Dissociation and Recombination of the Subunits of the Cholera Enterotoxin (Choleragen)

Abstract

By gel filtration under dissociating conditions in acid urea, the cholera enterotoxin (choleragen) is separated into two fractions, A and B. Fraction A, which elutes first from columns of Bio-Gel P-60, is unique to the toxin and is not found in the spontaneously formed enterotoxoid (choleragenoid) after similar treatment. Neither A nor B alone has significant toxicity in either the infant rabbit model or adult rabbit skin tests. However, after neutralization and dialysis of mixtures of A and B (regardless of whether the latter is derived from choleragen or choleragenoid) the toxin activity in both systems is restored as is the characteristic electrophoretic mobility of the original toxin. Antiserum against the holotoxin precipitates both fragments but A and B give reactions of non-identity. Antiserum against choleragenoid does not precipitate A. Peptide mapping indicates that choleragen contains a number of peptides not present in choleragenoid. The A fragment can further be dissociated into two parts by treatment with thiol reagents. It now seems evident that the cholera enterotoxin consists of two major kinds of non-covalently associated subunits. Of these, the B type, comprising choleragenoid, is likely to be responsible for interaction with G_M1 ganglioside-like host cell receptors and the A fragment for biologic activity, i.e., the activation of adenyl cyclase.