Cyclic ADP-ribose (cADPR) has been recently shown to be generated in pancreatic beta-cells by glucose stimulation, serving as a second messenger for Ca2+ mobilization in the endoplasmic reticulum in the process of insulin secretion (Takasawa, S., Nata, K., Yonekura, H., and Okamoto, H. (1993) Science 259, 370-373). In the present study, we isolated a cDNA for CD38, which has been reported to be a human leukocyte antigen, from a human insulinoma and expressed the cDNA in COS-7 cells. CD38 expression was observed in the plasma membrane and the microsome fractions of the COS-7 cells. When we incubated the plasma membrane fraction with NAD+ and analyzed the reaction products by high pressure liquid chromatography, the formation of cADPR was observed in addition to the ADP-ribose (ADPR) formation. When the plasma membrane fraction was incubated with cADPR, cADPR was converted to ADPR stoichiometrically. These results suggest that CD38 has both cADPR-forming and -hydrolyzing activities. Moreover, we found that ATP (2-10 mM), generated in the glucose metabolism in beta-cells, inhibited the cADPR-hydrolyzing activity, resulting in the increased formation of cADPR. These findings indicate a role for CD38 in the synthesis and hydrolysis of cADPR in the process of insulin secretion in pancreatic beta-cells.