Spirohydantoin inhibitors of aldose reductase inhibit iron- and copper-catalysed ascorbate oxidation in vitro

Apparent superoxide dismutase-like activity of immunoglobulin

Using various superoxide generating systems and nitroblue tetrazolium or cytochrome c as superoxide detector molecules it is possible to assess the superoxide dismutase activity of proteins. Intact antibodies raised to different antigens, the Fab' fragment of anti-TNF [M632] and well-characterized recombinant Fv fragments of the murine antibody NQ11.7.22 appear to possess superoxide dismutase (SOD)-like activity. Kinetic characteristics of the SOD-like activity of NQ11.7.22-Fv fragments suggest an enzymatic property and these fragments behave in an analogous manner to human erythrocyte Cu-Zn SOD. Furthermore, the SOD-like activity of the NQ11.7.22-Fv fragment is affected by certain single-point mutations in the amino acid composition and has a pH optimum of 6.2-6.6 which is unlike Cu-Zn SOD (pH 7.8-8.2). A change in tyrosine at the 32 position in the heavy chain and histidine at position 27 of the light chain of the NQ11.7.22-Fv fragment results in a profound reduction in SOD-like activity. Tyrosine at the 32 position in the heavy chain is known to play a significant role in antigen binding suggesting that the SOD-like activity occurs at the antigen-binding site itself. Single-point mutations at the periphery of the antigen combining site on the NQ11.7.22-Fv fragment had little or no effect on SOD-like activity. Further studies show that immunoglobin (lgG-1), a commercially available murine monoclonal antibody, can also enhance the generation of hydrogen peroxide, the product of superoxide dismutation, when present in superoxide producing systems. The generation of hydrogen peroxide was increased by low pH (pH 6.25) with lgG-1 but reduced with Cu-Zn SOD.

Polyol pathway and diabetic peripheral 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 informaiton about the location and structure of their genes, their alleles, and the possible links of each enzyme and its alleles to diabetic neuropathy. Inhibitors of pathway enzyme 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 odel of the in vivo relationship between nerve sorbitol concentration and metabolic flux through aldose reuctase. 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 pathogeneic factor in diabetic preipheral nerve. Finally, key questions and future directions for bsic 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.

Chelating activity of advanced glycation end-product inhibitors

The advanced glycation end-product (AGE) hypothesis proposes that accelerated chemical modification of proteins by glucose during hyperglycemia contributes to the pathogenesis of diabetic complications. The two most commonly measured AGEs, N(epsilon)-(carboxymethyl)lysine and pentosidine, are glycoxidation products, formed from glucose by sequential glycation and autoxidation reactions. Although several compounds have been developed as AGE inhibitors and are being tested in animal models of diabetes and in clinical trials, the mechanism of action of these inhibitors is poorly understood. In general, they are thought to function as nucleophilic traps for reactive carbonyl intermediates in the formation of AGEs; however alternative mechanisms of actions, such as chelation, have not been rigorously examined. To distinguish between the carbonyl trapping and antioxidant activity of AGE inhibitors, we have measured the chelating activity of the inhibitors by determining the concentration required for 50% inhibition of the rate of copper-catalyzed autoxidation of ascorbic acid in phosphate buffer. All AGE inhibitors studied were chelators of copper, as measured by inhibition of metal-catalyzed autoxidation of ascorbate. Apparent binding constants for copper ranged from approximately 2 mm for aminoguanidine and pyridoxamine, to 10-100 microm for carnosine, phenazinediamine, OPB-9195 and tenilsetam. The AGE-breakers, phenacylthiazolium and phenacyldimethylthiazolium bromide, and their hydrolysis products, were among the most potent inhibitors of ascorbate oxidation. We conclude that, at millimolar concentrations of AGE inhibitors used in many in vitro studies, inhibition of AGE formation results primarily from the chelating or antioxidant activity of the AGE inhibitors, rather than their carbonyl trapping activity. Further, at therapeutic concentrations, the chelating activity of AGE inhibitors and AGE-breakers may contribute to their inhibition of AGE formation and protection against development of diabetic complications.

A new approach against sugar cataract through aldose reductase inhibitors

Aldose reductase inhibition is one of the therapeutic strategies that has been proposed to prevent or ameliorate long term diabetic complications including retinopathy and sugar cataract. Rats were fed with a galactose rich diet and the aldose reductase inhibitor Tolrestat was topically delivered by ocular instillation. The levels of lens aldose reductase activity, galactitol and the onset of cataract were evaluated during and after treatment with the inhibitor. Topical application of 1-3% Tolrestat (10 microl) four times daily resulted, after 9 days, in a significant decrease in the enzyme activity. Well after interrupting treatment with the drug, the enzyme activity remained impaired and galactose induced cataract was prevented. Our findings may represent the basis for therapeutic plans to prevent sugar cataract by long term cyclic treatments with aldose reductase inhibitors, with reduction in drug doses and side effects.

