KINETIK UND CHARAKTERISIERUNG DER ÖSTRADIOLSENSITIVEN 17β-HYDROXYSTEROIDDEHYDROGENASEN IN DER MENSCHLICHEN LEBER

Abstract

ABSTRACT NAD+- and NADP + -linked 17β-hydroxysteroid dehydrogenases from human liver preparations are able to convert 17β-oestradiol to oestrone as well as testosterone to androstenedione without any substrate specificity. The NAD + -sensitive enzyme occurs in the microsomal fraction (14 % of the total activity) and in the soluble fraction (86 % of the total activity). The pH of maximum activity was 10.3. The Km value for 17β-oestradiol was 3.5 × 10−6 Mol/l in the microsomal and 9.8 × 10−6 Mol/l in the soluble fraction. In contrast the NADP + -linked 17β-hydroxysteroid dehydrogenase was only present in the soluble fraction with a pH-optimum at 9.8 and a Km for 17β-oestradiol of 5.1 × 10−6 Mol/l. Both NAD+-and NADP+-sensitive enzymes reached maximum activity at 45°C. These data indicate that 17β-hydroxy-C18-steroid dehydrogenases from human placenta and human pregnant blood serum which can be used as a test in late pregnancy are different from those of human liver.