AtNHX8, a member of the monovalent cation: proton antiporter-1 family in Arabidopsis thaliana, encodes a putative Li/H antiporter

The Arabidopsis monovalent cation:proton antiporter-1 (CPA1) family includes eight members, AtNHX1-8. AtNHX1 and AtNHX7/SOS1 have been well characterized as tonoplast and plasma membrane Na+/H+ antiporters, respectively. The proteins AtNHX2-6 have been phylogenetically linked to AtNHX1, while AtNHX8 appears to be related to AtNHX7/SOS1. Here we report functional characterization of AtNHX8. AtNHX8 T-DNA insertion mutants are hypersensitive to lithium ions (Li+) relative to wild-type plants, but not to the other metal ions such as sodium (Na+), potassium (K+) and caesium (Cs+). AtNHX8 overexpression in a triple-deletion yeast mutant AXT3 that exhibits defective Na+/Li+ transport specifically suppresses sensitivity to Li+, but does not affect Na+ sensitivity. Likewise, AtNHX8 overexpression complemented sensitivity to Li+, but not Na+, in sos1-1 mutant seedlings, and increased Li+ tolerance of both the sos1-1 mutant and wild-type seedlings. Results of Li+ and K+ measurement of loss-of-function and gain-of-function mutants indicate that AtNHX8 may be responsible for Li+ extrusion, and may be able to maintain K+ acquisition and intracellular ion homeostasis. Subcellular localization of the AtNHX8-enhanced green fluorescent protein (EGFP) fusion protein suggested that AtNHX8 protein is targeted to the plasma membrane. Taken together, our findings suggest that AtNHX8 encodes a putative plasma membrane Li+/H+ antiporter that functions in Li detoxification and ion homeostasis in Arabidopsis.     

Kinetic characterization of wild-type and proton transfer-impaired variants of beta-carbonic anhydrase from Arabidopsis thaliana

We have cloned and overexpressed a truncated, recombinant form of beta-carbonic anhydrase from Arabidopsis thaliana. The wild-type enzyme and two site-directed variants, H216N and Y212F, have been kinetically characterized both at steady state by stopped-flow spectrophotometry and at chemical equilibrium by (18)O isotope exchange methods. The wild-type enzyme has a maximal k(cat) for CO2 hydration of 320 ms(-1) and is rate limited by proton transfer involving two residues with apparent pK(a) values of 6.0 and 8.7. The mutant enzyme H216N has a maximal k(cat) at high pH that is 43% that of wild type, but is only 5% that of wild type at pH 7.0. (18)O exchange studies reveal that the effect of the mutations H216N or Y212F is primarily on proton transfer steps in the catalytic mechanism and not in the rate of CO2-HCO3- exchange. These results suggest that residues His-216 and Tyr-212 are both important for efficient proton transfer in A. thaliana carbonic anhydrase.     

Studies of the enzymic mechanism of Candida tenuis xylose reductase (AKR 2B5): X-ray structure and catalytic reaction profile for the H113A mutant

