Evidence for the presence of specific prolactin binding protein in rat seminal vesicle fluid

The presence of a specific prolactin binding in rat seminal vesicle fluid has been demonstrated. The binding phenomenon was saturable and pH dependent, the optimum pH being 7.2. The prolactin binding protein (PBP, 1800 x g supernatant) was characterized by a single order affinity binding sites with an association constant (Ka) 1.52 x 10(7) mol-1. The specificity of the prolactin binding was demonstrated by the fact that other proteohormones such as rFSH, rLH, rTSH and hTSH failed to inhibit the binding of labelled hormone to PBP, while unlabelled oPRL gave a dose-response inhibition curve. Among the rats in the age group 45 to 180 days, PBP obtained from animals of 90 day old showed a maximum binding of radioiodinated rPRL. Following castration for five days the percent binding of 125I-rPRL to PBP decreased markedly. The treatment of castrated rats with testosterone propionate for five days returned the 125I-rPRL binding to near intact values. Following a centrifugation of 1800 x g supernatant at 360 000 x g for 3 h, prolactin binding activity still remained in the supernatant thus implicating that the binding protein is in a soluble state.