Isolation and characterization of the normal crossreacting antigen: homology of its NH2-terminal amino acid sequence with that of carcinoembryonic antigen.

The normal antigen, NCA, which crossreacts with the carcinoembryonic antigen, CEA, was purified from normal spleen tissue by an immunochemical purification method using insolubilized antibodies to either CEA or NCA. The highly purified NCA obtained was extensively characterized by immunological tests. The molecular weight of NCA determined by chromatography on Sephadex G-200 was approximately 100,000. The total amount of carbohydrate in NCA was 30%, compared to 60% in CEA. NCA and CEA also differed in sugar composition. The amino acid composition of NCA was nearly identical to that of CEA, except for the apparent presence of methionine in NCA but not in CEA. The sequence of the first 26 NH2-terminal amino acids in NCA was identical to that of CEA except at position 21, where alanine was found in NCA instead of valine in CEA.