Immunoreactive Inhibinα-Subunit in Human Serum: Implications for Radioimmunoassay*

Abstract

The specificity of the most widely used heterologous RIA for human serum inhibition was examined with regard to its cross-reactivity with the alpha-subunit of inhibin. Using a mixture of recombinant alpha inhibin proteins, including precursor and mature forms, the epitopic specificity of this antibody was localized entirely to the alpha-subunit. Furthermore, varying degrees of alpha inhibin immunoactivity were demonstrated in serum from normal subjects and patients with various reproductive disorders by both Western blot and RIA. These results demonstrate that data obtained using this RIA should be interpreted with caution as to the degree to which they reflect the presence of intact dimeric inhibin vs. alpha inhibin proteins in the circulation.

The Functional Significance of FSH in Spermatogenesis and the Control of Its Secretion in Male Primates

“…The inhibin hypothesis suffered a further set back when specific ELISAs for inhibin A, one of two dimeric forms of the mature hormone, failed to detect this molecule in the circulation of normal men (205). It is now known, however, that the original RIA used for infertile men recognizes circulating inhibin ␣-subunit as well as dimeric inhibin (206,207), and that dimeric inhibin in the circulation of the human male is accounted for entirely by inhibin B (208 -210). Moreover, inhibin B in men is present in substantial concentrations (300 -500 pg/ml), is of testicular origin, and is inversely related to circulating FSH levels (209,210).…”

Differential Control of Gonadotropin Secretion in the Human: Endocrine Role of Inhibin1

“…The pioneering heterologous polyclonal RIA generated to bovine inhibin (5) used in the initial studies of inhibin in the human suffered from problems with specificity. As the antisera exhibit complete epitopic specificity for amino acids 93-108 in the C-terminus of the ␣-subunit (6), this RIA, which is commonly known as the Monash assay, is incapable of distinguishing ␣-subunit precursor proteins from biologically active dimeric inhibins (7,8). Although this (9) and other (10) assays of total inhibin may have an important role as markers of granulosa cell or mucinous epithelial tumors of the ovary, they fail to reflect the dynamic secretion of the dimeric inhibins.…”

“…When the inhibin hypothesis was initially tested in various clinical models of primary and secondary hypogonadism, a reciprocal relationship between serum inhibin and FSH was not observed; indeed, inhibin levels were shown to be normal or even raised in men with severe spermatogenic failure (19). This raised doubts about the physiological relevance of inhibin in the male until it was recognized that the Monash RIA used in these studies had cross-reactivity of up to 288% with the biologically inactive ␣-subunit (7,8).…”

Plasma Concentrations of Immunoreactive (ir)-Inhibin, Gonadotropins and Steroid Hormones during the Ovulatory Cycle of the Duck

“…T h e a n t i b o d y u s e d i n t h i s i n h i b i n radioimmunoassay has a high degree of crossreactivity to the pro-αC and α subunits of inhibin [27,28]. Our measurements of the inhibin in duck plasma may include the free α subunit and biologically active in tact inhibin dimmer.…”