Recent advances in the human physiology of inhibin secretion

Abstract

The past three years have seen considerable pro­ gress in the field of inhibin physiology and also the emergence of more complex issues. This review provides the background to enable the clinician to appreciate the advances of the field, the emerging problems and the likely relevance of this protein to clinical medicine. Several extensive reviews have been published recently which provide more detail concerning the basic developments in the field (1­ 4). (i) Structure of inhibin and related proteins Although the term "inhibin" was proposed in 1932 (5) to describe a water soluble substance that in­ hibited the development of castration cells within the pituitary gland, the name later became equated with the substance responsible for specific feedback control of FSH secretion. In 1985, inhibin was iso­ lated and shown to be a glycoprotein consisting of two dissimilar subunits, termed a and B, joined by disulphide bonds and which exist in two molecular weight forms of 58 kDa and 32 kDa (6, 7). Subse­ quent cloning of the genes (8, 9) that independently code for the a and B subunits demonstrated that the 32 kDa form was a truncated product of the 58 kDa variety due to cleavage of an NH2-terminal segment of the a subunit. In addition to these mo­ lecular weight forms of what is now termed inhibin [A (aB A ) ] , Ling and colleagues (10) isolated another type of inhibin termed inhibin B which consisted of the identical a subunit joined to a protein which is homologous but structurally distinct from the Bsub­ unit termed BA. Both inhibin A and B appear to be equipotent in their inhibition of FSH secretion in vitro. More recently, dimers of the Bsubunit termed Activin A (BAB A) and Activin AB (BAB s ) have been isolated