The presence of androgen-binding protein in the guinea-pig testis, epididymis and epididymal fluid.

Androgen-binding protein (ABP) is present in the guinea-pig testis, epididymis and epididymal fluid. Guinea-pig ABP sediments as an approx. 4.6S species on sucrose gradients containing 0.01 M KCl. Electrophoresis on non-denaturing polyacrylamide gels indicated that specific androgen binding was present in epididymal cytosol, but not in plasma. Time-course studies indicated that binding equilibrium is approached in about 2.5 h; the dissociation half-time of [3H]5 alpha-DHT from guinea-pig ABP is 5.64 +/- 0.62 h (n = 6) at 4 degrees C. The relative affinities of some steroids for guinea-pig ABP in relation to 5 alpha-DHT = 1 are: testosterone = 0.55 +/- 0.13 (n = 4), estradiol = 0.14 +/- 0.03 (n = 4), the anti-androgen cyproterone acetate = 0.0025 +/- 0.0002 (n = 3). Guinea-pig ABP exhibited an equilibrium dissociation constant of 6.34 +/- 0.52 nM (n = 3) at 4 degrees C and there were 3.43 +/- 0.78 (n = 3) pmoles of binding sites per mg of protein when homogenates of the whole epididymis were assayed. The concentration of ABP was lowest in the caput-corpus region of the epididymis, highest in the proximal cauda, and intermediate in the distal cauda. Essentially all of the ABP present in the distal cauda was intraluminal, as evidenced by the fact that flushing of the duct eliminated most of the [3H]5 alpha-DHT binding activity.