Androgen binding in the skin of mature male brown trout, Salmo trutta L.

Specific, saturable binding of [3H]testosterone (Kd = 12.5 nM, Nmax = 1.1 pmol mg-1 protein) has been identified in skin cytosol of mature male brown trout. Binding with this affinity and capacity resembles more closely that observed in the plasma of mature male brown trout (Kd = 20.6 nM, Nmax = 6.6 pmol mg-1 protein) than that expected of a specific steroid receptor. However, the fraction of skin cytosol precipitating with 30% ammonium sulphate has a higher affinity for testosterone (Kd = 1.9 nM) and a lower capacity (Nmax = 96.3 fmol mg-1 protein) than does whole cytosol. Furthermore, the high-salt extract of crude skin nuclear pellet binds [3H]testosterone with affinity (Kd = 1.3 nM) and capacity (Nmax = 69.4 fmol mg-1 protein) similar to those of ammonium sulphate-precipitated cytosol. Specific binding in all three fractions of skin is abolished by treatment with proteolytic enzymes and a component of both cytosol and nuclear extract, which binds [3H]testosterone specifically, is retained on DNA-cellulose columns, eluting with high-salt buffer. Specifically bound [3H]testosterone is displaced most efficiently by testosterone. 5 alpha-dihydrotestosterone, estradiol-17 beta, and 11-ketotestosterone also compete but are 5- to 10-fold less potent. Consistent and reproducible binding of [3H]11-ketotestosterone to skin cytosol fractions or nuclear extract could not be demonstrated. It is concluded that testosterone binding with the above characteristics partially fulfills the criteria required of a putative steroid hormone receptor. The results are discussed with reference to the relative roles of testosterone and 11-ketotestosterone during sexual maturation.