ESCHERICHIA-COLI HEAT-LABILE ENTEROTOXIN BINDS TO GLYCOSYLATED PROTEINS WITH LACTOSE BY AMINO CARBONYL REACTION

The binding of Escherichia coli heat-labile enterotoxin (LT) type I to glycosylated proteins with lactose (Gal beta 1-4Glc) by amino carbony l reaction was studied by the Western blot assay and by the microtiter well binding assay. LT bound to a lactose-alpha-lactalbumin amino car bonyl product (Lac-LA), whereas cholera toxin did not. The binding abi lity of Lac-LA was abolished by beta-galactosidase treatment, indicati ng that the terminal galactose is essential for the binding of LT. The binding of LT to Lac-LA was inhibited by galactose and lactose, and m ost effectively inhibited by lactulose (Gal beta 1-4Fru), which is a s tructural analog of the Amadori rearrangement product of the amino car bonyl reaction between lactose and an epsilon-amino group of a lysine residue (lactuloselysine). The results suggest that LT recognizes the portion of lactuloselysine in Lac-LA. LT also bound to a melibiose (Ga l alpha 1-6Glc)-alpha-lactalbumin amino carbonyl product (Mel-LA), but the binding ability of Mel-LA was weaker than that of Lac-LA, suggest ing that the beta 1-4 linked terminal galactose is dispensable but pre ferable for the binding. Furthermore, LT bound to the amino carbonyl p roducts of lactose with beta-lactoglobulin, caseins, bovine serum albu min, and ovalbumin. These results indicate that LT binds to the amino carbonyl products between proteins and sugars containing the terminal galactose, such as lactose.