Formation of advanced glycation end (AGE) products in diabetes: prevention by pyruvate and alpha-keto glutarate

Inhibition of tumor suppressor p53 preserves glycation-serum induced pancreatic beta-cell demise

Tumor suppressor p53 is a transcriptional factor that determines cell fate in response to multiple stressors, such as oxidative stress and endoplasmic reticulum stress, in the majority of cells. However, its role in pancreatic beta cells is not well documented. Our previous research has revealed that glycation-serum (GS) induced pancreatic beta-cell demise through the AGEs-RAGE pathway. In the present study, we investigated the role of p53 in GS-related beta-cell demise. Using pancreatic islets beta-cell line INS-1 cells, we found that with GS treatment, the transcriptional activity of p53 was significantly evoked due to the increased amount of nuclear p53 protein. Resveratrol (RSV) was capable of further enhancing this transcriptional ability and consequently increased the population of dead beta cells under GS exposure. In contrast, inhibiting this transcriptional activity via p53 interference greatly protected beta cells from the damage provoked by GS, as well as damage strengthened by RSV. However, the pharmacological activation of PPARγ with troglitazone (TRO) only suppressed GS-induced, not RSV-induced, p53 activity. Moreover, the activation of PPARγ greatly preserved beta cells from GS-induced death. This protective effect recurred due to improved mitochondrial function with Bcl2 overexpression. Further, p53 activation could induce cellular apoptosis in primary rat islets. Our findings explore the broader role of p53 in regulating pancreatic beta-cell demise in the presence of GS and may provide a therapeutic target for the treatment and prevention of diabetes.

The domino effect: Role of hypoxia in malignant transformation of oral submucous fibrosis

INTRODUCTION

Oral submucous fibrosis (OSMF) is a precancerous condition predominantly seen in people of Asian descent. About 7-12% OSMF patients develop oral squamous cell carcinoma (OSCC). Morphological features of OSMF especially fibrosis suggests a possibility of the hypoxic environment in diseased tissues. Oral cancer usually develops from hyperplasia through dysplasia to carcinoma. Neovascularization and increased glycolysis, represent adaptations to a hypoxic microenvironment that are correlated with tumor invasion and metastasis. The adaptation of cells to hypoxia appears to be mediated via hypoxia inducible factor-1α (HIF-1α). HIF-1α is said to be associated with malignant transformation of epithelium in other sites. It appears that HIF-1α plays a significant role in both prostate and cervical carcinogenesis at early stages. We hypothesize that progression of OSMF and malignant transformation in the background of fibrosis mediates via HIF-1α either by up- or down-regulation of various such molecules. Therefore, the main objective of this study was to investigate the relationship between the expression of HIF-1α in OSMF, OSCC and OSCC with OSMF.

AIM

To investigate the relationship between the expression of HIF-1α in OSMF, OSCC and OSCC with OSMF.

MATERIALS AND METHODS

The study group consists of histopathologically diagnosed 20 cases of OSCC, oral submucous fibrosis and OSCC with OSMF each. The immunohistochemistry was carried out on neutral buffered formalin-fixed paraffin-embedded tissue sections by using the monoclonal antibody of HIF-1α.

RESULTS

A rise in the expression of HIF-1α from OSMF to OSCC to OSCC with OSMF is observed.

Metabolomics of oxidative stress in recent studies of endogenous and exogenously administered intermediate metabolites

