Radiolabeled Toxin for Studying Binding of Cholera Toxin and Toxoids to Intestinal Mucosal Receptor Sites

Recent evidence suggests that the mode of action of the cholera toxin involves at least two steps, which include binding to a membrane receptor and stimulation of the membrane-bound enzyme adenyl cyclase. A sensitive in vitro technique for studying toxin binding to tissue receptor sites is described and appears to have considerable potential usefulness in the study of toxin interaction with mucosal tissue. Highly purified ¹²⁵I-labeled cholera toxin was found to bind specifically to insoluble mucosal homogenates from the small intestines of normal adult guinea pigs. Binding of labeled toxin could be inhibited by nanogram levels of both cholera toxin and choleragenoid (a spontaneously formed toxoid), but not by heterologous rabbit serum proteins. Artificial cholera toxoids prepared by treatment of purified toxin with formaldehyde or glutaraldehyde failed to inhibit radiolabeled toxin binding to mucosal homogenates. The data indicate that cholera toxin binding is highly specific for the toxin and is not inhibited by heterologous proteins. In contrast, artificial toxoids, currently under investigation as potential human vaccines, appear to have lost their affinity for the tissue receptor.