Xylose reductase from the yeast Candida tenuis (CtXR) is a family 2 member of the aldo-keto reductase (AKR) superfamily of proteins and enzymes. Active site His-113 is conserved among AKRs, but a unified mechanism of how it affects catalytic activity is outstanding. We have replaced His-113 by alanine using site-directed mutagenesis, determined a 2.2 A structure of H113A mutant bound to NADP(+), and compared catalytic reaction profiles of NADH-dependent reduction of different aldehydes catalyzed by the wild type and the mutant. Deuterium kinetic isotope effects (KIEs) on k(cat) and k(cat)/K(m xylose) show that, relative to the wild type, the hydride transfer rate constant (k(7) approximately 0.16 s(-1)) has decreased about 1000-fold in H113A whereas xylose binding was not strongly affected. No solvent isotope effect was seen on k(cat) and k(cat)/K(m xylose) for H113A, suggesting that proton transfer has not become rate-limiting as a result of the mutation. The pH profiles of log(k(cat)/K(m xylose)) for the wild type and H113A decreased above apparent pK(a) values of 8.85 and 7.63, respectively. The DeltapK(a) of -1.2 pH units likely reflects a proximally disruptive character of the mutation, affecting the position of Asp-50. A steady-state kinetic analysis for H113A-catalyzed reduction of a homologous series of meta-substituted benzaldehyde derivatives was carried out, and quantitative structure-reactivity correlations were used to factor the observed kinetic substituent effect on k(cat) and k(cat)/K(m aldehyde) into an electronic effect and bonding effects (which are lacking in the wild type). Using the Hammett sigma scale, electronic parameter coefficients (rho) of +0.64 (k(cat)) and +0.78 (k(cat)/K(m aldehyde)) were calculated and clearly differ from rho(k(cat)/K(aldehyde)) and rho(k(cat)) values of +1.67 and approximately 0.0, respectively, for the wild-type enzyme. Hydride transfer rate constants of H113A, calculated from kinetic parameters and KIE data, display a substituent dependence not seen in the corresponding wild-type enzyme rate constants. An enzymic mechanism is proposed in which His-113, through a hydrogen bond from Nepsilon2 to aldehyde O1, assists in catalysis by optimizing the C=O bond charge separation and orbital alignment in the ternary complex.     

Involvement of the H3O+-Lys-164 -Gln-161-Glu-345 charge transfer pathway in proton transport of gastric H+,K+-ATPase

Gastric H(+),K(+)-ATPase is shown to transport 2 mol of H(+)/mol of ATP hydrolysis in isolated hog gastric vesicles. We studied whether the H(+) transport mechanism is due to charge transfer and/or transfer of hydronium ion (H(3)O(+)). From transport of [(18)O]H(2)O, 1.8 mol of water molecule/mol of ATP hydrolysis was found to be transported. We performed a molecular dynamics simulation of the three-dimensional structure model of the H(+),K(+)-ATPase alpha-subunit at E(1) conformation. It predicts the presence of a charge transfer pathway from hydronium ion in cytosolic medium to Glu-345 in cation binding site 2 (H(3)O(+)-Lys-164 -Gln-161-Glu-345). No charge transport pathway was formed in mutant Q161L, E345L, and E345D. Alternative pathways (H(3)O(+)-Gln-161-Glu-345) in mutant K164L and (H(3)O(+)-Arg-105-Gln-161-Gln-345) in mutant E345Q were formed. The H(+),K(+)-ATPase activity in these mutants reflected the presence and absence of charge transfer pathways. We also found charge transfer from sites 2 to 1 via a water wire and a charge transfer pathway (H(3)O(+)-Asn-794 -Glu-797). These results suggest that protons are charge-transferred from the cytosolic side to H(2)O in sites 2 and 1, the H(2)O comes from cytosolic medium, and H(3)O(+) in the sites are transported into lumen during the conformational transition from E(1)PtoE(2)P.     

The tomato ethylene receptor LE-ETR3 (NR) is not involved in mediating ozone sensitivity: causal relationships among ethylene emission, oxidative burst and tissue damage

The causal relationships among ethylene emission, oxidative burst and tissue damage, and the temporal expression patterns of some ethylene biosynthetic and responsive genes, were examined in the Never ripe (Nr) tomato (Lycopersicon esculentum) mutant and its isogenic wild type (cv. Pearson), to investigate the role played by the ethylene receptor LE-ETR3 (NR) in mediating the plant response to ozone (O(3)). Tomato plants were used in a time-course experiment in which they were exposed to acute O(3) fumigation with 200 nl l(-1) O(3) for 4 h. The pattern of leaf lesions indicated similar sensitivities to O(3) for cv. Pearson and Nr. In both genotypes, O(3) activated a hydrogen peroxide (H(2)O(2))-dependent oxidative burst, which was also ethylene-driven in Nr leaves. Ozone induced some ethylene and jasmonate biosynthetic and inducible genes, although with different timings and to different extents in the two genotypes. The overall data indicate that Nr retains partial sensitivity to ethylene, suggesting only a marginal role of the NR receptor in mediating the complex response of tomato plants to O(3).     

