Two CEA and three NCA species, although distinguishable by monoclonal antibodies, have nearly identical peptide patterns.
In perchloric acid extracts of normal lung and colonic tumors, 3 NCA molecules were identified by monoclonal antibodies that cross-reacted with CEA, which itself gave 2 bands in SDS-PAGE. The proteins had molecular weights of 50, 75 and 97 kd, while the 2 CEA molecules banded at 180 and 160 kd in SDS-PAGE. No MAb recognized only one molecule, with the exception of MAb 3/13 which precipitated solely the upper CEA band. Analysis of the biochemical relationship of the cross-reactive antigens showed that none of them contained any internal methionine. Furthermore, after digestion by thermolysin, the peptide maps of the immunoprecipitated molecules showed very close similarities, if not identity. When the cross-reactive and the CEA were compared, the only differences found were in the upper CEA band, which apparently lacked one hydrophobic peptide, while the 97 kd cross-reacting protein showed one extra peptide. We conclude from our results that CEA and the cross-reacting molecules are composed of nearly identical, small (i.e. less than 50 kd) polypeptide chains.