Aldose reductase: a window to the treatment of diabetic complications?

Kinetic studies on the aldose reductase protein (AR2) have shown that it does not behave as a classical enzyme in relation to ring aldose sugars. These results have been confirmed by X-ray crystallography studies, which have pinpointed binding sites for pharmacological "aklose reductase inhibitors" (ARIs). As with non-enzymic glycation reactions, there is probably a free-radical element involved derived from monosaccharide autoxidation. In the case of AR2, there is free radical oxidation of NADPH by autoxidising monosaccharides, enhanced in the presence of the NADPH-binding protein. Whatever the behaviour of AR2, many studies have showed that sorbitol production is not an initiating aetiological factor in the development of diabetic complications in humans. Vitamin E (alpha-tocopherol), other antioxidants and high fat diets can delay or prevent cataract in diabetic animals even though sorbitol and fructose levels are not modified; vitamin C acts as an AR1 in humans. Protein post-translational modification by glyc-oxidation or other events is probably the key factor in the aetiology of diabetic complications. There is now no need to invoke AR2 in xylitol biosynthesis. Xylitol can be produced in the lens from glucose, via a pathway involving the enzymes myo-inositol-oxygen oxidoreductase, D-glucuronate reductase. L-gulonate NAD(+)-3-oxidoreductase and L-iditol-NAD(+)-5-oxidoreductase, all of which have recently been found in bovine and rat lens. This chapter investigates the molecular events underlying AR2 and its binding and kinetics. Induction of the protein by osmotic response elements is discussed, with detailed analysis of recent in vitro and in vivo experiments on numerous ARIs. These have a number of actions in the cell which are not specific, and which do not involve them binding to AR2. These include peroxy-radical scavenging and recently discovered effects of metal ion chelation. In controlled experiments, it has been found that incubation of rat lens homogenate with glucose and the copper chelator o-phenanthroline abolishes production of sorbitol. Taken together, these results suggest AR2 is a vestigial NADPH-binding protein, perhaps similar in function to a number of non-mammalian crystallins which have been recruited into the lens. There is mounting evidence for the binding of reactive aldehyde moieties to the protein, and the involvement of AR2 either as a 'housekeeping' protein, or in a free-radial-mediated 'catalytic' role. Interfering with the NADPH binding and flux levels--possibly involving free radicals and metal ions--has a deleterious effect. We have yet to determine whether aldose reductase is the black sheep of the aldehyde reductase family, or whether it is a skeleton in the cupboard, waiting to be clothed in the flesh of new revelations in the interactions between proteins, metal ions and redox metabolites.

Glucose-modified low density lipoprotein enhances human monocyte chemotaxis

In diabetes mellitus the progression of atherosclerosis is accelerated. The interaction of glucose with atherogenic lipoproteins may be relevant to the mechanisms responsible for this vascular damage. The aim of this study was to examine the effect of glucose-modified low density lipoprotein (LDL) on human monocyte chemotaxis and to investigate the roles of oxidation and glycation in the generation of chemotactic LDL. Cu(II)-mediated LDL oxidation was potentiated by glucose in a dose-dependent manner and increased its chemotactic activity. Incubation with glucose alone, under conditions where very little oxidation was observed, also increased the chemotactic property of LDL. Neither diethylenetriamine pentaacetic acid (DETAPAC) nor aminoguanidine, which both inhibited LDL oxidation, completely inhibited the chemotactic activity of glycated oxidised LDL. The results suggest that both oxidation and glycation contribute to increased chemotactic activity.

Micellar acceleration of oxygen-dependent reactions and its potential use in the study of human low density lipoprotein

The reaction rate between superoxide and nitro-blue tetrazolium (NBT) is known to be accelerated/catalysed by micellar systems. Previous reports suggest that an accelerated rate of NBT reduction by micellar systems may be the result of either the binding of organic substrates such as NBT to the micellar phase giving a more favourable environment for superoxide reduction (an orientation effect), or the electrostatic interaction between micelles and superoxide. Here we show, using three different superoxide generating systems, that micelles composed of a number of different lipids or human low density lipoprotein (LDL) accelerates the apparent reaction between superoxide and NBT. Evidence in favour of an accelerated production of superoxide as opposed to the accelerated reduction of NBT is provided and we propose that the accelerated production of superoxide is a consequence of increased oxygen solubility in the lipid, rather than aqueous, phase. This is supported by: 1. The absence of any spectrophotometric changes due to interaction between lipid or LDL and reagents used. 2. The ability of micelles composed of a number of different fatty substances, including LDL, to accelerate superoxide generation, assessed by NBT reduction. 3. The behaviour of micelles, which appears to be one of substrate rather than catalyst, during the acceleration of NBT reduction. This is confirmed by the use of a known micellar catalyst, Triton-X100. This suggests that lipids contribute to the reaction as a substrate rather than a catalyst. 4. The inability of LDL to accelerate NBT reduction by potassium superoxide, a reaction which is independent of bimolecular oxygen. 5. The inability of LDL to accelerate NBT reduction when added after superoxide generation. 6. Studies that show LDL can enhance an NBT-independent monitor of oxidation, namely the transition metal-catalysed oxidation of vitamin C. 7. Estimations of the solubility of oxygen in LDL which appear to be consistent with reported physical measurements. Furthermore, we show that LDL modification can alter LDL-mediated micellar acceleration of superoxide generation. Extensive oxidation of LDL decreases micellar acceleration and minimal oxidation enhances it. We suggest that LDL micellar acceleration might serve as a novel approach to studying human LDL.

