Binding of [3H]methyltrienolone to androgen receptor in rat liver

The synthetic androgen methyltrienolone is superior to testosterone and androstenedione for the measurement of androgen receptor in tissues where the native ligands are metabolized into inactive derivatives. [3H]Methyltrienolone binds with a high affinity to androgen receptor in cytosol prepared from male rat livers, as the Scatchard analysis revealed that the Kd value was 3.3 X 10(-8) M and the number of binding sites was 35.5 fmol/mg protein. Since methyltrienolone also binds glucocorticoid receptor which exists in rat liver, the apparent binding of androgen receptor is faulty when measured in the presence of glucocorticoid receptor. The binding of methyltrienolone to glucocorticoid receptor can be blocked by the presence of a 100-fold molar excess of unlabeled synthetic glucocorticoid, triamcinolone acetonide, without interfering in its binding to androgen receptor, because triamcinolone does not bind to androgen receptor. Triamcinolone-blocked cytosol exhibited that the Kd value was 2.5 X 10(-8) M and the number of binding sites was 26.3 fmol/mg protein, indicating a reduction to 3/4 of that in the untreated cytosol. The profile of glycerol gradient centrifugation indicated that [3H]methyltrienolone-bound receptor migrated in the 8-9 S region in both untreated and triamcinolone-blocked cytosols, but the 8-9 S peak in triamcinolone-blocked cytosol was reduced to about 3/4 of that of untreated cytosol.