Catalyzes the hydrolysis of nucleoside triphosphates and diphosphates in a calcium- or magnesium-dependent manner. Has a strong preference for nucleoside diphosphates, preferentially hydrolyzes GDP, IDP, and UDP, with slower hydrolysis of CDP, ITP, GTP, CTP, ADP, and UTP and virtually no hydrolysis of ATP (PubMed:10948193, PubMed:14529283, PubMed:11041856). The membrane bound form might support glycosylation reactions in the Golgi apparatus and, when released from cells, might catalyze the hydrolysis of extracellular nucleotides (PubMed:10948193, PubMed:14529283, PubMed:11041856). a ribonucleoside 5'-diphosphate + H2O = a ribonucleoside 5'-phosphate + H(+) + phosphate H2O + IDP = H(+) + IMP + phosphate GDP + H2O = GMP + H(+) + phosphate H2O + UDP = H(+) + phosphate + UMP Strongly and equally activated by either Ca(2+) or Mg(2+). Glycosylation does not appear to be required for enzymatic activity. Optimum pH is 7.0 to 7.4. Monomer. Exists as a secreted and membrane-bound forms in the medium of transfected cells, the secreted form is predominant. Expressed in most tissues, but predominantly in heart. The secreted form may be produced by intracellular processing. N-glycosylated. Belongs to the GDA1/CD39 NTPase family.