Activation of aldose reductase in rat lens and metal-ion chelation by aldose reductase inhibitors and lipoic acid

Sorbitol formation in rat lenses incubated with high levels of glucose was related to activation of aldose reductase (AR). The hyperglycaemia-activated aldose reductase was inhibited by alpha-lipoic (thioctic) acid, O-phenanthroline and aldose reductase inhibitors (ARIs) including Zeopolastat (ZPLS), Sorbinil (SBN) and AL-1576. This study also examined ARIs for the ability to chelate metal ions. We found that ARIs suppress copper-dependent ascorbate oxidation, lipid peroxidation and hydrogen peroxide production in erythrocytes. ARIs also increased partition of copper ions into noctanol, which indicates formation of lipophilic complexes. Our data support the hypothesis that transition metals may be involved in activation of the polyol (aldose reductase) pathway. Also, ARIs function as metal-chelating antioxidants that may contribute to their therapeutic role for diabetic complications.

Thioctic (lipoic) acid: a therapeutic metal-chelating antioxidant?

Thioctic (alpha-lipoic) acid (TA) is a drug used for the treatment of diabetic polyneuropathy in Germany. It has been proposed that TA acts as an antioxidant and interferes with the pathogenesis of diabetic polyneuropathy. We suggest that one component of its antioxidant activity requiring study is the direct transition metal-chelating activity of the drug. We found that TA had a profound dose-dependent inhibitory effect upon Cu(2+)-catalysed ascorbic acid oxidation (monitored by O2 uptake and spectrophotometrically at 265 nm) and also increased the partition of Cu2+ into n-octanol from an aqueous solution suggesting that TA forms a lipophilic complex with Cu2+. TA also inhibited Cu(2+)-catalysed liposomal peroxidation. Furthermore, TA inhibited intracellular H2O2 production in erythrocytes challenged with ascorbate, a process thought to be mediated by loosely chelated Cu2+ within the erythrocyte. These data, taken together, suggest that prior intracellular reduction of TA to dihydrolipoic acid is not an obligatory mechanism for an antioxidant effect of the drug, which may also operate via Cu(2+)-chelation. The R-enantiomer and racemic mixture of the drug (alpha-TA) generally seemed more effective than the S-enantiomer in these assays of metal chelation.

Erythrocyte catalase inactivation (H2O2 production) by ascorbic acid and glucose in the presence of aminotriazole: role of transition metals and relevance to diabetes

Erythrocytes exposed to ascorbic acid in the presence of aminotriazole undergo a dose- and time-dependent inactivation of endogenous catalase which is proportional to environmental hydrogen peroxide (H2O2) concentrations. The production of H2O2 seems to be dependent upon the availability of transition metal chelatable by o-phenanthroline (OPT), although the kinetics of catalase inactivation and H2O2 production by externally added copper ions in the presence of OPT is complex. Furthermore, although glucose is also able to undergo a transition-metal-catalysed oxidation yielding H2O2, the production of H2O2 by glucose seems to be a minor process by comparison with ascorbic acid oxidation. Indeed, on the basis of these data, transition-metal-catalysed ascorbic acid oxidation is likely to be a more important source of oxidative stress in the diabetic state than hyperglycaemia.

Selective loss of vascular smooth muscle cells in the retinal microcirculation of diabetic dogs

This study was undertaken to further characterise the fine structural changes occurring in the retinal circulation in early diabetes. The eyes of eight alloxan/streptozotocin and three spontaneously diabetic dogs were examined by trypsin digest and electron microscopy after durations of diabetes of between 1 and 7 years. Basement membrane (BM) thickening in the retinal capillaries was the only obvious fine structural change identified during the first 3 years of diabetes and was established within 1 year of induction. Widespread pericyte loss was noted after 4 years of diabetes and was paralleled by loss of smooth muscle (SM) cells, in the retinal arterioles. SM cell loss was most obvious in the smaller arterioles of the central retina. No microaneurysms were noted in the experimental diabetic dogs with up to 5 years' duration of diabetes but were widespread in a spontaneously diabetic animal at 7 years. This study has shown that SM cell loss, a hitherto unrecognised feature of diabetic microangiopathy, accompanies pericyte loss in the retinal circulation of diabetic dogs.