[The effect of cofactors on the binding of steroid substrates with extrophilic hydroxysteroid dehydrogenase in the rabbit liver].

The isolated NADP (H)-dependent extrophilic hydroxysteroid dehydrogenase (EHSD) catalyzes oxidative-reductive reactions of 3 alpha-, 3 beta-, 17 beta-, and 20 alpha-hydroxy groups of androgens and progestagens, and binds these steroids and estrogens with relatively high affinities. In three series of experiments of the enzymatic kinetics, binding of 3H steroids to the protein, and inhibitory analysis an influence of cofactors on the affinity of steroid substrates and their analogs to the protein was demonstrated. A degree and direction of this influence are dependent on the steroid structure and cofactor form. It is proposed that cofactors may be important physiological regulators of enzymatic and steromoduline functions of the isolated EHSD.