FIGURE 1.

Crystal structures of cADPR and NAADP. The structure of cADPR was obtained from crystals of its free acid, and that of NAADP was from the crystals of the complex of NAADP and CD38. The cyclization site in cADPR is indicated by an ellipse. The C8 of cADPR is indicated by a white asterisk. Attachment of a bromo (8-bromo-cADPR) or an amino (8-NH₂-cADPR) group at this position converts the compound to a specific antagonist of cADPR. The structure of NAADP is identical to that of its parent NADP, except that the amide nitrogen of the nicotinamide group of NADP is changed to oxygen as indicated by the circle. Blue, nitrogen; red, oxygen; yellow, phosphorus; green, carbon.