Relative binding of testosterone and estradiol to testosterone-estradiol-binding globulin

The binding of estradiol (E₂) and testosterone (T) to testosterone-estradiol-binding globulin (TeBG) was studied in vivo at 37 C by three independent methods: equilibrium dialysis, steady state polyacrylamide gel electrophoresis, and TeBG-ligand dissociation kinetics. Equilibrium dialysis was performed at 37 C with the dialysate containing human serum albumin in amounts equivalent to that of the plasma dialysand. Scatchard analysis indicated that under these conditions E₂ does not measurably bind to TeBG, while T has a K_d of 3.7 x 10^‑10 M. Similarly, Scatchard-type analysis of E₂ binding to TeBG in steady state polyacrylamide gel electrophoresis at 37 C revealed no high affinity saturable binding, while dihydrotestosterone was bound with a K_d of 2.7 X 10^‑10 M. Examination of the dissociation kinetics of T and E₂ from TeBG revealed that the mean (±SD) T_1/2 of dissociation of T from plasma at 37 C (10.8 ± 2.4 min) was significantly shortened to 3.5 ± 0.4 min by saturation of plasma with dihydrotestosterone (P < 0.01), whereas that of E₂ (8.9 ± 1.4 min) was not changed (9.6 ± 3.0 min). These data suggest that TeBG is not an important binder of plasma E₂ at physiological temperatures and explain the observation that in diseases characterized by high TeBG levels, such as hyperthyroidism and liver disease, the MCR and free E₂ levels have generally been normal.