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Type II heat-labile enterotoxin (LT-II) from Escherichia coli causes characteristic morphological changes and accumulation of cyclic AMP in Y-1 adrenal cells, but it is not neutralized by antisera against choleragen (CT) or the classical type I heat-labile enterotoxin (LT-1) from E. coli. The action of purified LT-II on CT- and LT-I-responsive human fibroblasts was investigated and compared with that of CT. Fibroblasts incubated with LT-II or CT had an increased cyclic AMP content as well as a fourfold elevation of membrane adenylate cyclase activity. In membranes, activation of cyclase by toxin was enhanced by NAD, GTP, and dithiothreitol. The effect of LT-II on intact fibroblasts or membranes was increased by trypsin treatment of toxin. Since activation of adenylate cyclase by LT-II was stimulated by NAD, the ability of LT-II to catalyze the [32P]ADP-ribosylation of membrane proteins in the presence of [32P]NAD from control and LT-II- and CT-treated fibroblasts was investigated. Similar proteins were [32P]ADP-ribosylated in membranes exposed to LT-II or CT; LT-II- and CT-specific labeling was significantly decreased in membranes prepared from cells preincubated with either LT-II or CT. These studies are consistent with the hypothesis that LT-II, similar to CT and LT-I, increases cyclic AMP by activating adenylate cyclase through the GTP-dependent ADP-ribosylation of specific membrane proteins.

Type II heat-labile enterotoxin of Escherichia coli activates adenylate cyclase in human fibroblasts by ADP ribosylation