BINDING OF CORTICOSTEROIDS BY PLASMA PROTEINS. I. DIALYSIS EQUILIBRIUM AND RENAL CLEARANCE STUDIES 12

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Abstract
Dialysis equilibrium experiments were carried out on human plasma obtained 2 hours after the ingestion of oral cortisone acetate. An average of 94% of the 17-OH corticosteroids and 65% of the 17-OH corticosteroid glucuronides were loosely bound to plasma proteins. Human plasma Fraction V and to a lesser extent Fraction IV-4 possessed significant steroid binding activity. The binding observed in Fraction IV-4 could be attributed to its contained albumin. The binding affinity of human Fraction V increased as the concentration of hydrocortisone decreased. This was not noted in bovine serum albumin. Low renal clearance of 17-OH corticosteroids after cortisone acetate ingestion can be related to low concentration of non-protein bound steroid available for filtration. The clearance of 17-OH corticosteroid glucuronides is higher than can be explained by simple filtration of the non-protein bound component. This observation plus depression of clearance of the glucuronide fraction with probenecid suggest a tubular excretory process.