alpha-crystallin protects glucose 6-phosphate dehydrogenase against inactivation by malondialdehyde

Inactivation of human glucose 6-phosphate dehydrogenase (G6PDH) by peroxyl radicals is strongly modulated by its substrate and cofactor

Glucose 6-phosphate dehydrogenase (G6PDH) is the rate-limiting enzyme of the pentose phosphate pathway (PPP). This enzyme catalyzes the oxidation of glucose 6-phosphate (G6P) into 6-phosphogluconolactone with concomitant reduction of NADP+ to NADPH. Despite the link between the PPP and oxidative stress, the oxidation and consequences on the activity of the human G6PDH (hG6PDH) has not been investigated. In the present work we report the oxidative inactivation of hG6PDH mediated by peroxyl radicals (ROO•) generated by AAPH (2,2'-azobis(2-methylpropionamidine) dihydrochloride) thermolysis. hG6PDH (46.4 μM, monomers) was incubated at 37 °C with 10 or 100 mM AAPH. At defined times, enzyme activity was determined (NADPH release followed at 340 nm), mapping of modifications studied by LC-MS, structural changes analyzed by circular dichroism, and results rationalized by in silico analysis of the three-dimensional structure of the enzyme. Analogous experiments were developed in the presence of NADP+ or G6P at excess or 1:1 (hG6PDH:NADP+ or G6P) molar ratios. High susceptibility to inactivation by ROO• was observed, 3.6 mol of ROO• inactivated 1 mol of hG6PDH. This behavior is rationalized, at least in part, by oxidation at Trp349 which is located close to the structural site of NADP+. The presence of G6P significantly increased the ROO•-mediated inactivation of hG6PDH, while an opposite effect was observed in the presence of NADP+ where, despite oxidation at different sites, the enzyme activity was practically unaltered by ROO•. These results demonstrate that hG6PDH is highly susceptible to inactivation mediated by ROO• with these processes strongly modulated by G6P and NADP+.

The enzymes of the oxidative phase of the pentose phosphate pathway as targets of reactive species: consequences for NADPH production

The pentose phosphate pathway (PPP) is a key metabolic pathway. The oxidative phase of this process involves three reactions catalyzed by glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconolactonase (6PGL) and 6-phosphogluconate dehydrogenase (6PGDH) enzymes. The first and third steps (catalyzed by G6PDH and 6PGDH, respectively) are responsible for generating reduced nicotinamide adenine dinucleotide phosphate (NAPDH), a key cofactor for maintaining the reducing power of cells and detoxification of both endogenous and exogenous oxidants and electrophiles. Despite the importance of these enzymes, little attention has been paid to the fact that these proteins are targets of oxidants. In response to oxidative stimuli metabolic pathways are modulated, with the PPP often up-regulated in order to enhance or maintain the reductive capacity of cells. Under such circumstances, oxidation and inactivation of the PPP enzymes could be detrimental. Damage to the PPP enzymes may result in a downward spiral, as depending on the extent and sites of modification, these alterations may result in a loss of enzymatic activity and therefore increased oxidative damage due to NADPH depletion. In recent years, it has become evident that the three enzymes of the oxidative phase of the PPP have different susceptibilities to inactivation on exposure to different oxidants. In this review, we discuss existing knowledge on the role that these enzymes play in the metabolism of cells, and their susceptibility to oxidation and inactivation with special emphasis on NADPH production. Perspectives on achieving a better understanding of the molecular basis of the oxidation these enzymes within cellular environments are given.

The effect of systemic amantadine sulfate on malondialdehyde and total thiol levels in rat corneas

Purpose:

To evaluate the malondialdehyde (MDA) and total thiol (sulfhydryl, SH) levels in rat corneas after intraperitoneal injection of amantadine sulfate.

