Characterisation of D-arabinono-1,4-lactone oxidase from Candida albicans ATCC 10231

Structure, mechanism, and evolution of the last step in vitamin C biosynthesis

Photosynthetic organisms, fungi, and animals comprise distinct pathways for vitamin C biosynthesis. Besides this diversity, the final biosynthetic step consistently involves an oxidation reaction carried out by the aldonolactone oxidoreductases. Here, we study the origin and evolution of the diversified activities and substrate preferences featured by these flavoenzymes using molecular phylogeny, kinetics, mutagenesis, and crystallographic experiments. We find clear evidence that they share a common ancestor. A flavin-interacting amino acid modulates the reactivity with the electron acceptors, including oxygen, and determines whether an enzyme functions as an oxidase or a dehydrogenase. We show that a few side chains in the catalytic cavity impart the reaction stereoselectivity. Ancestral sequence reconstruction outlines how these critical positions were affixed to specific amino acids along the evolution of the major eukaryotic clades. During Eukarya evolution, the aldonolactone oxidoreductases adapted to the varying metabolic demands while retaining their overarching vitamin C-generating function.

Central Carbon Metabolism in Candida albicans Biofilms Is Altered by Dimethyl Sulfoxide

The effect of dimethyl sulfoxide (DMSO) on fungal metabolism has not been well studied. This study aimed to evaluate, by metabolomics, the impact of DMSO on the central carbon metabolism of Candida albicans. Biofilms of C. albicans SC5314 were grown on paper discs, using minimum mineral (MM) medium, in a dynamic continuous flow system. The two experimental conditions were control and 0.03% DMSO (v/v). After 72 h of incubation (37 °C), the biofilms were collected and the metabolites were extracted. The extracted metabolites were subjected to gas chromatography-mass spectrometry (GC/MS). The experiment was conducted using five replicates on three independent occasions. The GC/MS analysis identified 88 compounds. Among the 88 compounds, the levels of 27 compounds were markedly different between the two groups. The DMSO group exhibited enhanced levels of putrescine and glutathione and decreased levels of methionine and lysine. Additionally, the DMSO group exhibited alterations in 13 metabolic pathways involved in primary and secondary cellular metabolism. Among the 13 altered pathways, seven were downregulated and six were upregulated in the DMSO group. These results indicated a differential intracellular metabolic profile between the untreated and DMSO-treated biofilms. Hence, DMSO was demonstrated to affect the metabolic pathways of C. albicans. These results suggest that DMSO may influence the results of laboratory tests when it is used as a solvent. Hence, the use of DMSO as a solvent must be carefully considered in drug research, as the effect of the researched drugs may not be reliably translated into clinical practice.

Transport of Vitamin C in Cancer

Significance: Vitamin C is a powerful antioxidant that has an intricate relationship with cancer and has been studied for more than 60 years. However, the specific mechanisms that allow malignant cells to uptake, metabolize, and compartmentalize vitamin C remain unclear. In normal human cells, two different transporter systems are responsible for its acquisition: glucose transporters (GLUTs) transport the oxidized form of vitamin C (dehydroascorbic acid) and sodium-coupled ascorbic acid transporters (SVCTs) transport the reduced form (ascorbic acid [AA]). In this study, we review the mechanisms described for vitamin C uptake and metabolization in cancer. Recent Advances: Several studies performed recently in vivo and in vitro have provided the scientific community a better understanding of the differential capacities of cancer cells to acquire vitamin C: tumors from different origins do not express SVCTs in the plasma membrane and are only able to acquire vitamin C in its oxidized form. Interestingly, cancer cells differentially express a mitochondrial form of SVCT2. Critical Issues: Why tumors have reduced AA uptake capacity at the plasma membrane, but develop the capacity of AA transport within mitochondria, remains a mystery. However, it shows that understanding vitamin C physiology in tumor survival might be key to decipher the controversies in its relationship with cancer. Future Directions: A comprehensive analysis of the mechanisms by which cancer cells acquire, compartmentalize, and use vitamin C will allow the design of new therapeutic approaches in human cancer. Antioxid. Redox Signal. 35, 61-74.

One-Step Biosynthesis of Vitamin C in Saccharomyces cerevisiae

Vitamin C (VC) is comprehensively applied in foods, cosmetics, pharmaceuticals, and especially clinical medicine. Nowadays, the industrial production of VC mainly relies on the classic two-step fermentation route, and researchers have explored the way for one-step fermentation of VC in recent years. In this study, a VC biosynthesis pathway that directly produced VC from glucose was reconstructed in Saccharomyces cerevisiae, and the protein engineering and metabolic engineering strategies were adopted to improve it. First, five exogenous modules from Arabidopsis were introduced into the chassis cells by synthetic biology approaches to obtain the strain YLAA harboring VC biosynthesis. In addition, L-galactose dehydrogenase (L-GalDH) and L-galactono-1,4-lactone dehydrogenase (L-GLDH) were fused and expressed in S. cerevisiae cells for the first time, which increased the intracellular VC accumulation by 2.78-fold, reaching 9.97 ± 0.09 mg/L. Through copy number engineering, it was further confirmed that the last step catalyzed by L-GLDH is the rate-limiting step. GDP-L-galactose phosphorylase (GPP) encoded by vtc2 is another rate-limiting enzyme confirmed by GAL1p overexpression results. Finally, by balancing gene expression and cell growth, the highest production strain with overexpressing vtc2 by multicopy plasmids was constructed. The VC accumulation reached 24.94 ± 1.16 mg/L, which was currently the highest production from glucose in S. cerevisiae. The production of the recombinant strain reached nearly 44 mg/L with the exogenous addition of L-galactose or glutathione. The results further emphasized the importance of the step catalyzed by GPP. The investigation provided experience for the efficient biosynthesis of VC and the determination of rate-limiting steps.

Characterization of Two Arabidopsis L-Gulono-1,4-lactone Oxidases, AtGulLO3 and AtGulLO5, Involved in Ascorbate Biosynthesis

L-Ascorbic acid (AsA, vitamin C) is an essential antioxidant for plants and animals. There are four known ascorbate biosynthetic pathways in plants: the L-galactose, L-gulose, D-galacturonate, and myo-inositol routes. These pathways converge into two AsA precursors: L-galactono-1,4-lactone and L-gulono-1,4-lactone (L-GulL). This work focuses on the study of L-gulono-1,4-lactone oxidase (GulLO), the enzyme that works at the intersect of the gulose and inositol pathways. Previous studies have shown that feeding L-gulono-1,4-lactone to multiple plants leads to increased AsA. There are also reports showing GulLO activity in plants. We describe the first detailed characterization of a plant enzyme specific to oxidize L-GulL to AsA. We successfully purified a recombinant Arabidopsis GulLO enzyme (called AtGulLO5) in a transient expression system. The biochemical properties of this enzyme are similar to the ones of bacterial isozymes in terms of substrate specificity, subcellular localization, use of flavin adenine dinucleotide (FAD) as electron acceptor, and specific activity. AtGulLO5 is an exclusive dehydrogenase with an absolute specificity for L-GulL as substrate thus differing from the existing plant L-galactono-1,4-lactone dehydrogenases and mammalian GulLOs. Feeding L-GulL to N. benthamiana leaves expressing AtGulLO5 constructs led to increased foliar AsA content, but it was not different from that of controls, most likely due to the observed low catalytic efficiency of AtGulLO5. Similar results were also obtained with another member of the AtGulLO family (AtGulLO3) that appears to have a rapid protein turnover. We propose that AsA synthesis through L-GulL in plants is regulated at the post-transcriptional level by limiting GulLO enzyme availability.

