BINDING OF CORTICOSTEROIDS BY PLASMA PROTEINS. III. THE BINDING OF CORTICOSTEROID AND RELATED HORMONES BY HUMAN PLASMA AND PLASMA PROTEIN FRACTIONS AS MEASURED BY EQUILIBRIUM DIALYSIS12

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Abstract
The binding of cortisol-4-C14, corticosterone-4-C", progesterone-4-Cl4, testosterone-4-c[image] and estrone-16-Cl4 by human plasma has been compared in dialysis equilibrium experiments. Whereas the binding of estrone is influenced relatively little by the amount of steroid in the dialysis system, the affinity of plasma for cortisol and corticosterone is greatly affected by the amount of steroid added. With additions of less than 1 /[image]g/10 ml of plasma, about 99% of cortisol in plasma is protein bound. The affinity of plasma decreases abruptly with further additions of steroid. The behavior of testosterone and progesterone was intermediate between that of the corticosteroid and estrogenic hormones. The ability of related corticosteroids to displace cortisol-4-Cl4 and corticosterone-4-Cl4 from the binding sites of high affinity in plasma has been examined. The presence of at least two hydroxyl groups at the 11 [beta] 17 [alpha], or 21 positions, and an intact delta4-3-ketone in ring A are required for maximum displacement of the radioactive hormone. Competition is greatly decreased by the presence of the 11-keto, the ll[alpha]-hydroxyl, the 18-aldehydic or the 9[alpha]-fluoro groups on a corticosteroid molecule. Dialysis equilibrium experiments with plasma protein fractions using low concentrations of cortisol-4-Cl4 demonstrated that Fraction IV-4 had the highest affinity for cortisol and corticosterone. The ability of albumin to bind these hormones under these conditions was markedly less. Fraction IV-4 had less affinity for progesterone, testosterone, estrone and estradiol than for the two corticosteroid hormones.