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, a radioimmunoassay was devised in which anti A bodies were coupled to either Sepharose or Sephadex beads. The following observations were made. The monosaccharide, N acetyl D galactosamine was capable of inhibiting the interaction between ¹²⁵I CEA and a preparation of anti A antibodies. 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.