A Study of the Binding Capacity of Corticosteroid-Binding Globulin in Plasma

Abstract

A method was developed for the determination of CBG in terms of the amount of cortisol required to “saturate” it at 9 C and at 37.5 C. At low concentrations of cortisol it was shown that there was no unbound cortisol present in plasma. After a certain amount of cortisol was added, the percentage of unbound cortisol increased with further additions of steroid in proportion to the amount of binding protein present. A linear relationship between the percentage of unbound and the log total cortisol was demonstrated. Extrapolation of the straight line so obtained back to 0% unbound gave the amount of cortisol required to saturate the protein under the conditions used. Determinations of CBG binding capacity made at 37.5 C were technically less satisfactory and, since they were consistently 9% of those made at 9 C, the measurement at 9 C was adopted for routine use. Values of CBG binding capacity at 9 C in human plasma gave a mean of 31.8 ±5.5 μg cortisol capacity/100 ml in healthy men, 30.9 ±4.5 in healthy women and 24.3 ±5.1 in male subjects with Laennec's cirrhosis. With estrogen administration, values rose to approximately twice the previous levels. No change was found after the administration of ACTH. Unlike plasma corticoid levels, CBG levels did not undergo a diurnal variation. A good correlation was found between CBG binding capacity and plasma corticosteroid levels when the latter were determined at a specific time of day. CBG values were found to be clinically useful in the assessment of adrenal and hepatic disease.