Isolation and characterization of two proteins copurifying with carcinoembryonic antigen

Using common purification procedures CEA was eluted as a symmetrical peak after gel-chromatography with a molecular weight (mw) of 180,000. The purity was assessed by the Ouchterlony test and immunoelectrophoresis, and by SDS-PAGE, where only one precipitation line and one band were obtained. However, two weak bands with a mw of 45,000 and 58,000 appeared, when iodinated CEA preparations were analyzed on SDS-PAGE. It was impossible to separate these proteins from CEA by a great variety of purification procedures. Out of several different immune sera, only two, anti-alpha-antitrypsin and anti-alpha-1-antichymotrypsin, reacted with the proteins. Furthermore, antisera against these protease inhibitors also immunoprecipitated the typical 180,000 mw band of CEA. This was also true for CEA prepared by other laboratories. We have purified and partially characterized both proteins. Although reacting with antisera against alpha-1-antitrypsin and alpha-1-antichymotrypsin they were not identical to the protease inhibitors, because they possess a different N-terminal amino acid sequence than published for them. However, the comparison of their sequences to 1900 total protein sequences made by computer search revealed a strong homology of the N-terminal sequence of the 45,000 mw protein with an internal sequence of alpha-1-antitrypsin. For the 58,000 mw protein no significant homology was found.