Immunochemical Studies of the Intramolecular Heterogeneity of the Carcinoembryonic Antigen (CEA) of the Human Digestive System

Previous studies have shown that in addition to the tumor-specific site on the carcinoembryonic antigen (CEA), this molecule also contains a blood group A-like grouping. To study the A-like site on the CEA molecule, we devised a radioimmunoassay in which anti-A antibodies were coupled to either Sepharose or Sephadex beads. The following observations were made. 1) The monosaccharide, N-acetyl-D-galactosamine was capable of inhibiting the interaction between 125I-CEA and a preparation of anti-A antibodies. 2) A glycopeptide (GP-1), containing the tumor-specific antigenic site of the CEA, which was obtained by the enzymatic degradation of the CEA molecule was capable of binding to anti-A antibodies. The ratio, by weight, of GP-1 to N-acetyl-D-galactosamine required to achieve equivalent binding was 10-3 to 10-4 to 1. GP-1 has a molecular weight of about 4000 daltons, and although it contains N-acetyl-D-glucosamine, D-mannose, D-galactose, and L-fucose, the glycopeptide is apparently devoid of N-acetyl-D-galactosamine.