Methods:

A total of 12 Wistar albino rats were divided into two groups: control group (n = 6) and amantadine group (n = 6). Balanced salt solution (1 mL, 0.9% NaCl, twice/day) was injected into rats in control group. Amantadine sulfate (2 mg/1 mL, twice/day) was injected into rats in amantadine group. In each group, two rats were injected for 1 week, two received injections for 1 month, and two rats received injections for 3 months. The corneas were homogenized and MDA and SH levels were measured spectroflourometrically.

Results:

In control group, median MDA and SH levels were 2.37 (range, 0.92-3.60) and 25.35 (range, 6.30-54.0) nmol/mg, respectively. In amantadine group, median MDA and SH levels were 3.57 (range, 1.25-5.92) and 32.65 (range, 3.30-48.3) nmol/mg, respectively. The difference between this two groups regarding MDA (P = 0.14) and SH (P = 1.0) levels was statistically insignificant.

Conclusion:

Systemically administered amantadine sulfate seems not to cause MDA and SH imbalance in rat corneas.

GAPDH in anesthesia

Thus far, two independent laboratories have shown that inhaled anesthetics directly affect GAPDH structure and function. Additionally, it has been demonstrated that GAPDH normally regulates the function of GABA (type A) receptor. In light of these literature observations and some less direct findings, there is a discussion on the putative role of GAPDH in anesthesia. The binding site of inhaled anesthetics is described from literature reports on model proteins, such as human serum albumin and apoferritin. In addition to the expected hydrophobic residues that occupy the binding cavity, there are hydrophilic residues at or in very close proximity to the site of anesthetic binding. A putative binding site in the bacterial analog of the human GABA (type A) receptor is also described. Additionally, GAPDH may also play a role in anesthetic preconditioning, a phenomenon that confers protection of cells and tissues to future challenges by noxious stimuli. The central thesis regarding this paradigm is that inhaled anesthetics evoke an intra-molecular protein dehydration that is recognized by the cell, eliciting a very specific burst of chaperone gene expression. The chaperones that are implicated are associated with conferring protection against dehydration-induced protein aggregation.

Identification of the preferentially targeted proteins by carbamylation during whole lens incubation by using radio-labelled potassium cyanate and mass spectrometry

AIM

To attempt to identify the primary targets of carbamylation in bovine lenses incubated under physiological condition.

METHODS

Fresh intact bovine lenses were incubated with [(14)C]-labelled potassium cyanate for seven days. The water-soluble proteins (WSP) of both cortex and nucleus lens were isolated by size-exclusion chromatography on a Sephacryl S-300HR column. The higher radioactive fractions were pooled and freeze-dried, and separated further by loading on an Affinity Blue column to separate some enzymes. In addition, WSP from cortex was separated directly by affinity chromatography. The most reactive fractions with higher radioactivity from [(14)C]-cyanate were further analyzed by SDS-gels and mass spectrometry.

RESULTS

The majority of protein incorporating [(14)C]-labelled potassium cyanate was in the water-soluble fractions, and much more in the cortex than in the nucleus. Chromatography results demonstrated that the major incorporated [(14)C]-carbamylated crystallins were fractions corresponding to α-crystallin, β-crystallin and ξ-crystallin in the cortex, but β-crystallin and γ-crystallin in the nucleus. The SDS gels showed that bound fractions of cortex crystallins after Affi-Gel Blue separation were abundant with 20 and 35kDa proteins. However, the bound fractions of nucleus crystallins mainly showed 20kDa proteins. Mass spectrometry analysis of these higher radioactivity fractions and a database search revealed that the proteins were originated from bovine α-crystallin A and B chains and ξ-crystallin in the cortex; βA1 and αB-crystallins with a little γB-crystallin in the nucleus respectively. Further analysis suggested the location of this carbamylation of αB-crystallin in the nucleus to be at Lys 92 and 103.

CONCLUSION

α-and ξ-crystallin from cortex can be preferentially targeted by carbamylation during whole lens incubations. Carbamylation of these crystallins at the earlier stage may result in further unfolding and misfolding of lens proteins, leading to aggregation of crystallins and eventually to cataract formation.

