Further characterization of estrogen binding to rat testis cytosol.

Binding of estradiol (E2), estriol (E3), RU16117, and moxestrol to testis cytosol from adult male rats was investigated. High-affinity binding sites were identified in the 8-9S region of sucrose density gradients; a second, high-capacity binding component in the 4S region was probably due to contamination with serum. Thermodynamic properties of the testicular estrogen binding site were quite similar to those of the uterine receptor. E2 had the highest affinity for testicular cytosol binding sites (Ka: E2 much greater than moxestrol greater than E3 greater than RU16117). Comparison of association rate (E2 greater than E3 greater than moxestrol = RU16117) and dissociation rate constants (E3 = RU16117 greater than E2 much greater than moxestrol) as well as studies in vivo revealed moxestrol as a long-acting and RU16117 as a short-acting compound. This difference may be useful for evaluation of the mediation of estrogen effects in the rat testis.