CHARACTERIZATION OF A HEPATIC PROTEIN IN NONHUMAN-PRIMATES THAT BINDSMIBOLERONE BUT NOT DIHYDROTESTOSTERONE OR METHYLTRIENOLONE

During our studies of the hepatic androgen receptor in cynomolgus monk eys, tritiated mibolerone +/- a 200-fold excess of unlabeled miboleron e has been used to determine specific binding in cytosol. During time- course studies, high-capacity, unsaturable binding of [H-3]mibolerone was noted after short-term incubations (4 h, 4 degrees C). When hepati c cytosol from male monkeys was incubated for 18 h at 4 degrees C, the high-capacity binding disappeared; saturable, high-affinity binding w ith characteristics consistent with the androgen receptor then could b e identified. The characterization of [3H]mibolerone binding in molybd ate-stabilized hepatic cytosol using sucrose density gradients and gel filtration yielded an unstable binding peak in addition to that of th e androgen receptor. This lower molecular weight protein identified by gel filtration did not bind other androgens, including methyltrienolo ne, and did nor have characteristics of other binding proteins that ha ve been identified previously. This protein was not precipitated from 30% ammonium sulfate, which allowed it to be separated from the androg en receptor. Binding to this protein in ovariectomized female monkeys did not disappear with extended incubation at 4 degrees C, suggesting greater stability or a higher capacity. The function of this protein i s not known, but both triamcinolone acetonide and contraceptive proges tins appeared to displace tritiated mibolerone that was bound to it. T his high-capacity binding of mibolerone interferes in the assessment o f androgen receptor levels in these females unless it is eliminated. T he synthetic androgen methyltrienolone does not bind to this protein a nd is a better choice for defining binding to the androgen receptor in these tissues.