Phenotype of a mechanosensitive channel mutant, mid-1, in a filamentous fungus, Neurospora crassa

In the yeast Saccharomyces cerevisiae, the MID1 (mating-induced death) gene encodes a stretch-activated channel which is required for successful mating; the mutant phenotype is rescued by elevated extracellular calcium. Homologs of the MID1 gene are found in fungi that are morphologically complex compared to yeast, both Basidiomycetes and Ascomycetes. We explored the phenotype of a mid-1 knockout mutant in the filamentous ascomycete Neurospora crassa. The mutant exhibits lower growth vigor than the wild type (which is not rescued by replete calcium) and mates successfully. Thus, the role of the MID-1 protein differs from that of the homologous gene product in yeast. Hyphal cytology, growth on diverse carbon sources, turgor regulation, and circadian rhythms of the mid-1 mutant are all similar to those of the wild type. However, basal turgor is lower than wild type, as is the activity of the plasma membrane H(+)-ATPase (measured by cyanide [CN(-)]-induced depolarization of the energy-dependent component of the membrane potential). In addition, the mutant is unable to grow at low extracellular Ca(2+) levels or when cytoplasmic Ca(2+) is elevated with the Ca(2+) ionophore A23187. We conclude that the MID-1 protein plays a role in regulation of ion transport via Ca(2+) homeostasis and signaling. In the absence of normal ion transport activity, the mutant exhibits poorer growth.     

The role of methionine recycling for ethylene synthesis in Arabidopsis

The methionine (Met) cycle contributes to sulfur metabolism through the conversion of methylthioadenosine (MTA) to Met at the expense of ATP. MTA is released as a by-product of ethylene synthesis from S-adenosylmethionine (AdoMet). Disruption of the Met cycle in the Arabidopsis mtk mutant resulted in an imbalance of AdoMet homeostasis at sulfur-limiting conditions, irrespective of the sulfur source supplied to the plants. At a low concentration of 100 mum sulfate, the mtk mutant had reduced AdoMet levels and growth was retarded as compared with wild type. An elevated production of ethylene was measured in seedlings of the ethylene-overproducing eto3 mutant. When Met cycle knockout and ethylene overproduction were combined in the mtk/eto3 double mutant, a reduced capacity for ethylene synthesis was observed in seedlings. Even though mature eto3 plants did not produce elevated ethylene levels, and AdoMet homeostasis in eto3 plants did not differ from that in wild type, shoot growth was severely retarded. The mtk/eto3 double mutant displayed a metabolic plant phenotype that was similar to mtk with reduced AdoMet levels at sulfur-limiting conditions. We conclude from our data that the Met cycle contributes to the maintenance of AdoMet homeostasis, especially when de novo AdoMet synthesis is limited. Our data further showed that the Met cycle is required to sustain high rates of ethylene synthesis. Expression of the Met cycle genes AtMTN1, AtMTN2, AtMTK, AtARD1, AtARD2, AtARD3 and AtARD4 was not regulated by ethylene. This result is in contrast to that found in rice where OsARD1 and OsMTK are induced in response to ethylene. We hypothesize that the regulation of the Met cycle by ethylene may be restricted to plants that naturally produce high quantities of ethylene for a prolonged period of time.     

