Non-specific crossreacting antigen (NCA) does not contain methionine.

The presence of methionine in non-specific cross-reacting antigen (NCA) is one of the properties distinguishing it from carcinoembryonic antigen (CEA). In this paper we show that the appearance of methionine in NCA is due to regularly copurified materials, which were immunologically identified as alpha-1-antitrypsin and alpha-1-antichymotrypsin-like proteins. NCA is itself not cleavable by cyanogen bromide, which means that the molecule is devoid of any internal methionine. This proved to be true for biochemically-purified NCA as well as for NCA purified by immunoprecipitation with anti-serum followed by perchloric acid extraction.