Physiological thiols as promoters of glutathione oxidation and modifying agents in protein S-thiolation

Characterization of leucine aminopeptidase (LAP) activity in sweet pepper fruits during ripening and its inhibition by nitration and reducing events

KEY MESSAGE

Pepper fruits contain two leucine aminopeptidase (LAP) genes which are differentially modulated during ripening and by nitric oxide. The LAP activity increases during ripening but is negatively modulated by nitration. Leucine aminopeptidase (LAP) is an essential metalloenzyme that cleaves N-terminal leucine residues from proteins but also metabolizes dipeptides and tripeptides. LAPs play a fundamental role in cell protein turnover and participate in physiological processes such as defense mechanisms against biotic and abiotic stresses, but little is known about their involvement in fruit physiology. This study aims to identify and characterize genes encoding LAP and evaluate their role during the ripening of pepper (Capsicum annuum L.) fruits and under a nitric oxide (NO)-enriched environment. Using a data-mining approach of the pepper plant genome and fruit transcriptome (RNA-seq), two LAP genes, designated CaLAP1 and CaLAP2, were identified. The time course expression analysis of these genes during different fruit ripening stages showed that whereas CaLAP1 decreased, CaLAP2 was upregulated. However, under an exogenous NO treatment of fruits, both genes were downregulated. On the contrary, it was shown that during fruit ripening LAP activity increased by 81%. An in vitro assay of the LAP activity in the presence of different modulating compounds including peroxynitrite (ONOO-), NO donors (S-nitrosoglutathione and nitrosocyteine), reducing agents such as reduced glutathione (GSH), L-cysteine (L-Cys), and cyanide triggered a differential response. Thus, peroxynitrite and reducing compounds provoked around 50% inhibition of the LAP activity in green immature fruits, whereas cyanide upregulated it 1.5 folds. To our knowledge, this is the first characterization of LAP in pepper fruits as well as of its regulation by diverse modulating compounds. Based on the capacity of LAP to metabolize dipeptides and tripeptides, it could be hypothesized that the LAP might be involved in the GSH recycling during the ripening process.

Fruit ripening-associated leucylaminopeptidase with cysteinylglycine dipeptidase activity from durian suggests its involvement in glutathione recycling

BACKGROUND

Durian (Durio zibethinus L.) is a highly popular fruit in Thailand and several other Southeast Asian countries. It is abundant in essential nutrients and sulphur-containing compounds such as glutathione (GSH) and γ-glutamylcysteine (γ-EC). Cysteinylglycine (Cys-Gly) is produced by GSH catabolism and occurs in durian fruit pulp. Cysteine (Cys) is a precursor of sulphur-containing volatiles generated during fruit ripening. The aforementioned substances contribute to the strong odour and flavour of the ripe fruit. However, the genes encoding plant Cys-Gly dipeptidases are unknown. The aim of this study was to measure leucylaminopeptidase (LAP) activity in durian fruit pulp.

RESULTS

We identified DzLAP1 and DzLAP2, which the former was highly expressed in the fruit pulp. DzLAP1 was expressed at various ripening stages and in response to ethephon/1-MCP treatment. Hence, DzLAP1 is active at the early stages of fruit ripening. DzLAP1 is a metalloenzyme ~ 63 kDa in size. It is activated by Mg2+ or Mn2+ and, like other LAPs, its optimal alkaline pH is 9.5. Kinetic studies revealed that DzLAP1 has Km = 1.62 mM for its preferred substrate Cys-Gly. DzLAP1-GFP was localised to the cytosol and targeted the plastids. In planta Cys-Gly hydrolysis was confirmed for Nicotiana benthamiana leaves co-infiltrated with Cys-Gly and expressing DzLAP1.

CONCLUSIONS

DzLAP1 has Cys-Gly dipeptidase activity in the γ-glutamyl cycle. The present study revealed that the LAPs account for the high sulphur-containing compound levels identified in fully ripened durian fruit pulp.

