Summary A specific antiserum has been generated against inhibin-like material (ILM) of prostatic origin. Using the immunoperoxidase technique, localization of ILM has been examined in a total of 114 prostates including normal (4 specimens), malignant (46) and hyperplastic (55) In the present study, we report immunocytochemical localization of inhibin-like material (ILM) in prostatic tissue. Inhibin is involved in the suppression of FSH synthesis and secretion. Of the various forms of inhibin-like materials (ILM) present in human seminal plasma, a peptide having a molecular weight of 10,400 daltons has been isolated in purified form from this laboratory (Thakur et al., 1981) the amino acid sequence of which has been reported (Sheth et al., 1984a; Johnson et al., 1984;Seidah et al., 1984). This peptide-ILM has been shown to be of prostatic origin (Sheth et al., 1984b;Beksac et al., 1984) and chemically similar (Johansson et al., 1984)

The complete sequence of a 94 amino acid human seminal plasma polypeptide exhibiting inhibin-like activity is presented. This molecule, called /I-inhibin, selectively and specifically suppresses the release of pituitary FSH in vivo as well as in vitro. It does not affect the secretion of LH. Such a novel acidic protein contains a very basic C-terminal segment which is easily cleaved by mild tryptic digestion. It is predicted that the FSH inhibiting activity may reside within this region of the molecule. This would imply a post Gln-Arg cleavage to release the basic C-terminal active moiety.

The amino acid sequence of a large form of inhibin‐like peptide in human seminal plasma was determined, and compared with structures reported for similar inhibin preparations and a seminal plasma globulin. The data confirm and correlate previous reports on this form of inhibin‐like peptide. The structural comparisons further suggest that the peptide is closely similar to or possibly identical to a sperm‐coating antigen reported to be synthesized from prostatic epithelium. This may correlate with non‐gonadal origins of inhibin‐like material and will help to elucidate the biological roles of inhibin(s).

A polypeptide in preparations of ‘large’ form (M r ∼14 000) material with inhibin‐like activity (inhibiting FSH secretion) has been isolated from human seminal plasma. Its amino acid composition, cleavage pattern with CNBr, N‐terminal sequence, and properties on reverse‐phase high‐performance liquid chromatography establish this inhibin‐like preparation to be homogeneous. The polypeptide contains close to 130 residues, has a free N‐terminal serine residue, a methionine residue in position 19, and a dibasic structure (Arg—Lys) in positions 16–17.