Studies on the carboxyl terminal peptides of human seminal plasma inhibin (HSPI). Chemical synthesis and in vivo biological activity of the disulfide loop peptide 67-94 of HSPI.

Observation of contradictory results with the in vitro assays for inhibin-like activity of the carboxyl terminal 28 amino acid peptide 67-94 with a disulfide loop, of human seminal plasma inhibin (HSPI), prompted us to synthesize both the linear and the cyclic peptides and test their ability to suppress the circulating levels of follicle stimulating hormone (FSH) in vivo in adult male rats. The linear peptide [Cys(Acm)73,87] 67-94 of HSPI was synthesized by solid-phase peptide synthesis using fluorenylmethyloxycarbonyl (Fmoc) chemistry and a continuous-flow technology. The peptide was cyclized by direct iodine oxidation of the S-diacetamidomethyl peptide in dilute solution. In the in vivo assay the linear peptide did not affect the levels of FSH, whereas the cyclic peptide suppressed the levels of FSH significantly. Thus, the carboxyl terminal region of HSPI does have inhibin-like activity and perhaps has the active core of the protein.