In vivo role of catalase-peroxidase in synechocystis sp. strain PCC 6803

The katG gene coding for the only catalase-peroxidase in the cyanobacterium Synechocystis sp. strain PCC 6803 was deleted in this organism. Although the rate of H2O2 decomposition was about 30 times lower in the DeltakatG mutant than in the wild type, the strain had a normal phenotype and its doubling time as well as its resistance to H2O2 and methyl viologen were indistinguishable from those of the wild type. The residual H2O2-scavenging capacity was more than sufficient to deal with the rate of H2O2 production by the cell, estimated to be less than 1% of the maximum rate of photosynthetic electron transport in vivo. We propose that catalase-peroxidase has a protective role against environmental H2O2 generated by algae or bacteria in the ecosystem (for example, in mats). This protective role is most apparent at a high cell density of the cyanobacterium. The residual H2O2-scavenging activity in the DeltakatG mutant was a light-dependent peroxidase activity. However, neither glutathione peroxidase nor ascorbate peroxidase accounted for a significant part of this H2O2-scavenging activity. When a small thiol such as dithiothreitol was added to the medium, the rate of H2O2 decomposition in the DeltakatG mutant increased more than 10-fold, indicating that a thiol-specific peroxidase, for which thioredoxin may be the physiological electron donor, is present. Oxidized thioredoxin is likely to be reduced again by photosynthetic electron transport. Therefore, under laboratory conditions, there are only two enzymatic mechanisms for H2O2 decomposition present in Synechocystis sp. strain PCC 6803. One is catalyzed by a catalase-peroxidase, and the other is catalyzed by thiol-specific peroxidase.     

Site-directed mutation of arginine 282 to glutamate uncouples the movement of peptides and protons by the rabbit proton-peptide cotransporter PepT1

A conserved positive residue in the seventh transmembrane domain of the mammalian proton-coupled di- and tripeptide transporter PepT1 has been shown by site-directed mutagenesis to be a key residue for protein function. Substitution of arginine 282 with a glutamate residue (R282E-PepT1) gave a protein at the plasma membrane of Xenopus laevis oocytes that was able to transport the non-hydrolyzable dipeptide [3H]d-Phe-l-Gln, although unlike the wild type, the rate of transport by R282E-PepT1 was independent of the extracellular pH level, and the substrate could not be accumulated above equilibrium. The binding affinity of the mutant transport protein was unchanged from the wild type. Thus, R282E-Pept1 appears to have been changed from a proton-driven to a facilitated transporter for peptides. In addition, peptide transport by R282E-PepT1 still induced depolarization as measured by microelectrode recordings of membrane potential. A more detailed study by two-electrode voltage clamping revealed that R282E-PepT1 behaved as a peptide-gated non-selective cation channel with the ion selectivity series lithium > sodium > N-methyl-d-glucamine at pH 7.4. There was also a proton conductance (comparing pH 7.4 and 8.4), and at pH 5.5 the predominant conductance was for potassium ions. Therefore, it can be concluded that changing arginine 282 to a glutamate not only uncouples the cotransport of protons and peptides of the wild-type PepT1 but also creates a peptide-gated cation channel in the protein.     

The Ethylene-insensitive sickle mutant of Medicago truncatula shows altered auxin transport regulation during nodulation

We studied the ethylene-insensitive, hypernodulating mutant, sickle (skl), to investigate the interaction of ethylene with auxin transport during root nodulation in Medicago truncatula. Grafting experiments demonstrated that hypernodulation in skl is root controlled. Long distance transport of auxin from shoot to root was reduced by rhizobia after 24 h in wild type but not in skl. Similarly, the ethylene precursor 1-amino cyclopropane-1-carboxylic acid inhibited auxin transport in wild type but not in skl. Auxin transport at the nodule initiation zone was significantly reduced by rhizobia after 4 h in both wild type and skl. After 24 h, auxin transport significantly increased at the nodule initiation zone in skl compared to wild type, accompanied by an increase in the expression of the MtPIN1 and MtPIN2 (pin formed) auxin efflux transporters. Response assays to different auxins did not show any phenotype that would suggest a defect of auxin uptake in skl. The auxin transport inhibitor N-1-naphthylphtalamic acid inhibited nodulation in wild type but not skl, even though N-1-naphthylphtalamic acid still inhibited auxin transport in skl. Our results suggest that ethylene signaling modulates auxin transport regulation at certain stages of nodule development, partially through PIN gene expression, and that an increase in auxin transport relative to the wild type is correlated with higher nodule numbers. We also discuss the regulation of auxin transport in skl in comparison to previously published data on the autoregulation mutant, super numerary nodules (van Noorden et al., 2006).     

