Rabbit intestinal and colonic mucins: isolation, partial characterization, and measurement of secretion using an enzyme-linked immunoassay.

Mucin was purified separately from the upper, mid, and distal small intestine and the proximal colon of the rabbit. The carbohydrate profiles of the three intestinal mucins were the same but differed from that of the colonic mucin, which contained less N-acetylgalactosamine but more sialic acid. All four mucins had similar polymeric structures composed of large, heterogeneous glycoprotein monomers and a smaller protein of Mr approximately 120,000, held together by disulphide bonds. The three intestinal mucins, however, were more resistant to dissociation by thiol reduction or degradation by proteolysis than colonic mucin. An antibody against a pool of the purified mucins was developed in guinea pigs and used to establish an enzyme-linked immunosorbent assay (ELISA). The antibody was highly specific for rabbit intestinal and colonic mucins, showing no cross-reactivity with nonmucin components of rabbit intestinal and colonic tissue, and very little or no reactivity with purified intestinal mucins from other species. The four purified rabbit mucins had the same affinity for the antibody and similar numbers of antigenic determinants. Antigenicity was apparently associated with the protein moeity of the mucins and was critically dependent on three-dimensional conformation, since proteolysis decreased the number of antigenic determinants while thiol reduction abolished antigen-antibody affinity. Using the ELISA, the tissue mucin content and the rate of mucin secretion were found to be significantly higher in the proximal colon that in the three regions of the intestine, which were the same.