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Background Aldose reductase (AR) and Advanced glycation end product (AGE) are known to play important roles in the development of diabetic complications. The inhibitors of AR and AGE would be potential agents for the prevention of diabetic complications. Objective The present study was aimed to evaluate the aldose reductase (AR) and advanced glycation end product (AGE) inhibitory potential of pterostilbene for its possible role in the treatment of diabetic complications such as cataract. Materials and methods The compound was studied for its inhibitory activity against rat lens AR (RLAR) and rat kidney AR (RKAR) in vitro along with its ability to inhibit the formation of AGEs. Anticataract activity of pterostilbene was demonstrated using sugar induced lens opacity model in isolated cattle lens. Further, the involvement of pterostilbene in galactosemia in rats was investigated by assessing the key markers in the polyol pathway and the results were compared with that of a potent AR inhibitor, fidarestat. Results Pterostilbene exhibited inhibitory activity against RLAR and RKAR with IC50 values of 5.49 mg/ml (21.4 mM) and 6.40 mg/ml (25.02 mM), respectively. In sugar-induced lens opacity model, pterostilbene displayed a significant protective effect by preventing opacification and formation of polyols in cattle lens. Besides, the compound exhibited in vivo inhibition of galactitol accumulation in lens and sciatic nerves of galactose fed rats. Conclusion The results obtained in the study underline the potential of pterostilbene as possible therapeutic agent against long-term diabetic complications., Graphical abstract

Background : Lots of plants in genus Swertia are widely used in the traditional medicine as the treatment for T2DM and its complications in Asia. However, the pharmacodynamic material basis and underlying mechanisms of these plants are still incompletely studied. Inhibition of aldose reductase to regulate polyol pathway is considered to be one effective strategy to treat many complications arising from diabetes. Aim : The inhibitory effect against aldose reductase of typical components from the genus Swertia were examined, and further verified in db/db mice model to explore its in vivo renal protective ability and underlying mechanism. Methods : Human recombinant aldose reductase (AR) was applied to detect the inhibitory activity of these components from genus Swetia in vitro, and the most active one was further analyzed in molecular docking experiment. Then, db/db diabetic mice model were employed to investigate the renal protective effects in vivo. Oxidative stress and renal function related indicators were determined using biochemical kit. Histopathological examination of kidney was conducted to evaluate pathological variations. RT-PCR was applied to measure the level of AR mRNA in renal cortex. Expression of AR, TGF-β1, Collagen Ⅳ, Nephrin and Podocin in kidney tissues were assessed by Western blot. Results : It turned out that demethylbellidifolin (DMB) showed the strongest inhibitory activity against AR, with an IC50 of 1.29 ± 0.16 μM, by stabilizing the active site of AR. In vivo experiment confirmed that DMB could remarkably reduce blood glucose and albuminuria level, and alleviate oxidative stress response. Meanwhile, histopathological examination also displayed that the kidney damage in db/db mice was significantly improved after treatment with DMB. Further mechanism study indicated that DMB could prevent the accumulation of sorbitol, one of the polyol pathway products. Besides, DMB could also maintaining the integrity of podocytes structure by up-regulating the expression of Nephrin and Podocin, and exert renal protective effects by inhibiting the expression of AR, Collagen Ⅳ and TGF-β1 in the polyol pathway. Conclusions : These findings indicated that genus Swertia could be an important plant sources of aldose reductase inhibitors, and one of its typical components, DMB, exerted a therapeutic role in the treatment of diabetic nephropathy by regulating the polyol pathway to relieve the injury of cells in the kidney, and may represent a promising renal protective medication for diabetic nephropathy therapy.