Thiol oxidase ability of copper ion is specifically retained upon chelation by aldose reductase

Bovine lens aldose reductase is susceptible to a copper-mediated oxidation, leading to the generation of a disulfide bridge with the concomitant incorporation of two equivalents of the metal and inactivation of the enzyme. The metal complexed by the protein remains redox active, being able to catalyse the oxidation of different physiological thiol compounds. The thiol oxidase activity displayed by the enzymatic form carrying one equivalent of copper ion (Cu₁-AR) has been characterized. The efficacy of Cu₁-AR in catalysing thiol oxidation is essentially comparable to the free copper in terms of both thiol concentration and pH effect. On the contrary, the two catalysts are differently affected by temperature. The specificity of the AR-bound copper towards thiols is highlighted with Cu₁-AR being completely ineffective in promoting the oxidation of both low-density lipoprotein and ascorbic acid.

Purification and characterization of a Cys-Gly hydrolase from the gastropod mollusk, Patella caerulea

A magnesium-dependent cysteinyl-glycine hydrolyzing enzyme from the gastropod mollusk Patella caerulea was purified to electrophoretic homogeneity through a simple and rapid purification protocol. The molecular masses of the native protein and the subunit suggest that the enzyme has a homohexameric structure. Structural data in combination with kinetic parameters determined with Cys-Gly and compared with Leu-Gly as a substrate, indicate that the purified enzyme is a member of the peptidase family M17. The finding that an enzyme of the peptidase family M17 is responsible also in mollusks for the breakdown of Cys-Gly confirms the important role of this peptidase family in the glutathione metabolism.

Determination of 3-mercaptopropionic acid by HPLC: A sensitive method for environmental applications

The organic sulfur compound 3-mercaptopropionic acid (3-MPA) is an important thiol intermediate in organic sulfur metabolism in natural environments. It is generated during degradation of sulfur-containing amino acids (e.g. methionine) and from demethylation of dimethylsulfoniopropionate (DMSP). This pathway is an alternative enzymatic process in the DMSP catabolism that routes sulfur away from the climatically-active dimethyl sulfide (DMS). 3-MPA detection and subsequent quantification in different matrices is difficult due to its extreme reactivity. We therefore developed a sensitive method for determination of 3-MPA based on pre-column derivatization with monobromobimane and analysis by high-performance liquid chromatography (HPLC) with fluorescence detection. This methodology was first tested with 3-MPA standards under low (0.005-0.2μmolL(-1)) and high (1-25μmolL(-1)) concentrations. For the optimization of the reaction, CHES and, alternatively, Tris-HCl buffers were evaluated in the derivatization step, with Tris-HCl showing more effective separation of thiol derivatives and a better 3-MPA peak shape. The detection limit was 4.3nmolL(-1) with a 10μL sample injection, and mean recoveries of 3-MPA ranged from 97 to 105% in estuarine waters with different salinities (0.17 and 35.9ppt). The linearity (r>0.99) and repeatability of detector response, with intra- and inter-day precision (% CV) of 2.68-7.01% and 4.86-12.5%, respectively, confirmed the reliability of the method. Previous 3-MPA analytical methods required immediate analysis due to unstable derivatives, but in this method we achieved high stability of the derivatized samples when stored at 4°C, with only a 3-5% loss after more than one year of storage. This method was successfully applied to measure 3-MPA concentrations and rates of 3-MPA production in a variety of intertidal estuarine sediment slurries. Dissolved 3-MPA concentrations in these sediment slurries varied between 2 and 237μmolL(-1) and, 3-MPA net fluxes ranged in wet sediments between -3.6±1.7 and 30±5μmolL(-1)g(-1)h(-1). Thus, the application of this optimized methodology showed an efficient performance for measuring 3-MPA in environmental samples, with a straightforward sample derivatization and a simple analysis of stable 3-MPA derivatives.

