Abstract
NAD is a necessary cofactor for the activation of adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) by cholera toxin. Lysates of certain types of cell that hydrolyze their endogenous store of NAD after cell disruption respond poorly or not at all to cholera toxin. Lysates of pigeon erythrocytes, which lack enzymes that degrade NAD, provide a convenient and reproducible system for assaying the activity of cholera toxin in vitro and allow investigation of the mechanism of action of the toxin upon broken cells.