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 synthesis 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-ribose 2'-phosphate that paralleled the consumption of the substrate. However, under the same conditions, synthesis of cyclic ADP-ribose 2'-phosphate was not observed. By immunoprecipitation experiments, we identified CD38 as the enzyme responsible for the observed NADP⁺ glycohydrolase activity. The lack of detection of cyclic ADP-ribose 2'-phosphate was not due to its rapid hydrolysis, since direct incubation of platelet membranes with cyclic ADP-ribose 2'-phosphate did not result in the formation of ADP-ribose 2'-phosphate. By contrast, the same membrane samples expressed a significant ability to hydrolyze cyclic ADP-ribose to ADP-ribose. The absence of cyclic ADP-ribose 2'-phosphate hydrolase activity was also confirmed using high concentrations of substrate and by analysing both intact Jurkat T-lymphocytes and immunoprecipitated CD38. These results indicate that CD38, which is a multifunctional enzyme towards NAD⁺, displays exclusively a NADP⁺ glycohydrolase activity and is unable to catalyze both the synthesis and the hydrolysis of cyclic ADP-ribose 2'-phosphate. Copyright (C) 1999 Federation of European Biochemical Societies.