Identification of a crucial histidine involved in metal transport activity in the Arabidopsis cation/H+ exchanger CAX1

In plants, yeast, and bacteria, cation/H+ exchangers (CAXs) have been shown to translocate Ca2+ and other metal ions utilizing the H+ gradient. The best characterized of these related transporters is the plant vacuolar localized CAX1. We have used site-directed mutagenesis to assess the impact of altering the seven histidine residues to alanine within Arabidopsis CAX1. The mutants were expressed in a Saccharomyces cerevisiae strain that is sensitive to Ca2+ and other metals. By utilizing a yeast growth assay, the H338A mutant was the only mutation that appeared to alter Ca2+ transport activity. The CAX1 His338 residue is conserved among various CAX transporters and may be located within a filter for cation selection. We proceeded to mutate His338 to every other amino acid residue and utilized yeast growth assays to estimate the transport properties of the 19 CAX mutants. Expression of 16 of these His338 mutants could not rescue any of the metal sensitivities. However, expression of H338N, H338Q, and H338K allowed for some growth on media containing Ca2+. Most interestingly, H338N exhibited increased tolerance to Cd2+ and Zn2+. Endomembrane fractions from yeast cells were used to measure directly the transport of H338N. Although the H338N mutant demonstrated 25% of the wild type Ca2+/H+ transport, it showed an increase in transport for both Cd2+ and Zn2+ reflected in a decrease in the Km for these substrates. This study provides insights into the CAX cation filter and novel mechanisms by which metals may be partitioned across membranes.     

Deletion of a histidine-rich loop of AtMTP1, a vacuolar Zn(2+)/H(+) antiporter of Arabidopsis thaliana, stimulates the transport activity

Arabidopsis thaliana AtMTP1 belongs to the cation diffusion facilitator family and is localized on the vacuolar membrane. We investigated the enzymatic kinetics of AtMTP1 by a heterologous expression system in the yeast Saccharomyces cerevisiae, which lacked genes for vacuolar membrane zinc transporters ZRC1 and COT1. The yeast mutant expressing AtMTP1 heterologously was tolerant to 10 mm ZnCl(2). Active transport of zinc into vacuoles of living yeast cells expressing AtMTP1 was confirmed by the fluorescent zinc indicator FuraZin-1. Zinc transport was quantitatively analyzed by using vacuolar membrane vesicles prepared from AtMTP1-expressing yeast cells and radioisotope (65)Zn(2+). Active zinc uptake depended on a pH gradient generated by endogenous vacuolar H(+)-ATPase. The activity was inhibited by bafilomycin A(1), an inhibitor of the H(+)-ATPase. The K(m) for Zn(2+) and V(max) of AtMTP1 were determined to be 0.30 microm and 1.22 nmol/min/mg, respectively. We prepared a mutant AtMTP1 that lacked the major part (32 residues from 185 to 216) of a long histidine-rich hydrophilic loop in the central part of AtMTP1. Yeast cells expressing the mutant became hyperresistant to high concentrations of Zn(2+) and resistant to Co(2+). The K(m) and V(max) values were increased 2-11-fold. These results indicate that AtMTP1 functions as a Zn(2+)/H(+) antiporter in vacuoles and that a histidine-rich region is not essential for zinc transport. We propose that a histidine-rich loop functions as a buffering pocket of Zn(2+) and a sensor of the zinc level at the cytoplasmic surface. This loop may be involved in the maintenance of the level of cytoplasmic Zn(2+).     

