Natural cholera toxoid appears to act as a competitive inhibitor of cholera enterotoxin and is thus a useful tool for studying the interaction of cholera enterotoxin with cell membranes. Cholera enterotoxin binds to gut mucosa more rapidly than does its natural toxoid. Once binding occurs, however, it appears to be prolonged for both materials. Formalinized cholera toxoid has no inhibitory effect upon cholera enterotoxin. Enterotoxic activity, ability to bind to gut mucosa, and antitoxigenicity appear to be independent properties of cholera enterotoxin.
Natural cholera toxoid does not inhibit Escherichia coli enterotoxin, indicating that although the two enterotoxins activate the same mucosal secretory mechanism they occupy different binding sites in the mucosa. Ganglioside, which may be the mucosal receptor of cholera enterotoxin, is highly efficient in deactivating cholera enterotoxin. By contrast, ganglioside is relatively inefficient in deactivating heat-labile E. coli enterotoxin and is without effect upon the heat-stable component of E. coli enterotoxin. These findings suggest that ganglioside is not likely to be the mucosal receptor for E. coli enterotoxin. Differences in cellular binding of E. coli and cholera enterotoxins may explain, at least in part, the marked differences in the time of onset and duration of their effects upon gut secretion.