Oxyhalogen–sulfur chemistry — Kinetics and mechanism of the bromate oxidation of cysteamine

The kinetics and mechanism of the oxidation of the biologically important molecule, cysteamine, by acidic bromate and molecular bromine have been studied. In excess acidic bromate conditions, cysteamine is oxidized to N-brominated derivatives, and in excess cysteamine the oxidation product is taurine according to the following stoichiometry: BrO3–+ H2NCH2CH2SH → H2NCH2CH2SO3H + Br–. There is quantitative formation of taurine before N-bromination commences. Excess aqueous bromine oxidizes cysteamine to give dibromotaurine: 5Br2+ H2NCH2CH2SH + 3H2O → Br2NCH2CH2SO3H + 8Br–+ 8H+, while excess cysteamine conditions gave monobromotaurine. The oxidation of cysteamine by aqueous bromine is effectively diffusion-controlled all the way to the formation of monobromotaurine. Further formation of dibromotaurine is dependent on acid concentrations, with highly acidic conditions inhibiting further reaction towards formation of dibromotaurine. The formation of the N-brominated derivatives of taurine is reversible, with taurine regenerated in the presence of a reducing agent such as iodide. This feature makes it possible for taurine to moderate hypobromous acid toxicity in the physiological environment.Key words: cysteamine, hypobromous acid, toxicities, antioxidant.

Distribution, adaptation and physiological meaning of thiols from vertebrate hemoglobins

In the present review, the sequences of hemoglobins (Hb) of 267 adult vertebrate species belonging to eight major vertebrate taxa are examined for the presence and location of cysteinyl residues in an attempt at correlation with their ecophysiology. Essentially, all vertebrates have surface cysteinyl residues in Hb molecules whereby their thiol groups may become highly reactive. Thiol-rich Hbs may display eight or more thiols per tetramer. In vertebrates so far examined, the cysteinyl residues occur in 44 different sequence positions in alpha chains and 41 positions in beta chains. Most of them are conservatively located and occur in only a few positions in Teleostei, Aves and Mammalia, whereas they are dispersed in Amphibia. The internal cysteinyl residue alpha104 is ubiquitous in vertebrates. Residue beta93 is highly conserved in reptiles, birds and mammals. The number of cysteine residues per tetramer with solvent access varies in vertebrates, mammalians and bony fish having the lowest number of external residues, whereas nearly all external cysteine residues in Aves and Lepidosauria are of the surface crevice type. In cartilaginous fish, amphibians, Crocodylidae and fresh water turtles, a substantial portion of the solvent accessible thiols are of the totally external type. Recent evidence shows that some Hb thiol groups are highly reactive and undergo extensive and reversible S-thiolation, and that they may be implicated in interorgan redox equilibrium processes. Participation of thiol groups in nitric oxide ((*)NO) metabolism has also been proved. The evidence argues for a new physiologically relevant role for Hb via involvement in free radical and antioxidant metabolism.

Colorimetric coupled enzyme assay for γ-glutamyltransferase activity using glutathione as substrate

A colorimetric coupled enzyme assay for the determination of gamma-glutamyltransferase (GGT) activity using glutathione as substrate is described. The cysteine released from glutathione upon sequential action of GGT and leucine aminopeptidase is spectrophotometrically detected through its reaction with ninhydrin at 100 degrees C in acidic conditions. The method was applied to the determination of the activity of both bovine kidney and human serum GGT. In the described assay conditions with final GGT concentrations ranging from 0.18 to 4 mU/ml, a linear relationship between produced cysteine and incubation times up to 90 min was observed. When a standard chromogenic assay for GGT using L-gamma-glutamyl-3-carboxy-4-nitroanilide as substrate and the proposed assay were applied on the same serum sample a linear relationship between the two method was observed. Since the use of GSH as substrate, the proposed method can be usefully adopted for enzymological studies on GGT-related enzymes, a class of enzymes which is still waiting to be characterized.

L-gamma-Glutamyl-L-cysteinyl-glycine (glutathione; GSH) and GSH-related enzymes in the regulation of pro- and anti-inflammatory cytokines: a signaling transcriptional scenario for redox(y) immunologic sensor(s)?

