Glyoxalase II in Saccharomyces cerevisiae: in situ kinetics using the 5,5'-dithiobis(2-nitrobenzoic acid) assay

Glyoxalase I activity affects Arabidopsis sensitivity to ammonium nutrition

Elevated methylglyoxal levels contribute to ammonium-induced growth disorders in Arabidopsis thaliana. Methylglyoxal detoxification pathway limitation, mainly the glyoxalase I activity, leads to enhanced sensitivity of plants to ammonium nutrition. Ammonium applied to plants as the exclusive source of nitrogen often triggers multiple phenotypic effects, with severe growth inhibition being the most prominent symptom. Glycolytic flux increase, leading to overproduction of its toxic by-product methylglyoxal (MG), is one of the major metabolic consequences of long-term ammonium nutrition. This study aimed to evaluate the influence of MG metabolism on ammonium-dependent growth restriction in Arabidopsis thaliana plants. As the level of MG in plant cells is maintained by the glyoxalase (GLX) system, we analyzed MG-related metabolism in plants with a dysfunctional glyoxalase pathway. We report that MG detoxification, based on glutathione-dependent glyoxalases, is crucial for plants exposed to ammonium nutrition, and its essential role in ammonium sensitivity relays on glyoxalase I (GLXI) activity. Our results indicated that the accumulation of MG-derived advanced glycation end products significantly contributes to the incidence of ammonium toxicity symptoms. Using A. thaliana frostbite1 as a model plant that overcomes growth repression on ammonium, we have shown that its resistance to enhanced MG levels is based on increased GLXI activity and tolerance to elevated MG-derived advanced glycation end-product (MAGE) levels. Furthermore, our results show that glyoxalase pathway activity strongly affects cellular antioxidative systems. Under stress conditions, the disruption of the MG detoxification pathway limits the functioning of antioxidant defense. However, under optimal growth conditions, a defect in the MG detoxification route results in the activation of antioxidative systems.

Metabolic Shades of S-D-Lactoylglutathione

S-D-lactoylglutathione (SDL) is an intermediate of the glutathione-dependent metabolism of methylglyoxal (MGO) by glyoxalases. MGO is an electrophilic compound that is inevitably produced in conjunction with glucose breakdown and is essentially metabolized via the glyoxalase route. In the last decades, MGO metabolism and its cytotoxic effects have been under active investigation, while almost nothing is known about SDL. This article seeks to fill the gap by presenting an overview of the chemistry, biochemistry, physiological role and clinical importance of SDL. The effects of intracellular SDL are investigated in three main directions: as a substrate for post-translational protein modifications, as a reservoir for mitochondrial reduced glutathione and as an energy currency. In essence, all three approaches point to one direction, namely, a metabolism-related regulatory role, enhancing the cellular defense against insults. It is also suggested that an increased plasma concentration of SDL or its metabolites may possibly serve as marker molecules in hemolytic states, particularly when the cause of hemolysis is a disturbance of the pay-off phase of the glycolytic chain. Finally, SDL could also represent a useful marker in such metabolic disorders as diabetes mellitus or ketotic states, in which its formation is expected to be enhanced. Despite the lack of clear-cut evidence underlying the clinical and experimental findings, the investigation of SDL metabolism is a promising field of research.

Tracing the Evolution of Plant Glyoxalase III Enzymes for Structural and Functional Divergence

