Sorbitol dehydrogenase (SDH), a member of the medium-chain dehydrogenase/reductase protein family and the second enzyme of the polyol pathway of glucose metabolism, converts sorbitol to fructose strictly using NAD(+) as coenzyme. SDH is expressed almost ubiquitously in all mammalian tissues. The enzyme has attracted considerable interest due to its implication in the development of diabetic complications as the polyol pathway is particularly active in hyperglycemic states. Although SORD is closely related to the class I long-chain alcohol dehydrogenases, it differs in substrate specificity, catalyzing polyols such as sorbitol and xylitol but having no activity towards primary alcohols.