Although glucose is one of the most important physio-pathological substrates of aldose reductase, it is not an easy molecule for in vitro investigation into the enzyme. In many cases alternative aldoses have been used for kinetic characterization and inhibition studies. However these molecules do not completely match the structural features of glucose, thus possibly leading to results that are not fully applicable to glucose. We show how aldose reductase is able to act efficiently on L-idose, the C-5 epimer of D-glucose. This is verified using both the bovine lens and the human recombinant enzymes. While the K-cat, values obtained are essentially identical to those measured for D-glucose, a significant decrease in K-M was observed. This can be due to the significantly higher level of the free aldehyde form present in L-idose compared to o-glucose. We believe that L-idose is the best alternative to D-glucose in studies on aldose reductase. (C) 2014 Elsevier Inc. All rights reserved.