Saccharomyces cerevisiae BY4741 and W303-1A laboratory strains differ in salt tolerance

Saccharomyces cerevisiae yeast cells serve as a model to elucidate the bases of salt tolerance and potassium homeostasis regulation in eukaryotic cells. In this study, we show that two widely used laboratory strains, BY4741 and W303-1A, differ not only in cell size and volume but also in their relative plasma-membrane potential (estimated with a potentiometric fluorescent dye diS-C3(3) and as Hygromycin B sensitivity) and tolerance to alkali-metal cations. W303-1A cells and their mutant derivatives lacking either uptake (trk1 trk2) or efflux (nha1) systems for alkali-metal cations are more tolerant to toxic sodium and lithium cations but also more sensitive to higher external concentrations of potassium than BY4741 cells and their mutants. Moreover, our results suggest that though the two strains do not differ in the total potassium content, the regulation of intracellular potassium homeostasis is probably not the same in BY4741 and W303-1A cells.     

耐辐射球菌基因DRB0099缺失突变株的构建及逆境分析

耐辐射球菌(Deinococcus radiodurans R1)有着极强的辐射抗性.研究其抗辐射的机理对于处理放射性废料有着潜在的应用价值.在耐辐射球菌的基因组中,许多序列的功能未知.其中DRB0099尤为引人注意.将DRB0099缺失突变构建该基因的突变株.对野生型和突变体进行比较后发现,在正常生长条件下的前期阶段(0～16 h),突变体生长速度比野生型慢.16 h以后,野生型逐渐进入稳定生长期.这时,突变株的生长速度高于野生型.但是,野生型的浓度一直高于突变株.表明在DRB0099被删除后,耐辐射球菌的生长可能受到了阻滞.在紫外线照射的条件下,尽管野生型随着照射剂量的增加,存活率越来越低,但是要比突变体高许多.野生型具有比突变体更强的修复DNA双链断裂的能力.DRB0099可能直接参与了对DNA的修复.突变体对H2O2的敏感程度高于野生型,表明野生型耐辐射球菌在对抗活性氧保护其蛋白质、DNA或者DNA修复方面具有比突变体更强的功能.在低浓度H2O2处理条件下,尽管野生型和突变体的存活率都出现下降趋势,但二者的差值并不大.随着H2O2剂量的增加,二者的差值越来越大.表明随着活性氧浓度的增加,蛋白质和DNA损伤的数量增加,失去DRB0099基因功能的突变体比野生型更容易受到损伤.在紫外线照射处理或者H2O2处理条件下,DRB0099能够保护蛋白质和DNA.     

Concurrent activation of cell death-regulating signaling pathways by singlet oxygen in Arabidopsis thaliana

Upon a dark/light shift the conditional flu mutant of Arabidopsis starts to generate singlet oxygen ((1)O(2)), a non-radical reactive oxygen species that is restricted to the plastid compartment. Immediately after the shift, plants stop growing and develop necrotic lesions. We have established a protoplast system, which allows detection and characterization of the death response in flu induced by the release of (1)O(2). Vitamin B6 that quenches (1)O(2) in fungi was able to protect flu protoplasts from cell death. Blocking ethylene production was sufficient to partially inhibit the death reaction. Similarly, flu mutant seedlings expressing transgenic NahG were partially protected from the death provoked by the release of (1)O(2), indicating a requirement for salicylic acid (SA) in this process, whereas in cells depleted of both, ethylene and SA, the extent of cell death was reduced to the wild-type level. The flu mutant was also crossed with the jasmonic acid (JA)-depleted mutant opr3, and with the JA, OPDA and dinor OPDA (dnOPDA)-depleted dde2-2 mutant. Analysis of the resulting double mutants revealed that in contrast to the JA-induced suppression of H(2)O(2)/superoxide-dependent cell death reported earlier, JA promotes singlet oxygen-mediated cell death in flu, whereas other oxylipins such as OPDA and dnOPDA antagonize this death-inducing activity of JA.     

