ABSTRACT
In 1927 Aschheim and Zondek observed that the urine of pregnant women contained abundant quantities of a gonadotropin with luteinizing activity (reviewed by Li; I Bagshawe et al. 2). Although Aschheim and Zondek thought the pituitary might be the site of production, it was soon recognized that this hormone, human chorionic gonadotropin (hCG), is produced by the placenta and that it functions to maintain the corpus luteum after implantation has occurred. 2.3 Its concentrations rise during the first trimester of pregnancy, peak near the end of the first trimester and decline thereafter. 2-5
Like human luteinizing (hLH) , follicle-stimulating (hFSH), and thyroid-stimulating (hTSH) hormones, hCG is a heterodimeric glycoprotein. The amino acid sequences of its subunits (hCGa and hCG[3) are established.6-9 The common alpha subunit is thought to have an identical peptide sequence among these four hormones (Chapter 1) and to arise from a single gene. IO-12 The hLH[3/hCG[3 family is composed of at least eight genes. 13-16 The current thinking is that the family of seven or more hCG[3 genes (or pseudogenes) arose from the hLH[3 gene, which is present as a single copy in the genome, by a series of selected replacement changes with very little neutral drift. 9. 17 Hence, hCG[3 and hLH[3 exhibit a high degree of peptide sequence homology. However, hCG[3 possesses a carboxy terminal region which is approximately 30 amino acids longer than hLH[3.6.7.9 This extension is thought to have resulted from a single base deletion which places the termination codon found in hLH(3 out of frame and allows translation of the extra amino acids. 17.18
hCG possesses four asparagine-linked oligosaccharides, two on each subunit, and four serine-linked sugar chains which reside on the carboxy terminal region of hCG[3. Although the detailed structures of its sugar chains have been reviewed in Chapter 2, information relevant to particular topics will, in part, be reiterated below.
