Abstract
Binding of [ 3H]-5α-dihydrotestosterone and [ 3H]-17β-estradiol to cytosol fractions of ram hypothalamus has been studied. For both hormones, specific binding to cytosol macromolecules with high affinity (affinity constants 5.09–14.08 × 10 9 M −1 and 1.00–4.41 × 10 9M −1 for 5α-dihydrotestosterone and estradiol respectively) and low capacity were demonstrated. Binding was abolished by heating cytosol at 56°C for 30 min or by incubation with pronase. Macromolecules bound to radioactive steroids sedimented on a sucrose density gradient at 7 S ([ 3H]-5α-dihydrotestosterone) or 8.2 S ([ 3H]-17β-estradiol). Binding to macromolecules of serum or cerebral cortex appeared different or negligible. Competitive binding assays suggested moreover that cytosolic androgen receptor is different from estrogen receptor; 5α-dihydrotestosterone and other androgens had no affinity for the estrogen receptor whereas 17β-estradiol showed a 12% relative affinity for the androgen receptor. In hypothalamus of rams that had been castrated 24 h earlier, there were more binding sites in the sexual season (17.4fmol for 5α-dihydrotestosterone and 75.2 fmol, for 17β-estradiol) than in the non-sexual season (7.9 and 23.4 fmol).
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