Binding of 125I-hCG to rainbow trout (Salmo gairdneri) testis in vitro

Homogenates of maturing rainbow trout testes show specific binding sites for 125J-labeled hCG (= 125I-labeled hCG). The binding is competitively inhibited by unlabeled hCG and by a hypophyseal extract of rainbow trout. It could be demonstrated that the tissue 125J-hCG binding specificity is restricted to the gonadal preparation. The trout testis was characterized by determining affinity and capacity from Scatchard plot analysis giving a high constant of dissociation Kd 3.65 x 10(-10)/M and a low binding capacity of 0.88 x 10(-15) M/mg tissue. The test system is markedly dependent on temperature, incubation-time, and pH. The maximum binding was found at 37 degrees during 2 hr of incubation in a buffer of pH 7.5.