HETEROGENEITY OF GLUCOCORTICOID BINDERS - UNIQUE AND A CLASSICAL DEXAMETHASONE-BINDING SITE IN BOVINE-TISSUES

Bovine tissues are known to possess high affinity [3H]dexamethasone-binding sites. However, we found the properties of these sites to be unusual in that triamcinolone acetonide could only compete for a fraction of the [3H]dexamethasone binding. The dilemma was resolved when it became clear that multiple binding sites were present in bovine tissues. One site resembed the classical glucocorticoid (type II) receptor in rat kidney and other organs. Its characteristics in adrenally intact bovine thymus include an apparent Kd at 0 C for [3H]triamcinolone acetonide of 3 nM, a maximum number of binding sites of 210 fmol/mg protein, heat lability, and the usual sequence of steroid specificities: triamcinolone acetonide > dexamethasone > cortisol > corticosterone > progesterone > 17α-hydroxyprogesterone. The second site manifested several novel properties. This binder has an apparent Kd at 0 C for [3H]dexamethasone of 14 nM, ad a maximum number of binding sites of 600 fmol/mg protein, was stable to heating at 37.C for 24 h, and exhibited an unusual sequence of steroid specificities: dexamethasone > progesterone > cortisol > corticosterone > 17α-hydroxyprogesterone. Uniquely, the affinity for triamcinolone was negligible. The type II binder sedimented at 8S in hypotonic sucrose gradients, whereas the heat-resistant binder sedimented at 6.8S. Both binders were present in multiple bovine organs, including thymus, kidney, liver, adipose tissue, and adrenal cortex, but not in plasma. A heat-resistant, triamcinolone acetonideresistant binder could not be demonstrated in the rat. In summary, bovine tissues provide another example of glucocorticoidbinding site heterogeneity. We believe that the type II binder represents the glucocorticoid receptor; the function of the unique heat-resistant binder that fails to bind triamcinolone acetonide remains to be determined. © 1979 by The Endocrine Society.