ENTEROTOXIN-BINDING GLYCOPROTEINS IN A PROTEOSE-PEPTONE FRACTION OF HEATED BOVINE-MILK

The binding of Escherichia coli heat-labile enterotoxin to caseins, wh ey proteins, milk fat globule membrane, and proteose-peptone fraction from bovine milk was studied by using the Western blot technique. Two toxin-binding glycoproteins, pp16k and pp20k, with molecular weights o f 15,500 and 20,000, respectively, were detected only in a proteose-pe ptone fraction. These glycoproteins were partially purified by ammoniu m sulfate precipitation and Toyopearl HW 55 gel filtration chromatogra phy. The binding ability to the toxin was destroyed by periodate treat ment or beta-galactosidase treatment, indicating that a carbohydrate m oiety, particularly a terminal galactosyl residue, was essential for t he binding of the toxin. In contrast, the binding ability was not chan ged by mild acid treatment, and these glycoproteins did not bind chole ra toxin, which can bind to ganglioside GM1, suggesting that the carbo hydrate structure of the glycoproteins is different from that of GM1. The N-terminal amino acid sequence and immunoblot analysis indicated t hat the protein moieties of pp16k and pp20k are identical to alpha-lac talbumin and beta-lactoglobulin, respectively. These toxin-binding gly coproteins were not detected in whey proteins isolated from unheated s kim milk, suggesting that they are newly generated during heat treatme nt of skim milk before the preparation of a proteose-peptone fraction.