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

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Abstract
The binding of iodinated cholera and E. 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) .times. 109 M-1], but there appeared to be twice as many receptor sites/cell for E. coli toxin (.apprx. 4 .times. 105). 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. 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 for the A subunit. Evidently, there are receptor loci on cells for the A subunit. Conformational differences exist between the 2 toxins that might explain the greater sensivity 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.

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