Co-expression of NCED and ALO improves vitamin C level and tolerance to drought and chilling in transgenic tobacco and stylo plants

Abscisic acid (ABA) regulates plant adaptive responses to various environmental stresses, while L-ascorbic acid (AsA) that is also named vitamin C is an important antioxidant and involves in plant stress tolerance and the immune system in domestic animals. Transgenic tobacco (Nicotiana tabacum L.) and stylo [Stylosanthes guianensis (Aublet) Swartz], a forage legume, plants co-expressing stylo 9-cis-epoxycarotenoid dioxygenase (SgNCED1) and yeast D-arabinono-1,4-lactone oxidase (ALO) genes were generated in this study, and tolerance to drought and chilling was analysed in comparison with transgenic tobacco overexpressing SgNCED1 or ALO and the wild-type plants. Compared to the SgNCED1 or ALO transgenic plants, in which only ABA or AsA levels were increased, both ABA and AsA levels were increased in transgenic tobacco and stylo plants co-expressing SgNCED1 and ALO genes. Compared to the wild type, an enhanced drought tolerance was observed in SgNCED1 transgenic tobacco plants with induced expression of drought-responsive genes, but not in ALO plants, while an enhanced chilling tolerance was observed in ALO transgenic tobaccos with induced expression of cold-responsive genes, but not in SgNCED1 plants. Co-expression of SgNCED1 and ALO genes resulted in elevated tolerance to both drought and chilling in transgenic tobacco and stylo plants with induced expression of both drought and cold-responsive genes. Our result suggests that co-expression of SgNCED1 and ALO genes is an effective way for use in forage plant improvement for increased tolerance to drought and chilling and nutrition quality.

Recent progress on the characterization of aldonolactone oxidoreductases

L-Ascorbic acid (ascorbate, AsA, vitamin C) is essential for animal and plant health. Despite our dependence on fruits and vegetables to fulfill our requirement for this vitamin, the metabolic network leading to its formation in plants is just being fully elucidated. There is evidence supporting the operation of at least four biosynthetic pathways leading to AsA formation in plants. These routes use D-mannose/L-galactose, L-gulose, D-galacturonate, and myo-inositol as the main precursors. This review focuses on aldonolactone oxidoreductases, a subgroup of the vanillyl alcohol oxidase (VAO; EC 1.1.3.38) superfamily, enzymes that catalyze the terminal step in AsA biosynthesis in bacteria, protozoa, animals, and plants. In this report, we review the properties of well characterized aldonolactone oxidoreductases to date. A shared feature in these proteins is the presence of a flavin cofactor as well as a thiol group. The flavin cofactor in many cases is bound to the N terminus of the enzymes or to a recently discovered HWXK motif in the C terminus. The binding between the flavin moiety and the protein can be either covalent or non-covalent. Substrate specificity and subcellular localization differ among the isozymes of each kingdom. All oxidases among these enzymes possess dehydrogenase activity, however, exclusive dehydrogenases are also found. We also discuss recent evidence indicating that plants have both L-gulono-1,4-lactone oxidases and L-galactono-1,4-lactone dehydrogenases involved in AsA biosynthesis.

Candida albicans erythroascorbate peroxidase regulates intracellular methylglyoxal and reactive oxygen species independently of D-erythroascorbic acid

Candida albicans D-erythroascorbate peroxidase (EAPX1), which can catalyze the oxidation of D-erythroascorbic acid (EASC) to water, was observed to be inducible in EAPX1-deficient and EAPX1-overexpressing cells via activity staining. EAPX1-deficient cells have remarkably increased intracellular reactive oxygen species and methylglyoxal independent of the intracellular EASC content. The increased methylglyoxal caused EAPX1-deficient cells to activate catalase-peroxidase and cytochrome c peroxidase, which led to defects in cell growth, viability, mitochondrial respiration, filamentation and virulence. These findings indicate that EAPX1 mediates cell differentiation and virulence by regulating intracellular methylglyoxal along with oxidative stresses, regardless of endogenous EASC biosynthesis or alternative oxidase expression.

Evolution of alternative biosynthetic pathways for vitamin C following plastid acquisition in photosynthetic eukaryotes

Ascorbic acid (vitamin C) is an enzyme co-factor in eukaryotes that also plays a critical role in protecting photosynthetic eukaryotes against damaging reactive oxygen species derived from the chloroplast. Many animal lineages, including primates, have become ascorbate auxotrophs due to the loss of the terminal enzyme in their biosynthetic pathway, l-gulonolactone oxidase (GULO). The alternative pathways found in land plants and Euglena use a different terminal enzyme, l-galactonolactone dehydrogenase (GLDH). The evolutionary processes leading to these differing pathways and their contribution to the cellular roles of ascorbate remain unclear. Here we present molecular and biochemical evidence demonstrating that GULO was functionally replaced with GLDH in photosynthetic eukaryote lineages following plastid acquisition. GULO has therefore been lost repeatedly throughout eukaryote evolution. The formation of the alternative biosynthetic pathways in photosynthetic eukaryotes uncoupled ascorbate synthesis from hydrogen peroxide production and likely contributed to the rise of ascorbate as a major photoprotective antioxidant.

DOI:http://dx.doi.org/10.7554/eLife.06369.001

Animals, plants, algae and other eukaryotic organisms all need vitamin C to enable many of their enzymes to work properly. Vitamin C also protects plant and algal cells from damage by molecules called reactive oxygen species (ROS), which can be produced when these cells harvest energy from sunlight in a process called photosynthesis. Photosynthesis occurs inside structures called chloroplasts, and has evolved on multiple occasions in eukaryotes when non-photosynthetic organisms acquired chloroplasts from other algae and then had to develop improved defences against ROS.

There are several steps involved in the production of vitamin C. In many animals, an enzyme called GULO carries out the final step by converting a molecule known as an aldonolactone into vitamin C; this reaction also produces ROS as a waste product. The GULO enzyme is missing in humans, primates and some other groups of animals, so these organisms must get all the vitamin C they need from their diet.

Plants and algae use a different enzyme—called GLDH—to make vitamin C from aldonolactone. GLDH is very similar to GULO, but it does not produce ROS as a waste product. It is not clear how the different pathways have evolved, or why some animals have lost the ability to make their own vitamin C.

