Physical properties and binding capacity of testosterone-estradiol-binding globulin in human plasma, determined by polyacrylamide gel electrophoresis.

Testosterone-estradiol-binding globulin in human plasma was characterized and quantified by polyacrylamide gel electrophoresis. Testosterone and estradiol were shown to bind to a single homogeneous protein over a wide range of experimental conditions. Testosterone binding to cortisolbinding globulin (transcortin) and estradiol binding to albumin were clearly demonstrated; both transcortin and albumin were separated from testosterone-estradiol-binding globulin by polyacrylamide gel electrophoresis, as was transferrin, a serum protein similar to testosterone-estradiol-binding globulin with respect to size and charge. The molecular, radius, apparent molecular weight, and net charge of testosterone-estradiol-binding globulin were obtained at three different pH values at 0’. A molecular radius of 2.95 nm was estimated and a molecular weight of 98,000 was computed with the use of a partial specific volume of 0.66. These values were independent of the presence or absence of ligand and were also obtained in urea concentrations up to 4M. Testosterone-estradiol-binding globulin capacity was routinely determined with 5a-dihydrotestosterone-&H as a ligand because of its high affinity for the protein. Binding capacities in plasma expressed as micrograms of steroid bound per 100 ml were men, 0.49 & 0.04 (S.E.); women, 1.42 + 0.22; pregnancy, 10.91 f 0.74; estrogen-treated women, 10.70 & 1.04. The usefulness of polyacrylamide gel electrophoresis for the study of steroid-binding and other binding proteins in biological fluids or tissues is discussed.