Determination of the binding capacity and affinity of proteins from human pregnancy plasma for androstenediol

Proteins from late pregnancy plasma were able to bind androstenediol (3 β,17 β-dihydroxyandrost-5-ene). This steroid and testosterone compete for the binding sites, displacing each other with apparently similar affinity. Estradiol-17 β had only 89% of the displacing effect of equal amounts of cold androstenediol. 60% of the plasma binding activity was lost after heating at 60°C for 30′. The binding capacity of pregnancy plasma for androstenediol was: 0.93 ug/g of protein, with apparently two different binding components in the system. A purified globulin fraction had a capacity of 1.35 ug/g of protein. The combining affinity for 1% plasma at 4°C was: 3.49 × 10 −3 ml/ug ± 0.48 × 10 −3. For the globulin fraction the value was: 9.25 × 10 −3 ml/ug ± 0.31 × 10 −3. The evidence suggests that androstenediol is bound by the same protein already described as testosterone and estradiol binding globulin.