Widespread occurrence in animal tissues of an enzyme catalyzing the conversion of NAD+ into a cyclic metabolite with intracellular Ca2+-mobilizing activity

Abstract

A metabolite with intracellular Ca2+-mobilizing activity can be produced by incubating NAD+ with extracts from sea urchin eggs. Structural determination indicates it is a cyclized ADP-ribose, and we have proposed cyclic ADP-ribose as a common name for it. In this study, we addressed the question of how widespread is the occurrence of the synthesizing enzyme for this NAD+ metabolite. Incubation of NAD+ with extracts prepared from rabbit liver resulted in a progressive increase in Ca2+ release activity which was monitored by a biological assay using sea urchin egg homogenates. The half-maximal concentration of NAD+ required was about 1 mM. The reaction was stereospecific, and the extracts were sensitive to protease treatment and heat, as well as alkaline pH of about 9.0, indicating the reaction was catalyzed by a protein. The active metabolite was purified by an identical high pressure liquid chromatography (HPLC) procedure used for cyclic ADP-ribose. Functionally, the liver metabolite behaved similarly to cyclic ADP-ribose. Both discharged the same Ca2+ stores in sea urchin egg homogenates with the same half-maximal effective concentrations. Both were active in inducing the cortical exocytosis reaction when microinjected into sea urchin eggs. That they are indeed identical compounds was demonstrated by structural analyses showing that they coeluted on a Partisil 5 SAX HPLC column and had identical 1H NMR spectra. Mass spectrometry indicated a mass of 540.0529 for the molecular ion (M - H)- of the liver metabolite, which was identical to within 0.74 ppm of cyclic ADP-ribose. Furthermore, their collisional activated decomposition mass spectra were virtually superimposable. Extracts from rabbit brain, heart, spleen, and kidney were all active in producing similar Ca2+-releasing metabolites which could be isolated by the same HPLC procedure and had similar elution times on both the mixed mode and the Partisil 5 SAX column. It is therefore apparent that the synthesizing enzyme for cyclic ADP-ribose is a very common enzyme.