Glyoxalase pathway is the primary route for metabolism of methylglyoxal (MG), a toxic ubiquitous metabolite that affects redox homeostasis. It neutralizes MG using Glyoxalase I and Glyoxalase II (GLYI and GLYII) enzymes in the presence of reduced glutathione. In addition, there also exists a shorter route for the MG detoxification in the form of Glyoxalase III (GLYIII) enzymes, which can convert MG into D-lactate in a single-step without involving glutathione. GLYIII proteins in different systems demonstrate diverse functional capacities and play a vital role in oxidative stress response. To gain insight into their evolutionary patterns, here we studied the evolution of GLYIII enzymes across prokaryotes and eukaryotes, with special emphasis on plants. GLYIII proteins are characterized by the presence of DJ-1_PfpI domains thereby, belonging to the DJ-1_PfpI protein superfamily. Our analysis delineated evolution of double DJ-1_PfpI domains in plant GLYIII. Based on sequence and structural characteristics, plant GLYIII enzymes could be categorized into three different clusters, which followed different evolutionary trajectories. Importantly, GLYIII proteins from monocots and dicots group separately in each cluster and the each of the two domains of these proteins also cluster differentially. Overall, our findings suggested that GLYIII proteins have undergone significant evolutionary changes in plants, which is likely to confer diversity and flexibility in their functions.

Optimal Design of Experiments for Hybrid Nonlinear Models, with Applications to Extended Michaelis–Menten Kinetics

Biochemical mechanism studies often assume statistical models derived from Michaelis–Menten kinetics, which are used to approximate initial reaction rate data given the concentration level of a single substrate. In experiments dealing with industrial applications, however, there are typically a wide range of kinetic profiles where more than one factor is controlled. We focus on optimal design of such experiments requiring the use of multifactor hybrid nonlinear models, which presents a considerable computational challenge. We examine three different candidate models and search for tailor-made D- or weighted-A-optimal designs that can ensure the efficiency of nonlinear least squares estimation. We also study a compound design criterion for discriminating between two candidate models, which we recommend for design of advanced kinetic studies.

Supplementary materials accompanying this paper appear on-line

Enhanced Formation of Methylglyoxal-Derived Advanced Glycation End Products in Arabidopsis Under Ammonium Nutrition

Nitrate (NO₃^-) and ammonium (NH₄⁺) are prevalent nitrogen (N) sources for plants. Although NH₄⁺ should be the preferred form of N from the energetic point of view, ammonium nutrition often exhibits adverse effects on plant physiological functions and induces an important growth-limiting stress referred as ammonium syndrome. The effective incorporation of NH₄⁺ into amino acid structures requires high activity of the mitochondrial tricarboxylic acid cycle and the glycolytic pathway. An unavoidable consequence of glycolytic metabolism is the production of methylglyoxal (MG), which is very toxic and inhibits cell growth in all types of organisms. Here, we aimed to investigate MG metabolism in Arabidopsis thaliana plants grown on NH₄⁺ as a sole N source. We found that changes in activities of glycolytic enzymes enhanced MG production and that markedly elevated MG levels superseded the detoxification capability of the glyoxalase pathway. Consequently, the excessive accumulation of MG was directly involved in the induction of dicarbonyl stress by introducing MG-derived advanced glycation end products (MAGEs) to proteins. The severe damage to proteins was not within the repair capacity of proteolytic enzymes. Collectively, our results suggest the impact of MG (mediated by MAGEs formation in proteins) in the contribution to NH₄⁺ toxicity symptoms in Arabidopsis.

Mitochondrial involvement to methylglyoxal detoxification: D-Lactate/Malate antiporter in Saccharomyces cerevisiae

Research during the last years has accumulated a large body of data that suggest that a permanent high flux through the glycolytic pathway may be a source of intracellular toxicity via continuous generation of endogenous reactive dicarbonyl compound methylglyoxal (MG). MG detoxification by the action of the glyoxalase system produces D-lactate. Thus, this article extends our previous work and presents new insights concerning D-lactate fate in aerobically grown yeast cells. Biochemical studies using intact functional mitochondrial preparations derived from Saccharomyces cerevisiae show that D-lactate produced in the extramitochondrial phase can be taken up by mitochondria, metabolised inside the organelles with efflux of newly synthesized malate. Experiments were carried out photometrically and the rate of malate efflux was measured by use of NADP(+) and malic enzyme and it depended on the rate of transport across the mitochondrial membrane. It showed saturation characteristics (K(m) = 20 μM; V(max) = 6 nmol min(-1) mg(-1) of mitochondrial protein) and was inhibited by α-cyanocinnamate, a non-penetrant compound. Our data reveal that reducing equivalents export from mitochondria is due to the occurrence of a putative D-lactate/malate antiporter which differs from both D-lactate/pyruvate antiporter and D-lactate/H(+) symporter as shown by the different V(max) values, pH profile and inhibitor sensitivity. Based on these results we propose that D-lactate translocators and D-lactate dehydrogenases work together for decreasing the production of MG from the cytosol, thus mitochondria could play a pro-survival role in the metabolic stress response as well as for D-lactate-dependent gluconeogenesis.