Aerobic metabolism occurs in a background of oxygen radicals and reactive oxygen species (ROS) that originate from the incomplete reduction of molecular oxygen in electron transfer reactions. The essential role of aerobic metabolism, the generation and consumption of ATP and other high energy phosphates, sustains a balance of approximately 3000 essential human metabolites that serve not only as nutrients, but also as antioxidants, neurotransmitters, osmolytes, and participants in ligand-based and other cellular signaling. In hypoxia, ischemia, and oxidative stress, where pathological circumstances cause oxygen radicals to form at a rate greater than is possible for their consumption, changes in the composition of metabolite ensembles, or metabolomes, can be associated with physiological changes. Metabolomics and metabonomics are a scientific disciplines that focuse on quantifying dynamic metabolome responses, using multivariate analytical approaches derived from methods within genomics, a discipline that consolidated innovative analysis techniques for situations where the number of biomarkers (metabolites in our case) greatly exceeds the number of subjects. This review focuses on the behavior of cytosolic, mitochondrial, and redox metabolites in ameliorating or exacerbating oxidative stress. After reviewing work regarding a small number of metabolites—pyruvate, ethyl pyruvate, and fructose-1,6-bisphosphate—whose exogenous administration was found to ameliorate oxidative stress, a subsequent section reviews basic multivariate statistical methods common in metabolomics research, and their application in human and preclinical studies emphasizing oxidative stress. Particular attention is paid to new NMR spectroscopy methods in metabolomics and metabonomics. Because complex relationships connect oxidative stress to so many physiological processes, studies from different disciplines were reviewed. All, however, shared the common goal of ultimately developing “omics”-based, diagnostic tests to help influence therapies.

Inhibition of lipid peroxidation of herbal extracts (obtained from plant drug mixtures of Myrtilli folium, Phaseoli fructus sine seminibus and Salviae folium) used in type 2 diabetes mellitus

Plant drug mixtures are widely used in the adjuvant therapy of type 2 diabetes mellitus for the prevention of complications. The drug mixtures generally contribute to the efficiency of the therapy and may also reduce undesirable side effects. Two herbal extracts (lyophilized aqueous extracts of plant drug mixtures 1: Myrtilli folium, Phaseoli fructus sine seminibus and 2: Myrtilli folium, Phaseoli fructus sine seminibus, Salviae folium) were investigated in in vitro rat models. The content of bioactive constituents (polyphenol, flavonoid and vitamin C) in plant drug mixtures and lyophilized samples was evaluated. The antioxidant activity of lyophilized extracts was determined by measuring the ferric reducing ability of the plant, Fe2+ induced lipid peroxidation (LPO) in rat brain homogenates and NADPH (beta-nicotinamide adenine dinucleotide phosphate reduced form) induced LPO in cerebral microsomes. The antioxidant activity of lyophilized extracts was compared to that of quercetin and rutin. Both teas of lyophilized extracts had significant reducing ability (2694 and 2771 micromol/l) and inhibited LPO (IC50 28.0 and 20.6 microl in NADPH induced LPO, 17.3 and 8.7 microl in Fe2+ induced LPO). The high concentration of polyphenol/flavonoid (12.38-13.00 and 1.45-5.22 g/100 g, respectively) and vitamin C (0.099-0.165 g/100 g) in the herbal extracts is related to their significant antioxidant properties. The tea mixtures have significant nutritional value, since the consumption of 2 or 3 cups of tea a day covers 50% of the daily requirement of vitamin C and it is also relevant polyphenol source. The high polyphenol/flavonoid content may restore the redox imbalance and contribute to the prevention of diabetic complications.

Fructose-1,6-bisphosphate does not preserve ATP in hypoxic-ischemic neonatal cerebrocortical slices

Fructose-1,6-bisphosphate (FBP), an endogenous intracellular metabolite in glycolysis, was found in many preclinical studies to be neuroprotective during hypoxia-ischemia (HI) when administered exogenously. We looked for HI neuroprotection from FBP in a neonatal rat brain slice model, using 14.1 T (1)H/(31)P/(13)C NMR spectroscopy of perchloric acid slice extracts to ask: 1) if FBP preserves high energy phosphates during HI; and 2) if exogenous [1-(13)C]FBP enters cells and is glycolytically metabolized to [3-(13)C]lactate. We also asked: 3) if substantial superoxide production occurs during and after HI, thinking such might be treatable by exogenous FBP's antioxidant effects. Superfused P7 rat cerebrocortical slices (350 mum) were treated with 2 mM FBP before and during 30 min of HI, and then given 4 h of recovery with an FBP-free oxygenated superfusate. Slices were removed before HI, at the end of HI, and at 1 and 4 h after HI. FBP did not improve high energy phosphate levels or change (1)H metabolite profiles. Large increases in [3-(13)C]lactate were seen with (13)C NMR, but the lactate fractional enrichment was always (1.1+/-0.5)%, implying that all of lactate's (13)C was natural abundance (13)C, that none was from metabolism of (13)C-FBP. FBP had no effect on the fluorescence of ethidium produced from superoxide oxidation of hydroethidine. Compared to control slices, ethidium fluorescence was 25% higher during HI and 50% higher at the end of recovery. Exogenous FBP did not provide protection or enter glycolysis. Its use as an antioxidant might be worth studying at higher FBP concentrations.

