Radioimmunoassay for an inhibin-like peptide from human seminal plasma

Antisera raised in rabbits to a synthetic peptide consisting of 31 amino acids with a sequence identical to inhibin-like peptide (ILP) isolated from human seminal plasma afford a highly specific and sensitive radioimmunoassay. Synthetic ILP completely displaces antiserum binding of radioiodinated [Tyr4]-ILP, with half maximal displacement at 36 fmoles ILP/tube. ILP, [Tyr4]-ILP and ILP-(9-31) had essentially equal potency, while ILP-(1-25), ILP-(1-23) and ILP-(1-16) had reduced potency. No cross reactivity was found among a variety of peptide hormones and proteins. Human seminal plasma displaces 50% of [125I-Tyr4]-ILP at dilutions equivalent to 50-250 pl/tube, corresponding to immunoreactive ILP concentration of 0.5-2.5 mg/ml.