Binding Characteristics of an Androgen Receptor in the Ovaries of Coho Salmon, Oncorhynchus kisutch

Abstract

Androgen receptors were identified in the cytosol from ovaries of juvenile coho salmon, Oncorhynchus kisutch. The binding for the synthetic androgen mibolerone was specific and saturable ( K d = 0.32 ± 0.02 n M; Bmax = 15.31 ± 4.31 fmol/mg protein). Bound [ 3H]mibolerone was much higher in ovarian cytosol than in cytosolic extracts from heart, liver, and muscle. [ 3H]mibolerone specific binding was 50% lower in the plasma than in the ovarian cytosolic extracts. [ 3H]mibolerone binding was displaced most effectively by those 17α-methylated synthetic androgens (mibolerone, methyltestosterone, methylandrostanolone) that can induce functional masculinization in fish. The naturally occurring androgens 11 -ketotestosterone and 5α-dihydrotestosterone both displaced [ 3H]mibolerone binding, but they were 10- to 100-fold less effective than the 17α-methylated androgens. Testosterone, 11β-hydroxyandrostenedione, estradiol, progesterone, and 17α,20β-dihydroxyprogesterone were not potent competitors. [ 3H]mibolerone specific binding was reduced after preincubation with trypsin. About 25% of the binding in the cytosolic extract had DNA binding affinity under experimental conditions. The characteristics of this androgen binding site are consistent with a model of receptor-mediated steroid-induced sex inversion.