Future alternative therapies in a quest to halt aberrations in diabetes mellitus

Under normal physiological conditions, euglycaemia is maintained principally by the homeostatic balance of insulin and glucagon which are secreted from the pancreas. In both type 1 and type 2 diabetes mellitus there is a substantial and chronic increase in the circulating glucose concentration. This elevation in glucose levels is accompanied by a plethora of other biochemical disturbances, including disruption of carbohydrate, fat and protein metabolism. Clinical manifestations of diabetes, which arise from the metabolic disturbances vary between individuals but are often a serious threat to quality and length of life. Pancreas transplantation (Tx) and islet modifications are methods used to restore endogenous insulin secretion in insulin-dependent diabetic patients. In order for this to be achieved successfully, however, some of the problems such as hyperglycemia states (> 150 mg/dl), which may harm pancreatic graft beta cells, immunorejection, the effects of immunosuppression, for example, must be overcome. Considering these problems, therefore, it seems logical that the replacement of the islet tissue itself, either by transplanting a vascularised pancreatic allograft or by transplanting modified pancreatic islet cells, provides a better alternative therapeutic approach than simply replacing insulin that has been lost. This review will show the recent development in the use of pancreatic islets and their modification in a quest to halt the aberrations seen in diabetes mellitus.

Inhibition of in vitro pyrraline formation by l-arginine and polyamines

Glycation of biomolecules such as proteins, nucleic acids and lipids leading to the formation of advanced glycation end products (AGEs) may be a major contributor to the pathological manifestations of diabetes mellitus. Several studies have shown that the chemical inhibition of AGEs formation results in attenuation of diabetic complications. We tested the in vitro inhibition of pyrraline formation on bovine serum albumin and L-lysine by L-arginine and the polyamines putrescine, cadaverine, spermidine and spermine. Among the inhibitors, L-arginine and spermine potently inhibited pyrraline formation. This effect could be related to the presence of the guanidino group in L-arginine and four amino groups in spermine, but this inhibitory effect was also shown by putrescine, cadaverine and spermidine, suggesting that these natural compounds may have a novel therapeutic potential in preventing diabetic complications. A significant unexpected observation emerged when experiments were carried out with aminoguanidine. It showed increased absorbance produced by a non-identified compound whose peak appears at 285 nm, but this aspect remains to be investigated.

Possible involvement of pyruvate kinase in acquisition of tolerance to hypoxic stress in glial cells

Neurons are highly vulnerable to ischemic/hypoxic stress, while glial cells show tolerance to such stress. However, the mechanisms for tolerance acquisition in glial cells have yet to be established. We attempted to isolate and identify a stress protein that is upregulated in response to hypoxia in human astrocytoma CCF-STTG1 cells. In particular, pyruvate kinase (PK) was upregulated by hypoxia in CCF-STTG1 cells. Hypoxia-inducible factor 1 (HIF-1), the primary transcription factor that is responsible for multiple gene activation under hypoxia, plays a critical role in PK expression during hypoxic challenge. To determine whether newly synthesized PK is involved in tolerance to hypoxic stress, we established the PK-overexpressing neuronal cells. Overexpression of the wild-type, but not the kinase-negative mutant, resulted in attenuation of the loss of cell viability and the typical apoptotic features by hypoxia or oxidative stress in SK-N-MC cells. These findings suggest that upregulation of PK may result in acquisition of tolerance against hypoxic stress, and that the antioxidant effect may be involved in the protective effect of PK.

