Aldose reductase (AR) has been implicated in the etiology of the secondary complications of diabetes. This enzyme catalyzes the reduction of glucose to sorbitol using nicotinamide adenine dinucleotide phosphate as an essential cofactor. AR has been localized at the sites of tissue damage, and inhibitors of this enzyme prevent the development of neuropathy, nephropathy, retinopathy, and cataract formation in animal models of diabetes. The crystal structure of AR complexed with zopolrestat, a potent inhibitor of AR, has been described.(1) We have generated a model of the AR-inhibitor complex based on the reported Calpha coordinates of the protein and results of a structure-activity relationship study using four structurally distinct classes of inhibitors, recombinant human AR, and four single-site-directed mutants of this enzyme. The effects of the site-directed mutations on residues within the active site of the enzyme were evaluated by average interaction energy calculations and by calculations of carbon atom surface area changes. These values correlated well with the IC(50) values for zopolrestat with the wild-type and mutant enzymes, validating the model. On the basis of the zopolrestat-binding model, we have proposed binding models for 10 other AR inhibitors. Our models have enabled us to gain a qualitative understanding of the binding domains of the enzyme and how different inhibitors impact the size and shape of the binding site.