Preliminary characterization of a binding protein for androgen in rabbit serum. Comparison with the testosterone-binding globulin (TeBG) in human serum

Rabbit serum contains a specific androgen-binding protein (TeBG) with high affinity for 5a-dihydrotestosterone (DHT), 5α-androstane- 3α,17β-diol (Adiol) and testosterone (T). Rabbit TeBG was slightly smaller than human TeBG and had a sedimentation constant of 4-5S (mean 4.4S), a Stokes radius of 43–44 Ågiving a molecular weight of 73,000 and a frictional ratio of 1.6. Measurements of retardation coefficients in polyacrylamide gels of different pore size gave a mean molecular radius of 2.74 nm, indicating a molecular weight of 75,000. Binding to rabbit TeBG was inactivated at pH below 5 or by heating above 50 C for 30 min. Rabbit TeBG was slightly more negatively charged than was human TeBG as demonstrated by ion exchange chromatography. Isoelectric focusing in sucrose gradients or polyacrylamide gels indicated a pi of 5.2 for rabbit TeBG and 5.5 for human TeBG. At 0 C the dissociation of DHT from rabbit TeBG (t_1/2 = 5.2 min) was much faster than the dissociation of DHT from human TeBG (t_1/2 = 67 min). The steroid specificity of rabbit TeBG was similar to that of human TeBG: DHT > A-diol > T >>>progesterone, androst-4-ene-3,17-dione and cortisol. The equilibrium constant (K_a) for binding of DHT to TeBG at 0 C was 1.9 Å– 10⁹ M^-1. Binding affinity of rabbit TeBG for DHT was markedly lowered by increase in temperature (K_a 37 C = 1.2 Å– 10⁸ M^-1) As in human, higher concentrations of rabbit TeBG were found in adult females than in males (p < 0.01). In immature rabbits, TeBG concentrations were much higher than in adult.