MOLECULAR HETEROGENEITY OF THE BETA-CORE FRAGMENT OF HUMAN CHORIONIC-GONADOTROPIN

We have analysed the structure and composition of the beta-core fragme nt of human chorionic gonadotrophin (beta C-hCG) from fresh urine spec imens obtained from pregnant women and compared our findings with thos e previously proposed by other groups using different protocols. SDS-P AGE separation of reduced beta C-hCG demonstrated two major bands with apparent molecular weights of M(r) 8900 and M(r) 7500. The molecular weight of the agalacto beta C-hCG was estimated to be M(r) 10 218 from the amino acid analysis after high-performance liquid chromatography (HPLC) separation. Moreover, HPLC separation of its reduced and S-carb oxymethylated peptides resulted in three peaks, but only two of them c ould be sequenced and demonstrated to be the previously reported beta 6-40 (M(r) 5000) and beta 55-92 (M(r) 5300) peptides of the beta hCG s ubunit. The results showed that 56-78% of beta C-hCG molecules of mole cular weight M(r) 12 800 were able to bind Concanavalin A (Con A). Whi le most were lacking all the peripheral monosaccharides and terminated in mannose, some retained other sugar residues on their antennae. Dir ect carbohydrate analysis showed the following molar content normalize d to six mannose molecules: galactose 2.8, glucosamine 5.3, galactosam ine 0.3, fucose 1.7 and sialic acid 3.0. Approximately 22-44% of the b eta C-hCG molecules did not bind Con A (Con A non-reactive forms), of which 88% were totally deprived of sugar units and had an apparent mol ecular weight of approximately M(r) 10 000, and 12% were weakly reacti ve to Con A and reactive to anion exchange (negatively charged forms), being incompletely trimmed of their oligosaccharide chains. Compariso n of our results with those of two other groups have indicated that th e differences noted among preparations are due to either the source or the methods used to purify and characterize this fragment. In additio n, our results showed significant microheterogeneity on the N-linked o ligosaccharide moieties with some molecules apparently having no sugar molecules. These results have implications for the origins of beta C- hCG, suggesting secretion of some molecules without sugar chains and i n other cases possible metabolism of hCG in the peripheral tissues.