The function of ENTPD5 (ectonucleoside triphosphate diphosphohydrolase 5 (inactive), ENSG00000187097) is as follows. Hydrolyzes nucleoside diphosphates with a preference for GDP, IDP and UDP compared to ADP and CDP (PubMed:10400613, PubMed:15698960). 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). {ECO:0000250|UniProtKB:Q9WUZ9, ECO:0000269|PubMed:10400613, ECO:0000269|PubMed:15698960}.