Aldose reductase, a key enzyme in the oxidative deamination of norepinephrine in rats

Natural Compounds as Source of Aldose Reductase (AR) Inhibitors for the Treatment of Diabetic Complications: A Mini Review

BACKGROUND

Aldol reductase (AR) is the polyol pathway's main enzyme that portrays a crucial part in developing 'complications of diabetes' involving cataract, retinopathy, nephropathy, and neuropathy. These diabetic abnormalities are triggered tremendously via aggregation of sorbitol formation (catalyzed by AR) in the polyol pathway. Consequently, it represents an admirable therapeutic target and vast research was done for the discovery of novel molecules as potential AR inhibitors for diabetic complications.

OBJECTIVE

This review article has been planned to discuss an outline of diabetic complications, AR and its role in diabetic complications, natural compounds reported as AR inhibitors, and benefits of natural/plant derived AR inhibitors for the management of diabetic abnormalities.

RESULTS

The goal of AR inhibition remedy is to stabilize the increased flux of blood glucose and sorbitol via the 'polyol pathway' in the affected tissues. A variety of synthetic inhibitors of AR have been established such as tolrestat and sorbinil, but both of these face limitations including low permeability and health problems. Pharmaceutical industries and other scientists were also undertaking work to develop newer, active, and 'safe' AR inhibitors from natural sources. Therefore, several naturally found molecules were documented to possess a potent inhibitory action on AR activity.

CONCLUSION

Natural inhibitors of AR appeared as harmless pharmacological agents for controlling diabetic complications. The detailed literature throughout this article shows the significance of herbal extracts and phytochemicals as prospective useful AR inhibitors in treating diabetic complications.

Activities of biotransformation enzymes and flubendazole metabolism in lambs (Ovis aries): effect of gender and flubendazole therapy

The effect of flubendazole (FLU) therapy on in vitro FLU biotransformation and the activities of selected biotransformation enzymes were investigated in male and female lambs. Four experimental groups were used: control (untreated) ewes and rams and FLU-treated ewes and rams (orally, 15 mg/kg per day, for three consecutive days). Subcellular fractions were prepared from liver and intestinal mucosa 24 h after the final dosage was administered. Activities of cytochromes P450 (CYP), flavine monooxygenases (FMO), carbonyl reducing enzymes, UDP-glucuronosyl transferase (UGT) and glutathione S-transferase were tested. Significant gender differences were observed for FMO-mediated activity (2-fold higher in ram lambs) and UGT activity (up to 30% higher in ewe lambs), but no gender differences were observed in FLU metabolism. FLU-treatment of lambs moderately changed the activities of some CYPs, FMO, and UGT in liver microsomes. In vitro FLU reduction was not altered in the liver, but was slightly higher in the small intestine of FLU pre-treated lambs. This correlated with the higher carbonyl reductase activities measured in the gut mucosa of these animals.

Biotransformation of flubendazole and selected model xenobiotics in Haemonchus contortus

Haemonchus contortus is one of the most pathogenic parasites of small ruminants (e.g., sheep and goat). The treatment of haemonchosis is complicated because of frequent resistance of H. contortus to common anthelmintics. The development of resistance can be facilitated by the action of drug metabolizing enzymes of parasites that can deactivate anthelmintics and thus protect parasites against the toxic effect of the drug. The aim of this project was to investigate the Phase I biotransformation of benzimidazole anthelmintic flubendazole in H. contortus and to determine the biotransformation of other model xenobiotics. For this purpose, in vitro (subcellular fractions of H. contortus homogenate) as well as ex vivo (live nematodes cultivated in flasks with medium) experiments were used. The results showed that cytosolic NADPH-dependent enzymes of H. contortus metabolize flubendazole via reduction of its carbonyl group. The apparent kinetic parameters of this reaction were determined (V'max=39.8+/-2.1 nM min(-1), K'm=1.5+/-0.3 microM). The reduction of flubendazole in H. contortus is stereospecific, the ratio of (-):(+) enantiomers of reduced flubendazole formed was 90:10. Reduced flubendazole was the only Phase I metabolite found. Effective reduction of other xenobiotics with carbonyl group (metyrapon, daunorubicin, and oracin) was also found. Significant activity of carbonyl-reducing enzymes may be important for H. contortus to survive the attacks of anthelmintics or other xenobiotics with carbonyl group.

