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.