Binding of androgens in dog prostate cytosol and in plasma

Abstract

When dog prostate cytosol is incubated with the synthetic androgen, methyltrienolone (R1881), specific binding is observed in the 9S region of sucrose gradients. This binding shows the classical features of androgen receptors: specificity, affinity and sedimentation coefficient. Under identical experimental conditions, the natural hormone 5α-dihydrotestosterone (DHT) exhibits an additional saturable binding peak in the 3–4S region. This binding is different from the 9S receptor and is saturated by very low concentrations of diethylstilbestrol. Chromatographic analysis reveals that the 4S peak radioactivity consists mainly of 5α-androstane-3β, 17β-diol while DHT is predominant in the 9S peak. When tritiated 5α-androstane-3α, 17β-diol is incubated with prostate cytosol, some 9S binding is observed along with binding to slower sedimenting components. The 9S binding is attributed to a conversion of 5α-androstane-3α, 17β-diol into DHT during the in vitro incubation and not to a different androgen receptor. Tritiated 5α-androstane-3β, 17β-diol sediments exclusively in the 4S region of sucrose gradients. [ 3H]-Testosterone (T), which is not metabolized during in vitro incubations, is associated with both 9S and 4S components saturable by low concentrations of DHT and R1881. Thus, there seems to be a single androgen receptor which is specific for DHT and T. The level of androgen receptors was measured with the use of tritiated R1881 in immature, mature and hypertrophie dog prostates. No apparent differences could be found between the different groups. The mean concentration of binding sites in 22 prostates was 83 ± 7 (S.E.M.) fmol/mg prot. and the association constant ( K A ) at 0°C was 3.2 ± 0.4 × 10 9 M −1. An androgen binding protein is also present in male and female dog plasma. It is different from the prostate receptor in many respects: sedimentation coefficient, steroid specificity and affinity. It is concluded that dog plasma binding protein is either present only in low amounts in dog prostate cytosol or does not intefere with androgen receptor determination by usual charcoal assays.