Structural determination of a cyclic metabolite of NAD⁺ with intracellular Ca²⁺-mobilizing activity

Incubation of NAD⁺ with extracts from sea urchin eggs resulted in production of a metabolite which could mobilize intracellular Ca²⁺ stores of the eggs. In this study we present structural evidence indicating that the metabolite is a cyclized ADP-ribose having an N-glycosyl linkage between the anomeric carbon of the terminal ribose unit and the N⁶-amino group of the adenine moiety. In view of this structure we propose cyclic ADP-ribose as the common name for the metabolite. The purification procedure for the metabolite consisted of deproteinizing the incubated egg extracts and sequentially chromatographing the extracts through three different high pressure liquid chromatography (HPLC) columns. The homogeneity of the purified metabolite was further verified by HPLC on a Partisil 5 SAX column. Using radioactive precursor NAD⁺ with label at various positions it was demonstrated that the metabolite was indeed derived from NAD⁺ and that the adenine ring as well as the adenylate α-phosphate were retained in the metabolite whereas the nicotinamide group was removed. This was confirmed by ¹H NMR and two-dimensional COSY experiments, which also allowed the identification of all 12 protons on the two ribosyl units as well as the two protons on the adenine ring. From the chemical shifts of the two anomeric protons it was concluded that the C-1 carbons of both ribosyl units were still bonded to nitrogen. The positive and negative ion fast atom bombardment mass spectra showed (M + Na)⁺, (M - H + 2Na)⁺, (M - H)^-, and (M - 2H + Na)^- peaks at m/z 564, 586, 540, and 562, respectively. Exact mass measurements indicated a molecular weight of 540.0526 for (M - H)^-. This together with the constraints imposed by the results from NMR, radioactive labeling, and total phosphate determination uniquely specified a molecular composition of C₁₅H₂₁N₅O₁₃P₂. Analysis by ¹H NMR and mass spectroscopy of the only major breakdown product of the metabolite after prolonged incubation at room temperature established that it was ADP-ribose, thus providing strong support for the cyclic structure.