Transcortin: A corticosteroid-binding protein of plasma—XIV. Effects of time of equilibration of serum with cortisol on transcortin determinations by gel filtration

Abstract

The results of determinations of plasma transcortin levels by gel filtration are subject to assay conditions such as duration of incubation and the amount of cortisol added. We investigated the cause for the dependency of results on assay conditions by incubating samples over prolonged periods of time. Transcortin concentration and the rate of dissociation of the cortisol-transcortin complex remained constant over many weeks of storage at 4°C. while irreversible binding to unidentified macromolecules increased continuously. Irreversible binding occurs faster at 37°C than at 4°C. In addition, with time, an ether-extractable. less polar metabolite of cortisol appears in the serum. The data suggest that a highly reactive metabolite of cortisol enters into covalent binding with macromolecules during prolonged incubations.