2′-phospho-cyclic ADP-ribose, a calcium-mobilizing agent derived from NADP

Cyclic adenosine diphosphoribose (cADPR), a metabolite of NAD, appears to modulate changes in intracellular free Ca²⁺ levels by activation of ryanodine-sensitive Ca²⁺ channels. We report here that an ADPR cyclase purified from Aplysia ealifornica readily catalyzes the conversion of NADP to 2′-phospho-cyclic adenosine diphosphoribose (2′-P-cADPR), cyclized at N-1 of the adenine moiety. An enzyme from canine spleen previously shown to contain NAD glycohydrolase, ADPR cyclase, and cADPR hydrolase activities also utilized NADP and 2′-P-cADPR as substrates. The apparent K_m value for NADP was 1.6 μM compared with 9.9 μM 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-ADPR 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 mechanisms as microsomes desensitized to IP₃ still released Ca²⁺ in response to 2′-P-cADPR and vice versa. Also, inhibition of IP₃-induced Ca²⁺ 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 similar kinetics of Ca²⁺ release, similar dose-response curves, cross-desensitization, 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.