Hydrolyzes nucleoside diphosphates with a preference for GDP, IDP and UDP compared to ADP and CDP (By similarity). In the lumen of the endoplasmic reticulum, hydrolyzes UDP that acts as an end-product feedback inhibitor of the UDP-Glc:glycoprotein glucosyltransferases. UMP can be transported back by an UDP-sugar antiporter to the cytosol where it is consumed to regenerate UDP-glucose. Therefore, it positively regulates protein reglucosylation by clearing UDP from the ER lumen and by promoting the regeneration of UDP-glucose. Protein reglucosylation is essential to proper glycoprotein folding and quality control in the ER (By similarity). a ribonucleoside 5'-diphosphate + H2O = a ribonucleoside 5'-phosphate + H(+) + phosphate GDP + H2O = GMP + H(+) + phosphate H2O + UDP = H(+) + phosphate + UMP H2O + IDP = H(+) + IMP + phosphate CDP + H2O = CMP + H(+) + phosphate ADP + H2O = AMP + H(+) + phosphate Protein modification; protein glycosylation. Monomer; active form. Homodimer; disulfide-linked. Homodimers are enzymatically inactive. Belongs to the GDA1/CD39 NTPase family.