Testosterone-binding protein in a seasonally breeding amphibian.

The concentration of testosterone and dihydrotestosterone (DHT) in the plasma of Taricha granulosa rises markedly during the season when androgen-dependent epigamic structures and behaviors recrudesce. During this season (June-October), the physiological characteristics of the testosterone-binding globulin (TeBG) remain unchanged. Competitive binding studies indicate that TeBG binds fairly specifically to testosterone and DHT. The dissociation constant (5 X 10(-8) M) and binding capacities (ranging from 6 to 12 ng androgen bound/ml plasma) were determined using the charcoal adsorption procedures. Because the binding capacity of TeBG remains unchanged when the concentrations of androgens increase, the proportion of testosterone and DHT bound to TeBG is drastically lower during the breeding season, compared to the nonbreeding season. Furthermore, bilateral castration of male newts apparently reduces the binding capacity of TeBG, but Silastic implants of androgen fail to restore the binding capacity.