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Summary: The Carcinoembryonic antigen (CEA) is a glycoprotein which is defined mainly by its immunochemical properties. Essential biochemical properties of CEA are: its molecular weight of 180000—2, the alkali stable bond of the oligosaccharide unit to the peptide (GlcNAc-Asn), and the solubility in 0.6—1.0 mol/1 perchloric acid. Most of the immunochemical and biochemical properties of CEA relate to preparations from liver metastases of colonic carcinomas. Biochemical heterogeneity of CEA preparations can be shown by several biochemical procedures e.g. isoelectric focussing, ion exchange chromatography and lectin affinity chromatography. CEA preparations that show differences with respect to biochemical parameters, but behave idenfically when immunochemical methods are applied, are called 'isomeric species' or 'isoantigens'. Substaiices which react with anti-CEA sera but do not fulfill the criteria given for CEA are called 'CEA-related antigens'. The CEA-related antigens can be further subclassified into the cross reacting antigens, the membrane variant of CEA (CEA-M), and more or less well defined CEA related antigens which were isolated from sources different from colonic carcinoma or its liver metastases. It is still cqntroversial whether CEA is mainly a membrane glycoprotein or a secretory glycoprotein. The finding that additional antigenic determinants are present on a membrane variant of CEA suggests that a CEA precursor or a distinct CEA membrane variant may exist. CEA^M is only partially soluble in 0.6—0.9 mol/1 perchloric acid. Many findings indicate that membrane glycoproteins from normal and tumour cells are glycosylated differently. It has been suggested that glycosylation of CEA and CEA related antigens may be organ or phase specific. As perchloric acid extractipn is a generally used step in the Isolation procedure of CEA, variants of CEA with a low perchloric acid solubility due to incomplete glycosylation may escape the Isolation (e.g. CEA-M). Recent results on the Isolation of CEA variants from sources other than liver metastases of colonic carcinomas are contradictory. CEA-related antigens shpwing a close similarity to CEA, as well as CEA variants exhibiting considerable differences with respect to moleeular weight and perchlorie acid solubility have been described.

Biochemical and Immunochemical Aspects of Carcinoembryonic Antigen (CEA) and CEA Related Antigens: Current Status and Future Perspectives