CD38 AND ADP-RIBOSYL CYCLASE CATALYZE THE SYNTHESIS OF A DIMERIC ADP-RIBOSE THAT POTENTIATES THE CALCIUM-MOBILIZING ACTIVITY OF CYCLIC ADP-RIBOSE

CD38, a lymphocyte differentiation antigen, is also a bifunctional enz yme catalyzing the synthesis of cyclic ADP-ribose (cADPR) from NAD(+) and its hydrolysis to ADP-ribose (ADPR), An additional enzymatic activ ity of CD38 shared by monofunctional ADP ribosyl cyclase from Aplysia californica is the exchange of the base group of NAD(+) (nicotinamide) with various nucleophiles, Both human CD38 (either recombinant or pur ified from erythrocyte membranes) and Aplysia cyclase were found to ca talyze the exchange of ADPR with the nicotinamide group of NAD(+) lead ing to the formation of a dimeric ADPR ((ADPR)(2)), The dimeric struct ure of the enzymatic product, which was generated by recombinant CD38 and by CD38(+) Namalwa cells from as low as 10 mu M NAD(+), was demons trated using specific enzyme treatments (dinucleotide pyrophosphatase and 5'-nucleotidase) and mass spectrometry analyses of the resulting p roducts, The linkage between the two ADPR units of (ADPR)(2) was ident ified as that between the N-1 of the adenine nucleus of one ADPR unit and the anomeric carbon of the terminal ribose of the second ADPR mole cule by enzymatic analyses and by comparison with patterns of cADPR cl eavage with Me2SO:tert-butoxide. Although (ADPR)(2) itself did not rel ease Ca2+ from sea urchin egg microsomal vesicles, it specifically pot entiated the Ca2+-releasing activity of subthreshold concentrations of cADPR, Therefore, (ADPR)(2) is a new product of CD38 that amplifies t he Ca2+-mobilizing activity of cADPR.