NAD(+) consumption in carcinogen-treated hamster cells overexpressing a dominant negative mutant of poly(ADP-ribose) polymerase

We found that 82- and 76-kDa proteins in the outer-membrane fractions were overproduced in the hfq::cat mutant cells when grown in synthetic media. Expression of these proteins was repressed by addition of FeCl(3) in the mutant as well as in the wild type. It was revealed that these are FepA and FhuE proteins involved in iron transport. The hfq::cat mutant was more susceptible to killing by hydrogen peroxide, probably due to the excess incorporation of iron, which potentially generates hydroxyl radicals. Increased incorporation of iron in the hfq::cat mutant was also confirmed by the suppressive effect on the ftsH1 mutation. These results suggest that the hfq gene product is involved in the defense mechanism against oxidative stress.     

Ozone-sensitive arabidopsis rcd1 mutant reveals opposite roles for ethylene and jasmonate signaling pathways in regulating superoxide-dependent cell death

We have isolated a codominant Arabidopsis mutant, radical-induced cell death1 (rcd1), in which ozone (O(3)) and extracellular superoxide (O(2)(*)-), but not hydrogen peroxide, induce cellular O(2)(*)- accumulation and transient spreading lesions. The cellular O(2)(*)- accumulation is ethylene dependent, occurs ahead of the expanding lesions before visible symptoms appear, and is required for lesion propagation. Exogenous ethylene increased O(2)(*)--dependent cell death, whereas impairment of ethylene perception by norbornadiene in rcd1 or ethylene insensitivity in the ethylene-insensitive mutant ein2 and in the rcd1 ein2 double mutant blocked O(2)(*)- accumulation and lesion propagation. Exogenous methyl jasmonate inhibited propagation of cell death in rcd1. Accordingly, the O(3)-exposed jasmonate-insensitive mutant jar1 displayed spreading cell death and a prolonged O(2)(*)- accumulation pattern. These results suggest that ethylene acts as a promoting factor during the propagation phase of developing oxyradical-dependent lesions, whereas jasmonates have a role in lesion containment. Interaction and balance between these pathways may serve to fine-tune propagation and containment processes, resulting in alternate lesion size and formation kinetics.     

Overexpression of peroxisomal malate dehydrogenase MDH3 gene enhances cell death on H2O2 stress in the ald5 mutant of Saccharomyces cerevisiae

Mitochondrial aldehyde dehydrogenase ALD5 of Saccharomyces cerevisiae is involved in the biosynthesis of mitochondrial electron transport chain, and the ald5 mutant is incompetent for respiration. With use of the mutant, we examined the detoxication of H2O2 generation by fatty acid beta-oxidation in peroxisome. The ald5 mutant (AKD321), as well as the 746 rho0 mutant, was more resistant to H2O2 stress than the wild type. However, overexpression of the MDH3 gene that was involved in the reoxidation of NADH during fatty acid beta-oxidation caused a decrease in cell viability of AKD321 to H2O2 stress, while the 746 rho0 mutant had no such effect. Intracellular H2O2 concentration increased approximately fourfold in MDH3 overexpressing ald5 strain (MD3-AKD321), compared with AKD321. The peroxisomal catalase activity of MD3-AKD321 decreased by 83% to that of AKD321. And also, the overexpression of MDH3 had only a weak effect in MDH3 overexpressing 746 rho0 strain, decreasing by 14% to that of 746 rho0 mutant. The increased palmitoyl CoA oxidation by overexpression of MDH3 gene was the same in both strains. Under conditions of MDH3 overexpression, peroxisomal catalase (CTA1) appears to be a limiting factor to oxidative stress. These observations point to an important, as yet unidentified, role of mitochondrial aldehyde dehydrogenase (ALD5) to endogeneous oxidative stress in peroxisome.