Of the antioxidant/prooxidant mechanisms mediating the regulation of inflammatory mediators, particularly cytokines, oxidative stress-related pathways remain a cornerstone. It is conspicuous that there is a strong association between free radical accumulation (ROS/RNS; oxidative stress) and the evolution of inflammation and inflammatory-related responses. The scenario that upholds a consensus on the aforementioned is still evolving to unravel, from an immunologic perspective, the molecular mechanisms associated with ROS/RNS-dependent inflammation. Cytokines are keynote players when it comes to defining an intimate relationship among reduction-oxidation (redox) signals, oxidative stress and inflammation. How close we are to identifying the molecular basis of this intricate association should be weighed against the involvement of specific signaling molecules and, potentially, transcription factors. L-gamma-Glutamyl-L-cysteinyl-glycine, or glutathione (GSH), an antioxidant thiol, has shaped, and still is refining, the face of oxidative signaling in terms of regulating the milieu of inflammatory mediators, ostensibly via the modulation (expression/repression) of oxygen- and redox-responsive transcription factors, hence termed redox(y)-sensitive cofactors. When it comes to the arena of oxygen sensing, oxidative stress and inflammation, nuclear factor-kappaB (NF-kappaB) and hypoxia-inducible factor-1alpha (HIF-1alpha) are key players that determine antioxidant/prooxidant responses with oxidative challenge. It is the theme therein to underlie current understanding of the molecular association hanging between oxidative stress and the evolution of inflammation, walked through an elaborate discussion on the role of transcription factors and cofactors. Would that classify glutathione and other redox signaling cofactors as potential anti-inflammatory molecules emphatically remains of particular interest, especially in the light of identifying upstream and downstream molecular pathways for conceiving therapeutic, alleviating strategy for oxidant-mediated, inflammatory-related disease conditions.

Decreased cardiac mitochondrial NADP+-isocitrate dehydrogenase activity and expression: a marker of oxidative stress in hypertrophy development

Mitochondrial dysfunction subsequent to increased oxidative stress and alterations in energy metabolism is considered to play a role in the development of cardiac hypertrophy and its progression to failure, although the sequence of events remains to be elucidated. This study aimed at characterizing the impact of hypertrophy development on the activity and expression of mitochondrial NADP+-isocitrate dehydrogenase (mNADP+-ICDH), a metabolic enzyme that controls redox and energy status. We expanded on our previous finding of its inactivation through posttranslational modification by the lipid peroxidation product 4-hydroxynonenal (HNE) in 7-wk-old spontaneously hypertensive rat (SHR) hearts before hypertrophy development (Benderdour et al. J Biol Chem 278: 45154-45159, 2003). In this study, we used 7-, 15-, and 30-wk-old SHR and Sprague-Dawley (SD) rats with abdominal aortic coarctation. Compared with age-matched control Wistar-Kyoto (WKY) rats, SHR hearts showed a significant 25% decrease of mNADP+-ICDH activity, which preceded in time 1) the decline in its protein and mRNA expression levels (between 10% and 35%) and 2) the increase in hypertrophy markers. The chronic and persistent loss of mNADP+-ICDH activity in SHR was associated with enhanced tissue accumulation of HNE-mNADP+-ICDH and total HNE-protein adducts at all ages and contrasted with the profile of changes in the activity of other mitochondrial enzymes involved in antioxidant or energy metabolism. Two-way ANOVA of the data also revealed a significant effect of age on most parameters measured in SHR and WKY hearts. The mNADP+-ICDH activity, protein, and mRNA expression were reduced between 25% and 35% in coarctated SD rats and were normalized by treatment of SHR or coarctated SD rats with renin-angiotensin system inhibitors, which prevented or attenuated hypertrophy. Altogether, our data show that cardiac mNADP+-ICDH activity and expression are differentially and sequentially affected in hypertrophy development and, to a lesser extent, with aging. Decreased cardiac mNADP+-ICDH activity, which is attributed at least in part to HNE adduct formation, appears to be a relevant early and persistent marker of mitochondrial oxidative stress-related alterations in hypertrophy development. Potentially, this could also contribute to the aetiology of cardiomyopathy.