Exposure to chrysotile asbestos causes carbonylation of glucose 6-phosphate dehydrogenase through a reaction with lipid peroxidation products in human lung epithelial cells

Exposure to asbestos is known to lead to a reduction in glucose 6-phosphate dehydrogenase (G6PDH) activity and to cause oxidative damage to cells. In the present study, we exposed the human lung carcinoma cell line A549 to chrysotile. We observed an increase in the production of thiobarbituric acid-reactive substances (TBARS, the breakdown products of lipid peroxide) along with a significant decrease in G6PDH activity. Alternatively, when chrysotile was added directly to the cell extract obtained by removing the cell membrane, no loss of G6PDH activity was observed. To elucidate the mechanism of G6PDH inactivation due to exposure to chrysotile, we focused on the TBARS responsible for protein modification via carbonylation. When malondialdehyde or 4-hydroxy-2-nonenal was added to a membrane-free A549 cell extract, G6PDH activity was reduced markedly. However, when t-butylhydroperoxide was added to the extract, there was no significant decrease in G6PDH activity. Western blot analysis and immunoprecipitation of the carbonylated proteins in the A549 cell lysate that was prepared after exposure to chrysotile demonstrated that G6PDH had been carbonylated. Our findings indicate that the decrease in G6PDH activity that occurs after exposure of the cultured cells to chrysotile results from the carbonylation of G6PDH by TBARS.

Glutathione-Related Enzymes and the Eye

Glutathione and the related enzymes belong to the defence system protecting the eye against chemical and oxidative stress. This review focuses on GSH and two key enzymes, glutathione reductase and glucose-6-phosphate dehydrogenase in lens, cornea, and retina. Lens contains a high concentration of reduced glutathione, which maintains the thiol groups in the reduced form. These contribute to lens complete transparency as well as to the transparent and refractive properties of the mammalian cornea, which are essential for proper image formation on the retina. In cornea, gluthatione also plays an important role in maintaining normal hydration level, and in protecting cellular membrane integrity. In retina, glutathione is distributed in the different types of retinal cells. Intracellular enzyme, glutathione reductase, involved in reducing the oxidized glutathione has been found at highest activity in human and primate lenses, as compared to other species. Besides the enzymes directly involved in maintaining the normal redox status of the cell, glucose-6-phosphate dehydrogenase which catalyzes the first reaction of the pentose phosphate pathway, plays a key role in protection of the eye against reactive oxygen species. Cornea has a high activity of the pentose phosphate pathway and glucose-6-phosphate dehydrogenase activity. Glycation, the non-enzymic reaction between a free amino group in proteins and a reducing sugar, slowly inactivates gluthathione-related and other enzymes. In addition, glutathione can be also glycated. The presence of glutathione, and of the related enzymes has been also reported in other parts of the eye, such as ciliary body and trabecular meshwork, suggesting that the same enzyme systems are present in all tissues of the eye to generate NADPH and to maintain gluthatione in the reduced form. Changes of glutathione and related enzymes activity in lens, cornea, retina and other eye tissues, occur with ageing, cataract, diabetes, irradiation and administration of some drugs.

Thermally induced and developmentally regulated expression of a small heat shock protein in Trichinella spiralis

A cDNA encoding a small heat shock protein of Trichinella spiralis, Ts-sHsp, was cloned and expressed and is herein characterized. This cDNA encoded a predicted protein of 165 amino acids, which had a high sequence identity in alpha crystallin domain with various small heat shock proteins of other organisms. A Western blot analysis indicated that anti-Ts-sHsp recombinant antibody recognized the protein of adults and larvae migrating at about 19 kDa. An in situ localization study showed the protein to be abundantly present in the body wall muscle cells, hypodermis, stichocytes, and esophagus of muscle larvae. The Ts-sHsp recombinant protein possessed chaperone activity to suppress the thermally-induced aggregation of citrate synthase. This sHsp was expressed at various developmental stages of T. spiralis, but at different levels. A high level was observed in mature muscle larvae (infective larvae), which was much higher than the levels seen in adults, newborn larvae, or immature muscle larvae. The expression of the sHsp gene was thermal inducible, thus responding to both cold (0 degrees C) and heat shock (43 degrees C) stress; however, at different patterns. The expression of Ts-sHsp increased gradually from 3 to 72 h after cold stress, while the expression was elevated to its highest after 3 h heat stress and then decreased. These results suggest that this small heat shock protein likely plays a role in the tolerance to both chemical and physical stresses, thereby enhancing the survival ability of Trichinella muscle larvae.

