Enterotoxin-binding glycoproteins in a proteose-peptone fraction of heated bovine milk.
The binding of Escherichia coli heat-labile enterotoxin to caseins, whey 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 of 15,500 and 20,000, respectively, were detected only in a proteose-peptone fraction. These glycoproteins were partially purified by ammonium sulfate precipitation and Toyopearl HW 55 gel filtration chromatography. The binding ability to the toxin was destroyed by periodate treatment or beta-galactosidase treatment, indicating that a carbohydrate moiety, particularly a terminal galactosyl residue, was essential for the binding of the toxin. In contrast, the binding ability was not changed by mild acid treatment, and these glycoproteins did not bind cholera toxin, which can bind to ganglioside GM1, suggesting that the carbohydrate structure of the glycoproteins is different from that of GM1. The N-terminal amino acid sequence and immunoblot analysis indicated that the protein moieties of pp16k and pp20k are identical to alpha-lactalbumin and beta-lactoglobulin, respectively. These toxin-binding glycoproteins were not detected in whey proteins isolated from unheated skim milk, suggesting that they are newly generated during heat treatment of skim milk before the preparation of a proteose-peptone fraction.