Background Alteration of polyol pathway is a critical event in serials of disease, including inflammatory bowel disease (IBD). Shikonin, an inhibitor of PKM2, has been reported to exert a wide range of biological and pharmacological properties. However, the function of shikonin-mediated polyol pathway in IBD remained to be addressed. Methods We conducted WB and qPCR to analyze the effect of shikonin on intestinal epithelial cell differentiation. IF, IP, luciferase assay and ChIP were employed to demonstrate the mechanism that shikonin regulated cell differentiation through polyol pathway. Elisa was used to measure the serum sorbitol in subjects enrolled in this study, and the clinical sample was collected to detect PKM2, AR and NRF2 expression. Results Herein, we showed that shikonin treatment in HT-29 and CaCO2 cells not only contributed to CDX2 and its targets expression, including MUC2 and villin, leading to cell differentiation, but also induced NRF2-mediated AKR1B1 and AKR1B10 expression, resulting in increasing AR activity. A synergistic effect on CDX2-mediated cell differentiation was observed in CaCO2 cells in response to cotreatment with shikonin and AR agonist bortezomib for 24 hours, while aldose reductase inhibitor fidarestat treatment in CaCO2 cells reversed the enhanced CDX2 expression caused by shikonin. Mechanistically, PKM2 interacted with NRF2, and this interaction was enhanced in response to shikonin stimulation, resulting in NRF2 phosphorylation and nuclear translocation, which further increased AKR1B1 and AKR1B10 transactivation. Moreover, both AKR1B1 and AKR1B10 expression were decreased, while PKM2 was increased in intestinal epithelial cells labeled with E-cadherin in patients with IBD, and a negative clinical relationship was observed between PKM2 and AKR1B1/AKR1B10, respectively. What's more, the serum sorbitol level was decreased in patients with IBD. Conclusion These results suggested that shikonin could promote intestinal cell differentiation through NRF2-mediated polyol pathway, implying that shikonin may be a promising therapeutic drug for the treatment of IBD.

Bioactive natural products are enriched with inherently immense structural diversity than synthetic compounds, continuously used as safe therapeutic agents to combat with number of deadly health ailments, and will continually play leading role in discovering new drugs. Natural bioactive agents are considered privileged structures as they have the diversity space in which chemical scaffolds embody characteristics that promote binding to multiple macromolecular targets. An analysis of the origin of the drugs that were launched in the last 25 years showed that both natural products and their derived semi-synthetic compounds composed 34% of all new chemical entities, while 18% of them were synthetic mimics of natural compounds. Long-term hyperglycemia in diabetics causes acute reversible and chronic cumulative irreversible changes, includes damage to blood vessels and peripheral nerves which eventually leads to diabetic complications such as vasculopathy, nephropathy, neuropathy, retinopathy, and cataracts; greatly increasing the risk of atherosclerosis, heart attack, stroke, blindness, amputation, and kidney failure. Aldose reductase (AR, ALR2) inhibition is the most legitimate approach for the management of diabetic complications. In hyperglycemic condition, the excess of glucose firstly enters into polyol pathway via rate-limiting enzyme AR. The present review focused on exploring the journey of natural aldose reductase inhibitors such as curcumin, quercetin, berberine, etc., and their semi synthetic analogs used for the treatment of diabetic complications including their synthetic routes, stereochemistry, proposed mechanism of action, mode of binding interactions, SAR and QSAR studies in relation to elucidate the pharmacophoric structural requisite for diabetic complication prevention potential.

Diabetes is characterized by impaired pancreatic insulin production or an insufficient cellular response to insulin. Hyperglycemia is the main cause of diabetic complications affecting vascular system, kidneys, eyes, peripheral nerves, liver and gastrointestinal system. Hyperglycemia induces overproduction of reactive oxygen species, resulting in impaired cellular function and altered cell signaling. High intracellular glucose levels activate the formation of advanced glycation end-products, the polyol pathway, the hexosamine pathway, and the protein kinase C pathway, affecting gene expression and activity of different regulator proteins. Medicinal mushrooms as source of metabolites (minerals, vitamins, polyphenols, flavonoids, terpenes, alkaloids) have been used in traditional medicine in diabetes treatment for centuries. Lactarius species exhibit a variety of biological activities, including antioxidant, antitumor, and immunostimulatory actions. In this chapter the antioxidant capacity of Lactarius deterrimus and its potential to attenuate the processes that underlie the development and progression of diabetes and diabetes-induced complications will be examined. Preedy VR, editor. Diabetes: Oxidative Stress and Dietary Antioxidants. 2nd ed. San Diego : Elsevier Science & Technology; 2020. p. 265-73.