Here, Wheeler et al. used genetics and biochemistry to investigate the evolutionary origins of vitamin C production in a variety of eukaryotic organisms. This investigation revealed that although GULO is missing from the insects and several other groups of animals, it is present in the sponges and many other eukaryotes. This suggests that GULO evolved in early eukaryotic organisms and has since been lost by the different groups of animals. On the other hand, GLDH is only found in plants and the other eukaryotes that can photosynthesize.

Wheeler et al.'s findings suggest that GULO has been lost and replaced by GLDH in all plants and algae following their acquisition of chloroplasts. GDLH allows plants and algae to make vitamin C without also producing ROS, which could explain why vitamin C has been able to take on an extra role in these organisms. The results allow us to better understand the functions of vitamin C in photosynthetic organisms and the processes associated with the acquisition of chloroplasts during evolution.

DOI:http://dx.doi.org/10.7554/eLife.06369.002

Aldonolactone oxidoreductases

Vitamin C is a widely used vitamin. Here we review the occurrence and properties of aldonolactone oxidoreductases, an important group of flavoenzymes responsible for the ultimate production of vitamin C and its analogs in animals, plants, and single-cell organisms.

Manipulation of L-ascorbic acid biosynthesis pathways in Solanum lycopersicum: elevated GDP-mannose pyrophosphorylase activity enhances L-ascorbate levels in red fruit

Ascorbate (AsA) plays a fundamental role in redox homeostasis in plants and animals, primarily by scavenging reactive oxygen species. Three genes, representing diverse steps putatively involved in plant AsA biosynthesis pathways, were cloned and independently expressed in Solanum lycopersicum (tomato) under the control of the CaMV 35S promoter. Yeast-derived GDP-mannose pyrophosphorylase (GMPase) and arabinono-1,4-lactone oxidase (ALO), as well as myo-inositol oxygenase 2 (MIOX2) from Arabidopsis thaliana, were targeted. Increases in GMPase activity were concomitant with increased AsA levels of up to 70% in leaves, 50% in green fruit, and 35% in red fruit. Expression of ALO significantly pulled biosynthetic flux towards AsA in leaves and green fruit by up to 54 and 25%, respectively. Changes in AsA content in plants transcribing the MIOX2 gene were inconsistent in different tissue. On the other hand, MIOX activity was strongly correlated with cell wall uronic acid levels, suggesting that MIOX may be a useful tool for the manipulation of cell wall composition. In conclusion, the Smirnoff-Wheeler pathway showed great promise as a target for biotechnological manipulation of ascorbate levels in tomato.

What's in a covalent bond? On the role and formation of covalently bound flavin cofactors

Many enzymes use one or more cofactors, such as biotin, heme, or flavin. These cofactors may be bound to the enzyme in a noncovalent or covalent manner. Although most flavoproteins contain a noncovalently bound flavin cofactor (FMN or FAD), a large number have these cofactors covalently linked to the polypeptide chain. Most covalent flavin-protein linkages involve a single cofactor attachment via a histidyl, tyrosyl, cysteinyl or threonyl linkage. However, some flavoproteins contain a flavin that is tethered to two amino acids. In the last decade, many studies have focused on elucidating the mechanism(s) of covalent flavin incorporation (flavinylation) and the possible role(s) of covalent protein-flavin bonds. These endeavors have revealed that covalent flavinylation is a post-translational and self-catalytic process. This review presents an overview of the known types of covalent flavin bonds and the proposed mechanisms and roles of covalent flavinylation.

The Hog1 MAP kinase controls respiratory metabolism in the fungal pathogen Candida albicans

Signal transduction pathways mediated by mitogen-activated protein kinases (MAPKs) play crucial roles in eukaryotic cells. In the pathogenic fungus Candida albicans the HOG MAPK pathway regulates the response to external stresses (osmotic and oxidative among others) and is involved in morphogenesis and virulence. We show here that the lack of the Hog1 MAPK increases the sensitivity of this fungus to inhibitors of the respiratory chain. hog1 mutants also show an enhanced basal respiratory rate compared to parental strains, and higher levels of intracellular reactive oxygen species despite an increased expression of detoxifying enzymes. We also demonstrate that although oxidative phosphorylation is essentially unaffected, hog1 mutants have an altered mitochondrial membrane potential. Data indicate that hog1-defective mutants are more dependent on mitochondrial ATP synthesis, probably due to an increased cellular ATP demand. Our results therefore link a MAPK pathway with respiratory metabolism in pathogenic fungi.

The growing VAO flavoprotein family

The VAO flavoprotein family is a rapidly growing family of oxidoreductases that favor the covalent binding of the FAD cofactor. In this review we report on the catalytic properties of some newly discovered VAO family members and their mode of flavin binding. Covalent binding of the flavin is a self-catalytic post-translational modification primarily taking place in oxidases. Covalent flavinylation increases the redox potential of the cofactor and thus its oxidation power. Recent findings have revealed that some members of the VAO family anchor the flavin via a dual covalent linkage (6-S-cysteinyl-8alpha-N1-histidyl FAD). Some VAO-type aldonolactone oxidoreductases favor the non-covalent binding of the flavin cofactor. These enzymes act as dehydrogenases, using cytochrome c as electron acceptor.

l-Galactono-gamma-lactone dehydrogenase from Arabidopsis thaliana, a flavoprotein involved in vitamin C biosynthesis

l-Galactono-1,4-lactone dehydrogenase (GALDH; ferricytochrome c oxidoreductase; EC 1.3.2.3) is a mitochondrial flavoenzyme that catalyzes the final step in the biosynthesis of vitamin C (l-ascorbic acid) in plants. In the present study, we report on the biochemical properties of recombinant Arabidopsis thaliana GALDH (AtGALDH). AtGALDH oxidizes, in addition to l-galactono-1,4-lactone (K(m) = 0.17 mm, k(cat) = 134 s(-1)), l-gulono-1,4-lactone (K(m) = 13.1 mm, k(cat) = 4.0 s(-1)) using cytochrome c as an electron acceptor. Aerobic reduction of AtGALDH with the lactone substrate generates the flavin hydroquinone. The two-electron reduced enzyme reacts poorly with molecular oxygen (k(ox) = 6 x 10(2) m(-1).s(-1)). Unlike most flavoprotein dehydrogenases, AtGALDH forms a flavin N5 sulfite adduct. Anaerobic photoreduction involves the transient stabilization of the anionic flavin semiquinone. Most aldonolactone oxidoreductases contain a histidyl-FAD as a covalently bound prosthetic group. AtGALDH lacks the histidine involved in covalent FAD binding, but contains a leucine instead (Leu56). Leu56 replacements did not result in covalent flavinylation but revealed the importance of Leu56 for both FAD-binding and catalysis. The Leu56 variants showed remarkable differences in Michaelis constants for both l-galactono-1,4-lactone and l-gulono-1,4-lactone and released their FAD cofactor more easily than wild-type AtGALDH. The present study provides the first biochemical characterization of AtGALDH and some active site variants. The role of GALDH and the possible involvement of other aldonolactone oxidoreductases in the biosynthesis of vitamin C in A. thaliana are also discussed.

