Characteristics of glucocorticoid-binding sites of rat liver: different effects of adrenalectomy on the binding

Enhancement of affinity to receptors in the esterified glucocorticoid, hydrocortisone 17-butyrate 21-propionate (HBP), in the rat liver

To investigate the affinity of glucocorticoid (GC) to its receptor, the binding of [3H]hydrocortisone 17-butyrate 21-propionate ([3H]HBP) and [3H]dexamethasone ([3H]DEX) in rat liver was analyzed kinetically. Scatchard analysis of [3H]hydrocortisone ([3H]HC) binding yielded a curvilinear plot with upward concavity with high and low affinities. The dissociation constant (Kd) value of high affinity was 1.9 nM and of low affinity 68.7 nM. A Scatchard plot of [3H]HBP binding showed a straight line with high affinity. The Kd value was 9.8 nM. The Kd values for the low affinity site of HC were in good agreement with the Ki values obtained from displacement experiments of [3H]DEX and [3H]HBP binding. The Ki values of HC for [3H]DEX and [3H]HBP were 51.9 and 42.3 nM respectively. The association rate constant for HBP to the GC receptor was 2.9 times lower than that for HC. The dissociation rate constant for HBP was 6.1 to 8.3 times lower than that for HC. The Kd values for [3H]HBP (9.5 nM) and [3H]HC (30.0 nM) obtained from the above two rate constants were approximately the same as the Ki and Kd values (in the case of HC, the value of the low affinity site). These results suggest that esterification of the hydroxyl group(s) in the side chain of GC by butyrate and propionate increased the affinity to the GC receptor, and that a decrease in the dissociation rate from the receptor caused the increase in the affinity to the GC receptor.

Characteristics of binding of a new anti-inflammatory glucocorticoid, hydrocortisone 17-butyrate 21-propionate (HBP), to glucocorticoid receptors of rat liver

Characteristics of the specific binding of hydrocortisone 17-butyrate 21-propionate (HBP) to the cytoplasmic fraction from rat liver were investigated. The inhibition constant (Ki) of HBP for the site of 3H-dexamethasone (3H-DM) binding was approximately equal to the value of DM and significantly smaller than that of hydrocortisone (HC). The maximum number of binding sites (Bmax) and dissociation constant (Kd) for 3H-HBP were also approximately equal to those for 3H-DM. The Scatchard and Hofstee plots analyses of 3H-HC binding indicated that the HC binding sites consisted of three components with different affinity, while that of 3H-DM had only one site with an intermediate affinity. HBP and hydrocortisone 17-butyrate (HB) bound to other binding sites of HC in addition to the site for DM. The IC50 value for synthetic glucocorticoids determined from the inhibition curves of 3H-HC binding in the first phase agreed with the values determined by the displacement study of 3H-DM binding. Furthermore, the first phase of HBP in the inhibition curve of 3H-HC binding disappeared from the curve, and only the second phase remained following the addition of DM. These results indicate that the esterification of C-17 and C-21 OH increases the affinity of the binding site for synthetic glucocorticoid and attenuates the affinity for the other binding sites of HC.

Heterogeneity of binding sites for glucocorticoid and the glucocorticoid-receptor complex in rat livers

Glucocorticoid binding to cytoplasmic and nuclear fractions and glucocorticoid-receptor complex binding to the nuclear fraction were investigated using rat liver. The glucocorticoid-receptor complex binding to the nuclear fraction was temperature-dependent, saturable, small in amount and of high affinity. The affinity and number of the glucocorticoid-receptor complex binding to the nuclear fraction were altered according to the glucocorticoid. Both the Bmax of nuclear glucocorticoid-receptor complex binding and the affinity of glucocorticoid to the cytoplasmic fraction were correlated with the relative anti-inflammatory potencies of glucocorticoids reported by Hynes and Murad (1980) and Fried et al. (1958). These results suggest that the number of nuclear binding sites of the glucocorticoid-receptor complex depends on the ligand steroid which is bound to the receptor of the cytoplasmic fraction and may be involved in physiological and pharmacological potencies of the glucocorticoid in addition to the affinity of the glucocorticoid to the receptor.