This chapter outlines the pathophysiological sequel of persistent hyperglycaemia under type 2 diabetes (T2D). The microvascular changes including endothelial cells and associated pericytes along with macrovascular changes leading to diabetes complications such as diabetic nephropathy, retinopathy, neuropathy, and diabetic foot ulcers are discussed. On the molecular mechanisms of T2D complications, the biochemistry of advanced glycated end products, the cascades of oxidative stress and inflammatory mechanisms, and the polyol pathway are among those outlined. The role of these cellular and biochemical changes as pathological hallmark to the common acute and chronic T2D emergencies as well as therapeutic target options are presented.

It is well known that increased cardiovascular risk factors in obesity and diabetes are associated with insulin resistance and endothelial dysfunction. In obesity, molecular mechanisms by which insulin resistance contributes to endothelial dysfunction includes a downregulation of insulin signaling pathway through IRSs/PI3K/Akt pathway and an increased activation of ERK pathway. Angiotensin II, endoplasmic reticulum stress and activation of inflammatory pathways can have an important role in downregulation of insulin signaling through activation of multiple mechanisms that counteract IRS/PI3K/Akt pathway and induce insulin resistance. In diabetes, endothelial dysfunction has a key contribution of the mechanisms described for obesity, but hyperglycemia also has a pivotal role through an increase in ROS generation, activation of PKC and polyol pathway, AGEs, and attenuation of the pentose phosphate pathway. These mechanisms induce endothelial dysfunction, but they also take place in other cells like monocytes and platelets, contributing in a fundamental way to accelerate atherosclerosis.

Cataract is the leading cause of blindness worldwide and accounts for approximately half of all forms of vision loss. Currently, the only way to treat cataracts is by surgery. However, with an aging population and the increasing prevalence of diabetes, the demand for cataract surgery and the need for cost-effective solutions are growing. To reduce the need for cataract surgery, alternative medical therapies to delay cataracts are required. However, given the difficulty in accessing intact human cataractous lenses, due to new cataract procedures that result in phacoemulsification of the lenses, understanding the pathogenesis of this disease and testing the efficacy of potential therapies in humans is problematic. Therefore, researchers have tried to create suitable animal models of cataract to identify therapeutic options. Here we provide an overview of the two most common types of cataract: age-related nuclear cataract and diabetic cataract, with a specific focus on diabetic cataract and diabetic cataract animal models. Early studies using diabetic animal models provided us with extensive knowledge of the polyol pathway and the role of aldose reductase and sorbitol accumulation in the initiation of osmotic stress. However, the inability of aldose reductase inhibitors to ameliorate cataracts in humans, as it did in rats, highlights the importance of selecting appropriate animal models that mimic the human condition. To facilitate the researcher with this, we describe key changes to the diabetic cataractous lens in terms of lens physiology, cellular structure, and biochemistry. Using this set of human lens parameters, we then assess existing animal cataract models and their suitability for mimicking the processes associated with the development of diabetic cataract in humans and for testing of potential therapeutic interventions that are can be translated and effective in humans.

Type 2 diabetes mellitus (T2DM), by definition is a heterogeneous, multifactorial, polygenic syndrome which results from insulin receptor dysfunction. It is an outcome of oxidative stress caused by interactions of reactive metabolites (RMs) interactions with lipids, proteins and other mechanisms of human body. Production of RMs mainly superoxide (O2−) has been found in a variety of predominating cellular enzyme systems including NAD(P)H oxidase, xanthine oxidase (XO), cyclooxygenase (COX), uncoupled endothelial nitric oxide synthase (eNOS) and myeloperoxidase (MPO). The four main RM related molecular mechanisms are: increased polyol pathway flux; increased advanced glycation end-product (AGE) formation; activation of protein kinase C (PKC) isoforms and increased hexosamine pathway flux which have been implicated in glucose-mediated vascular damage. Superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione-S-transferase (GST), nitric oxide synthase (NOS) are antioxidant enzymes involved in scavenging RMs in normal individuals. Functional polymorphisms of these antioxidant enzymes have been reported to be involved in pathogenesis of T2DM individuals. The low levels of antioxidant enzymes or their non-functionality results in excessive RMs which initiate stress related pathways thereby leading to insulin resistance and T2DM. An attempt has been made to review the role of RMs and antioxidant enzymes in oxidative stress resulting in T2DM., Graphical abstract, Highlights • Four main molecular mechanisms are implicated in glucose-mediated vascular damage. • Impaired antioxidant defense contributes to T2DM and related complications. • SNPs in antioxidant enzymes are associated with pathogenesis of type 2 diabetes. • Genotyping of gene variants in populations will help identify individuals at risk.

