Requirement for guanosine triphosphate for cholera-toxin-catalysed incorporation of adenosine diphosphate ribose into rat liver plasma membranes and for activation of adenylate cyclase.

1. Cholera toxin was shown to require the presence of GTP to activate rat liver plasma-membrane adenylate cyclase. ATP did not affect the activation process. 2. Cholera toxin catalysed the incorporation of 32P from NAD labelled in the alpha-phosphate group of the ADP moiety into a rat liver plasma-membrane protein with a subunit mol.wt. of 42 500. This is taken to demonstrate ADP-ribosylation. The ADP-ribosylation of this protein also required GTP and was unaffected by ATP. 3. Nicotinamide inhibited both the activation of adenylate cyclase by cholera toxin and the ADP-ribosylation of the protein of 42 500 subunit mol wt. Neither the activation nor the ADP-ribosylation could be reversed by treatment with nicotinamide in the presence of cholera toxin.