The terminal step in vitamin C biosynthesis in Trypanosoma cruzi is mediated by a FMN-dependent galactonolactone oxidase

Humans lack the ability to synthesize vitamin C (ascorbate) due to the absence of gulonolactone oxidase, the last enzyme in the biosynthetic pathway in most other mammals. The corresponding oxidoreductase in trypanosomes therefore represents a target that may be therapeutically exploitable. This is reinforced by our observation that Trypanosoma cruzi, the causative agent of Chagas' disease, lacks the capacity to scavenge ascorbate from its environment and is therefore dependent on biosynthesis to maintain intracellular levels of this vitamin. Here, we show that T. cruzi galactonolactone oxidase (TcGAL) can utilize both L-galactono-gamma-lactone and D-arabinono-gamma-lactone as substrates for synthesis of vitamin C, in reactions that obey Michaelis-Menten kinetics. It is >20-fold more active than the analogous enzyme from the African trypanosome Trypanosoma brucei. FMN is an essential cofactor for enzyme activity and binds to TcGAL non-covalently. In other flavoproteins, a histidine residue located within the N-terminal flavin-binding motif has been shown to be crucial for cofactor binding. Using site-directed mutagenesis, we show that the corresponding residue in TcGAL (Lys-55) is not essential for this interaction. In contrast, we find that histidine and tryptophan residues (His-447 and Trp-448), localized within a C-terminal motif (HWXK) that is a feature of ascorbate-synthesizing enzymes, are necessary for the FMN association. The conserved lysine residue within this motif (Lys-450) is not required for cofactor binding, but its replacement by glycine renders the protein completely inactive.

Electron paramagnetic resonance and Mössbauer spectroscopy of intact mitochondria from respiring Saccharomyces cerevisiae

Mitochondria from respiring cells were isolated under anaerobic conditions. Microscopic images were largely devoid of contaminants, and samples consumed O(2) in an NADH-dependent manner. Protein and metal concentrations of packed mitochondria were determined, as was the percentage of external void volume. Samples were similarly packed into electron paramagnetic resonance tubes, either in the as-isolated state or after exposure to various reagents. Analyses revealed two signals originating from species that could be removed by chelation, including rhombic Fe(3+) (g = 4.3) and aqueous Mn(2+) ions (g = 2.00 with Mn-based hyperfine). Three S = 5/2 signals from Fe(3+) hemes were observed, probably arising from cytochrome c peroxidase and the a(3):Cu(b) site of cytochrome c oxidase. Three Fe/S-based signals were observed, with averaged g values of 1.94, 1.90 and 2.01. These probably arise, respectively, from the [Fe(2)S(2)](+) cluster of succinate dehydrogenase, the [Fe(2)S(2)](+) cluster of the Rieske protein of cytochrome bc (1), and the [Fe(3)S(4)](+) cluster of aconitase, homoaconitase or succinate dehydrogenase. Also observed was a low-intensity isotropic g = 2.00 signal arising from organic-based radicals, and a broad signal with g (ave) = 2.02. Mössbauer spectra of intact mitochondria were dominated by signals from Fe(4)S(4) clusters (60-85% of Fe). The major feature in as-isolated samples, and in samples treated with ethylenebis(oxyethylenenitrilo)tetraacetic acid, dithionite or O(2), was a quadrupole doublet with DeltaE (Q) = 1.15 mm/s and delta = 0.45 mm/s, assigned to [Fe(4)S(4)](2+) clusters. Substantial high-spin non-heme Fe(2+) (up to 20%) and Fe(3+) (up to 15%) species were observed. The distribution of Fe was qualitatively similar to that suggested by the mitochondrial proteome.

Mycobacterium tuberculosispossesses a functional enzyme for the synthesis of vitamin C,L-gulono-1,4-lactone dehydrogenase

The last step of the biosynthesis of L-ascorbic acid (vitamin C) in plants and animals is catalyzed by L-gulono-1,4-lactone oxidoreductases, which use both L-gulono-1,4-lactone and L-galactono-1,4-lactone as substrates. L-gulono-1,4-lactone oxidase is missing in scurvy-prone, vitamin C-deficient animals, such as humans and guinea pigs, which are also highly susceptible to tuberculosis. A blast search using the rat L-gulono-1,4-lactone oxidase sequence revealed the presence of closely related orthologs in a limited number of bacterial species, including several pathogens of human lungs, such as Mycobacterium tuberculosis, Pseudomonas aeruginosa, Burkholderia cepacia and Bacillus anthracis. The genome of M. tuberculosis, the etiologic agent of tuberculosis, encodes a protein (Rv1771) that shows 32% identity with the rat L-gulono-1,4-lactone oxidase protein. The Rv1771 gene was cloned and expressed in Escherichia coli, and the corresponding protein was affinity-purified and characterized. The FAD-binding motif-containing Rv1771 protein is a metalloenzyme that oxidizes L-gulono-1,4-lactone (Km 5.5 mm) but not L-galactono-1,4-lactone. The enzyme has a dehydrogenase activity and can use both cytochrome c (Km 4.7 microm) and phenazine methosulfate as exogenous electron acceptors. Molecular oxygen does not serve as a substrate for the Rv1771 protein. Dehydrogenase activity was measured in cellular extracts of a Mycobacterium bovis BCG strain. In conclusion, M. tuberculosis produces a novel, highly specific L-gulono-1,4-lactone dehydrogenase (Rv1771) and has the capacity to synthesize vitamin C.

The use of microorganisms in l-ascorbic acid production

L-Ascorbic acid has been industrially produced for around 70 years. Over the past two decades, several innovative bioconversion systems have been proposed in order to simplify the long time market-dominating Reichstein method, a largely chemical synthesis by which still a considerable part of L-ascorbic acid is produced. Here, we describe the current state of biotechnological alternatives using bacteria, yeasts, and microalgae. We also discuss the potential for direct production of l-ascorbic acid exploiting novel bacterial pathways. The advantages of these novel approaches competing with current chemical and biotechnological processes are outlined.

Vitamin C biosynthesis in trypanosomes: a role for the glycosome

The capacity to synthesize vitamin C (ascorbate) is widespread in eukaryotes but is absent from humans. The last step in the biosynthetic pathway involves the conversion of an aldonolactone substrate to ascorbate, a reaction catalyzed by members of an FAD-dependent family of oxidoreductases. Here we demonstrate that both the African trypanosome, Trypanosoma brucei, and the American trypanosome, Trypanosoma cruzi, have the capacity to synthesize vitamin C and show that this reaction occurs in a unique single-membrane organelle, the glycosome. The corresponding T. brucei flavoprotein (TbALO) obeys Michaelis-Menten kinetics and can utilize both L-galactono-gamma-lactone and D-arabinono-gamma-lactone as substrate, properties characteristic of plant and fungal enzymes. We could detect no activity toward the mammalian enzyme substrate L-gulono-gamma-lactone. TbALO null mutants (bloodstream form) were found to display a transient growth defect, a trait that was enhanced when they were cultured in medium in which the essential serum component had been pretreated with ascorbate oxidase to deplete vitamin C. It is implicit, therefore, that bloodstream-form trypanosomes also possess a capacity for ascorbate transport.