Inhibition of the mechanistic target of rapamycin (mTOR) by rapamycin is an approved clinical approach for organ transplant patients to suppress immune rejection. This therapeutic approach, unfortunately, often leads to new onset of diabetes after chronic administration of rapamycin. Hence, rapamycin has been regarded as a diabetogenic agent. Accordingly, there has been increasing interest in elucidating the mechanisms of rapamycin diabetogenesis. As diabetes is a redox imbalance disease driven by excess NADH (reduced nicotinamide adenine dinucleotide) derived from hyperglycemia-activated polyol pathway and diminished NAD+ (nicotinamide adenine dinucleotide) caused by activation of poly adenosine diphosphate ribose polymerase, we propose that mitochondrial complex I, an enzyme responsible for NAD+ regeneration from NADH, plays an important role in rapamycin diabetes. Studying complex I dysfunction downstream of the mTOR signaling pathway using the rapamycin diabetes platform not only will provide insights into the redox mechanisms of rapamycin diabetogenesis, but may also help devise novel strategies to reduce rapamycin toxicity during therapy.

Neuropathy is the earliest and commonest complication of diabetes. With increasing duration of diabetes, frequency and severity of neuropathy are worsened. Long-term hyperglycemia is therefore implicated in the development of this disorder. Nerve tissues require glucose energy to function and survive. Upon excessive glucose entry into the peripheral nerve, the glycolytic pathway and collateral glucose-utilizing pathways are overactivated and initiate adverse effects on nerve tissues. During hyperglycemia, flux through the polyol pathway, formation of advanced glycation end-products, production of free radicals, flux into the glucosamine pathway, and protein kinase C activity are all enhanced to negatively influence nerve function and structure. Suppression of these aberrant metabolic pathways has succeeded in prevention and inhibition of the development of neuropathy in animal models with diabetes. Satisfactory results were not attained, however, in patients with diabetes and further clinical trials are required. In this review, the author summarizes the hitherto proposed theories on the pathogenesis of diabetic neuropathy related to glucose metabolism and future prospects for the effective treatment of neuropathy.

We investigated the chronic functional and histopathological changes in the sciatic nerve and lens of streptozotocin (STZ)-diabetic rats and evaluated the preventive effects of ranirestat (AS-3201), a potent aldose reductase inhibitor, on these changes. Sorbitol levels in the sciatic nerve and lens, motor nerve conduction velocity (MNCV), and development of cataracts were measured in STZ-diabetic rats given a ranirestat-admixed diet (0.0005%) for 35 weeks. Ranirestat reduced sorbitol accumulation in the sciatic nerve and improved the decrease in MNCV of STZ-diabetic rats. Morphological and morphometric examination of changes in sural nerve revealed that treatment with ranirestat prevented both the deformity of myelinated fibers and the decrease in their axonal and myelin areas (atrophy). Ranirestat also averted the changes in the size frequency histogram of myelinated fibers. Finally, STZ-diabetic rats developed early lens opacities 8 weeks after STZ injection and had cataract by the end of the experimental period. However, in the ranirestat-treated diabetic rats, no lens opacity was observed in any rat throughout the entire experimental period. This study suggests that the polyol pathway plays an important role in the progress of diabetic neuropathy and cataract formation in STZ-diabetic rats. Ranirestat should be a promising agent for the treatment of complications associated with diabetes, especially neuropathy. Keywords:: ranirestat (AS-3201), diabetic neuropathy, cataract, aldose reductase inhibition

