In enzymology, a glucuronolactone reductase (EC1.1.1.20) is an enzyme that catalyzes the chemical reaction

Thus, the two substrates of this enzyme are L-gulono-1,4-lactone and NADP, whereas its 3 products are D-glucurono-3,6-lactone, NADPH, and H.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD or NADP as acceptor. The systematic name of this enzyme class is L-gulono-1,4-lactone:NADP 1-oxidoreductase. Other names in common use include GRase, and gulonolactone dehydrogenase. This enzyme participates in ascorbate and aldarate metabolism.