NADP improves the efficiency of cholera toxin catalyzed ADP-ribosylation in liver and heart membranes.

Guanine nucleotide binding proteins (G-proteins) can be identified by their ability to be ADP-ribosylated using [32P]NAD as the substrate and bacterial toxins as catalysts. This labelling, when performed in liver and sarcolemma membrane preparations, can be complicated by competing enzymes which degrade NAD, making it unavailable to participate in the desired reaction. The addition of NADP in reaction mixtures markedly slows the degradation of NAD, but does not compete with NAD in cholera toxin labelling of stimulatory G-protein. The efficiency of cholera toxin labelling is improved to the extent that saturation curves may be constructed, allowing the quantitation of ADP-ribosylation sites in membranes.