Polycyclic aromatic hydrocarbon quinone-mediated oxidation reduction cycling catalysed by a human placental 17beta-hydroxysteroid dehydrogenase.

The human placental 17beta-hydroxysteroid dehydrogenase reduces a number of polycyclic aromatic hydrocarbon (PAH) o-quinones; some of the quinones undergo redox cycling at rates that approach or exceed the rate of reduction of estrone by the enzyme. The non-K-region o-quinone, 7,8-benzo[a]pyrenequinone, is the best o-quinone substrate tested. Cycling of all the quinone substrates is inhibited by superoxide dismutase; cycling is also inhibited by 17beta-estradiol and other estrogens. Since 19 alpha-estradiol is a competitive inhibitor of 9,10-phenanthrenequinone by the 17beta-hydroxysteroid dehydrogenase, it is likely that both reactions occur at the same active site on the enzyme. In the presence of the 17beta-hydroxysteroid dehydrogenase, the equilibrium between 17beta-estradiol, estrone, NADP, and NADPH is shifted by 7,8-benzo[a]pyrenequinone because the rapid redox cycling of this quinone results in the oxidation of NADPH. Unlike a number of hydroxysteroid dehydrogenases, the placental 17beta-hydroxysteroid dehydrogenase does not oxidize any of the six PAH trans-dihydrodiols tested.