Optimization of time-course experiments for kinetic model discrimination

Systems biology relies heavily on the construction of quantitative models of biochemical networks. These models must have predictive power to help unveiling the underlying molecular mechanisms of cellular physiology, but it is also paramount that they are consistent with the data resulting from key experiments. Often, it is possible to find several models that describe the data equally well, but provide significantly different quantitative predictions regarding particular variables of the network. In those cases, one is faced with a problem of model discrimination, the procedure of rejecting inappropriate models from a set of candidates in order to elect one as the best model to use for prediction.

In this work, a method is proposed to optimize the design of enzyme kinetic assays with the goal of selecting a model among a set of candidates. We focus on models with systems of ordinary differential equations as the underlying mathematical description. The method provides a design where an extension of the Kullback-Leibler distance, computed over the time courses predicted by the models, is maximized. Given the asymmetric nature this measure, a generalized differential evolution algorithm for multi-objective optimization problems was used.

The kinetics of yeast glyoxalase I (EC 4.4.1.5) was chosen as a difficult test case to evaluate the method. Although a single-substrate kinetic model is usually considered, a two-substrate mechanism has also been proposed for this enzyme. We designed an experiment capable of discriminating between the two models by optimizing the initial substrate concentrations of glyoxalase I, in the presence of the subsequent pathway enzyme, glyoxalase II (EC 3.1.2.6). This discriminatory experiment was conducted in the laboratory and the results indicate a two-substrate mechanism for the kinetics of yeast glyoxalase I.

Insulin resistance induced by high-fructose diet potentiates carbon tetrachloride hepatotoxicity

Insulin resistance (IR) is recognized as a contributory factor for a variety of liver diseases. The present study investigates the susceptibility of liver to the toxic actions of carbon tetrachloride (CCl(4)) in a rat model of IR, induced by feeding a high-fructose diet (60 g/100 g) for 30 days. A sub-lethal dose of CCl(4) (2 mL/kg intraperitoneally [i.p.], in corn oil) was administered and the outcome of hepatotoxicity was assessed at 0 hour and at 6, 12, 24 and 36 hours after CCl(4) administration. After 30 days of fructose feeding, the rats showed IR, decline in liver antioxidant status and rise in lipid peroxidation. Liver dysfunction in fructose-fed rats was evident from a rise in transaminases, total bilirubin and a decrease in albumin/globulin ratio in plasma and decreases in nitrite, arginase and increase in protein carbonyl and nitrosothiol content in liver. Increased staining for 3-nitro tyrosine (3-NT) antibody was observed in fructose-fed rat liver as compared to control. CCl(4) (2 mL/kg) caused 100% mortality in fructose-fed rats within 48 hours, while no death of animals occurred in control. CCl(4) caused liver damage in both control and fructose-fed rats. Time-based studies showed that progressive liver injury occurred only in fructose-fed rats from 0, 6, 12, 24 hours, with a peak at 36 hours. In control diet-fed rats, the extent of damage was maximum at 24 hours, which declined at 36 hours. Thus, the toxic effects of CCl(4) are potentiated due to compromised liver function in the setting of IR.

Genistein improves liver function and attenuates non-alcoholic fatty liver disease in a rat model of insulin resistance

BACKGROUND

The high fructose-fed rat is widely used as a model of insulin resistance. Genistein, a soy isoflavone, has been shown to improve insulin sensitivity in this model. The present study investigated whether genistein could prevent fatty liver disease in this model.

