Characterization of spironolactone binding sites distinct from aldosterone receptors in rat kidney homogenates

The binding of [3H]spironolactone to kidney homogenates from adrenalectomized rats was studied by dextran-charcoal absorption methods. [3H]Spironolactone binds with high affinity and low capacity (KD = 12.9 +/- 0.6 nM; Bmax = 93.4 +/- 3.8 fmoles/mg protein) at low temperatures (0 degrees-2 degrees). Its hormone specificity, as measured by relative binding affinity (RBA) is spironolactone greater than prorenone greater than methyltrienolone greater than testosterone greater than progesterone greater than aldosterone greater than dexamethasone. In the same tissue preparation, specific spironolactone binding sites and classical mineralocorticoid receptor sites labelled with [3H]aldosterone differ in their thermal stability, binding parameters and hormone specificities, whereas their tissue distributions are similar. In conclusion, [3H]spironolactone binds specifically to kidney homogenates from adrenalectomized rats and these binding sites, apparently, are different from the classical mineralocorticoid receptors. The theoretical and practical aspects of this finding are discussed.