Abstract
1. Cholera toxin (1 microgram/ml) abolished net fluid absorption by everted sacs of rabbit ileum, leading to net fluid secretion. This action occurred via the toxin-catalysed ADP ribosylation of the stimulatory GTP-binding protein Gs which is linked to adenylate cyclase. Nicotinamide (10 mM), a reaction product of ADP ribosylation, reversed cholera toxin-induced secretion, restoring absorption. Lower concentrations of nicotinamide induced partial reversal. 2. Nicotinamide (1 mM) blocked the secretory action of cholera toxin applied to ileal sacs. This inhibitory action was more effective in the presence of methionine (1 mM). 3. Other inhibitors of ADP ribosylation, benzamide and adenine, blocked the secretory action of cholera toxin. Hypoxanthine, an analogue and metabolite of adenine, was similarly effective. 4. Nicotinamide was not, however, effective in blocking or reversing the secretory action of theophylline (10 mM) or of heat-stable E. coli enterotoxin STa. This indicates that nicotinamide has a highly specific action against ADP ribosylating toxins. 5. It is proposed that nicotinamide reverses the secretory action of cholera toxin by reversing ADP ribosylation, simply by the law of mass action. This counters the established idea that the effects of cholera and other ADP-ribosylating toxins are irreversible under physiological conditions.