Characterisation and biosynthesis of D-erythroascorbic acid in Phycomyces blakesleeanus

D-Erythroascorbate and D-erythroascorbate glucoside have been identified in the Zygomycete fungus Phycomyces blakesleeanus. Ascomycete and Basidiomycete fungi also synthesise D-erythroascorbate instead of l-ascorbate, suggesting that D-erythroascorbate synthesis evolved in the common ancestor of the fungi. Both compounds accumulate in P. blakesleeanus at higher levels than observed in other fungal species. D-Erythroascorbate glucoside reduced dichlorophenolindophenol as effectively as L-ascorbate, but was more stable to autoxidation. D-Erythroascorbate glucoside predominated in spores and stationary phase mycelium. Free D-erythroascorbate accumulated during the exponential phase of mycelial growth and decreased to very low levels in the stationary phase. This suggests an association between growth and free D-erythroascorbate. P. blakesleeanus converted exogenous D-arabinose to D-erythroascorbate and its glucoside. A monomeric NAD-dependent D-arabinose dehydrogenase of 41 kDa was purified to near homogeneity. The enzyme oxidised D-arabinose, L-galactose, and L-fucose. Correspondingly, mycelium converted exogenous L-galactose and L-fucose to L-ascorbate and 6-deoxyascorbate, respectively. The antioxidant role of D-erythroascorbate and its glucoside is discussed.

Production of L-ascorbic acid by metabolically engineered Saccharomyces cerevisiae and Zygosaccharomyces bailii

Yeasts do not possess an endogenous biochemical pathway for the synthesis of vitamin C. However, incubated with l-galactose, L-galactono-1,4-lactone, or L-gulono-1,4-lactone intermediates from the plant or animal pathway leading to l-ascorbic acid, Saccharomyces cerevisiae and Zygosaccharomyces bailii cells accumulate the vitamin intracellularly. Overexpression of the S. cerevisiae enzymes d-arabinose dehydrogenase and D-arabinono-1,4-lactone oxidase enhances this ability significantly. In fact, the respective recombinant yeast strains even gain the capability to accumulate the vitamin in the culture medium. An even better result is obtainable by expression of the plant enzyme L-galactose dehydrogenase from Arabidopsis thaliana. Budding yeast cells overexpressing the endogenous D-arabinono-1,4-lactone oxidase as well as L-galactose dehydrogenase are capable of producing about 100 mg of L-ascorbic acid liter(-1), converting 40% (wt/vol) of the starting compound L-galactose.

Cloning and characterization of gluconolactone oxidase of Penicillium cyaneo-fulvum ATCC 10431 and evaluation of its use for production of D-erythorbic acid in recombinant Pichia pastoris

A D-erythorbic acid-forming soluble flavoprotein, gluconolactone oxidase (GLO), was purified from Penicillium cyaneo-fulvum strain ATCC 10431 and partially sequenced. Peptide sequences were used to isolate a cDNA clone encoding the enzyme. The cloned gene exhibits high levels of similarity with the genes encoding other known eukaryotic lactone oxidases and also with the genes encoding some putative prokaryotic lactone oxidases. Analysis of the coding sequence of the GLO gene indicated the presence of a typical secretion signal sequence at the N terminus of GLO. No other targeting or anchoring signals were found, suggesting that GLO is the first known lactone oxidase that is secreted rather than targeted to the membranes of the endoplasmic reticulum or mitochondria. Experimental evidence, including the N-terminal sequence of mature GLO and data on glycosylation and localization of the enzyme in native and recombinant hosts, supports this analysis. The GLO gene was expressed in Pichia pastoris, and recombinant GLO was produced by using the strong methanol-induced AOX1 promoter. In order to evaluate the suitability of purified GLO for production of D-erythorbic acid, we immobilized it on N-hydroxysuccinimide-activated Sepharose and found that the immobilized GLO retained full activity during immobilization but was rather unstable under reaction conditions. Our results show that both soluble and immobilized forms of GLO can, in principle, be used for production of D-erythorbic acid from D-glucono-delta-lactone or (in combination with glucose oxidase and catalase) from glucose. We also demonstrated the feasibility of glucose-D-erythorbic acid fermentation with recombinant strains coexpressing GLO and glucose oxidase genes, and we analyzed problems associated with construction of efficient D-erythorbic acid-producing hosts.

Protective roles of mitochondrial manganese-containing superoxide dismutase against various stresses in Candida albicans

Candida albicans contains copper- and zinc-containing superoxide dismutase but also two manganese-containing superoxide dismutases (MnSODs), one in the cytosol and the other in the mitochondria. Among these, the SOD2 gene encoding mitochondrial MnSOD was disrupted and overexpressed to investigate its roles in C. albicans. The null mutant lacking mitochondrial MnSOD was more sensitive than wild-type cells to various stresses, such as redox-cycling agents, heating, ethanol, high concentration of sodium or potassium and 99.9% O2. Interestingly, the sod2/sod2 mutant was rather more resistant to lithium and diamide than the wild-type, whereas overexpression of SOD2 increased susceptibility of C. albicans to these compounds. The inverse effect of mitochondrial MnSOD on lithium toxicity was relieved when the sod2/sod2 and SOD2-overexpressing cells were grown on the synthetic dextrose medium containing sulphur compounds such as methionine, cysteine, glutathione or sulphite, indicating that mitochondrial MnSOD may affect lithium toxicity through sulphur metabolism. Moreover, disruption or overexpression of SOD2 increased or decreased glutathione reductase activity and cyanide-resistant respiration by alternative oxidase, respectively. Taken together, these findings suggest that mitochondrial MnSOD is important for stress responses, lithium toxicity and cyanide-resistant respiration of C. albicans.

Biotechnological approaches for L-ascorbic acid production

Over the past decade there has been increasing pressure to develop alternatives to the Reichstein process, a largely chemical synthesis by which the vast majority of world vitamin C (L-ascorbic acid, L-AA) is produced. The pressures include increasing environmental concerns and legislation, and the need to increase process efficiency and reduce capital costs. The development of efficient fermentation processes in the past ten years has also represented a catalyst for change. Here, we describe the development of biotechnological alternatives for the synthesis of Reichstein intermediates by industrial microorganisms. The recent elucidation of the plant biosynthetic pathway represents new opportunities not only for the direct synthesis of L-AA by fermentation but also for the production of human crop plants and animal fodder with enhanced nutritional value. We discuss the potential for these developments in the light of recent findings concerning L-AA biosynthesis in plants.

