Physicochemical and immunological characterization of an HCG-like substance from human pituitary glands

Abstract

Human chorionic gonadotropin (HCG), or an HCG-like substance (HCG′), has been reported to be produced more widely in human than merely by placental tissue and by certain tumours^1,2. It has been difficult to detect low levels of HCG in the presence of human luteinizing hormone (LH) because the two are structurally similar and react similarly in most assays. Each is composed of two glycoprotein subunits (α and β), and the major difference between the hormones is an extra 35 amino acid residue at the carboxyl terminal of HCG β-subunit³. The availability of a radioimmunoassay using antibodies directed towards this sequence made possible the distinction of HCG′ from LH in extracts of urine from normal postmenopausal women and in pooled human pituitaries^4,5. Although the existence of HCG′ in either urinary extracts or pituitary fractions has been investigated extensively, on the basis of immunological, biological and physicochemical characteristics typical of HCG, the possibility that some proteases mimic the biological and immunological activities of HCG^6,7 must be carefully considered. We describe here a series of isolation procedures, including a specific immunoaffinity Chromatographie step and isoelectrofocusing, which not only separate HCG′ from LH but also eliminate the effect of protease contamination. Furthermore, analyses of our purest HCG′ have revealed that its isoelectric points, molecular size and immunological and biological activities are similar to those of HCG but distinct from those of LH.