The “Polyol Pathway” is responsible for conversion of glucose to fructose in diabetic cataract, and in diabetic nerve cells. Sorbitol dehydrogenase is present and aldose reductase absent from liver tissue. Phosphofructokinase (PFK) catalyzes a key regulatory reaction in glycolysis. ATP and citrate powerfully inhibit PFK, while ADP, insulin and NH3 stimulate its activity. Glucagon inhibits hepatic PFK. Epinephrine stimulates PFK activity in muscle tissue.

Diabetic neuropathy is a serious complication in 60–70% of diabetic patients involving both type 2 and type 1. Several signaling pathways have been assumed to contribute to its development, including increased activation of the polyol pathway, oxidative stress, advanced glycation end product (AGE) formation, nerve hypoxia/ischemia, activation of protein kinase C and reduction of nerve growth factors. As a central integrating mechanism, overproduction of reactive oxygen species in peripheral nerves leads to oxidative stress, mitochondrial dysfunction, neuronal damage and finally apoptosis. Oxidative stress results in an increase in substrate for AGEs and in precursors of glycoxidation and lipoxidation products, and accelerates the free radical formation that may be accompanied by or caused by a deficiency of antioxidant and detoxification pathways. Oxidative stress is accompanied by the activation of the NF-κB, and then mitogen-activated protein kinases (MAPK) in Schwann cells as well as in neurons, which will lead to the activation of various inflammatory cytokines. Therefore, antioxidant treatment may improve neuronal repair in patients with diabetic neuropathy. However, the true mechanisms by which increased oxidative stress affects downstream signaling mechanisms, and how this results in the development of diabetic neuropathy have not yet been thoroughly studied. Neither is it known whether antioxidant supplementation could help the damaged neurons to regenerate, and thus enable patients with diabetic neuropathy to overcome this devastating, life-threatening situation.

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Diabetes is the world's most prevalent endocrine disorder. It is multifactorial, and both genetic and environmental factors are known to initiate/aggravate the pathological process. The conventionally used antidiabetic agents possess untoward side effects. This has necessitated the need for alternatives that are effective, cheap, and safe. Antidiabetic agents of the dietary sources are highly preferred as their regular consumption does not exert any toxic effects and are easily acceptable to all humans. Scientific studies carried out in the recent past indicate that ginger ( Zingiber officinale Roscoe) is effective in both type I and type II preclinical models of diabetes mellitus. Administering ginger caused hypoglycemic effects and reduced the secondary complications induced by diabetes like hyperlipidemia, nephropathy, neuropathy, and infertility. Mechanistic studies suggest that the protective effects are mediated through the antioxidant: correction of dyslipidemia, inhibition of polyol pathway of sorbitol formation, prevention of formation of advanced glycation end products, and the digestive enzyme alpha glucosidases. Ginger is also observed to possess an insulinotropic effect and to sensitize the peripheral tissues to insulin. Ginger is also reported to possess an antagonizing effect on serotonin-mediated suppression of insulin release. This chapter collates the scientific observations on the antidiabetic effects of ginger, the mechanisms responsible, and on aspects that merit future investigations.

Diabetic neuropathy is the most common complication of diabetes mellitus. This chapter focuses on the classification, pathogenesis, and treatment of diabetic neuropathy. Various classifications have been proposed in the chapter based on clinical presentations and anatomic findings. The primary types of diabetic neuropathy are sensorimotor and autonomic. The etiopathogenesis of diabetic neuropathy is not yet completely understood; however, evidence suggests that the pathogenesis of diabetic neuropathy is multifactorial. Metabolic factors include persistent hyperglycemia (or glucose toxicity), increased oxidative stress (OS), alteration of the polyol or sorbitol pathway, abnormalities in lipid metabolism, deficiencies of dihomo-y-linolenic acid (GLA) and N -acetyl-L-carnitine glycation, and diabetes mellitus-induced growth factor defects. Despite many experimental and clinical studies that have been performed to determine the most optimal strategy for neuropathy, glycemic control remains the gold-standard method for prevention and treatment of diabetic neuropathy. Patients with advanced diabetic neuropathy suffer from pain, burning sensation, and numbness in the distal extremities. The classic approach used to relieve these symptoms consists of two steps: maintaining metabolic control by achieving a normoglycemic state and using certain drugs. Surgery is generally considered only when ulceration, infection, and necrosis occur.

