Human Chorionic Gonadotropin and its Subunits as Tumor Markers

Abstract

Human chorionic gonadotropin (hCG) shares a common quaternary structure of two different subunits with the three pituitary glycoprotein hormones — thyroid stimulating hormone, luteinizing hormone (LH), and follicle stimulating hormone. Those hormones possess a common alpha subunit but their beta subunits differ, conferring immunologic and biologic specificities (1). The alpha subunit of hCG is comprised of 91 amino acids and two branched oligosaccharide side chains, usually terminating in sialic acid (2). The beta sub-unit of hCG contains 145 amino acids and six extensively branched oligosaccharide side chains, most terminating with sialic acid (2,3). Human CG is secreted in relatively large quantities by the syncytiotrophoblastic cells of the placenta but may be synthesized at low but significant levels by a wide array of normal tissues (4–6). Human luteinizing hormone (hLH) and hCG share indistinguishable biologic activities. That biologic similarity undoubtedly reflects the presence of a common alpha subunit on those two glycoprotein hormones as well as the extensive structural homology within the beta subunits of those two hormones. The beta subunit of LH contains 115 amino acids and shares at least 80% structural homology with the beta subunit of hCG (7).