Purification and Composition of a Novel Gastrointestinal Tumor-associated Glycoprotein Expressing Sialylated Lacto-N-fucopentaose II (CA 19-9)

Monoclonal antibody 1116NS 19-9 (Mab 19-9) exhibits selective reactivity with human gastrointestinal carcinomas and recognizes a carbohydrate determinant (CA 19-9) defined as sialylated lacto- N -fucopentaose II. A scheme was devised for the purification of a human gastrointestinal tumor-associated glycoprotein antigen expressing CA 19-9 from colorectal carcinoma cell line SW1116 culture media in high yield. The key steps in the purification were immunoaffinity column chromatography with Mab 19-9 followed by reduction and alkylation of the specifically bound proteins in the presence of 6 m guanidine hydrochloride and a second Mab 19-9 immunoaffinity fractionation. The purified CA 19-9 containing glycoprotein ran as a single band on sodium dodecyl sulfatepolyacrylamide gradient gels with an apparent molecular mass of 210 kilodaltons. In the absence of detergents, this purified glycoprotein apparently reassociated to form aggregates of 600–2000 kilodaltons molecular mass as determined by size-exclusion chromatography. Amino acid analysis of CA 19-9 containing glycoprotein revealed that serine, threonine, and proline together accounted for greater than 35% of the amino acid residues, consistent with a mucin-like structure for the protein. Carbohydrate compositional analysis, however, was in contrast to a typical mucin with a fucose:mannose:galactose: N -acetylgalactosamine: N -acetylglucosamine: N -acetylneuraminic acid molar ratio of 4:1:12:2.5:5:5. The presence of both N -acetylgalactosamine and mannose suggested that both O - and N -linked oligosaccharides may exist on CA 19-9 containing glycoprotein. Protein and carbohydrate analyses indicated that this novel tumor-associated glycoprotein was 85% carbohydrate by weight. This purification procedure may be applicable to the isolation of other epithelial tumor-associated antigens.