35S]PAPS). After stopping the enzymatic reaction with methanol and KCl, the 35S-labelled steroid sulfates are readily extracted into an organic phase. Optimal conditions for the sulfurylation of the two steroids were compared; optimum pH is 8.7 for DHA and 9.8 for desmosterol. Sulfoconjugation of desmosterol increases with magnesium concentrations up to 6 mM, while 40 mM concentrations of the divalent ion are required for the optimal sulfurylation of DHA. Maximum sulfurylation of these steroids requires the presence of 15 mM cysteine. Michaelis–Menten kinetics are observed with DHA which has an apparent Km of 32 μM, while desmosterol inhibits sulfotransferase activity at high concentrations. Saturation of the enzyme with PAPS results in an allosteric behaviour. Only the 3β-hydroxyl function of the steroid nucleus appears to be an appropriate sulfate acceptor for the epididymal hydroxysteroid sulfotransferase.