An NADPH-independent mechanism enhances oxidative and nitrosative stress tolerance in yeast cells lacking glucose-6-phosphate dehydrogenase activity

The Key Enzymes of Carbon Metabolism and the Glutathione Antioxidant System Protect Yarrowia lipolytica Yeast Against pH-Induced Stress

In this study, we first thoroughly assayed the response of the key enzymes of energy metabolism and the antioxidant system in Yarrowia lipolytica yeast at extreme pH. The activity of the tricarboxylic acid cycle enzymes, namely NAD-dependent isocitrate dehydrogenase, aconitate hydratase, NAD-dependent malate dehydrogenase, and fumarate hydratase, NADPH-producing enzymes of glucose-6-P dehydrogenase and NADP-dependent isocitrate dehydrogenase, and the enzymes of the glutathione system was assessed. All the enzymes that were tested showed a significant induction contrary to some decrease in the aconitate hydratase activity with acidic and alkaline stress. It is probable that a change in the enzyme activity in the mitochondria matrix is involved in the regulation of the cellular metabolism of Y. lipolytica, which allows the species to prosper at an extreme ambient pH. It distinguishes it from any other type of ascomycete. A close relationship between the induction of the Krebs cycle enzymes and the key enzymes of the glutathione system accompanied by an increased level of reduced glutathione was shown. The assumption that the increased activity of the Krebs cycle dehydrogenases and promotion of the pentose phosphate pathway at pH stress launches a set of events determining the adaptive response of Y. lipolytica yeast.

Fungal glyceraldehyde 3-phosphate dehydrogenase GpdC maintains glycolytic mechanism against reactive nitrogen stress-induced damage

Highly reactive nitrogen species (RNS) damage proteins, lipids, and nucleotides, and induce disordered intracellular metabolism. Microorganisms that respond to and defend against RNS include fungal pathogens that invade host tissues. However, the full picture of their mechanisms remains unknown. We identified a novel glyceraldehyde 3-phosphate dehydrogenase (GAPDH) isozyme (GpdC) in the fungus Aspergillus nidulans. This isozyme preferred NADP+, which was unlike glycolytic GpdA that uses NAD+ as a cofactor. Exogenous RNS induced expression of the encoding gpdC gene, which when disrupted, decreased intracellular GAPDH activity, mycelial proliferation, and ethanol fermentation under RNS stress. Under these conditions, fungal growth requires glucose instead of non-fermentable carbon sources, and intact pyruvate decarboxylase (pdcA) and alcohol dehydrogenase (alcC) genes indicated that fungal metabolism shifts from respiratory to glycolytic and ethanolic fermentation. These results indicated that GpdC is an alternative GAPDH isozyme that facilitates NADP+-dependent glycolysis and energy conservation, which constitutes a fungal mechanism of stress tolerance via metabolic adaptation.

Metallothionein Cup1 attenuates nitrosative stress in the yeast Saccharomyces cerevisiae

Metallothionein (MT), which is a small metal-binding protein with cysteine-rich motifs, functions in the detoxification of heavy metals in a variety of organisms. Even though previous studies suggest that MT is involved in the tolerance mechanisms against nitrosative stress induced by toxic levels of nitric oxide (NO) in mammalian cells, the physiological functions of MT in relation to NO have not been fully understood. In this study, we analyzed the functions of MT in nitrosative stress tolerance in the yeast Saccharomyces cerevisiae. Our phenotypic analyses showed that deletion or overexpression of the MT-encoding gene, CUP1, led to higher sensitivity or tolerance to nitrosative stress in S. cerevisiae cells, respectively. We further examined whether the yeast MT Cup1 in the cell-free lysate scavenges NO. These results showed that the cell-free lysate containing a higher level of Cup1 degraded NO more efficiently. On the other hand, the transcription level of CUP1 was not affected by nitrosative stress treatment. Our findings suggest that the yeast MT Cup1 contributes to nitrosative stress tolerance, possibly as a constitutive rather than an inducible defense mechanism.

Acetaldehyde reacts with a fluorescent nitric oxide probe harboring an o-phenylenediamine structure that interferes with fluorometry

Nitric oxide (NO) is a ubiquitous signaling molecule, and thus a variety of methods have been developed for its detection and quantification. Fluorometric analyses using a fluorescent NO probe harboring an o-phenylenediamine (OPD) structure are widely used for NO analyses in various organisms, including yeast. Here, we discovered that an NO-independent fluorophore (UNK436) was generated from a fluorescent NO probe 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM), which has an OPD structure, in yeast cells. The molecules responsible for this undesirable fluorescence and their reaction mechanisms were analyzed. Our mass spectrometric analysis showed that two carbon atoms from glucose were incorporated into UNK436. Subsequent analyses indicated that a non-proteinous small compound leads to the synthesis of UNK436 through an oxidative reaction. Furthermore, our LC/MS/MS analysis of the reaction mixture of DAF-FM with acetaldehyde in combination with stable isotope labeling demonstrated that acetaldehyde reacts with DAF-FM oxidatively, generating UNK436. Another NO probe with an OPD structure, diaminorhodamine-4M, reacted with acetaldehyde in the same way to emit fluorescence. Based on our findings, we recommend that in researches using OPD-based fluorescent NO probes, alternative analyses also be performed to identify the reaction products of the probes with NO to avoid false-positives.

Oxidative stress response pathways in fungi

Fungal response to any stress is intricate, specific, and multilayered, though it employs only a few evolutionarily conserved regulators. This comes with the assumption that one regulator operates more than one stress-specific response. Although the assumption holds true, the current understanding of molecular mechanisms that drive response specificity and adequacy remains rudimentary. Deciphering the response of fungi to oxidative stress may help fill those knowledge gaps since it is one of the most encountered stress types in any kind of fungal niche. Data have been accumulating on the roles of the HOG pathway and Yap1- and Skn7-related pathways in mounting distinct and robust responses in fungi upon exposure to oxidative stress. Herein, we review recent and most relevant studies reporting the contribution of each of these pathways in response to oxidative stress in pathogenic and opportunistic fungi after giving a paralleled overview in two divergent models, the budding and fission yeasts. With the concept of stress-specific response and the importance of reactive oxygen species in fungal development, we first present a preface on the expanding domain of redox biology and oxidative stress.

NADPH is important for isobutanol tolerance in a minimal medium of Saccharomyces cerevisiae

We showed that the isobutanol sensitivity in glucose-6-phosphate dehydrogenase-deficient cells of the yeast Saccharomyces cerevisiae was rescued by an alternative NADPH producer, acetaldehyde dehydrogenase, but not in the cells lacking 6-phosphogluconate dehydrogenase. This phenotype correlated with the intracellular NADPH/NADP+ ratio in yeast strains. Our findings indicate the importance of NADPH for the isobutanol tolerance of yeast cells.