Mechanism of action of cholera toxin: September 2007, 2009

Ans. A: Increase in c-AMP The debilitating loss of intestinal fluid is caused primarily by the release of cholera toxin, the main virulence factor of the pathogen Vibrio cholerae. Cholera toxin consists of an A subunit coupled to a B subunit: the A subunit consists of an Al domain containing the enzymatic active site, and an A2 domain while the B subunit contains five identical peptides that assemble into a pentameric ring surrounding a central pore. This AB; structure is closely related to the heat-labile enterotoxin (LT) from enterotoxigenic Escherichia coli, which causes diarrhoea, often in infants. Symptoms begin when the massive secretion of water and sodium in the small intestine, a consequence of the activation of cyclic AMP, exceeds the resorptive capacity of the colon.