Kishiko Sugino

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.

The Role of Follistatin in Activin Signal Transduction

Inhibin (1–4) and follistatin (FS) (5, 6) have been isolated from follicular fluids of various mammalian ovaries and characterized by their ability to suppress follicle-stimulating hormone (FSH) synthesis and secretion by the pituitary. The activity of FS resembles that of inhibin, but its structure is quite different: FS is a glycosylated single-chain protein (5, 6). We demonstrated that FS is an activin-binding protein (7) that neutralizes diverse activin bioactivities in various systems by forming stoichiometrically an inactive complex with activin (8). We also showed that FS is capable of associating with cell-surface heparan sulfate proteoglycans and proposed that FS participates in the regulation of the multiple actions of activin (9).

Activin, Activin-Binding Protein (Follistatin) and Activin Receptor

The principal regulators of the pituitary gonadotropins LH and FSH in mammals are the gonadotropin-releasing hormone (GnRH) from the hypothalamus and the gonadal steroids.