CARBOHYDRATE STRUCTURES OF A NORMAL COUNTERPART OF THE CARCINOEMBRYONIC ANTIGEN PRODUCED BY COLON EPITHELIAL-CELLS OF NORMAL ADULTS

Normal faecal antigen-2 (NFA-2) and non-specific crossreacting antigen-2 (NCA-2), cross-reacting with anticarcinoembryonic antigen (CEA) antibodies, were found in normal human faeces and meconium, respectively. Because NFA-2, NCA-2 and CEA are considered as the same gene products, NFA-2 and NCA-2 should be normal counterparts of CEA produced by colon epithelial cells of normal adults and fetuses, respectively. Comparison of sugar chain structures of these three antigens is indispensable in order to unravel the structural alteration induced by malignant transformation and development of colon epithelial cells. The sugar chain structures of CEA (Yamashita,K. et al., Cancer Res., 47, 3451-3459, 1987) and NCA-2 (Yamashita,K. et al., J. Biol. Chem., 264, 17873-17881, 1989) were previously reported. In this paper, the structures of the oligosaccharides released from four NFA-2 samples by hydrazinolysis were studied by means of lectin-affinity column chromatography, endo- and exo-glycosidase digestion, methylation analysis, hydrazinolysis-nitrous acid deamination and electrospray ionization mass spectrometry. NFA-2 contains 24-27 mol of N-linked sugar chains/molecule, which is similar to NCA-2 (27 mol) and CEA (24-27 mol). In contrast to CEA, which contains similar to 8% high-mannose-type sugar chains, all sugar chains of NFA-2 are mono- to tetra-antennary complex-type chains having four types of tri-mannosyl cores, with or without bisecting N-acetylglucosamine and fucose residues. The structures of their outer chain moieties comprise Gal beta 1-->3(HSO3--->6)GlcNAc, Neu5Ac alpha 2-->3Gal beta 1-->3GlcNAc, Type 1, repeating chain, Type 2, Type 2H, Type 1H, Le(x), Le(a) and Le(b) antigenic determinants. Approximately 50% of the outer chain moieties of the oligosaccharides of NFA-2 contain Type 1 chain, in contrast to those of CEAs produced by the liver metastases of colon tumours in which only a trace amount of Type 1 chain was detected.