Hydrolysis of NADP(+) by platelet CD38 in the absence of synthesis and degradation of cyclic ADP-ribose 2 '-phosphate

CD38 is a multifunctional cell surface ectoenzyme that catalyzes both the s ynthesis of cyclic ADP-ribose from NAD(+) and its hydrolysis to ADP-ribose, In this work, we investigated the metabolism of NADP(+) by CD38 expressed on human platelets. Incubation of either platelet membranes or intact cells with NADP+ resulted in the rapid and time-dependent accumulation of ADP-ri bose 2'-phosphate that paralleled the consumption of the substrate. However , under the same conditions, synthesis of cyclic ADP-ribose 2'-phosphate wa s not observed, By immunoprecipitation experiments, we identified CD38 as t he enzyme responsible for the observed NADP(+) glycohydrolase activity. The lack of detection of cyclic ADP-ribose 2'-phosphate was not due to its rap id hydrolysis, since direct incubation of platelet membranes with cyclic AD P-ribose 2'-phosphate did not result in the formation of ADP-ribose 2'-phos phate, By contrast, the same membrane samples expressed a significant abili ty to hydrolyze cyclic ADP-ribose to ADP-ribose, The absence of cyclic ADP- ribose 2'-phosphate hydrolase activity was also confirmed using high concen trations of substrate and by analysing both intact Jurkat T-lymphocytes and immunoprecipitated CD38, These results indicate that CD38, which is a mult ifunctional enzyme towards NAD+, displays exclusively a NADP+ glycohydrolas e activity and is unable to catalyze both the synthesis and the hydrolysis of cyclic ADP-ribose 2'-phosphate, (C) 1999 Federation of European Biochemi cal Societies.