Hypoxia-inducible factor 1 activation by aerobic glycolysis implicates the Warburg effect in carcinogenesis

Cancer cells display high rates of aerobic glycolysis, a phenomenon known historically as the Warburg effect. Lactate and pyruvate, the end products of glycolysis, are highly produced by cancer cells even in the presence of oxygen. Hypoxia-induced gene expression in cancer cells has been linked to malignant transformation. Here we provide evidence that lactate and pyruvate regulate hypoxia-inducible gene expression independently of hypoxia by stimulating the accumulation of hypoxia-inducible Factor 1alpha (HIF-1alpha). In human gliomas and other cancer cell lines, the accumulation of HIF-1alpha protein under aerobic conditions requires the metabolism of glucose to pyruvate that prevents the aerobic degradation of HIF-1alpha protein, activates HIF-1 DNA binding activity, and enhances the expression of several HIF-1-activated genes including erythropoietin, vascular endothelial growth factor, glucose transporter 3, and aldolase A. Our findings support a novel role for pyruvate in metabolic signaling and suggest a mechanism by which high rates of aerobic glycolysis can promote the malignant transformation and survival of cancer cells.

Inhibitors of advanced glycation end product-associated protein cross-linking

The reaction of lens proteins with sugars over time results in the formation of protein-bound advanced glycation end products (AGEs). The most damaging element of AGE formation may be the synthesis of protein-protein cross-links in long-lived proteins, such as collagen or lens crystallins. A quantitative cross-linking assay, involving the sugar-dependent incorporation of [U-(14)C]lysine into protein, was employed to determine the efficacy of a variety of potential cross-linking inhibitors. Reaction mixtures contained 5.0 mM L-threose, 2.5 microCi [(14)C]lysine (1.0 mCi/mmole), 5.0 mg/ml bovine lens proteins, 0-10 mM inhibitor and 1.0 mM DTPA in 100 mM phosphate buffer, pH 7.0. Of 17 potential inhibitors tested, 11 showed 50% inhibition or less at 10 mM. The dicarbonyl-reactive compounds 2-aminoguanidine, semicarbazide and o-phenylenediamine inhibited 50% at 2.0 mM, whereas 10 mM dimethylguanidine had no effect. Several amino acids failed to compete effectively with [(14)C]lysine in the cross-linking assay; however, cysteine inhibited 50% at 1.0 mM. This was likely due to the sulfhydryl group of cysteine, because 3-mercaptopropionic acid and reduced glutathione exhibited similar activity. Sodium metabisulfite had the highest activity, inhibiting 50% at only 0.1-0.2 mM. Protein dimer formation, as determined by SDS-PAGE, was inhibited in a quantitatively similar manner. The dicarbonyl-reactive inhibitors and the sulfur-containing compounds produced similar inhibition curves for [(14)C]lysine incorporation over a 3 week assay with 250 mM glucose. A much lesser effect was observed on either the incorporation of [(14)C]glucose, or on fluorophore formation (360/420 nm), suggesting that non-cross-link fluorophores were also formed. The inhibitor data were consistent with cross-linking by a dicarbonyl intermediate. This was supported by the fact that the inhibitors were uniformly less effective when the 5.0 mM threose was replaced by either 3.0 mM 3-deoxythreosone or 3.0 mM threosone.

Fructose induced deactivation of antioxidant enzymes: preventive effect of pyruvate

Glycation initiated changes in tissue proteins, which are triggered by the Schiff base formation between the sugar carbonyl and the protein -NH2, have been suggested to play an important role in the development of diabetes-related pathological changes such as the formation of cataracts. While the initial reaction takes place by the interaction of >C=O of the parent sugars with the -NH2 of proteins, reactive oxygen species (ROS) dependent generation of more reactive dicarbonyl derivatives from the oxidation of sugars also plays a significant role in these changes, altering the structural as well as functional properties of proteins. The purpose of this study was to examine whether the activities of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), catalase and superoxide dismutase (SOD) could be affected by the high levels of fructose prevalent in diabetic lenses. Incubation of the enzymes with this sugar led to a significant loss of their activities. GAPDH was inactivated within a day. This was followed by the inactivation of catalase (3-4 days) and SOD (6 days). The loss of the activities was prevented significantly by incorporation of pyruvate in the incubation mixture. The protective effect is ascribable to its ability to competitively inhibit glycation as well as to its ROS scavenging activity. Hence, it could play a significant role in the maintenance of lens physiology and cataract prevention.