Neurotoxicity and metabolism of the catecholamine-derived 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde: the role of aldehyde dehydrogenase

Aldehydes are highly reactive molecules formed during the biotransformation of numerous endogenous and exogenous compounds, including biogenic amines. 3,4-Dihydroxyphenylacetaldehyde is the aldehyde metabolite of dopamine, and 3,4-dihydroxyphenylglycolaldehyde is the aldehyde metabolite of both norepinephrine and epinephrine. There is an increasing body of evidence suggesting that these compounds are neurotoxic, and it has been recently hypothesized that neurodegenerative disorders may be associated with increased levels of these biogenic aldehydes. Aldehyde dehydrogenases are a group of NAD(P)+ -dependent enzymes that catalyze the oxidation of aldehydes, such as those derived from catecholamines, to their corresponding carboxylic acids. To date, 19 aldehyde dehydrogenase genes have been identified in the human genome. Mutations in these genes and subsequent inborn errors in aldehyde metabolism are the molecular basis of several diseases, including Sjögren-Larsson syndrome, type II hyperprolinemia, gamma-hydroxybutyric aciduria, and pyridoxine-dependent seizures, most of which are characterized by neurological abnormalities. Several pharmaceutical agents and environmental toxins are also known to disrupt or inhibit aldehyde dehydrogenase function. It is, therefore, possible to speculate that reduced detoxification of 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde from impaired or deficient aldehyde dehydrogenase function may be a contributing factor in the suggested neurotoxicity of these compounds. This article presents a comprehensive review of what is currently known of both the neurotoxicity and respective metabolism pathways of 3,4-dihydroxyphenylacetaldehyde and 3,4-dihydroxyphenylglycolaldehyde with an emphasis on the role that aldehyde dehydrogenase enzymes play in the detoxification of these two aldehydes.

Novel sites of aldose reductase immunolocalization in normal and streptozotocin-diabetic rats

Glucose metabolism by aldose reductase (AR) is implicated in the pathogenesis of many diabetic complications, including neuropathy. We have re-evaluated the distribution of AR in the sciatic nerve and dorsal root ganglion (DRG) of normal rats, expanded these observations to describe the location of AR in the spinal cord and footpad skin, and investigated whether diabetes alters the distribution of AR. In sciatic nerve, AR was restricted to cytoplasm of myelinated Schwann cells and endothelial cells of epineurial, but not endoneurial, blood vessels. AR immunoreactivity (IR) was present in satellite cells in the DRG. In skin, AR-IR was detected in vascular endothelial cells, Schwann cells of myelinated fibers, and axons of perivascular sympathetic nerves. AR was localized selectively to oligodendrocytes of the white matter of spinal cord. The distribution of AR-IR in sciatic nerve, DRG, skin, and spinal cord was not altered by up to 12 weeks of streptozotocin-induced diabetes. Identification of perineuronal satellite cells, oligodendrocytes, and perivascular sympathetic nerves as AR-expressing cells reveals them as cellular sites with the potential to contribute to diabetic neuropathy.

Structure of a glutathione conjugate bound to the active site of aldose reductase

