Aldose reductase does catalyse the reduction of glyceraldehyde through a stoichiometric oxidation of NADPH.

In order to define the ability of bovine lens aldose reductase (ALR2) to generate polyols from aldoses, the quantitative determination of glycerol in the presence of glyceraldehyde was performed by gas chromatography after derivatization with trifluoroacetic anhydride. The proposed method appears to be useful in quantifying low amounts of glycerol in the presence of relatively high concentrations of glyceraldehyde and in following glycerol formation in enzyme assay conditions. The generation of one equivalent of glycerol in the presence of ALR2, is paralleled by the oxidation of one equivalent of NADPH. A similar result was obtained when S-glutathionyl-modified ALR2 was used, instead of the native enzyme, as a catalyst of glyceraldehyde reduction. Sorbinil, a classical ALR2 inhibitor, present in the enzyme assay mixture, inhibits to the same extent both NADPH oxidation and glycerol formation. The demonstration of the stoichiometric ratio of 1:1 occurring in the presence of bovine lens ALR2 between the synthesis of glycerol from D, L -glyceraldehyde and the oxidation of NADPH, rules out doubts concerning the ability of the enzyme to catalyse the reduction of aldoses to the corresponding polyalcohols. Possible autooxidation processes of glyceraldehyde, in the enzyme assay conditions, appear to be irrelevant with respect to the enzyme-catalysed reduction of the aldose. This would indicate that the spectrophotometric monitoring of NADPH oxidation at 340 nm, in the presence of ALR2, is a reliable method to assay the enzyme activity.