HUMAN INHIBIN A

Description

Inhibin is a glycoprotein, a member of the transforming growth factor (TGF)-β superfamily. Inhibin is secreted mainly from the gonads and inhibits follicle-stimulating hormone (FSH) secretion from the anterior pituitary and in turn regulates gonadal function and development. The presence of FSH-inhibiting activity had been reported in the gonads since the 1970s, and the FSHinhibiting factor “inhibin” was first isolated in 1985 from porcine1 and bovine follicular fluid.

Clinical Use

Inhibin levels in the circulation are a reliable marker for granulosa cell tumors and serous and mucinous epithelial carcinomas. This may be true for granulosa cell tumors in mares. Evaluation of plasma inhibin A levels until the second trimester of pregnancy is useful for screening for Down’s syndrome. The circulating inhibin B is a good predictor for the conditions of spermatogenesis. High levels of inhibin are noted in humans, rats, and dogs with Leydig cell tumors, whereas a low level clinically suggests a premature ovarian failure.

Structure and conformation

Inhibins and activins are structurally related glycoprotein hormones. Inhibins are disulfide-linked heterodimers composed of an α-subunit and either a βA-subunit (inhibin A) or a βB-subunit (inhibin B), whereas activins are homodimers or heterodimers made up of βA- and βB-subunits. The monomeric α-subunit, devoid of FSHsuppressing activity, has also been identified in follicular fluid. The α-subunit has N-linked glycosylation sites and their degree of glycosylation modifies biological activity. An approximate 80% identity is seen in the sequences of the human, porcine, bovine, and rat α-subunits . The mature βA-subunit shares the same aa sequence among the above species while the βB-subunit shows an approximate 90% identity. The human βA- and βB-subunits share 64% aa sequence identity.