The enterotoxin C molecule produced by strain 137 undergoes considerable unfolding upon treatment with 5 m guanidine hydrochloride as indicated by a large change in intrinsic viscosity from 3.4 ml/g for the native toxin in 0.05 m sodium phosphate buffer (pH 6.8) to a value of 22.1 ml/g for the expanded configuration. The effect of 8 m urea on the molecular domain is not as dramatic as that produced by 5 m guanidine hydrochloride. On removal of guanidine or urea from enterotoxin C solutions, the resulting viscosity data resemble those given by the untreated enterotoxin. Guanidine-treated and urea-treated enterotoxin C give identical or nearly identical precipitin reactions as that exhibited by the native toxin and also evoke emesis at concentrations identical with that of the untreated toxin. Acetylation of five tyrosyl groups of enterotoxin C does not affect the immunological and toxic properties, which suggests that the “free” tyrosines are not required for serological and emetic activities. Acetylation of all the 21 tyrosyl residues results in an almost total loss of precipitating capacity and ability to induce vomiting in monkeys. The effective average number of determinants per molecule of enterotoxin C which can bind antibody molecules simultaneously was found to be three. © 1969, American Chemical Society. All rights reserved.