The advanced Maillard reaction was investigated in lens proteins of galactosemic rats and dogs. The relationship between the degree of metabolic control and advanced Maillard reactions was investigated in long-term diabetes in dogs. The effect of polyol pathway activation on advanced Maillard reactions was studied by treating the animals with an aldose reductase inhibitor, sorbinil. Advanced Maillard reactions were studied using three different parameters: (1) protein-bound fluorescence, (2) pentosidine, and (3) fluorophore LM-1. In galactosemic rats, both protein-bound fluorescence and pentosidine accumulated significantly in comparison to controls, and sorbinil treatment almost completely prevented these changes. A close association between the degree of metabolic control and advanced Maillard reactions tested by all three parameters was noted in diabetic dogs. In addition, in diabetic dogs, pentosidine formation revealed the requirement of a glycemic threshold for enhancement of advanced Maillard reactions.

Publisher Summary This chapter describes the diabetic neuropathy, one of the most common and debilitating effects of diabetes, and reviews nerve fiber pathology in human and experimental diabetic neuropathy. The chapter discusses the different ways that exaggerated polyol pathway flux, oxidative stress, increased advanced glycosylation end product formation, and deficient neurotrophic support compromise Schwann cells and the axons they ensheath. The chapter concludes that reported nerve fiber pathology resulting from diabetes mellitus ranges from subtle Schwann cell changes and axonal atrophy to complete fiber degeneration. Sensitivity of myelinated fiber injury to aldose reductase inhibition suggests a link to exaggerated polyol pathway flux in Schwann cells, vessels, and target tissue. Current thought emphasizes the role of oxidative stress, enhanced by polyol pathway activity and advanced glycation end product formation, in the vascular dysfunction of the early metabolic phase of human and experimental diabetic neuropathy. The sensitivity of markers of oxidative stress and antioxidant defense mechanisms to aldose reductase inhibition highlights the potential contribution of oxidative stress to the Schwann cell injury characteristic of human and experimental diabetic neuropathy.

This chapter critically examines the concept of the polyol pathway and how it relates to the pathogenesis of diabetic peripheral neuropathy. The two enzymes of the polyol pathway, aldose reductase and sorbitol dehydrogenase, are reviewed. The structure, biochemistry, physiological role, tissue distribution, and localization in peripheral nerve of each enzyme are summarized, along with current information about the location and structure of their genes, their alleles, and the possible links of each enzymes and its alleles to diabetic neuropathy. Inhibitors of pathway enzymes and results obtained to date with pathway inhibitors in experimental models and human neuropathy trials are updated and discussed. Experimental and clinical data are analyzed in the context of a newly developed metabolic model of the in vivo relationship between nerve sorbitol concentration and metabolic flux through aldose reductase. Overall, the data will be interpreted as supporting the hypothesis that metabolic flux through the polyol pathway, rather than nerve concentration of sorbitol, is the predominant polyol pathway-linked pathogenic factor in diabetic peripheral nerve. Finally, key questions and future directions for basic and clinical research in this area are considered. It is concluded that robust inhibition of metabolic flux through the polyol pathway in peripheral nerve will likely result in substantial clinical benefit in treating and preventing the currently intractable condition of diabetic peripheral neuropathy. To accomplish this, it is imperative to develop and test a new generation of “super-potent” polyol pathway inhibitors.

