Escherichia coli heat-labile enterotoxin. Ganglioside specificity and ADP-ribosyltransferase activity.

Escherichia coli heat-labile enterotoxin (LT) bound to rat glioma C6 cells that had incorporated ganglioside GMl but not to cells that had taken up gangliosides GM2, GD~, or G D ~ ~ . The same specificity also was observed with choleragen, the enterotoxin of Vihrio cholem. The tryptophanyl fluorescence spectra of LT and its B component differed from those of choleragen and its B component, respectively. The oligosaccharide moiety of GMI “blue-shifted” the tryptophanyl fluorescence of LT and its B Component, whereas neuramin lactose and the oligosaccharide of GD~, did not. As reported previously (F’ishman, P. H., Moss, J., and Osborne, J. C., Jr. (1978) Biochemistry 17, 711-716), GMl-oligosaccharide also “blue-shifted” the fluorescence spectrum of choleragen at concentrations similar to those observed to be effective with LT. LT exhibited NAD glycohydrolase and ADP-ribosyltransferase activities, which were stimulated by dithiothreitol. Incubation of LT, but not choleragen, with trypsin increased the ADP-ribosyltransferase activity of LT over 200%. The specific enzymatic activity of trypsin-treated LT was similar to that of choleragen. The enzymatically active A component of LT is a single polypeptide chain, whereas the A component of choleragen consists of two polypeptide chains linked together by a disulfide bridge. It would appear that trypsin cleaves the A component of LT and generates a fragment with enhanced ADP-ribosyltransferase activity. Both LT and choleragen have the same specificity for G M ~ and its oligosaccharide and may share similar receptors. The two toxins, however, differ in that the catdytie activity of LT is not fully expressed without prior trypsinization. This may contribute to differences in the toxicity of LT and choleragen for different types of cells.