Inhibin alpha-subunit monomer is present in bovine follicular fluid

The 26 kDa form of the inhibin alpha-subunit monomer was isolated from bovine follicular fluid. Both sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting analysis revealed that the 26 kDa form of the alpha-subunit monomer is composed of 6 kDa and 20 kDa fragments linked together by a disulfide bond(s). The NH2-terminal sequence and amino acid analyses of each fragment showed that those fragments are derived from residues 18 to 60 and 227 to 360, respectively, of the inhibin alpha-subunit precursor (residues 1 to 360) as deduced from the cDNA sequence. Thus, it is concluded that removal of the putative signal peptide (residues 1 to 17) and the middle portion (residues 61 to 226) from the alpha-subunit precursor generates the 26 kDa form of the alpha-subunit. In the in vitro rat pituitary assay, the 26 kDa alpha-subunit exhibited neither inhibin-like nor FSH-release stimulating activity.