CHARACTERIZATION OF THE BINDING-SITES FOR ESCHERICHIA-COLI HEAT-LABILE TOXIN TYPE-I IN INTESTINAL BRUSH-BORDERS

Intestinal brush borders from Wistar rats contained a total of 20-30-times more binding sites for Escherichia coli heat-labile enterotoxin (LT-1) than for cholera toxin (CT). The results suggest that LT-1 binds to sites in addition to ganglioside GM1, the binding site for CT. Brush border proteins were separated by SDS-PAGE, blotted to nitrocellulose and the filters incubated with I-125-labeled toxins. [I-125]LT-1 was shown to bind to a series of brush border galactoproteins ranging in size from 130-140 kDa. Binding was inhibited by unlabeled LT-1 (but not CT), and by ricin and free galactose. A number of brush border enzymes are large glycoproteins which can be solubilised by papain. The papain-solubilised sucrase-isomaltase complex was purified by affinity chromotography and shown to bind LT-1, as did the proteins in fractions enriched in maltase activity. However, such brush border galactoproteins do not account for all of the additional LT-1 binding sites. Thus, brush borders prepared from 1-15-day-old rabbits contained many more binding sites for LT-1 than CT despite the absence of any sucrase-isomaltase activity, and no [I-125]LT-1 binding proteins could be detected by blotting. There was a marked variation in the number of LT-1 binding sites in different strains of rat, and between different species.