Metals are directly involved in the redox interconversion of Saccharomyces cerevisiae glutathione reductase

Are we neglecting earth while conquering space? Effects of aluminized solid rocket fuel combustion on the physiology of a tropical freshwater invertebrate

Space launchers often use aluminized-solid fuel ("propergol") as propellant and its combustion releases tons of Al₂O₃ and HCl that sink in terrestrial and aquatic environments, polluting and decreasing water pH. We studied the impact of these events on the biochemical/physiological performance of the freshwater shrimp Macrobrachium jelskii, with wild specimens collected from a non-impacted site in French Guiana. In the laboratory, shrimps were exposed for one week to: i) undisturbed conditions; ii) Al₂O₃ exposure (0.5 mg L^-1) at normal pH (6.6); iii) decreased pH (4.5) (mimicking HCl release in the environment) with no Al₂O₃; or iv) Al₂O₃ 0.5 mg L^-1 and pH 4.5, representing the average conditions found in the water bodies around the Ariane 5 launch pad. Results showed that shrimps bioaccumulated aluminium (Al) regardless of water pH. The combined effect of Al₂O₃ and low pH caused the most impact: acetylcholinesterase and carboxylesterase activities decreased, indicating neurotoxicity and reduced detoxification capacity, respectively. Animal respiration was enhanced with Al₂O₃ and pH variations alone, but the synergic interaction of both stressors caused respiration to decrease, suggesting metabolic depression. Oxidative damage followed a similar pattern to respiration rates across conditions, suggesting free radical-mediation in Al toxicity. Antioxidant activities varied among enzymes, with glutathione reductase being the most impacted by Al₂O₃ exposure. This study shows the importance of addressing space ports' impact on the environment, setting the bases for selecting the most appropriate biomarkers for future monitoring programs using a widespread and sensitive crustacean in the context of an increasing space-oriented activity across the world.

Direct Determination of Glutathione Reductase in Cells Cultured in Microtitre Plates as a Biomarker for Oxidative Stress

— A new method was developed for the direct determination of glutathione reductase (GOR) activity in Vero cells cultured in microtitre plates, avoiding cell-free extract preparation. The cells in each well were washed twice with phosphate-buffered saline, lysed with Triton X-100, and assayed in 0.1M potassium phosphate, pH 7.0. After subtracting oxidase activity, which increased with NADPH concentration, the net GOR activity was similar at different oxidised glutathione (GSSG) and NADPH concentrations, thus confirming enzyme saturation. The optimised GOR assay used 2.5mM GSSG and 0.12mM NADPH; 5mM EDTA was also added to prevent the enzyme from redox inactivation. The GOR activity was directly proportional to the number of cells per well for a wide range of cell densities, thus supporting the assay's validity for use with cultured cells.

The effects on GOR activity of three chemicals which induce oxidative stress, namely, paraquat, iron (II) chloride and iron (III) chloride, were examined to validate the assay under experimental conditions. The specific enzymatic activity increased to 357% of untreated control activity in 5mM paraquat-treated cells, and to 407% of control activity in cells exposed to 7.5mM iron (II) chloride. By contrast, activity decreased to 56% of control activity in cells exposed to 5mM iron (III) chloride. In conclusion, the changes in GOR activity detected in Vero cells confirm that the new assay is suitable for routine in vitro screening of toxicants capable of inducing oxidative stress.

Reductive inactivation of yeast glutathione reductase by Fe(II) and NADPH

Glutathione reductase (GR) carries out the enzymatic reduction of glutathione disulfide (GSSG) to its reduced form (GSH) at the expense of the reducing power of NADPH. Previous studies have shown that GR from several species is progressively inactivated in the presence of NADPH, but that the mechanism of inactivation (especially in the presence of metals) has not been fully elucidated. We have investigated the involvement of iron ions in the inactivation of yeast (Saccharomyces cerevisiae) GR in the presence of NADPH. Even in the absence of added iron, inactivation of GR was partly blocked by the iron chelators, deferoxamine and ortho-phenanthroline, suggesting the involvement of trace amounts of contaminating iron in the mechanism of inhibition. Exogenously added antioxidants including ethanol, dimethylsulfoxide and 2-deoxyribose did not protect GR against NADPH-induced inactivation, whilst addition of exogenous Fe(II) (but not Fe(III)) potentiated the inactivation. Moreover, removal of oxygen from the medium led to increased inhibition of GR, whereas pre-incubation of the Fe(II)-containing medium for 30 min under normoxic conditions prior to the addition of GR abolished the enzyme inactivation by NADPH. Under these pre-incubation conditions, Fe(II) is fully oxidized to Fe(III) within 1 min. Furthermore, GR that had been previously inactivated in the presence of Fe(II) plus NADPH could be partially reactivated by treatment with ortho-phenanthroline and deferoxamine. In contrast, Fe(III) had no effect on GR reactivation. Together, these results indicate that yeast GR is inactivated by a reductive mechanism mediated by NADPH and Fe(II). According to this mechanism, GR is diverted from its normal redox cycling by the generation of an inactive reduced enzyme form in which both the FAD and thiol groups at the active site are likely in a reduced state.