Diabetes-associated oxidative stress is clearly manifest in peripheral nerve, dorsal root, and sympathetic ganglia of the peripheral nervous system and endothelial cells and is implicated in nerve blood flow and conduction deficits, impaired neurotrophic support, changes in signal transduction and metabolism, and morphological abnormalities characteristic of peripheral diabetic neuropathy (diabetic peripheral neuropathy). Hyperglycemia has a key role in oxidative stress in diabetic nerve, whereas the contribution of other factors, such as endoneurial hypoxia, transition metal imbalance, and hyperlipidemia, has not been rigorously proven. It has been suggested that oxidative stress, particularly mitochondrial superoxide production, is responsible for sorbitol pathway hyperactivity, nonenzymatic glycation/glycooxidation, and activation of protein kinase C. However, this concept is not supported by in vivo studies demonstrating the lack of any inhibition of the sorbitol pathway activity in peripheral nerve, retina, and lens by antioxidants, including potent superoxide scavengers. It has been also hypothesized that aldose reductase (AR) detoxifies lipid peroxidation products, and therefore, the enzyme inhibition in diabetes is detrimental rather than beneficial. However, the role for AR in lipid peroxidation product metabolism has never been demonstrated in vivo , and the effects of aldose reductase inhibitors and antioxidants on diabetic peripheral neuropathy are unidirectional, i.e., both classes of agents prevent and correct functional, metabolic, neuropathic, and morphological changes in diabetic nerve. Growing evidence indicates that AR has a key role in oxidative stress in the peripheral nerve and contributes to superoxide production by the vascular endothelium. The potential mechanism of this phenomenon are discussed.

Publisher Summary Several excellent reviews of the recent advances in the understanding of the biochemical events contributing to the pathophysiology of diabetic complications have focused upon aldose reductase (AR) inhibitors. The biological rationale and chemical progress toward design of AR inhibitors constitute the major discussion of this chapter. Diabetes mellitus (DM) that consists of an absolute (Type I or insulin dependent, IDDM) or relative (Type II or non-insulin-dependent, NIDDM) lack of insulin is a disease of increasing incidence. Diabetics also have a three or four times greater chance of developing peripheral arterial disease and a five times greater chance than the general population of developing gangrenes that leads to amputation. Recently, theories and data have been emerging that implicate aldose reductase (AR), increased flux through the polyol pathway due to hyperglycemia, and accumulation of sorbitol and fructose in the pathophysiology of diabetic complications. Because of the high Km of AR for glucose, it is hypothesized that under normoglycemic conditions there is very little flux through the polyol pathway. On the other hand, under conditions of hyperglycemia, as in diabetes, the flux through this pathway can be considerable and the products of the pathway, sorbitol and fructose, are slowly metabolized and not freely diffusible through the cell plasma membrane, and thus tend to accumulate in the tissues. The chapter in its early phase discusses the extensive biological data based on this agent as a probe for the role of AR inhibition in the treatment of diabetic complications. Significant differences in susceptibility of AR enzymes to inhibition have been observed by the researchers. No trends can be predicted other than that in general human placental AR (HPAR) is less susceptible to inhibition than other AR enzymes.

Publisher Summary This chapter describes an assay method for the synthesis of aldose reductase from human tissues. Aldose reductase and polyol dehydrogenase constitute the sorbitol pathway converting glucose to fructose in extrahepatic tissues. Aldose reductase catalyzes the reduction of other sugar aldehydes and of several aliphatic and aromatic aldehydes. Aldose reductase is assayed spectrophotometrically by recording the decrease in nicotinamide adenine dinucleotide phosphate dehydrogenase (NADPH) absorbance at 340 nm. With crude enzyme solutions, blank reactions occur with NADPH in the absence of any aldehyde substrate; consequently the rate of the reaction is recorded before the addition of exogenous aldehyde. Depending on the substrate and on the tissue analyzed, other dehydrogenases, notably aldehyde reductase, may interfere. Interference with aldehyde reductase is diminished by the addition of diphenylhydantoin or phenobarbitone to the assay medium. The ratio of activities measured with D-xylose, DL-glyceraldehyde, and D-glucuronate is used to distinguish between the two enzymes. The chapter discusses the procedure used to isolate and purify carbonyl reducing enzymes from human tissues, along with the preparation of brain aldose reductase. Brains, either whole or without cortex and cerebellum, are obtained from legal medical autopsies.

