Abstract
17β-Hydroxysteroid oxidoreductase, as well as estrone sulfate and dehydroepiandrosterone sulfate sulfatases, were found in the plasma membrane of microvilli of the fetal syncytiotrophoblast. Because of their location, these enzymes may influence feto—maternal transfer of steroids circulating as sulfates, the utilization of sulfated estrogen precursors and the proportion of estrone and estradiol delivered towards fetal and maternal circulations. Microvillar vesicles isolated from human term placentas were disrupted in hypotonic medium to obtain a membrane preparation. A fraction of the estradiol 17β-oxidoreductase (E 2DH) activity in the vesicle remained associated to the membrane after disruption and treatment with 2 M NaCl. The membrane-associated activity was resistant to inhibition with trypsin and did not react with a polyclonal antibody which neutralized cytosolic E 2DH activity. The membrane-associated enzyme was solubilized with a cholate-glycerol buffer solution and purified on Sephadex G-100. The estimated molecular weight of the solubilized enzyme (137kDa) appears to correspond to a tetramer since it was found to be about twice the size of the cytosolic enzyme. Both enzymes focused in polyacrylamide gels at pH 5.2. The K m relative to E 2 of the membrane-associated E 2DH (1.3 μM) diners from those of mitochondrial (0.43 μM), microsomal (0.69 μM) and cytosolic (11 μM) fractions. The cytosolic and the microvillar membrane associated 17β-hydroxysteroid oxidoreductases also differ in their specificity for C 18 and C 19 steroid substrates and in their pH dependence patterns. Sulfatases acting on estrone sulfate and dehydroepiandrosterone sulfate in microvillar membranes were insensitive to trypsin and as resistant to washes with 2 M NaCl as alkaline phosphatase. This data indicated that steroid sulfatases are also microvillar membrane associated enzymes of potential physiologic importance in the hydrolysis of estrogen precursors.
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