TOXIN-A AND TOXIN-B FROM CLOSTRIDIUM-DIFFICILE DIFFER WITH RESPECT TOENZYMATIC POTENCIES, CELLULAR SUBSTRATE SPECIFICITIES, AND SURFACE BINDING TO CULTURED-CELLS

Clostridium difficile toxins A and B together are responsible for the symptoms of pseudomembranous colitis. Both toxins intoxicate cultured cells by the same mechanism but they differ in cytotoxic potency, toxi n B being generally 1,000 times more potent than toxin A. Don and T84 cells were used to determine differences in the intoxication process e xerted by both toxins. Three main differences were identified: (a) the specific binding of radiolabeled toxins to the cell surfaces correlat ed with the cytotoxic potency, (b) toxin B was found to have a 100-fol d higher enzymatic activity than toxin A, and (c) toxin A was found to modify an additional substrate, Rap. The relative contribution of (a) and (b) to the difference in cytotoxic potency was determined by micr oinjection of the toxins. The differing enzymatic activities turned ou t to be the main determinant of the difference in cytotoxic potency, w hereas the difference in binding contributes to a lesser degree. These findings are discussed in the context of the pathophysiological role of the toxins.