Characterization of the gene family encoding alternative oxidase from Candida albicans

Candida albicans possesses a cyanide-resistant respiratory pathway mediated by alternative oxidase (AOX), which seems to be encoded by a gene family with two members. Cloning and expression of AOX1a, one of the genes encoding alternative oxidase from C. albicans, has previously been reported [Huh and Kang (1999) J. Bacteriol. 181, 4098-4102]. In the present study we report the isolation of another gene coding for alternative oxidase, designated AOX1b. AOX1b contains a continuous open reading frame that encodes a polypeptide consisting of 365 amino acids. Interestingly, AOX1a and AOX1b were found to be located in tandem on one of the chromosomes of C. albicans. The presence of cyanide in the culture medium remarkably retarded the growth of the aox1a/aox1a mutants. The growth of the aox1b/aox1b mutants and the aox1a/aox1a aox1b/aox1b double mutants was almost completely inhibited in the same medium. beta-Galactosidase reporter assays indicated that, whereas AOX1a was expressed constitutively, the expression of AOX1b was dependent on growth phase and was induced by treatment with cyanide, antimycin A, H(2)O(2), menadione and paraquat. Growth of the cells in media with non-fermentable carbon sources also enhanced the expression of AOX1b. CaSLN1, which encodes a histidine kinase, seems to be involved in the regulation of AOX expression in C. albicans on the basis of the observation that the activity of cyanide-resistant respiration and the expression level of AOX in the casln1/casln1 mutants were found to be significantly low under normal conditions and slightly increased in the presence of respiratory inhibitors compared with the wild-type strain. Like AOX1a, AOX1b could also be functionally expressed in AOX-deficient Saccharomyces cerevisiae and confer cyanide-resistant respiration on the organism.

Mitochondrial NADH-cytochrome b(5) reductase plays a crucial role in the reduction of D-erythroascorbyl free radical in Saccharomyces cerevisiae

The relevance of NADH-cytochrome b(5) reductase to the NADH-dependent reduction of D-erythroascorbyl free radical was investigated in Saccharomyces cerevisiae. MCR1, which is known to encode NADH-cytochrome b(5) reductase in S. cerevisiae, was disrupted by the insertion of URA3 gene into the gene of MCR1. In the mcr1 disruptant cells, the activity of NADH-D-erythroascorbyl free radical reductase almost disappeared and the intracellular level of D-erythroascorbic acid was about 11% of that of the congenic wild-type strain. In the transformant cells carrying MCR1 in multicopy plasmid, the intracellular level of D-erythroascorbic acid and the activity of NADH-D-erythroascorbyl free radical reductase increased up to 1.7-fold and 2.1-fold, respectively. Therefore, it indicated that the MCR1 product, mitochondrial NADH-cytochrome b(5) reductase, plays a key role in the NADH-dependent reduction of D-erythroascorbyl free radical in S. cerevisiae. On the other hand, the mcr1 disruptant cells were hypersensitive to hydrogen peroxide and menadione, and overexpression of MCR1 made the cells more resistant against oxidative stress. These results suggested that the mitochondrial NADH-cytochrome b(5) reductase functions as NADH-D-erythroascorbyl free radical reductase and plays an important role in the response to oxidative damage in S. cerevisiae.

Deficiency of D-erythroascorbic acid attenuates hyphal growth and virulence of Candida albicans

In some lower eukaryotes, D-erythroascorbic acid, a five-carbon analog of L-ascorbic acid, is present instead of L-ascorbic acid. We have cloned ALO1, the gene encoding D-arabinono-1,4-lactone oxidase, which catalyzes the final step of D-erythroascorbic acid biosynthesis in Candida albicans. The ALO1 gene contained a continuous open reading frame of 1,671 bp that encodes a polypeptide consisting of 557 amino acids with a calculated molecular mass of 63,428 Da. To investigate the functional roles of D-erythroascorbic acid in C. albicans, we disrupted or overexpressed the ALO1 gene. In the alo1/alo1 null mutants, the activity of D-arabinono-1,4-lactone oxidase was completely lost and D-erythroascorbic acid could not be detected. When ALO1 on a multicopy plasmid was transformed in C. albicans, the enzyme activity and the intracellular D-erythroascorbic acid level were increased up to 3.4-fold and 4.0-fold, respectively. The alo1/alo1 null mutants of C. albicans showed increased sensitivity towards oxidative stress. Overexpression of ALO1 made the cells more resistant to the same stress. The alo1/alo1 mutants showed defective hyphal growth and attenuated virulence. Taken together, our results suggest that D-erythroascorbic acid functions as an important antioxidant and can be considered one of the virulence factors enhancing the pathogenicity of C. albicans.

BIOSYNTHESIS OF ASCORBIC ACID IN PLANTS: A Renaissance

The structure of the familiar antioxidant L-ascorbic acid (vitamin C) was described in 1933 yet remarkably, its biosynthesis in plants remained elusive until only recently. It became clear from radioisotopic labeling studies in the 1950s that plant ascorbic acid biosynthesis does not proceed in toto via a route similar to that in mammals. The description in 1996 of an Arabidopsis thaliana mutant deficient in ascorbic acid prompted renewed research effort in this area, and subsequently in 1998 a new pathway was discovered that is backed by strong biochemical and molecular genetic evidence. This pathway proceeds through the intermediates GDP-D-mannose, L-galactose, and L-galactono-1,4-lactone. Much research has focused on the properties of the terminal enzyme responsible for conversion of the aldonolactone to ascorbate, and on related enzymes in both mammals and fungi. Two of the plant biosynthetic genes have been studied at the molecular level and additional ascorbate-deficient A. thaliana mutants may hold the key to other proteins involved in plant ascorbate metabolism. An analysis of the biosynthesis of ascorbate and its analogues in algae and fungi as well as the study of alternative proposed pathways should broaden our understanding of ascorbate metabolism in plants. With a biosynthetic pathway in hand, research on areas such as the control of ascorbate biosynthesis and the physiological roles of ascorbate should progress rapidly.

