Inhibins and activins: chemical properties and biological activity

The long-sought, nonsteroidal, gonadal inhibitor of the secretion of FSH has been isolated, characterized, and the primary structure in several species (human, porcine, bovine, murine) has been deduced. Inhibins are proteins consisting of two subunits (18-kDa alpha- and 14-kDa beta-subunits) linked by disulfide bridges and two forms of inhibins were observed in human, porcine, and murine, but only one in bovine. Each form of inhibin (A and B) has a common alpha-subunit, but a highly homologous, distinct beta-subunit (beta A and beta B). The beta-subunits and the alpha-subunit are linked to form inhibins A and B which exert an inhibitory effect on basal FSH secretion, but the dimer formed by either two beta A-subunits or two distinct beta A- and two beta B-subunits (homoactivin-A and activin, respectively) possess FSH-stimulating activity. Inhibin secreted in response to FSH from the pituitary originates primarily from the granulosa cells of the ovary and the Sertoli cells of the testes, thus demonstrating a reciprocal feedback relationship.