Catalyzes the reduction of 2,5-diketo-D-gluconic acid (25DKG) to 2-keto-L-gulonic acid (2KLG). It is also capable of stereoselective -keto ester reductions on ethyl acetoacetate and other 2-substituted derivatives. 2-dehydro-D-gluconate + NADP(+) = 2,5-didehydro-D-gluconate + H(+) + NADPH Optimum pH is 7.5. Monomer. 2-keto-L-gulonic acid is a key intermediate in the production of L-ascorbic acid (vitamin C). Belongs to the aldo/keto reductase family.