L-ascorbic acid biosynthesis

Biosynthesis of L-ascorbate (vitamin C) occurs by different pathways in plants and mammals. Yeast contain D-erythroascorbate, a C5 analog of ascorbate. UDP-D-glucuronic acid is the precursor in mammals. Loss of UDP forms glucuronic acid/glucuronolactone. Reduction of these at C-1 then forms L-gulonic acid/L-gulono-1,4-lactone. The lactone is oxidized by a microsomal L-gulono-1,4-lactone oxidase to ascorbate. Only the L-gulono-1,4-lactone oxidase has been purified and cloned, and very little is known about the properties of the other enzymes. Plants form ascorbate from GDP-D-mannose via GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone. The final oxidation of L-galactono-1,4-lactone to ascorbate is catalyzed by a mitochondrial L-galactono-1,4-lactone dehydrogenase located on the inner membrane and using cytochrome c as electron acceptor. GDP-mannose pyrophosphorylase and L-galactono-1,4-lactone dehydrogenase have been cloned. Yeast synthesizes D-erythroascorbate from D-arabinose and D-arabinono-1,4-lactone in a pathway analogous to that in plants. The plant, mammalian, and yeast aldonolactone oxidase/dehydrogenases that catalyze the last step in each pathway have significant sequence homology. L-Gulono-1,4-lactone oxidase is mutated and not expressed in animals, such as primates, that have lost ascorbate biosynthesis capacity. Assessment of the literature reveals that little is known about many of the enzymes involved in ascorbate biosynthesis or about the factors controlling flux through the pathways. There is also a possibility that minor alternative pathways exist in plants and mammals.

Ascorbic acid in plants: biosynthesis and function

Ascorbic acid (vitamin C) is an abundant component of plants. It reaches a concentration of over 20 mM in chloroplasts and occurs in all cell compartments, including the cell wall. It has proposed functions in photosynthesis as an enzyme cofactor (including synthesis of ethylene, gibberellins and anthocyanins) and in control of cell growth. A biosynthetic pathway via GDP-mannose, GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone has been proposed only recently and is supported by molecular genetic evidence from the ascorbate-deficient vtc 1 mutant of Arabidopsis thaliana. Other pathways via uronic acids could provide minor sources of ascorbate. Ascorbate, at least in some species, is a precursor of tartrate and oxalate. It has a major role in photosynthesis, acting in the Mehler peroxidase reaction with ascorbate peroxidase to regulate the redox state of photosynthetic electron carriers and as a cofactor for violaxanthin de-epoxidase, an enzyme involved in xanthophyll cycle-mediated photoprotection. The hypersensitivity of some of the vtc mutants to ozone and UV-B radiation, the rapid response of ascorbate peroxidase expression to (photo)-oxidative stress, and the properties of transgenic plants with altered ascorbate peroxidase activity all support an important antioxidative role for ascorbate. In relation to cell growth, ascorbate is a cofactor for prolyl hydroxylase that posttranslationally hydroxylates proline residues in cell wall hydroxyproline-rich glycoproteins required for cell division and expansion. Additionally, high ascorbate oxidase activity in the cell wall is correlated with areas of rapid cell expansion. It remains to be determined if this is a causal relationship and, if so, what is the mechanism. Identification of the biosynthetic pathway now opens the way to manipulating ascorbate biosynthesis in plants, and, along with the vtc mutants, this should contribute to a deeper understanding of the proposed functions of this multifaceted molecule.

The biosynthesis of erythroascorbate in Saccharomyces cerevisiae and its role as an antioxidant

This study investigated the ability of the yeast Saccharomyces cerevisiae to synthesize ascorbate and its 5-carbon analogue erythroascorbate from a variety of precursors, and their importance as antioxidants in this organism. Studies of ascorbate and analogues in micro-organisms have been reported previously, but their function as antioxidants have been largely ignored. Ascorbate and erythroascorbate concentrations in yeast extracts were measured spectrophotometrically, and their levels and identity were checked using liquid chromatography-electrospray mass spectrometry. The yeast was readily able to synthesize ascorbate from L-galactono-1,4-lactone or erythroascorbate from D-arabinose and D-arabino-1,4-lactone, whereas L-gulono-1,4-lactone was a much poorer substrate for ascorbate biosynthesis. In untreated cells, the concentration of ascorbate-like compounds was below the level of detection of the methods of analysis used in this study (approximately 0.1 mM). Intracellular ascorbate and erythroascorbate were oxidized at high concentrations of tert-butylhydroperoxide, but not hydrogen peroxide. Their synthesis was not increased in response to low levels of stress, however, and preloading with erythroascorbate did not protect glutathione levels during oxidative stress. This study provides new information on the metabolism of ascorbate and erythroascorbate in S. cerevisiae, and suggests that erythroascorbate is of limited importance as an antioxidant in S. cerevisiae.

Bacterial production of D-erythroascorbic acid and L-ascorbic acid through functional expression of Saccharomyces cerevisiae D-arabinono-1,4-lactone oxidase in Escherichia coli

D-Arabinono-1,4-lactone oxidase, which catalyzes the terminal step in the biosynthesis of D-erythroascorbic acid in Saccharomyces cerevisiae, was functionally expressed in Escherichia coli inherently lacking the enzyme. The recombinant E. coli strain expressing the enzyme could overproduce D-erythroascorbic acid and L-ascorbic acid when supplied with D-arabinono-1,4-lactone and L-galactono-1,4-lactone, respectively.

Direct measurement of ascorbic acid biosynthesis in Arabidopsis cell suspension culture using capillary electrophoresis

We describe procedures to directly measure the biosynthesis of vitamin C (L-ascorbic acid, L-AA) in crude extracts of an Arabidopsis thaliana cell suspension culture by capillary electrophoresis. Optimal conditions have been established for the quantitation of L-AA formed by the oxidation of three different substrates: L-galactose, L-galactono-1,4-lactone, and L-gulono-1,4-lactone. We also demonstrate that L-galactono-1,4-lactone dehydrogenase activity does not require exogenous cofactor. The minimal sample handling requirements, the high selectivity, and short analysis times represent significant advantages over existing protocols.

Molecular cloning and functional expression of alternative oxidase from Candida albicans

The AOX1 gene, which encodes an alternative oxidase, was isolated from the genomic DNA library of Candida albicans. The gene encodes a polypeptide consisting of 379 amino acids with a calculated molecular mass of 43,975 Da. The aox1/aox1 mutant strain did not show cyanide-resistant respiration under normal conditions but could still induce cyanide-resistant respiration when treated with antimycin A. The measurement of respiratory activity and Western blot analysis suggested the presence of another AOX. When C. albicans AOX1 was expressed in alternative oxidase-deficient Saccharomyces cerevisiae, it could confer cyanide-resistant respiration on S. cerevisiae.

