Aldose reductase: monosaccharide autoxidation and NADPH binding.

Introduction Aldose reductase (AR2) is a 37 kDa protein found in human erythrocytes and lenses of various species (see refs. [l-31 for reviews). It has been suggested that the protein is one of the NADPH-dependent monomeric aldehyde reductases present in many mammalian tissues [4], although the proteins from this ‘family’ do not cross-react immunochemically [5,6]. AR2 is therefore not identical with the other reductases that have been purified and characterized from brain, liver and kidney from various species (see, e.g., [1,2]). Many of these preparations have been purified to apparent homogeneity, but whereas their steady-state kinetic properties have often been examined in detail, their rate enhancements, k,,,/K,, values and stoichiometry have been less well studied or ignored. It is clear that the protein isolated as AR2 from the lens is capable of binding NADPH, and that histidine and arginine residues located at or near nucleotide-binding sites may be important in the binding or orientation of the reduced nucleotide [7]. Removal of the proton from the NADPH does appear to be stereospecific for the ‘A’ side [8]. While there is no doubt as to the function of AR2 as an NADPH-binding protein, there must be doubt about its catalytic role within the cell. The three major characteristics of an enzyme reaction are its rate enhancement, its stereospecificity and its susceptibility to control. AR2 is remarkably deficient in all these areas. A measure of the ‘catalytic power’ of an enzyme is the k,,JKm value. This is the lower limit of the rate constant for the association of enzyme and substrate, the apparent second-order rate constant that refers to the properties and reactions of the free enzyme and free substrate. The values for AR2 are nearly 109-fold lower than the diffusion-controlled encounter frequency, and about 105-fold lower than those for lens aldehyde dehydrogenase. The substrate specificity shows a lack of stereospecificity with regard to some monosaccharides [9]. Dand