Diabetes-induced biochemical changes in rat lens: attenuation of cataractogenesis by pyruvate

AIM

Studies have been conducted to determine the effect of pyruvate administration on the biochemistry of rat lens and the status of its transparency as affected by diabetic conditions.

METHODS

Sprague-Dawley rats were rendered diabetic by intravenous (i.v.) injection of streptozotocin (55 mg/kg body weight (b.w.)) and treated with sodium pyruvate (2%) in drinking water. The levels of glucose, fructose, sorbitol, ATP, GSH, MDA as well as glycated proteins in the lenses were determined at various intervals after the onset of diabetes and the values compared with untreated diabetic controls. The progress of cataract formation and associated histological changes in the tissue were also monitored.

RESULTS

Studies show that the pyruvate treatment decreased the extent of several biochemical changes known to be associated with cataract formation, such as the elevation in the levels of glycated proteins, sorbitol, lipid peroxidation (MDA) and inhibition of the cation pump. The progress of cataract was also significantly delayed.

CONCLUSION

Exogenous administration of this compound hence was found to exert an overall protective effect against cataract formation induced by the diabetic conditions.

Fructose-mediated damage to lens alpha-crystallin: prevention by pyruvate

Post-translational modifications in lens crystallins due to glycation and oxidation have been suggested to play a significant role in the development of cataracts associated with aging and diabetes. We have previously shown that alpha-keto acids, like pyruvate, can protect the lens against oxidation. We hypothesize that they can also prevent the glycation of proteins competitively by forming a Schiff base between their free keto groups and the free -NH(2) groups of protein as well as subsequently inhibit the oxidative conversion of the initial glycation product to advanced glycation end products (AGE). The purpose of this study was to investigate these possibilities using purified crystallins. The crystallins isolated from bovine lenses were incubated with fructose in the absence and presence of pyruvate. The post-incubation mixtures were analyzed for fructose binding to the crystallins, AGE formation, and the generation of high molecular weight (HMW) proteins. In parallel experiments, the keto acid was replaced by catalase, superoxide dismutase (SOD), or diethylene triaminepentaacetic acid (DTPA). This was done to ascertain oxidative mode of pyruvate effects. Interestingly, the glycation and consequent formation of AGE from alpha-crystallin was more pronounced than from beta-, and gamma-crystallins. The changes in the crystallins brought about by incubation with fructose were prevented by pyruvate. Catalase, SOD, and DTPA were also effective. The results suggest that pyruvate prevents against fructose-mediated changes by inhibiting the initial glycation reaction as well as the conversion of the initial glycated product to AGE. Hence it is effective in early as well as late phases of the reactions associated with the formation of HMW crystallin aggregates.

Attenuation of sugar cataract by ethyl pyruvate

Studies describe an attenuation of sugar cataract formation by topical administration of ethyl pyruvate. Cataract formation was induced by feeding young rats a 30% galactose diet. Mature cataracts appeared in about thirty days. Instillation of the eye drops containing 5% ethyl pyruvate decelerated the process significantly. Biochemically, the effect was reflected by lowering in the contents of dulcitol and glycated proteins. The ATP levels were also higher in comparison to the placebo treated group. The effects are hence attributable to the effect of pyruvate in inhibiting dulcitol synthesis and protein glycation, in addition to its antioxidant properties and metabolic support. The use of esterified pyruvate instead of the unesterified pyruvate was preferred because of its greater penetration through the cornea and consequently a higher concentration attained in the aqueous humor.

