Estrogen-binding sites in the liver of sexually mature male and female brown trout, Salmo trutta L

Abstract

Binding sites with characteristics conforming to those of a putative estrogen receptor have been identified in the liver of sexually mature male and female brown trout. [ 3H]Estradiol is bound with high affinity ( k D in the 10 −9, M range) and limited capacity (<400 fmol mg −1 protein) to a proteinaceous moiety in liver cytosol. Binding is highly specific, only estradiol and estrone displace specifically bound [ 3H]estradiol. Cortisol, testosterone, 11-ketotestosterone, and 17α, 20β-dihydroxyprogesterone are inactive. Labeled cytosol is retained on DNA-cellulose columns, eluting with 0.1 M NaCl. The liver of a sexually mature female brown trout contains more than twice as many binding sites as that of the male (168 ± 15 fmol mg − protein cf. 69 ± 4 fmol mg −1 protein), and no difference in k D is observed between males and females (2.6 ± 0.2 10 −9, M and 2.9 ± 0.3 10 −9, M, respectively). Plasma from both male and female brown trout was also found to bind [ 3H]estradiol, but with lower affinity ( k D in 10 −8, M range) and higher capacity (4000 – 11,000 fmol mg −1 protein) and with less specificity. Estradiol, testosterone, and 11-ketotestosterone all displaced plasma-bound [ 3H]estradiol. It is concluded that cytosol and plasma binding of estradiol in brown trout are distinct, and indicative of the presence of an intracellular hepatic estrogen receptor and a plasma sex-steroid-binding protein.