METHODS

Male Wistar rats were fed a diet containing starch (control) or 60% fructose (insulin-resistant model). Fifteen days later, rats in each dietary group were divided into two groups and were treated with either genistein (1 mg/kg per day) in dimethylsulfoxide (DMSO) or 30% DMSO alone. After 60 days, markers of liver injury, oxidative stress, interleukin (IL)-6, tumor necrosis factor (TNF)-α, lipids, lipoprotein profile, nitrite, and nitrosothiol in the plasma and liver were quantified. Liver sections were examined for 3-nitrotyrosine (3-NT) expression and pathological lesions.

RESULTS

Fructose-fed rats displayed hyperlipidemia, significant changes in plasma lipoprotein profile, and increases in IL-6 and TNF-α levels compared with control. In addition, the accumulation of lipids, liver injury, a decline in liver function, inactivation of the glyoxalase system, depletion of antioxidants, and increased 3-NT expression were observed in the fructose-fed group. Administration of genistein to fructose-fed rats significantly reduced these biochemical and histological abnormalities.

CONCLUSIONS

Genistein activates the antioxidant profile, decreases IL-6 and TNF-α concentrations, prevents oxidative damage, and ameliorates fatty liver in insulin-resistant rats.

Taurine enhances the metabolism and detoxification of ethanol and prevents hepatic fibrosis in rats treated with iron and alcohol

The study examines the effects of taurine on the metabolism and detoxification of ethanol in liver fibrosis induced by simultaneous administration of iron carbonyl (0.5%, w/w) and ethanol (6g/(kgday)). Ethanol and iron administration caused liver damage and fibrosis as evidenced by liver histology and biochemical profile in plasma. Over accumulation of iron and a loss in taurine in hepatic tissue was observed in fibrotic animals. The activities of alcohol dehydrogenase and aldehyde dehydrogenase were significantly reduced in these rats compared to control. Adaptive induction of activities of Cytochrome P4502E1 (CYP2E1) and aniline hydroxylase accompanied by the reduction in glutathione-S-transferase, DT-diaphorase and glyoxalases I and II was observed. Taurine administration (2% in drinking water) ameliorated the effects of ethanol and iron. Hepatic damage and fibrosis were reduced in taurine-supplemented rats. Thus taurine has the potential for the treatment of alcoholic liver fibrosis.

A comparative study of methylglyoxal metabolism in trypanosomatids

The glyoxalase system, comprising the metalloenzymes glyoxalase I (GLO1) and glyoxalase II (GLO2), is an almost universal metabolic pathway involved in the detoxification of the glycolytic byproduct methylglyoxal to d-lactate. In contrast to the situation with the trypanosomatid parasites Leishmania major and Trypanosoma cruzi, this trypanothione-dependent pathway is less well understood in the African trypanosome, Trypanosoma brucei. Although this organism possesses a functional GLO2, no apparent GLO1 gene could be identified in the T. brucei genome. The absence of GLO1 in T. brucei was confirmed by the lack of GLO1 activity in whole cell extracts, failure to detect a GLO1-like protein on immunoblots of cell lysates, and lack of d-lactate formation from methylglyoxal as compared to L. major and T. cruzi. T. brucei procyclics were found to be 2.4-fold and 5.7-fold more sensitive to methylglyoxal toxicity than T. cruzi and L. major, respectively. T. brucei also proved to be the least adept of the ‘Tritryp’ parasites in metabolizing methylglyoxal, producing l-lactate rather than d-lactate. Restoration of a functional glyoxalase system by expression of T. cruzi GLO1 in T. brucei resulted in increased resistance to methylglyoxal and increased conversion of methylglyoxal to d-lactate, demonstrating that GLO2 is functional in vivo. Procyclic forms of T. brucei possess NADPH-dependent methylglyoxal reductase and NAD⁺-dependent l-lactaldehyde dehydrogenase activities sufficient to account for all of the methylglyoxal metabolized by these cells. We propose that the predominant mechanism for methylglyoxal detoxification in the African trypanosome is via the methylglyoxal reductase pathway to l-lactate.