Glucose-induced oxidative stress and programmed cell death in diabetic neuropathy

The Diabetes Control and Complications Trial (DCCT) established the importance of hyperglyemia and other consequences of insulin deficiency in the pathogenesis of diabetic neuropathy, but the precise mechanisms by which metabolic alterations produce peripheral nerve fiber damage and loss remain unclear. Emerging data from human and animal studies suggest that glucose-derived oxidative stress may play a central role, linking together many of the other currently invoked pathogenetic mechanisms such as the aldose reductase and glycation pathways, vascular dysfunction, and impaired neurotrophic support. These relationships suggest combinations of pharmacological interventions that may synergistically protect the peripheral nervous system (PNS) against the metabolic derangements of diabetes mellitus.

Non-enzymatic glycation of lens proteins and haemoglobin-inhibition by pyruvate: an in-vivo study

AIM

Previous studies have demonstrated that pyruvate can prevent protein glycation and oxidative stress under in-vitro conditions. The aim of this study was to examine the in-vivo effectiveness of this metabolite against glycation of lens crystallins and haemoglobin in galactosemic rats.

METHODS

Sprague-Dawley rats were maintained on a 30% or 50% galactose-containing diet in the absence or presence of 2% or 5% pyruvate in food and water, respectively. The animals were killed subsequently and the extent of glycation of lens crystallins and haemoglobin was determined using an affinity column chromatograpic technique.

RESULTS

Maintenance of rats on the high galactose diet resulted in a significant increase in glycation of both the proteins. The increase was faster and more substantial in the animals maintained on the 50% galactose diet than that in the animals fed a 30% galactose diet. The increase in the latter was also very significant. Supplementation with pyruvate inhibited the process.

CONCLUSION

The inhibition is attributable to a competitive binding of pyruvate to the protein NH2 groups as well as to the antioxidant effect of the compound. The studies therefore suggest that this and other alpha-keto-acids may be physiologically useful in minimizing glycation and oxidative stress induced tissue pathology by the hyperglycaemic conditions, such as diabetes and galactosemia. The results are also considered pharmacologically significant.

Attenuation of galactose-induced cataract by pyruvate

Data in the present paper demonstrate a significant inhibition in the progress of sugar cataract formation by systemic administration of pyruvate. The formation of the cataract was induced by feeding young rats a diet containing 30% galactose. All animals fed this diet developed nuclear lens opacity by the end of 30 days. This was delayed if the diet and water contained, in addition, 2% sodium pyruvate. The incidence of cataract in the latter group was 0% at day 30 and only 25% at day 55. Physiologically, the inhibition was associated with the prevention of lens membrane damage as reflected by its ability to maintain transport of rubidium ions against a concentration gradient; decreased tissue hydration as indexed by the lens wet weight; inhibition of protein glycation, and higher levels of ATP. Since pyruvate, being a normal tissue metabolite, is likely to be non-toxic, the findings are considered useful for further pharmacological studies with this and other similar metabolites, relevant to protection against various secondary complications of diabetes and galactosemia.

A review of current research on the effect of diabetes mellitus on the eye

It is estimated that almost one million Australians will have diabetes by the year 2000. Of those with diabetes a significant proportion will have eye-related conditions, the most debilitating being diabetic retinopathy. Appropriate identification and treatment can result in prevention of visual loss and blindness. The importance of diabetes as a cause of blindness in our community is realised by the commencement of a national program by the National Health and Medical Research Council to develop clinical practice guidelines for the management of diabetic retinopathy. The development of these guidelines was based on available evidence following an extensive review of the literature up to May 1996. This review is a summary of our advances in research on the effect of diabetes on various aspects of the eye and vision over the past two years. This review is a compilation of articles of research on the effect of diabetes on various aspects of the eye and vision. As a result of the enormous amount of effort and work by scientists and clinicians around the world, as well as space restrictions, the review covers the past two years only. Although every effort has been made to include as many research articles as possible, not all articles of research are covered. It is intended that this review provide an overview of the latest trends in research, particularly relating to new techniques and methods in the study of diabetes in ocular tissue as well as the new theories in the development of ocular damage to each of the tissue.