Mechanisms involved in the protection of UV-induced protein inactivation by the corneal crystallin ALDH3A1

Various lines of evidence have shown that ALDH3A1 (aldehyde dehydrogenase 3A1) plays a critical and multifaceted role in protecting the cornea from UV-induced oxidative stress. ALDH3A1 is a corneal crystallin, which is defined as a protein recruited into the cornea for structural purposes without losing its primary function (i.e. metabolism). Although the primary role of ALDH3A1 in the metabolism of toxic aldehydes has been clearly demonstrated, including the detoxification of aldehydes produced during UV-induced lipid peroxidation, the structural role of ALDH3A1 in the cornea remains elusive. We therefore examined the potential contribution of ALDH3A1 in maintaining the optical integrity of the cornea by suppressing the aggregation and/or inactivation of other proteins through chaperone-like activity and other protective mechanisms. We found that ALDH3A1 underwent a structural transition near physiological temperatures to form a partially unfolded conformation that is suggestive of chaperone activity. Although this structural transition alone did not correlate with any protection, ALDH3A1 substantially reduced the inactivation of glucose-6-phosphate dehydrogenase by 4-hydroxy-2-nonenal and malondialdehyde when co-incubated with NADP(+), reinforcing the importance of the metabolic function of this corneal enzyme in the detoxification of toxic aldehydes. A large excess of ALDH3A1 also protected glucose-6-phosphate dehydrogenase from inactivation because of direct exposure to UVB light, which suggests that ALDH3A1 may shield other proteins from damaging UV rays. Collectively, these data demonstrate that ALDH3A1 can reduce protein inactivation and/or aggregation not only by detoxification of reactive aldehydes but also by directly absorbing UV energy. This study provides for the first time mechanistic evidence supporting the structural role of the corneal crystallin ALDH3A1 as a UV-absorbing constituent of the cornea.

Protection against glycation and similar post-translational modifications of proteins

Glycation and other non-enzymic post-translational modifications of proteins have been implicated in the complications of diabetes and other conditions. In recent years there has been extensive progress in the search for ways to prevent the modifications and prevent the consequences of the modifications. These areas are covered in this review together with newer ideas on possibilities of reversing the chemical modifications.

Verminderung der Lipidperoxidation und der Apoptoserate in kornealen Endothelzellen durch Vitamin A

PURPOSE

The goal of this study was to determine the effects of lipid peroxidation-mediated toxicity of iron ions on corneal endothelial cells leading to apoptosis.

METHODS

Murine corneal endothelial cells were maintained in tissue culture medium supplemented with free iron ions, known to lead to increased lipid peroxidation. Retinoic acid in the cell supernatant and cytoplasm of these cells was determined using HPLC. The rate of apoptosis was assessed by quantification of caspase-3-like activity. The lipid peroxidation was measured using the malondialdehyde method. Supplementation of retinoic acid was tested in the setting of apoptosis.

RESULTS

Free iron ions led to a rapid loss of retinoic acid in the supernatant and the corneal endothelial cells. This was correlated with rising levels of malondialdehyde following oxidative stress and increased apoptosis. Supplementation of retinoic acid alone significantly reduced oxidative stress and apoptosis in the respective cells.

CONCLUSION

In this study the authors present a novel in vitro model to test the direct influence of pro-oxidative species on corneal endothelial cells. The authors also prove that supplementing corneal endothelial cells with retinoic acid sufficiently prevents free radical injury and apoptosis.

