Purification and characterization of heat-stable enterotoxin produced by a strain of E. coli pathogenic for man

Escherichia coli heat-stable enterotoxin derived from a strain pathogenic for man has been purified 13,000-fold to apparent homogeneity. The purification scheme involved growth in a minimal medium. Amberlite XAD-2 chromatography, acetone fractionation, Sephadex G-25 filtration, DEAE-Sephacel ion exchange chromatography, and Sephadex G-25 gel filtration. This scheme resulted in a white flocculent material which was biologically active in 2.7-ng quantities. Heat-stable enterotoxin was homogeneous as determined by the following: (a) a single peak on gel filtration; (b) a single band on thin layer chromatography; (c) a single band on thin layer electrophoresis; (d) a single NH2-terminal amino acid residue, asparagine; and (e) an amino acid composition demonstrating a stoichiometric relationship among the amino acids. The molecule is composed of 10 different amino acids, a total of 18 amino acid residues, one-third of which are half-cystine. The molecule contains no detectable carbohydrate. Biological activity is promptly lost on treatment with the reducing reagents, 2-mercaptoethanol or dithiothreitol, or after performic acid oxidation, suggesting the presence of disulfide bridges which are required for biological activity.