Cycling assay for determining intracellular cyclic adp-ribose levels

Cyclic ADP-ribose (cADPR) is a Ca(2+)-mobilizing second messenger involved in the regulation of various physiological processes. The ability to detect changes in endogenous cADPR is a fundamental step in the identification of its role in signal transduction triggered by hormones and other stimuli. Because the intracellular concentration of cADPR can be very low, depending on the expression level of the ADP-ribosyl cyclase activity (forming cADPR and nicotinamide from NAD) in the cell type of interest, very sensitive and selective methods are required. The method presented here exploits the ability of the ADP-ribosyl cyclase to catalyze the reverse reaction (i.e., to synthesize NAD stoichiometrically starting from cADPR) in the presence of an excess of nicotinamide. The generation of NAD can be coupled to a cycling assay using the enzymes alcohol dehydrogenase and diaphorase. The former reduces NAD to NADH in the presence of ethanol and the latter oxidizes NADH to NAD in the presence of resazurin and flavin mononucleotide. The formation of the fluorescent reduced resazurin (resofurin) can be detected with a plate reader. Thus, this cycling assay for cADPR determination can be considered a high-throughput method, potentially screening cADPR concentration simultaneously in many samples.