2'-PHOSPHO-CYCLIC ADP-RIBOSE, A CALCIUM-MOBILIZING AGENT DERIVED FROMNADP

Cyclic adenosine diphosphoribose (cADPR), a metabolite of NAD, appears to modulate changes in intracellular free Ca2+ levels by activation o f ryanodine-sensitive Ca2+ channels. We report here that an ADPR cycla se purified from Aplysia californica readily catalyzes the conversion of NADP to 2'-phospho-cyclic adenosine diphosphoribose (2'-P-cADPR), c yclized at N-1 of the adenine moiety. An enzyme from canine spleen pre viously shown to contain NAD glycohydrolase, ADPR cyclase, and cADPR h ydrolase activities also utilized NADP and 2'-P-cADPR as substrates. T he apparent K-m value for NADP was 1.6 mu M compared with 9.9 mu M for NAD, and the V-max with NADP was twice that with NAD, indicating that 2'-P-cADPR is a likely metabolite in mammalian cells. 2'-P-cADPR was as active as cADPR in eliciting Ca2+ release from rat brain microsomes , but was unable to elicit Ca2+ release following conversion to 2'-P-A DPR by the action of canine spleen NAD glycohydrolase, 2'-P-cADPR and 1-D-myo-inositol 1,4,5-trisphosphate (IP3) appear to act by distinct m echanisms as microsomes desensitized to IP3 still released Ca2+ in res ponse to 2'-P-cADPR and vice versa. Also, inhibition of IP3-induced Ca 2+ release by heparin had no effect on release by 2'-P-cADPR. Both 2'- P-cADPR and cADPR appear to act by a similar mechanism based on simila r kinetics of Ca2+ release, similar dose-response curves, cross-desens itization, and partial inhibition of release by procaine. The results of this study suggest that 2'-P-cADPR may function as a new component of Ca2+ signaling and a possible link between NADP metabolism and Ca2 homeostasis.