Sex hormone binding globulin and transcortin in human and baboon males

TeBG and transcortin were identified in serum from adult male baboons. TeBG values were higher than the TeBG concentration in adult men. The relative competitive abilities of three natural steroids, a synthetic androgen and two synthetic estrogens, for binding sites on TeBG occupied by radioactive DHT were compared. At a 10-fold molar excess, using both baboon and human TeBG, DHT and T were the most effective competitors; E2 was intermediate and DES a very poor one. Using gel filtration, the transcortin concentrations (mg/L) in two baboons were 16.8 and 27.4; the normal range for men is 23--45. The serum transcortin value of a third baboon, determined by equilibrium dialysis, was 0.41 micron/L (21.5 mg/L). Competition studies, using both techniques, showed that progesterone has a slightly higher affinity for baboon transcortin than cortisol and that a synthetic progestin does not bind to transcortin. Plasma binding capacity for synthetic steroids should be tested before they are used for studies on specific tissue receptors contaminated with serum.