D-arabinose dehydrogenase and its gene from Saccharomyces cerevisiae

D-Arabinose dehydrogenase was purified 843-fold from the cytosolic fraction of Saccharomyces cerevisiae with a recovery of 9%. The purified enzyme gave two bands with a molecular mass of 40 and 39 kDa on SDS-PAGE. The native enzyme had a molecular mass of 74 kDa as estimated by Sephacryl S-200 chromatography. Therefore, this enzyme was considered to be a heterodimer. The purified enzyme exhibited maximum activity at pH 10.0 and around 30 degrees C. The enzyme catalysed the oxidation of D-arabinose, L-xylose, L-fucose and L-galactose in the presence of NADP+. The apparent Km values at pH 10.0 with 50 microM NADP+ for D-arabinose, L-xylose, L-fucose, and L-galactose were 161, 24, 98 and 180 mM, respectively. The pH profile of Vmax and kcat/Km showed one ionisable groups around pH 8.3. D-Erythroascorbic acid was formed in vitro from D-arabinose by D-arabinose dehydrogenase and D-arabinono-1,4-lactone oxidase. The N-terminal amino acid sequence of the heavy subunit was Ser-Thr-Glu-Asn-Ile-Val-Glu-Asn-Met-Leu-His-Pro-Lys-Thr-. The N-terminus of the light subunit was blocked. The obtained peptide sequence was identical to the translational product of an unknown open reading frame, YBR149W, in chromosome II of S. cerevisiae. When compared with the translational product of this open reading frame, the peptide sequence was identical to the amino acid sequences of residues 7 to 20. The first six amino acids of this open reading frame were lost in protein sequence, which may be modified post-translationally. The heavy subunit was composed of 344 amino acid residues and its deduced amino acid sequence contained the motifs I, II, and III of aldo-keto reductase and also leucine zipper motif. This enzyme is the first heterodimeric protein of aldo-keto reductase family. In the deletion mutant of this gene, D-arabinose dehydrogenase activity and D-erythroascorbic acid were not detected.

5-O-(alpha-D-galactopyranosyl)-D-glycero-pent-2-enono-1,4-lactone: characterization in the oxalate-producing fungus, Sclerotinia sclerotiorum

Extracts of sclerotia from Sclerotinia sclerotiorum, a fungal phytopathogen, contain two electrochemically-active constituents, D-glycero-pent-2-enono-1,4-lactone (trivial name: D-erythroascorbic acid), and a previously unidentified compound, here characterized as 5-O-(alpha-D-galactopyranosyl)-D-glycero-pent-2-enono-1,4-lactone on the basis of its physical and chemical properties and its two hydrolytic products, D-galactose and D-erythroascorbic acid. Treatment of this galactoside with alkaline hydrogen peroxide produces oxalic acid as observed earlier with erythroascorbic acid.

Oxidative stress responses of the yeast Saccharomyces cerevisiae

All aerobically growing organisms suffer exposure to oxidative stress, caused by partially reduced forms of molecular oxygen, known as reactive oxygen species (ROS). These are highly reactive and capable of damaging cellular constituents such as DNA, lipids and proteins. Consequently, cells from many different organisms have evolved mechanisms to protect their components against ROS. This review concentrates on the oxidant defence systems of the budding yeast Saccharomyces cerevisiae, which appears to have a number of inducible adaptive stress responses to oxidants, such as H2O2, superoxide anion and lipid peroxidation products. The oxidative stress responses appear to be regulated, at least in part, at the level of transcription and there is considerable overlap between them and many diverse stress responses, allowing the yeast cell to integrate its response towards environmental stress.

D-Erythroascorbic acid is an important antioxidant molecule in Saccharomyces cerevisiae

D-Arabinono-1,4-lactone oxidase catalysing the final step of D-erythroascorbic acid biosynthesis was purified from the mitochondrial fraction of Saccharomyces cerevisiae. Based on the amino acid sequence analysis of the enzyme, an unknown open reading frame (ORF), YML086C, was identified as the ALO1 gene encoding the enzyme. The ORF of ALO1 encoded a polypeptide consisting of 526 amino acids with a calculated molecular mass of 59493Da. The deduced amino acid sequence of the enzyme shared 32% and 21% identity with that of L-gulono-1,4-lactone oxidase from rat and L-galactono-1,4-lactone dehydrogenase from cauliflower, respectively, and contained a putative transmembrane segment and a covalent FAD binding site. Blot hybridization analyses showed that a single copy of the gene was present in the yeast genome and that mRNA of the ALO1 gene was 1.8kb in size. In the alo1 mutants, D-erythroascorbic acid and the activity of D-arabinono-1,4-lactone oxidase could not be detected. The intracellular concentration of D-erythroascorbic acid and the enzyme activity increased up to 6.9-fold and 7.3-fold, respectively, in the transformant cells carrying ALO1 in multicopy plasmid. The alo1 mutants showed increased sensitivity towards oxidative stress, but overexpression of ALO1 made the cells more resistant to oxidative stress.

Subcellular localization of vanillyl-alcohol oxidase in Penicillium simplicissimum

Growth of Penicillium simplicissimum on anisyl alcohol, veratryl alcohol or 4-(methoxymethyl)phenol, is associated with the synthesis of relatively large amounts of the hydrogen peroxide producing flavoprotein vanillyl-alcohol oxidase (VAO). Immunocytochemistry revealed that the enzyme has a dual location namely in peroxisomes and in the cytosol. The C-terminus of the primary structure of VAO displays a WKL-COOH sequence which might function as a peroxisomal targeting signal type 1 (PTS1). As VAO activity results in production of hydrogen peroxide also the subcellular location of a recently characterized co-inducible catalase-peroxidase was studied. As VAO, this hydroperoxidase is also distributed throughout the cytosol and peroxisomes.

D-arabinose dehydrogenase and biosynthesis of erythroascorbic acid in Candida albicans

D-Arabinose dehydrogenase was purified 2750-fold from the cytosolic fraction of Candida albicans to apparent homogeneity, with an overall yield of 3%, by a purification procedure consisting of ammonium sulfate precipitation and DEAE-Sepharose A-50, Sephacryl S-200, Cibacron blue and phenyl-Sepharose CL-4B chromatographies. Gel-filtration chromatography gave an apparent molecular mass of 41 kDa and SDS-PAGE showed only one protein band corresponding to a molecular mass of 42 kDa, indicating that the enzyme is a single polypeptide. The enzyme was optimally active at pH 8.0 and the pI value of the enzyme was 5.0. The enzyme was relatively stable from pH 4.5 to 7.5. The optimal temperature for the enzyme activity was 30 degrees C. The activity of the enzyme was inhibited by Hg2+, Fe2+, Zn2+, Cu2+, Mg2+, Mn2+, N-ethylmaleimide and p-chloromercuribenzoic acid. The enzyme catalysed the oxidation of D-arabinose, L-fucose, L-xylose and L-galactose, which have the same configurations of hydroxyl groups at C2- and C3-positions, with apparent K(m) values of 29.2, 28.9, 37.1 and 91.3 mM at pH 8.0, respectively, with 50 microM NADP+. The enzyme used NADP+ as a coenzyme. Apparent K(m) value at 60 mM D-arabinose for NADP+ was 44.6 microM. NADPH inhibited the enzyme activity competitively with respect to NADP+ (Ki = 78.6 microM). The amino-terminal sequence of the enzyme was Met-Lys-Leu-Ala-Thr-Glu-Ile-Asp-Phe-X-Leu-Asn-Asn-Gly-. The reaction product was D-arabinono-1,4-lactone, judged from gas-liquid chromatography/mass spectrometry. In C. albicans, D-erythroascorbic acid was formed from D-arabinose by D-arabinose dehydrogenase and D-arabinono-1,4-lactone oxidase.