Fluorescent analogs of cyclic ADP-ribose: synthesis, spectral characterization, and use

Cyclic ADP-ribose (cADPR) is a Ca(2+)-mobilizing cyclic nucleotide derived from NAD+. Accumulating evidence indicates that it is an endogenous modulator of the Ca(2+)-induced Ca2+ release mechanism in cells. In this study, we show that ADP-ribosyl cyclase catalyzes the cyclization of not only NAD+ but also several of its analogs with various purine bases (guanine, hypoxanthine, or xanthine) substituting for adenine. Unlike cADPR, the resulting cyclic products are fluorescent. Comparisons with various model compounds indicate that only 7-methyl substituted purine nucleosides and nucleotides are fluorescent, and the pH-dependence of their UV spectra is most similar to that of the fluorescent cADPR analogs, indicating that the site of cyclization of these analogs is at the N7-position of the purine ring. This finding is novel since the site of cyclization is at the N1-position for cADPR as determined by X-ray crystallography. That a single enzyme can cyclize a variety of substrates at two different sites has important implications mechanistically, and a model is proposed to account for these novel catalytic properties. Among the analogs synthesized, cyclic GDP-ribose is highly resistant to hydrolysis, while cyclic IDP-ribose can be readily hydrolyzed by CD38, a bifunctional enzyme involved in the metabolism of cADPR. These unique properties of the analogs can be used to develop fluorimetric assays for monitoring separately the cyclization and hydrolytic reactions catalyzed by the metabolic enzymes of cADPR. The convenience of the method in measuring kinetic parameters, pH-dependence, and modulator activity of the metabolic enzymes of cADPR is illustrated.