Charakterisierung und Kinetik einer mikrosomalen 17β-Hydroxysteroid-Oxydoreduktase der menschlichen Placenta

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Abstract
The microsomal fraction of human placenta contains a 17[beta]-hydroxy-steroid oxidoreductase which reacts with phenolic as well as with neutral steroids. Both NAD and NADP can serve as cofactors, the latter giving about one-fifth of the initial rate of the former. The oxidation of the 17B-hydroxyl group shows a maximum at pH 7. 8, whereas the reduction of the 17-oxo group is optimum at pH 6. 3, The Michaelis-Menten constant was found to be 2. 2/. 10-6m. The enzyme has a temperature optimum at 42[degree]C and an activation energy of 8 kcal/ Mol. The enzymic activity is inhibited by SH blocking agents. The specific activity of the microsomal 17[beta]-hydroxysteroid oxidoreductase is only one-tenth of that of the non-purified cytoplasmic 17[beta]-hydroxy-steroid oxidoreductase of human placenta. 16[alpha]-Hydroxyestrone is easily reduced to estriol by the enzyme, thus indicating that the microsomal enzyme takes part in the placental biosysnthesis of estriol.