Molecular heterogeneity of the beta-core fragment of human chorionic gonadotrophin

We have analysed the structure and composition of the beta-core fragment of human chorionic gonadotrophin (beta C-hCG) from fresh urine specimens obtained from pregnant women and compared our findings with those previously proposed by other groups using different protocols. SDS-PAGE 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-carboxymethylated peptides resulted in three peaks, but only two of them could 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 subunit. The results showed that 56-78% of beta C-hCG molecules of molecular weight M(r) 12,800 were able to bind Concanavalin A (Con A). While most were lacking all the peripheral monosaccharides and terminated in mannose, some retained other sugar residues on their antennae. Direct carbohydrate analysis showed the following molar content normalized to six mannose molecules: galactose 2.8, glucosamine 5.3, galactosamine 0.3, fucose 1.7 and sialic acid 3.0. Approximately 22-44% of the beta 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 molecular weight of approximately M(r) 10,000, and 12% were weakly reactive to Con A and reactive to anion exchange (negatively charged forms), being incompletely trimmed of their oligosaccharide chains. Comparison of our results with those of two other groups have indicated that the differences noted among preparations are due to either the source or the methods used to purify and characterize this fragment. In addition, our results showed significant microheterogeneity on the N-linked oligosaccharide 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 in other cases possible metabolism of hCG in the peripheral tissues.