Involvement of Cytosolic NAD+Glycohydrolase in Cyclic ADP-Ribose Metabolism

Abstract

The NAD+glycohydrolase homogeneously purified from bovine brain cytosol was found to catalyze the synthesis and hydrolysis of cyclic ADP-ribose (cADPR). Although the formation of cADPR from NAD+does not exceed about 2% of the reaction products, the cyclase activity is clearly evidenced by its conversion of NGD+to cyclic GDP-ribose (cGDPR), which cannot be hydrolyzed to GDPR. Importantly, a steep increase in cADPR hydrolytic activity was observed at cADPR concentrations above 60 μM, which could be reproduced on a Hill curve with a Hill coefficient of 2. Thus, the allosteric binding of cADPR to the NAD+glycohydrolase (E) molecule promotes the hydrolysis of cADPR. These results suggest that NAD+hydrolysis to ADPR and nicotinamide catalyzed by the NAD+glycohydrolase occurs through the formation of a cADPR · E · cADP-ribosyl complex. The low production of cADPR by NAD+glycohydrolase compared with invertebrate ADP-ribosyl cyclase is believed to be attributable to the fast hydrolysis of cADPR by the allosteric effect of cADPR bound to the same enzyme that produces it.