COMPARISON OF THE GLYCOLIPID-BINDING SPECIFICITIES OF CHOLERA-TOXIN AND PORCINE ESCHERICHIA-COLI HEAT-LABILE ENTEROTOXIN - IDENTIFICATION OF A RECEPTOR-ACTIVE NON-GANGLIOSIDE GLYCOLIPID FOR THE HEAT-LABILE TOXIN IN INFANT RABBIT SMALL-INTESTINE

The binding specificities of cholera toxin and Escherichia coli heat-l abile enterotoxin were investigated by binding of I-125-labelled toxin s to reference glycosphingolipids separated on thin-layer chromatogram s and coated in microtitre wells. The binding of cholera toxin was res tricted to the GM1 ganglioside. The heat-labile toxin showed the highe st affinity for GM1 but also bound, though less strongly, to the GM2, GD2 and GD1b gangliosides and to the non-acid glycosphingolipids gangl iotetraosylceramide and lactoneotetraosylceramide. The infant rabbit s mall intestine, a model system for diarrhoea induced by the toxins, wa s shown to contain two receptor-active glycosphingolipids for the heat -labile toxin, GM1 ganglioside and lactoneotetraosylceramide, whereas only the GM1 ganglioside was receptor-active for cholera toxin. Prelim inary evidence was obtained, indicating that epithelial cells of human small intestine also contain lactoneotetraosylceramide and similar se quences. By computer-based molecular modelling, lactoneotetraosylceram ide was docked into the active site of the heat-labile toxin, using th e known crystal structure of the toxin in complex with lactose. Intera ctions which may explain the relatively high toxin affinity for this r eceptor were found.