N-acetyl-L-cysteine combined with mesalamine in the treatment of ulcerative colitis: Randomized, placebo-controlled pilot study

AIM: To evaluate the effectiveness and safety of oral N-acetyl-L-cysteine (NAC) co-administration with mesalamine in ulcerative colitis (UC) patients.

METHODS: Thirty seven patients with mild to moderate UC were randomized to receive a four-wk course of oral mesalamine (2.4 g/d) plus N-acetyl-L-cysteine (0.8 g/d) (group A) or mesalamine plus placebo (group B). Patients were monitored using the Modified Truelove-Witts Severity Index (MTWSI). The primary endpoint was clinical remission (MTWSI ≤ 2) at 4 wk. Secondary endpoints were clinical response (defined as a reduction from baseline in the MTWSI of ≥ 2 points) and drug safety. The serum TNF-α, interleukin-6, interleukin-8 and MCP-1 were evaluated at baseline and at 4 wk of treatment.

RESULTS: Analysis per-protocol criteria showed clinical remission rates of 63% and 50% after 4 wk treatment with mesalamine plus N-acetyl-L-cysteine (group A) and mesalamine plus placebo (group B) respectively (OR = 1.71; 95% CI: 0.46 to 6.36; P = 0.19; NNT = 7.7). Analysis of variance (ANOVA) of data indicated a significant reduction of MTWSI in group A (P = 0.046) with respect to basal condition without significant changes in the group B (P = 0.735) during treatment. Clinical responses were 66% (group A) vs 44% (group B) after 4 wk of treatment (OR = 2.5; 95% CI: 0.64 to 9.65; P = 0.11; NNT = 4.5). Clinical improvement in group A correlated with a decrease of IL-8 and MCP-1. Rates of adverse events did not differ significantly between both groups.

CONCLUSION: In group A (oral NAC combined with mesalamine) contrarily to group B (mesalamine alone), the clinical improvement correlates with a decrease of chemokines such as MCP-1 and IL-8. NAC addition not produced any side effects.

Quantitative assessment of the glyoxalase pathway in Leishmania infantum as a therapeutic target by modelling and computer simulation

The glyoxalase pathway of Leishmania infantum was kinetically characterized as a trypanothione-dependent system. Using time course analysis based on parameter fitting with a genetic algorithm, kinetic parameters were estimated for both enzymes, with trypanothione derived substrates. A K(m) of 0.253 mm and a V of 0.21 micromol.min(-1).mg(-1)for glyoxalase I, and a K(m) of 0.098 mm and a V of 0.18 micromol.min(-1).mg(-1) for glyoxalase II, were obtained. Modelling and computer simulation were used for evaluating the relevance of the glyoxalase pathway as a potential therapeutic target by revealing the importance of critical parameters of this pathway in Leishmania infantum. A sensitivity analysis of the pathway was performed using experimentally validated kinetic models and experimentally determined metabolite concentrations and kinetic parameters. The measurement of metabolites in L. infantum involved the identification and quantification of methylglyoxal and intracellular thiols. Methylglyoxal formation in L. infantum is nonenzymatic. The sensitivity analysis revealed that the most critical parameters for controlling the intracellular concentration of methylglyoxal are its formation rate and the concentration of trypanothione. Glyoxalase I and II activities play only a minor role in maintaining a low intracellular methylglyoxal concentration. The importance of the glyoxalase pathway as a therapeutic target is very small, compared to the much greater effects caused by decreasing trypanothione concentration or increasing methylglyoxal concentration.

