Comparison of the binding of cholera and Escherichia coli enterotoxins to Y1 adrenal cells.

The binding of iodinated cholera and Escherichia coli (LT) enterotoxins to Y1 mouse adrenal cells was studied by using saturation analysis (Scatchard). Each toxin bound to Y1 cells with similar affinity [KA = (1.5--2.0) x 10(9)M-1], but there appeared to be twice as many receptor sites per cell for E. coli toxin (approximately 4 x 10(5). Despite the increased binding of E. coli toxin, Y1 cells respond sooner to, and to smaller concentrations of, cholera toxin. The binding of each toxin was inhibited competitively by both toxins, although twice as much E. coli toxin was required to inhibit 50% of the binding of cholera toxin as was needed for either homologous inhibition or the inhibition of E. coli toxin binding by cholera toxin. The B subunits of both toxins were equally effective in competing for the binding of both iodinated toxins. Whereas the A subunits of both toxins had little or no effect on the binding of E. coli toxin, they consistently inhibited 20--40% of the binding of cholera toxin to cells. These results suggest that there are receptor loci on cells for the A subunit and that conformational differences exist between the two toxins that might explain the greater sensitivity of Y1 cells to cholera toxin. A model is suggested in which cholera toxin exhibits a greater degree of multivalent ligand binding than does the E coli toxin, resulting in a more favorable situation for apposition of the A subunit to its receptor or for its insertion into the membrane.