Measurement of the Binding Capacity of Corticosteroid-Binding Globulin in Human Plasma

Abstract

Corticosteroid-binding globulin (CBG) of human plasma can be inactivated selectively by heating for 20 minutes at 60 C. Under these conditions cortisol binding by albumin is not altered significantly. These findings provide the basis of a quantitative measurement of plasma CBG binding capacity in which double equilibrium dialysis is conducted with native and heat-inactivated plasma with sufficient cortisol-4-C14 to permit equilibrium and the saturation of CBG binding sites. The difference in bound cortisol-4- C14 in the 2 bags represents binding due to CBG. Knowing the specific activity of cortisol in the system, we can express CBG binding capacity as μg of cortisol bound per 100 ml of plasma. The mean CBG binding capacity of normal human plasma was 20.1 ±sd 5.2 μg/100 ml. The CBG binding capacity rose to between 30 and 128 μg/100 ml in pregnancy and after the administration of high doses of estrogen. Abnormalities of CBG binding capacity were observed in a number of plasma samples from patients with dysproteinemias.