Activation of adenylate cyclase by heat-labile Escherichia coli enterotoxin. Evidence for ADP-ribosyltransferase activity similar to that of choleragen

low molecular weight Escherichia coli heat-labile enterotoxin (LT) catalyzed the hydrolysis of NAD to ADP-ribose and nicotinamide. This NAD glycohydro-lase activity was stimulated by dithiothreitol and was independent of cellular components. Nicotinamide formation was enhanced by arginine methyl ester> D-arginine-L-arginine-guanidine. A 20-fold in-crease in activity was noted with arginine methyl ester, and maximal activity again required dithio-threitol. When the reaction was initiated with toxin, a delay was observed before a constant rate was estab-lished. The reaction products found after incubation of [adenine-U-14C]NAD and L-[3H]arginine or unla-beled arginine methyl ester with the enterotoxin had mobilities on thin-layer chromatograms similar to the reaction products obtained after incubation of cholera-gen with these substrates and are consistent with the formation of ADP-ribose-L-arginine and ADP-ribose-L-arginine methyl ester, respectively. Both toxins, which catalyze the NAD-dependent activation of adenylate cyclase, thus appear to possess NAD glyco-hydrolase and ADP-ribosyltransferase activities. Al-though the activities of both toxins are dependent on dithiothreitol, Escherichia coli enterotoxin exhibited optimal activity in Tris(Cl-) (pH 7.5) and was inhi-bited by high concentrations of potassium phosphate (pH 7.0) or low pH (sodium acetate, pH 6.2). It appears Portions of this work were presented at the meetings o