Estrogen-binding proteins of male rat liver: Influences of hormonal changes

Abstract

The liver of male as well as female mammals responds to estrogen. When assayed for the presence of [ 3H]estradiol-binding proteins it was found that male rat liver cytosol contains two species of estrogen binders. The first type, the minor species (3% of total activity), sediments in sucrose gradients at 8–9 S and elutes in the void volume of a Sephadex G-100 column. This protein exhibits estrogen receptor-like properties: high affinity binding ( K d = 10 −10 m), low binding capacity (12 × 10 −15 mol/mg cytosol protein), saturability, and specificity for both steroidal and nonsteroidal estrogens, but not for other steroids. Castration of the male results in a modest increase in the level of this protein. A similar if not identical protein is the major estrogen binder of female rat liver cytosol and has been shown to be an estradiol receptor. The second type of estrogen-binding protein is unique to the male. It sediments in sucrose gradients at 3–4 S and elutes from Sephadex G-100 with an apparent molecular weight of approximately 25,000. This protein has properties which distinguish it from the 8–9 S receptor: moderate binding affinity ( K d = 10 −7–10 −8 m), high binding capacity (196 × 10 −15 mol/mg cytosol protein), and specificity for steroidal estrogens only. Castration or estrogen treatment of the male rat results in almost complete disappearance of this protein. Conversely, treatment of the castrate male rat with dihydrotestosterone results in the reappearance of this species of estrogen-binding protein. The function of this male-specific estrogen-binding protein is not known.