Adenosine diphosphate-ribosylation of adenylate cyclase catalyzed by heat-labile enterotoxin of Escherichia coli: comparison with cholera toxin.

The heat-labile enterotoxin of Escherichia coli, like cholera toxin, activates adenylate cyclase by catalyzing the transfer of adenosine diphosphate-ribose from HAD+ (oxidized nicotinamide adenine dinucleotide) to the guanyl nucleotide-dependent regulatory component of the cyclase. A preparation of enterotoxin that had been released from E. coli following exposure to polymyxin B and then partially purified was found to contain two enzymatically active peptides, one of about 29,000 and the other of about 24,000 daltons, which correspond in molecular size to the enzymatically active subunit A and fragment A1 of cholera toxin, respectively. As with cholera toxin, the enzymatic activity of E. coli enterotoxin was elevated by incubation with sodium dodecyl sulfate to release active peptides. Treatment with dithiothreitol, however, had no effect. Dithiothreitol activates subunit A of cholera toxin by reducing an internal disulfide bond, but no corresponding bond appears to be present in the partially purified E. coli enterotoxin.