Enzymatic reduction of 5,5'-dithiobis-(2-nitrobenzoic acid) by lysate of rat liver mitochondria

The oxidation of mitochondrial sulphydryl groups is known to increase the permeability of mitochondrial membranes and to be a key event in oxidative stress. Resistance to this damage is thought to involve thioredoxin reductase. In this study, the reduction of 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) by a lysate of rat liver mitochondria was used to assay the mitochondrial disulphide reducing capacity. NADPH-dependent reduction of DTNB was used to distinguish enzymatic reduction from the non-enzymatic reduction. Enzymatic reduction by the mitochondrial lysate was suppressed by DTNB at concentrations exceeding 0.25 mM and by pH above 7.0. It was strongly inhibited by Zn2+ and Mn2+ (IC50 about 2.5 and 20 microM, respectively) and was more weakly inhibited by Mg2+ and Ca2+ (IC50 about 1.8 and 2.1 mM, respectively). As a consequence of inhibition by divalent cations, the reaction was stimulated by both physiological (ATP, ADP, pyrophosphate and citrate) and non-physiological (EDTA and EGTA) chelators. Reduction of insulin disulphides by the mitochondrial lysate was dependent on the presence of a divalent cation chelator during the isolation of mitochondria and during the enzyme assay. Our results suggest that stimulation of mitochondrial disulphide reducing activity by lowered pH, as well as by increased levels of ATP, ADP and citrate, has the potential to contribute to the maintenance of mitochondrial sulphydryl groups under oxidative conditions.

Regulation of horse-liver glutathione reductase

1. The enzyme was rapidly inactivated by NAD(P)H, GSH, dithionite or borohydride, while activity increased in the presence of NAD(P)+ or GSSG. NADH was more efficient for inactivation than NADPH. Redox inactivation required neutral or alkaline pH, was maximal at pH 8.5, and depended on the presence of metal cations. 2. GSSG and dithiothreitol fully protected the enzyme from inactivation at concentrations stoichiometric with NAD(P)H. Ten-fold higher ferricyanide or GSH concentrations were required to obtain partial protection. NAD+ or NADP+ were quite ineffective. 3. GSSG fully reactivated the inactive enzyme at 38 degrees C and neutral to acidic pH (5.5-7.5). Reactivation by dithiothreitol was accomplished in short periods of time at pH 8.5 although the activity was progressively lost afterwards. Ferricyanide and GSH also reactivated the enzyme to different extents.

Horse-liver glutathione reductase: purification and characterization

1. Purification of horse-liver glutathione reductase was obtained by affinity chromatography on N6-(6-aminohexyl)-adenosine-1'5'-bisphosphate Sepharose (N6-2'5'-ADP-Sepharose) and Reactive Red-120-Agarose, and chromatography on DEAE-Sephadex and Sephacryl S-300. 2. The final preparation had 248 U/mg specific activity after 11,174-fold purification with 47% final recovery, and was homogeneous by SDS-electrophoresis. It showed charge heterogeneity in non-denaturing electrophoresis and chromatofocusing, with several peaks of pI between 5.7 and 6.7. 3. The enzyme was homodimeric (107,000 native MW), with S20w = 6.31 S, and 41.22 A of hydrodynamic radius. It showed absorption peaks at 270, 370 and 462 nm, a characteristic of flavoproteins. 4. When NADPH was substituted by deamino-NADPH or NADH the enzyme showed 69 and 8.5% activity, respectively, while with glutathione-CoA mixed disulfide the enzyme had 23% of the activity shown with GSSG. Apparent Km values of 8.8, 680, 59, and 560 microM were measured for NADPH, NADH, GSSG and ferricyanide, respectively.

Glutathione reductase from Saccharomyces cerevisiae undergoes redox interconversion in situ and in vivo

Redox interconversion of glutathione reductase was studied in situ with S. cerevisiae. The enzyme was more sensitive to redox inactivation in 24 hour-starved cells than in freshly-grown ones. While 5 microM NADPH or 100 microM NADH caused 50% inactivation in normal cells in 30 min, 0.75 microM NADPH or 50 microM NADH promoted a similar effect in starved cells. GSSG reactivated the enzyme previously inactivated by NADPH, ascertaining that the enzyme was subjected to redox interconversion. Low EDTA concentrations fully protected the enzyme from NADPH inactivation, thus confirming the participation of metals in such a process. Extensive inactivation was obtained in permeabilized cells incubated with glucose-6-phosphate or 6-phosphogluconate, in agreement with the very high specific activities of the corresponding dehydrogenases. Some inactivation was also observed with malate, L-lactate, gluconate or isocitrate in the presence of low NADP+ concentrations. The inactivation of yeast glutathione reductase has also been studied in vivo. The activity decreased to 75% after 2 hours of growth with glucono-delta-lactone as carbon source, while NADPH rose to 144% and NADPH+ fell to 86% of their initial values. Greater changes were observed in the presence of 1.5 microM rotenone: enzymatic activity descended to 23% of the control value, while the NADH/NAD+ and NADPH/NADP+ ratios rose to 171% and 262% of their initial values, respectively. Such results indicate that the lowered redox potential of the pyridine nucleotide pool existing when glucono-delta-lactone is oxidized promotes in vivo inactivation of glutathione reductase.