Transfer of ADP-ribose from NAD to choleragen: a subunit acts as catalyst and acceptor protein.

Choleragen selectively incorporates 3H from [3H]NAD labeled on the adenosine moiety and not 14C from [14C]NAD labeled on the nicotinamide moiety. This reaction does not require protein in addition to choleragen. Incorporation of isotope does not proceed at 4 degrees, requires dithiothreitol, is stable after extensive washing with cold trichloroacetic acid, and is decreased 80% by boiling in trichloroacetic acid. Studies with the A and B subunits of choleragen show that the A subunit catalyzes ADP-ribosylation and serves as an acceptor protein. The B subunit does not show catalytic or acceptor activity. We conclude that choleragen and its A subunit catalyze the hydrolysis of NAD and the enzymatic transfer of ADP-ribose to the A subunit.