Pathogenesis of shigella diarrhea: Rabbit intestinal cell microvillus membrane binding site for shigella toxin

This study examined the binding of purified ¹²⁵I-labeled shigella toxin to rabbit jejunal microvillus membranes (MVMs). Toxin binding was concentration dependent, saturable, reversible, and specifically inhibited by unlabeled toxin. The calculated number of toxin moleculars bound at 4°C was 7.9 x 10¹⁰ (3 x 10¹⁰ to 2 x 10¹¹)/μg of MVM protein or 1.2 x 10⁶ per enterocyte. Scatchard analysis showed the binding site to be of a single class with an equilibrium association constant, K, of 4.7 x 10⁹ M^-1 at 4°C. Binding was inversely related to the temperature of incubation. A total of 80% of the labeled toxin binding at 4°C dissociated from MVM when the temperature was raised to 37°C, but reassociated when the temperature was again brought to 4°C. There was no structural or functional change of MVM due to toxin as monitored by electron microscopy or assay of MVM sucrase activity. These studies demonstrate a specific binding site for shigella toxin on rabbit MVMs. The physiological relevance of this receptor remains to be determined.