[Hormone binding properties of estrophilic oxysteroid dehydrogenase in the rabbit liver].

The estrophilic fraction of hydroxysteroid dehydrogenase (HSD) from the soluble fraction of rabbit liver was purified by ammonium sulfate precipitation, gel filtration, ion-exchange chromatography and affinity chromatography on estradiol-Sepharose. The isolated protein preparation expresses the 3 alpha-, 3 beta-, 17 beta- and 20 alpha-HSD activities on androgen and progestogen substrates. Some characteristics of 3H-estradiol, 3H-testosterone and 3H-progesterone interaction with the protein, the mutual competition of these hormones and the competitive efficiency of 73 steroids and their analogs in experiments with 3H-progesterone were investigated. It was found that sex steroids of all the three groups interact with a moderate affinity with the isolated HSD (K alpha approximately 10(7) M-1 at 0-4 degrees C). This interaction is characterized by relatively high association and dissociation rates. The main structural determinants of steroid ligands which provide for their affinity for the protein were established. The experimental results suggest that androgens and progestogens interact with the same binding site of the protein, whereas estrogens interact with a different site. It is supposed that by virtue of its high affinity for steroids and high concentration in the cells, the isolated protein can accomplish not only the enzymatic, but also a steromodulin function via reversible interactions with steroid ligands.