Involvement of cytosolic NAD+ glycohydrolase in cyclic ADP-ribose metabolism

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 mu M, 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. (C) 1998 Academic Press.