Anti-oxidative vitamins prevent lipid-peroxidation and apoptosis in corneal endothelial cells

To determine the effects of vitamin supplementation on the lipid-peroxidation-mediated toxicity of iron-ions on corneal endothelial cells (CECs) leading to apoptosis, murine CECs were maintained in tissue culture medium supplemented with increasing concentrations of free iron-ions, a treatment known to lead to increased lipid-peroxidation. The concentration of anti-oxidative vitamins (ascorbic acid, tocopherol and retinoic acid) in the cell supernatant and in the cells was determined by high-pressure liquid chromatography. Apoptosis was assessed by quantification of caspase-3-like activity and by using annexin-V/propidium iodide stains for flow cytometry. Lipid-peroxidation was measured by the malondialdehyde method. Supplementation with anti-oxidative vitamins was tested for the ability to counteract the induction of apoptosis. The production of nitric oxide was assessed spectrophotometrically and the expression levels of inducible and endothelial nitric oxide synthase were determined by Western blot. Increasing levels of free iron led to a rapid loss of anti-oxidative vitamins in the supernatant and in the CECs. This was correlated with rising levels of malondialdehyde and increased apoptosis. Supplementation with ascorbic acid or alpha-tocopherol alone did not prevent lipid-peroxidation in the cells. A combination of vitamins C and E (ascorbic acid, tocopherol) or solitary supplementation with vitamin A (retinoic acid) prevented lipid-peroxidation. We thus present a novel in vitro model for testing the direct influence of pro-oxidative species on CECs. We also show that supplementation with anti-oxidative vitamins to CECs significantly prevents the generation of free-radical-induced oxidative injury and apoptosis. These findings may have important implications for the storage of human corneae prior to transplantation and for the prolongation of corneal graft survival.

Long-term lens organ culture system to determine age-related effects of UV irradiation on the eye lens

Aging of the eye lens represents the life-long accumulation of damage. Factors responsible for age-related cataract are unknown because medical evaluations of aged populations demonstrate a wide range of systemic diseases and medical disorders. There are some main suspected factors, which may contribute to accumulated age-related damage in the eye lens. (1) Diseases, such as diabetes, substantially increase the probability of cataract formation in the age group from 40 to 49, and double or triple this probability for ages 50 to 69. (2) Drugs, including systemic medications such as steroids. (3) Environmental factors, such as UV radiation, heat and electromagnetic radiation. Our study represents an effort to determine the effects of suspected cataractogenic factors on the eye lens. The experiments are performed using a unique long-term lens organ culture system of bovine lenses. In our system it is possible to give controlled amounts of insult and monitor changes in lens optical quality throughout the culture period of 8-15 days. The optical properties, monitored in association with biochemical analysis of lens epithelium, cortex and nuclear samples, help in determining the mechanisms of cataract formation. The present study investigates mechanisms by which UV-A radiation at 365 nm causes damage to the lens. It is believed that solar radiation is one of the major environmental factors involved in lens cataractogenesis. Bovine lenses were placed in our special culture cells for pre-incubation of 24 hr followed by irradiation of 29 or 33 J cm(-2). The lenses were maintained in the cells during irradiation. After irradiation, lens optical quality was monitored throughout the culture period and lens epithelium was taken for enzyme analysis. Using the culture system we learned that: (a) young lenses (less than one-year-old) are less sensitive to UV radiation than 3-year-old lenses; (b) the lenses have the ability to recover in organ culture conditions; (c) applying the insult in one step results in less damage than dividing the same insult in 4 steps with 24 hr interval between each one; and (d) the damage from UV is greater if the intervals between each irradiation stage are insufficient to permit full recovery.

