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Binding of glucocorticoids and related steroids to rat thymus cells in vitro has been measured by equilibrium and kinetic methods. Results are interpreted in terms of the previously established specific and nonspecific metabolic activities of glucocorticoids. Equilibrium binding, which is proportional to nonspecific activity, is largely accounted for kinetically by a fraction which at 37° dissociates with a time constant under 15 sec. A minor fraction, with a time constant of about 3 min, appears to consist of molecules responsible for specific glucocorticoid activity. This slowly dissociating or specific fraction becomes bound rapidly enough to account for the fastest glucocorticoid effects so far observed. In contrast to the rapidly dissociating or nonspecific fraction it becomes saturated at physiological concentrations, it depends on cellular ATP levels, and it is competed for by all steroids tested in rough proportion to glucocorticoid activity. Chromatographic analysis indicated that the specific fraction probably consists entirely of unaltered cortisol, adding to the evidence that the hormonally active form of cortisol is cortisol itself. The 9α-fluoro group increases specific binding, and possibly intrinsic activity. The 11β-hydroxyl, 17α-hydroxyl, and 4-ene-3-ketone groups also increase binding whereas the 11-ketone and 11α-hydroxyl groups decrease binding. Certain steroids that lack glucocorticoid activity nevertheless compete for specific binding. Antiglucocorticoid activity on glucose uptake in vitro has been shown with cortexolone.
Munck et al. (Fri,) studied this question.