Reduction of Aldehyde and Ketone Carbonyls

The aldo or keto groups of carbohydrates can be readily reduced in aqueous solution at pH values between 6 and 10 with sodium borohydride to give sugar alcohols (alditols, reaction 4.35). The rate of the reduction can be accelerated by heating at 50°C. The reduction of ketoses results in two epimeric alditols (reaction 4.36). Use of tritiated sodium borohydride gives a tritium atom attached to C-1. 

D-Glucose can be specifically reduced in the presence of other sugars by the enzyme, D-sorbitol dehydrogenase, using the reduced form of the coenzyme nicotinamide adenine dinucleotide (NADH). The reaction is reversible, and d-glucitol (D-sorbitol) can be specifically oxidized to D-glucose using the oxidized form of the coenzyme, NAD+. 

Sorbitol dehydrogenase is obtained commercially from sheep liver. It occurs naturally in humans in two specific locations, sperm and the eye. Its function in the eye is to keep the concentration of D-sorbitol low D-sorbitol is a factor in the formation of cataracts. It is, however, the reaction of sorbitol dehydrogenase in the reverse reaction with the high concentrations of D-glucose resulting from an uncontrolled diabetic condition that leads to diabetic cataracts. 

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