Characterization of a sexual steroid binding protein in Bufo arenarum

Reproductively active female Bufo arenarum possess a serum protein (SBP) that binds testosterone, dihydrotestosterone, and estradiol with high affinity and capacity. The binding capacity values are two orders of magnitude higher than those found in human serum. The specificity of B. arenarum SBP is comparable to human SBP with the exception that it has a high affinity for the synthetic androgen methyltrienolone (R1881). The physicochemical characterization of SBP indicates a Stokes radius of 43.5 A, a sedimentation coefficient of 5.2 S, and a molecular weight of 93,300.