Relative competition of corticosterone, cortisol, cortisone, 11-dexycortisol and prednisolone with (1,2-3H)-cortisol in various protein-binding radioassay systems.

The relative influence of some endogenous corticosteroids and of synthetic prednisolone on competitive protein-binding radioassay was compared with that of cortisol, using as a source of transcortin pooled plasmas from various species and at different dilutions. Human (different clinical situations), cow, dog, sheep, hog, rabbit and chicken plasma were examined. The ability of corticosterone, cortisone, 11-deoxycortisol and prednisolone to displace [1,2-3-H] cortisol from corticosteroid-binding globulin (CBG) was measured: (1) by assessing the amounts (ng/tube) which produce a displacement equal to 10 ng/tube of cortisol, and (2) by calculating the integrated areas of displacement defined by the binding curves and by expressing them as a percentage of the cortisol curve area. While corticosterone, 11-deoxycortisol and prednisolone have a binding potency not very different from that of cortisol in almost the entire set of competitive protein-binding assays tested, the binding ability of cortisone was found to be particularly dependent upon the species of diluted plasma. Differences in the relative specificity of binding are apparent also within the human species, depending on the source of diluted plasma, whereas the concentration of endogenous steroids does not seem to significantly affect binding curves under the conditions examined. It is suggested that binding proteins in various conditions differ from those of healthy adults not only quantitatively but also qualitatively.