Aldose reductase (AR) is a monomeric NADPH-dependent oxidoreductase that catalyzes the reduction of aldehydes, ketones, and aldo-sugars. AR has been linked to the development of hyperglycemic injury and is a clinical target for the treatment of secondary diabetic complications. In addition to reducing glucose, AR is key regulator of cell signaling through it's reduction of aldehydes derived from lipoproteins and membrane phospholipids. AR catalyzes the reduction of glutathione conjugates of unsaturated aldehydes with higher catalytic efficiency than free aldehydes. The X-ray structure of human AR holoenzyme in complex with the glutathione analogue S-(1,2-dicarboxyethyl) glutathione (DCEG) was determined at a resolution of 1.94 A. The distal carboxylate group of DCEG's dicarboxyethyl moiety interacted with the conserved AR anion binding site residues Tyr48, His110, and Trp111. The bound DCEG's glutathione backbone adopted the low-energy Y-shape form. The C-terminal carboxylate of DCEG glutathione's glycine formed hydrogen bonds to Leu301 and Ser302, while the remaining interactions between DCEG and AR were hydrophobic, permitting significant flexibility of the AR and glutathione (GS) analogue interaction. The observed conformation and interactions of DCEG with AR were consistent with our previously published molecular dynamics model of glutathionyl-propanal binding to AR. The current structure identifies major interactions of glutathione conjugates with the AR active-site residues.

High glucose-induced activation of the polyol pathway and changes of gene expression profiles in immortalized adult mouse Schwann cells IMS32

We investigated the polyol pathway activity and the gene expression profiles in immortalized adult mouse Schwann cells (IMS32) under normal (5.6 mM) and high (30 and 56 mM) glucose conditions for 7-14 days in culture. Messenger RNA and the protein expression of aldose reductase (AR) and the intracellular sorbitol and fructose contents were up-regulated in IMS32 under high glucose conditions compared with normal glucose conditions. By employing DNA microarray and subsequent RT-PCR/northern blot analyses, we observed significant up-regulation of the mRNA expressions for serum amyloid A3 (SAA3), angiopoietin-like 4 (ANGPTL4) and ecotropic viral integration site 3 (Evi3), and the down-regulation of aldehyde reductase (AKR1A4) mRNA expression in the cells under high glucose (30 mM) conditions. The application of an AR inhibitor, SNK-860, to the high glucose medium ameliorated the increased sorbitol and fructose contents and the reduced AKR1A4 mRNA expression, while it had no effect on mRNA expressions for SAA3, ANGPTL4 or Evi3. Considering that the exposure to the high glucose (>or= 30 mM) conditions mimicking hyperglycaemia in vivo accelerated the polyol pathway in IMS32, but not in other previously reported Schwann cells, the culture system of IMS32 under those conditions may provide novel findings about the polyol pathway-related abnormalities in diabetic neuropathy.

Catecholamine metabolism: a contemporary view with implications for physiology and medicine

This article provides an update about catecholamine metabolism, with emphasis on correcting common misconceptions relevant to catecholamine systems in health and disease. Importantly, most metabolism of catecholamines takes place within the same cells where the amines are synthesized. This mainly occurs secondary to leakage of catecholamines from vesicular stores into the cytoplasm. These stores exist in a highly dynamic equilibrium, with passive outward leakage counterbalanced by inward active transport controlled by vesicular monoamine transporters. In catecholaminergic neurons, the presence of monoamine oxidase leads to formation of reactive catecholaldehydes. Production of these toxic aldehydes depends on the dynamics of vesicular-axoplasmic monoamine exchange and enzyme-catalyzed conversion to nontoxic acids or alcohols. In sympathetic nerves, the aldehyde produced from norepinephrine is converted to 3,4-dihydroxyphenylglycol, not 3,4-dihydroxymandelic acid. Subsequent extraneuronal O-methylation consequently leads to production of 3-methoxy-4-hydroxyphenylglycol, not vanillylmandelic acid. Vanillylmandelic acid is instead formed in the liver by oxidation of 3-methoxy-4-hydroxyphenylglycol catalyzed by alcohol and aldehyde dehydrogenases. Compared to intraneuronal deamination, extraneuronal O-methylation of norepinephrine and epinephrine to metanephrines represent minor pathways of metabolism. The single largest source of metanephrines is the adrenal medulla. Similarly, pheochromocytoma tumor cells produce large amounts of metanephrines from catecholamines leaking from stores. Thus, these metabolites are particularly useful for detecting pheochromocytomas. The large contribution of intraneuronal deamination to catecholamine turnover, and dependence of this on the vesicular-axoplasmic monoamine exchange process, helps explain how synthesis, release, metabolism, turnover, and stores of catecholamines are regulated in a coordinated fashion during stress and in disease states.

