Binding of corticosteroids by plasma proteins. IV. The electrophoretic demonstration of corticosteroid binding globulin.

The usefulness of equilibrium paper electrophoresis to detect the binding of corticosteroids by plasma proteins was described in an earlier paper of this series (2). Plasma was dialyzed initially against the electrophoretic buffer to which carbon labeled corticosteroids had been added. Later this buffer was used to saturate the electrophoretic paper. Thus, the concentration relationship between the buffer steroid and the protein bound steroid was maintained throughout the development of the electrophoresis. Consequently, there was little tendency for the bound steroid to trail the advancing protein components as occurs with customary electrophoresis of steroids (3). In the experiments carried out with this method using barbital buffer, pH 8.8, both corticosterone4-C14 and cortisol-4-C14 appeared to migrate with the albumin peak of plasma. Subsequent experiments with carbon labeled steroids using dialysis equilibrium suggested the possibility that there were two important corticosteroid binding proteins of human plasma (4). The first of these binding systems probably accounts for nearly all the binding of cortisol at normal physiologic concentrations. Under such circumstances about 99 per cent of the cortisol in plasma is protein-bound. As the concentration of hormone in the plasma rises, the degree of binding falls rapidly at first and then very much more slowly. Like plasma, plasma Fraction IV-4 has also been demonstrated to have a high affinity for corticosterone and cortisol. The affinity of this fraction for progesterone, testosterone, estrone, and estradiol is much less than for cortisol and