[pH-dependence of hormone-binding and enzymatic activity of estrophilic hydroxysteroid dehydrogenase from the rabbit...

The effects of pH on the ability of NADP-dependent estrophilic hydroxysteroid dehydrogenase (HSD) from the soluble fraction of rabbit liver to bind steroids and catalyze their 3 alpha, 3 beta, 17 beta- and 20 alpha-oxidoreduction were studied. The pH optima for enzymatic transformations of various steroids were found to differ significantly by more than two units. These differences do not seem to be related to the localization of the modified group in the steroid molecule. Kinetic data suggest that pH influences the catalytic efficiency, steroid affinity for the protein and, perhaps, the degree of interdependence of steroid and cofactor binding to the protein. These assumptions were confirmed by the results of direct 3H-steroid-HSD binding studies. Furthermore, the maximal levels of binding of various steroids to the protein were found to occur at pH values differing by more than 4 units. Scatchard analysis revealed the effects of hydrogen ion concentrations both on the steroids affinity for the protein and on the concentration of steroid-binding sites of HSD. The data obtained are suggestive of some "superfluity" of the protein steroid-binding site which, in turn, ensures the multifunctionality of estrophilic HSD including a possibility of an alternative orientation of steroids in their binding site.