BIOLOGICAL AND IMMUNOLOGICAL CHARACTERIZATION OF INHIBIN FORMS IN HUMAN PLASMA

A number of immunoassay methods have been developed recently to detect specifically the bioactive alpha-beta A subunit inhibin dimer (inhibi n A) in human plasma. However, the specificity of these assays in term s of their ability to detect the range of inhibin forms found in plasm a and their relationship to bioactivity have not been investigated. In hibin was fractionated from human follicular fluid (hFF) and serum/ pl asma hom women stimulated with gonadotropins (IVF serum), and from pos tmenopausal and male plasma, using a combined immunoaffinity/preparati ve SDS-PAGE procedure. The molecular weight profile of inhibin was est ablished by inhibin in vitro bioassay, three alpha-beta A subunit spec ific immunoassays, and three alpha subunit-directed immunoassays that detect the alpha subunit as well as inhibin A and B forms. In hFF inhi bin forms of 33, 36, 55 and 66K were detected by in vitro bioassay and by most immunoassays except for 33 k inhibin, which was nondetectable by one alpha-beta A ELISA. The alpha subunit-directed assays also det ected activity in the 29-31K region, in some assays in considerably hi gh levels. In IVF serum in vitro bioactivity and immunoactivities were detected between 27 and 100K with the alpha-beta A assays failing to detect all bioactive forms. Alpha subunit-directed assays gave similar immunoactive profiles. Neither in vitro bioassay nor alpha-beta A ass ays detected activity in post-menopausal plasma or male plasma, while alpha subunit-directed assays showed peaks predominantly at 36 k, alth ough at low levels. It is concluded that dimeric inhibin A specific as says detected bioactive inhibin forms in hFF and to a lesser extent in IVF serum. Alpha subunit-directed assays correlated poorly with in vi tro bioassay in hFF because of the high alpha subunit levels in this s ample. The higher correlation between these assays in IVF serum sugges ted that there was little free alpha subunit. The 36K form in male pla sma may be free alpha subunit or inhibin B.