Mechanisms of maturation and ageing of collagen

The deleterious age-related changes in collagen that manifest in the stiffening of the joints, the vascular system and the renal and retinal capillaries are primarily due to the intermolecular cross-linking of the collagen molecules within the tissues. The formation of cross-links was elegantly demonstrated by Verzar over 40 years ago but the nature and mechanisms are only now being unravelled. Cross-linking involves two different mechanisms, one a precise enzymically controlled cross-linking during development and maturation and the other an adventitious non-enzymic mechanism following maturation of the tissue. It is this additional non-enzymic cross-linking, known as glycation, involving reaction with glucose and subsequent oxidation products of the complex, that is the major cause of dysfunction of collagenous tissues in old age. The process is accelerated in diabetic subjects due to the higher levels of glucose. The effect of glycation on cell-matrix interactions is now being studied and may be shown to be an equally important aspect of ageing of collagen. An understanding of these mechanisms is now leading to the development of inhibitors of glycation and compounds capable of cleaving the cross-links, thus alleviating the devastating effects of ageing.

Fructose induced deactivation of glucose-6-phosphate dehydrogenase activity and its prevention by pyruvate: implications in cataract prevention

Glucose-6-phosphate dehydrogenase (G6PDH) is an important lens enzyme diverting about 14% of the tissue glucose to the hexose monophosphate shunt pathway. The main function of such a pronounced activity of the enzyme is to support reductive biosyntheses, as well as to maintain a reducing environment in the tissue so as to prevent oxy-radical induced damage and consequent cataract formation. Sugars are one of the well-known cataractogenic agents. Several reports suggest that the cataractogenic effect of the sugars in diabetes as well as in normal aging is initiated by the glycation of the proteins including the enzymes and subsequent formation of more complex and biologically inactive or harmful structures. In a diabetic lens the concentration of fructose exceeds significantly the concentration of glucose, suggesting that the contribution of fructosylation may be greater than that of glucosylation. These studies were undertaken to examine further the possibility that in addition to glycation, generation of oxygen free radicals by fructose and consequent oxidative modifications in certain enzymes may be an important participant in the cataractogenic process. This hypothesis was tested by using G6PDH. The enzyme was incubated with various levels of fructose (0-20mM) and its activity determined as a function of time. This led to a significant loss of its activity, which was prevented by superoxide dismutase, catalase, mannitol and myoinositol. Most interestingly, pyruvate at levels between 0.2 and 1.0 mM also offered substantial protection. Hence, the results, while elucidating further the mechanism of enzyme deactivation by sugars such as fructose, also demonstrate the possibility of therapeutic prevention of cataracts by pyruvate and other such keto acids, in diabetes and other disabilities involving oxygen free radicals in the pathogenetic process.

Prevention of lens protein glycation by taurine

Modifications in lens protein structure and function due to nonenzymic glycosylation and oxidation have been suggested to play a significant role in the pathogenesis of sugar and senile cataracts. The glycation reaction involves an initial Schiff base formation between the protein NH2 groups and the carbonyl group of a reducing sugar. The Schiff base then undergoes several structural modifications, via some oxidative reactions involving oxygen free radicals. Hence certain endogenous tissue components that may inhibit the formation of protein-sugar adduct formation may have a sparing effect against the cataractogenic effects of sugars and reactive oxygen. The eye lens is endowed with significant concentration of taurine, a sulfonated amino acid, and its precursor hypotaurine. It is hypothesized that taurine and hypotaurine may have this purported function of protecting the lens proteins against glycation and subsequent denaturation, in addition to their other functions. The results presented herein suggest that these compounds are indeed capable of protecting glycation competitively by forming Schiff bases with sugar carbonyls, and thereby preventing the glycation of lens proteins per se. In addition, they appear to prevent oxidative damage by scavenging hydroxyl radicals. This was apparent by their preventive effect against the formation of the thiobarbituric acid reactive material generated from deoxy-ribose, when the later was exposed to hydroxyl radicals generated by the action of xanthine oxidase on hypoxanthine in presence of iron.