Binding of cholera toxin to mucins and inhibition by gastric mucin

The effects of mucin on cholera toxin induced intestinal secretion was studied in the rat small intestine. Gastric mucin inhibited secretion when premixed with cholera toxin on an equal dry-weight basis. Neither salivary nor intestinal mucin inhibited the toxin's action in the intestine. Inhibition by gastric mucin was not reversed by a mucolytic agent or by hexoses or agents that bind hexoses. Plasma and serum did not inhibit the effects of cholera toxin in the intestine. Choleragen was labeled with (14)C and its binding to mucins and ganglioside was studied. Gastric mucin was shown to bind to choleragen, but less binding was observed when choleragen was dissociated into subunits by acid pH. Salivary and intestinal mucins bound intact choleragen but not the subunits at acid pH. Salivary mucin binding was reversed by N-acetyl neuraminic acid and neuraminidase. Ganglioside bound choleragen in both the intact and dissociated forms. Binding to the dissociated form was reversed by wheat germ agglutinin. Gastric mucin and ganglioside competed for binding to choleragen with the binding of intact choleragen greater for ganglioside and the affinity of the subunits greater for gastric mucin. Electrophoresis of labeled choleragen showed uniform labeling of the subunits dissociated by acid pH, but a major part of cholera toxin was found not to be labeled when fractionated with sodium dodecyl sulfate and 2-mercaptoethanol.