Characterization of a cytosolic steroid-binding protein in the liver of the winter flounder, Pseudopleuronectes americanus.

The winter flounder liver contains significant concentrations of estrogen binding protein(s) that fulfill some of the criteria normally assigned to receptor proteins, in that these proteins are saturated at low substrate concentrations (less than 5 nM) and have a high affinity for ligand (Kd approximately 6 X 10(-10) M for [3H]17 beta-estradiol). Sedimentation analysis of the 17 beta-estradiol-binding protein complex on 5-20% sucrose density gradients detected an 8 S form of the binding protein under low salt conditions. Nonspecific sites sedimented in the 4 S region. These sedimentation characteristics are similar to those observed for the estrogen receptor and nonspecific binding sites in mammalian tissues. In contrast to mammalian estrogen receptor, which binds selectively to estrogens, the binding protein(s) investigated in our studies also binds other steroids. Competitive binding studies demonstrated that 10-fold excess of unlabeled testosterone and progesterone displaced 50 and 45% of [3H]17 beta-estradiol from the binding protein, respectively. These studies demonstrate the presence of steroid-binding protein in liver of female winter flounder which might play a role in regulating hepatic steroid hormone action.