Oxidative stress and seasonal coral bleaching

During the past two decades, coral reefs have experienced extensive degradation worldwide. One etiology for this global degradation is a syndrome known as coral bleaching. Mass coral bleaching events are correlated with increased sea-surface temperatures, however, the cellular mechanism underlying this phenomenon is uncertain. To determine if oxidative stress plays a mechanistic role in the process of sea-surface temperature-related coral bleaching, we examined corals along a depth transect in the Florida Keys over a single season that was characterized by unusually high sea-surface temperatures. We observed strong positive correlations between accumulation of oxidative damage products and bleaching in corals over a year of sampling. High levels of antioxidant enzymes and small heat-shock proteins were negatively correlated with levels of oxidative damage products. Corals that experienced oxidative stress had higher chaperonin levels and protein turnover activity. Our results indicate that coral bleaching is tightly coupled to the antioxidant and cellular stress capacity of the symbiotic coral, supporting the mechanistic model that coral bleaching (zooxanthellae loss) may be a final strategy to defend corals from oxidative stress.

Escherichia coli small heat shock proteins, IbpA and IbpB, protect enzymes from inactivation by heat and oxidants

To examine functions of two small heat shock proteins of Escherichia coli, IbpA and IbpB, we constructed His-IbpA and His-IbpB, in which a polyhistidine tag was fused to the N-terminals. Both purified His-IbpA and His-IbpB formed multimers, which have molecular masses of about 2.0-3.0 MDa and consist of about 100-150 subunits. They suppressed the inactivation of several enzymes including citrate synthase and 6-phosphogluconate dehydrogenase by heat, potassium superoxide, hydrogen peroxide and freeze-thawing, but not the inactivation of glyceraldehyde-3-phosphate dehydrogenase by hydrogen peroxide. Both His-IbpA and His-IbpB suppressed enzyme inactivation by various treatments and were also found to be associated with their non-native forms. However, both His-IbpA and His-IbpB were not able to reactivate enzymes inactivated by heat, oxidants or guanidine hydrochloride. When heated to 50 degrees C, each multimeric form of His-IbpA or His-IbpB was dissociated to form a monomer for His-IbpA, and an oligomer of about one-quarter size for His-IbpB. These structural changes were reversible, as both heated proteins regained the multimeric structures after incubation at 25 degrees C. However, when exposed to hydrogen peroxide or potassium superoxide, the large multimeric forms of His-IbpA and His-IbpB were maintained. The results suggest that His-IbpA and His-IbpB suppress the inactivation of enzymes and bind non-native proteins to protect their structures from heat and oxidants.

Evidence of oxidative stress in human corneal diseases

This study localized malondialdehyde (MDA, a toxic byproduct of lipid peroxidation), nitrotyrosine [NT, a cytotoxic byproduct of nitric oxide (NO)], and nitric oxide synthase isomers (NOS) in normal and diseased human corneas. Normal corneas (n=11) and those with clinical and histopathological diagnoses of keratoconus (n=26), bullous keratopathy (n=17), and Fuchs' endothelial dystrophy (n=12) were examined with antibodies specific for MDA, NT, eNOS (constitutive NOS), and iNOS (inducible NOS). Normal corneas showed little or no staining for MDA, NT, or iNOS, whereas eNOS was detected in the epithelium and endothelium. MDA was present in all disease groups, with each group displaying a distinct pattern of staining. NT was detected in all keratoconus and approximately one half of Fuchs' dystrophy corneas. iNOS and eNOS were evident in all the diseased corneas. Keratoconus corneas showed evidence of oxidative damage from cytotoxic byproducts generated by lipid peroxidation and the NO pathway. Bullous keratopathy corneas displayed byproducts of lipid peroxidation but not peroxynitrite (MDA but not NT). Conversely, Fuchs' dystrophy corneas displayed byproducts of peroxynitrite with little lipid peroxidation (NT >> MDA). These data suggest that oxidative damage occurs within each group of diseased corneas. However, each disease exhibits a distinctive profile, with only keratoconus showing prominent staining for both nitrotyrosine and MDA. These results suggest that keratoconus corneas do not process reactive oxygen species in a normal manner, which may play a major role in the pathogenesis of this disease.