Publisher Summary This chapter discusses the medicinal chemistry of aldose reductase inhibitors. Insulin therapy has been dramatically effective in eliminating keto-acidotic coma as a cause of death in diabetics. While prolonging life, insulin therapy does not prevent the occurrence of disabling complications of chronic diabetes, such as neuropathy, nephropathy, retinopathy, and cataracts. Insulin therapy is only partially effective in normalizing glucose levels, and the occurrence of diabetic complications appears to be related to the severity and duration of diabetic hyperglycemia. The tissues involved (nerve, kidney, retina, and lens) do not require insulin for glucose uptake and consequently are exposed to elevated glucose levels. Under normal conditions, glucose is metabolized via the energy-producing glycolytic pathway involving an initial phosphorylation by the enzyme hexokinase. As a result, in the presence of high glucose concentrations, hexokinase becomes saturated, and the polyol pathway becomes activated, resulting in the intracellular production of sorbitol and fructose in tissues where this pathway exists.

Publisher Summary This chapter discusses the polyol pathway activity in aorta. Insulin in vitro has no effect on the glucose space in aortic intima and media, and free intracellular glucose can be demonstrated in tissue from normal or alloxan-diabetic rabbits. Aortic tissue incubated in a medium containing glucose releases free fructose into the medium. The rate of fructose release can be used to provide a minimal estimate of glucose utilization via the polyol pathway. There is a significant turnover of sorbitol in aortic intima and media. Although the steady-state sorbitol concentration in aortic tissue incubated with 5 mM glucose is only 12.9 ± 0.87 nmoles/g, glucose utilization through the polyol pathway accounts for approximately 1.8% of the glucose uptake. Increasing the medium glucose concentration to 50 mM results in increased steady-state sorbitol and fructose concentrations and a sixfold increase in flux through the polyol pathway. The increased sorbitol concentration observed in aortic tissue exposed to elevated glucose concentrations reflects increased sorbitol synthesis.

Publisher Summary This chapter discusses the polyol pathway in the neuropathy of early diabetes. The rise of osmotic pressure within the nerve as a result of the accumulation of sorbitol and fructose will not exceed 5% if the products are uniformly distributed in the aqueous portion of the nerve. If these compounds accumulate within the Schwann cell itself, however, osmotic damage may occur. Evidence suggests that the sorbitol pathway and the Schwann cell are closely associated. In an experiment described in the chapter, nerves of animals killed at 3, 6, and 10 days after induction of diabetes were analyzed. The results showed that the sorbitol pathway is activated very quickly when the blood sugar rises. It was found that after 3 days of hyperglycemia, the sorbitol levels were not significantly different from those found in animals with well-established diabetes. Control of blood sugar with insulin results in a significant reduction in the levels of sorbitol and fructose, this reduction being most evident in animals killed when normoglycemic. The levels in animals killed 40 h after the administration of insulin were well above those found in nondiabetics.

Publisher Summary This chapter discusses the role of sorbitol pathway in neuropathy. The role of the sorbitol pathway in cataract formation has been proved by the finding that aldose reductase inhibitors, which block the increased formation of sugar alcohol in experimental diabetes and galactosemia, also prevent cataract development both in vitro and in vivo. In peripheral nerve aldose reductase has been localized to the Schwann cell, which is the cell responsible for the formation and maintenance of the myelin sheath. Recent investigations with the galactose-fed rat model provide evidence for a role of sugar alcohol in causing neuropathy. The chapter describes a study involving a sample of rats. In this study, the insulin dose was subsequently reduced from 10 to 4 U per rat daily, and in the following three weeks, an increase in blood sugar to a mean of 250 mg% was observed. A nerve conduction velocity defect again appeared. Insulin dosage was again increased to an average dose of 8 U per rat, a dose lower than was initially required to eliminate glycosuria but unexpectedly sufficient to eliminate several experimental animals secondary to hypoglycemia, as evidenced by a mean blood glucose value of 59 mg% in the surviving group.