Aldose reductase: an aldehyde scavenging enzyme in the intraneuronal metabolism of norepinephrine in human sympathetic ganglia

The neurotransmitter norepinephrine is metabolized by monoamine oxidase into an aldehyde intermediate that is further metabolized to the stable glycol derivative, 3,4-dihydroxyphenylglycol (DHPG). In this study, the possible role of aldose reductase in reducing this aldehyde intermediate in human sympathetic neurons has been examined. DHPG is formed when norepinephrine is incubated with aldose reductase in the presence of monoamine oxidase. DHPG metabolism is inhibited by the monoamine oxidase inhibitor, pargyline which prevents the deamination of norepinephrine, and by the aldose reductase inhibitor AL 1576, which inhibits DHPG formation without affecting the deamination of norepinephrine. Although similar formation of DHPG was observed with human liver aldehyde reductase, the production of DHPG was more effective with aldose reductase than aldehyde reductase. Two peaks of reductase activity corresponding to aldose reductase and aldehyde reductase were observed when sympathetic ganglia were chromatofocused. Molecular modeling studies indicate that the energy-minimized structure of 3,4-dihydroxymandelaldehyde bound to aldose reductase is similar to that of glyceraldehyde where the 2'-hydroxyl group forms hydrogen bonds with Trp111 and NADPH. These results suggest that aldose reductase may be important in metabolizing the potentially toxic aldehyde intermediate formed from norepinephrine in human sympathetic ganglia.

Side-chain metabolism of propranolol: involvement of monoamine oxidase and aldehyde reductase in the metabolism of N-desisopropylpropranolol to propranolol glycol in rat liver

The further metabolism of N-desisopropylpropranolol (NDP), a side-chain metabolite of propranolol (PL), was investigated in isolated rat hepatocytes. Propranolol glycol (PGL) was generated from NDP as a major metabolite. Naphtetrazole (NTE), a potent inhibitor of monoamine oxidase (MAO), significantly retarded the disappearance of NDP from the incubation medium, suggesting the involvement of MAO in the deamination of NDP to an aldehyde intermediate. In a reaction mixture of rat liver mitochondria and cytosol with NADPH, phenobarbital, a specific inhibitor of aldehyde reductase, and 4-nitrobenzaldehyde (4-NBA), a substrate inhibitor of aldehyde reductase, decreased the formation of PGL from NDP. 4-NBA was a competitive inhibitor of the enzyme responsible for the PGL formation. The optimal pH for the formation of PGL from NDP in the reaction mixture was approximately 8.0. Based on these results, we propose the possibility that, in the rat liver, MAO catalyzes the oxidative deamination of NDP to an aldehyde intermediate and the formed aldehyde intermediate is subsequently reduced to PGL by aldehyde reductase. Furthermore, the enantioselective metabolism of NDP to PGL was examined. In isolated rat hepatocytes, the amount of PGL formed from S-NDP [S(-)-form of NDP] was larger than that of PGL formed from R-NDP [R(+)-form of NDP].

A vertebrate aldo-keto reductase active with retinoids and ethanol

Enzymes of the short chain and medium chain dehydrogenase/reductase families have been demonstrated to participate in the oxidoreduction of ethanol and retinoids. Mammals and amphibians contain, in the upper digestive tract mucosa, alcohol dehydrogenases of the medium chain dehydrogenase/reductase family, active with ethanol and retinol. In the present work, we searched for a similar enzyme in an avian species (Gallus domesticus). We found that chicken does not contain the homologous enzyme from the medium chain dehydrogenase/reductase family but an oxidoreductase from the aldo-keto reductase family, with retinal reductase and alcohol dehydrogenase activities. The amino acid sequence shows 66-69% residue identity with the aldose reductase and aldose reductase-like enzymes. Chicken aldo-keto reductase is a monomer of M(r) 36,000 expressed in eye, tongue, and esophagus. The enzyme can oxidize aliphatic alcohols, such as ethanol, and it is very efficient in all-trans- and 9-cis-retinal reduction (k(cat)/K(m) = 5,300 and 32,000 mm(-1).min(-1), respectively). This finding represents the inclusion of the aldo-keto reductase family, with the (alpha/beta)(8) barrel structure, into the scenario of retinoid metabolism and, therefore, of the regulation of vertebrate development and tissue differentiation.