Two separate pathways for d-lactate oxidation by Saccharomyces cerevisiae mitochondria which differ in energy production and carrier involvement

In this work we looked at whether and how mitochondria isolated from Saccharomyces cerevisiae (SCM) oxidize d-lactate. We found that: (1). externally added d-lactate causes oxygen uptake by SCM with P/O ratio equal to 1.5; in the presence of antimycin A (AA), P/O ratio was 1.8, differently in the presence of the non-penetrant alpha-cyanocinnamate (alpha-CCN-) no P/O ratio could be measured. Consistently, mitochondrial electrical membrane potential (deltapsi) generation was found, due to externally added d-lactate in the presence of antimycin A, but not of alpha-CCN-. (2). SCM oxidize d-lactate in two different manners: (i). via inner membrane d-lactate dehydrogenase which leads to d-lactate oxidation without driving deltapsi generation and ATP synthesis and (ii). via the matrix d-lactate dehydrogenase, which drives deltapsi generation and ATP synthesis by using taken up d-lactate. (3). Pyruvate newly synthesised in the mitochondrial matrix is exported via the novel d-lactate/pyruvate antiporter. d-Lactate/pyruvate antiport proved to regulate the rate of pyruvate efflux in vitro. (4). The existence of the d-lactate/H+ symporter is also proposed as shown by mitochondrial swelling. The d-lactate carriers and d-lactate dehydrogenases could account for the removal of the toxic methylglyoxal from cytosol, as well as for the d-lactate-dependent gluconeogenesis.

In situ kinetic analysis of glyoxalase I and glyoxalase II in Saccharomyces cerevisiae

The kinetics of glyoxalase I [(R)-S-lactoylglutathione methylglyoxal-lyase; EC 4.4.1.5] and glyoxalase II (S-2-hydroxyacylglutathione hydrolase; EC 3.1.2.6) from Saccharomyces cerevisiae was studied in situ, in digitonin permeabilized cells, using two different approaches: initial rate analysis and progress curves analysis. Initial rate analysis was performed by hyperbolic regression of initial rates using the program HYPERFIT. Glyoxalase I exhibited saturation kinetics on 0.05-2.5 mM hemithioacetal concentration range, with kinetic parameters Km 0.53 +/- 0.07 mM and V (3.18 +/- 0.16) x 10(-2) mM.min(-1). Glyoxalase II also showed saturation kinetics in the SD-lactoylglutathione concentration range of 0.15-3 mM and Km 0.32 +/- 0.13 mM and V (1.03 +/- 0.10) x 10(-3) mM.min(-1) were obtained. The kinetic parameters of both enzymes were also estimated by nonlinear regression of progress curves using the raw absorbance data and integrated differential rate equations with the program GEPASI. Several optimization methods were used to minimize the sum of squares of residuals. The best parameter fit for the glyoxalase I reaction was obtained with a single curve analysis, using the irreversible Michaelis-Menten model. The kinetic parameters obtained, Km 0.62 +/- 0.18 mM and V (2.86 +/- 0.01) x 10(-2) mM.min(-1), were in agreement with those obtained by initial rate analysis. The results obtained for glyoxalase II, using either the irreversible Michaelis-Menten model or a phenomenological reversible hyperbolic model, showed a high correlation of residuals with time and/or high values of standard deviation associated with Km. The possible causes for the discrepancy between data obtained from initial rate analysis and progress curve analysis, for glyoxalase II, are discussed.

In situ analysis of methylglyoxal metabolism in Saccharomyces cerevisiae

Methylglyoxal metabolism was studied during Saccharomyces cerevisiae grown with D-glucose as the sole carbon and energy source. Using for the first time a specific assay for methylglyoxal in yeast, metabolic fluxes of its formation and D-lactate production were determined. D-Glucose consumption and ethanol production were determined during growth. Metabolic fluxes were also determined in situ, at the glycolytic triose phosphate levels and glyoxalase pathway. Maximum fluxes of ethanol production and glucose consumption correspond to maxima of methylglyoxal and D-lactate formation fluxes during growth. Methylglyoxal formation is quantitatively related to glycolysis, representing 0.3% of the total glycolytic flux in S. cerevisiae.