Elevated sorbitol concentration in the cerebrospinal fluid of patients with mood disorders

This study was undertaken to test the hypothesis that a specific pathophysiological mechanism of diabetic neuropathy, namely increased polyol pathway flux, could be operative in patients with bipolar and unipolar mood disorders. Numerous studies have shown abnormalities of carbohydrate metabolism, including high rates of diabetes mellitus, in patients with mood disorders. Several studies have found that peripheral neuropathy is a risk factor for depression in diabetics. Furthermore, increased polyol pathway flux results in elevated sorbitol concentrations in peripheral tissues and cerebrospinal fluid (CSF) of diabetics with neuropathy. The purpose of this study was to determine whether sorbitol concentration is elevated in the CSF of non-medically ill patients with mood disorders. Lumbar punctures were performed on 30 subjects - 10 with bipolar mood disorder, 10 with unipolar mood disorder, and 10 age-matched normal controls, and CSF sorbitol concentrations were measured, using a gas chromatographic-mass spectroscopic technique. The mean+/-standard deviation of CSF sorbitol concentrations differed among the three groups as follows: bipolar (22.9+/-4.6 micromoles/l) > unipolar (19.0+/-2.8 micromoles/l)>normal control (15. 6+/-1.9 micromoles/l). One-way ANOVA showed significant (P=0.0002) differences among the three groups. Post-hoc tests indicated a significant (P<0.05) difference between bipolars and normal controls, bipolars and unipolars, and unipolars and normal controls. Further investigation is needed to determine the pathophysiological significance of this novel finding of elevated sorbitol concentration in the CSF of patients with mood disorders.

Aldose reductase inhibitors: therapeutic implications for diabetic complications

The 'late complications' of diabetes mellitus, i.e., nephropathy, neuropathy and retinopathy are firmly rooted in inadequate control of blood glucose: hyperglycaemia. Hyperglycaemia causes elevated cytosolic glucose and/or rates of glucose metabolism, i.e., 'hyperglysolia,' within cells of vulnerable tissues. Although the molecular basis for the pathogenic effects of hyperglysolia remains to be proven, substantial evidence points to a key role for increased glucose metabolism through a cytosolic enzyme, aldose reductase (AR). Recent human genetic and biochemical data link polymorphisms of the AR gene (technically called the AR2 gene) and elevated tissue levels of AR with strongly altered risks for diabetic complications. Despite several genetic reports failing to confirm such an association, there are now ten concordant reports from five continents that certain polymorphisms of the AR gene are associated with an ~ 3- to 20-fold higher risk for diabetic complications. Moreover, in US and European diabetic study populations the principle allele of the AR gene associated with elevated disease risk, the Z-2 allele, correlates with an ~ 2- to 3-fold increase in AR expression. These results, together with recent clinical, experimental and pharmacological data, provide powerful new support for the rationale for research and development of aldose reductase inhibitors (ARIs) targeted at slowing the progression of diabetic complications. Although past clinical trials of ARIs have been disappointing, this has stemmed from overly optimistic expectations, inadequate trial designs and lack of pharmacological robustness and/or acceptable systemic toleration of the agents tested. However, a more realistic and encouraging perspective for therapeutic expectations for ARIs has arisen from recent data revealing that the seemingly modest short-term effects of intensified glycaemic control and of pancreatic transplantation are followed by substantial long-term benefits on diabetic complications. In addition, robust inhibition of AR in human nerve has recently yielded dose-dependent efficacy on nerve structure and function. Thus, the quest for well-tolerated, potent ARIs continues to be a worthy and more urgent objective than ever before.