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

The effect of DMSO on Saccharomyces cerevisiae yeast with different energy metabolism and antioxidant status

We studied the effect of dimethyl sulfoxide (DMSO) on the biochemical and physiological parameters of S. cerevisiae yeast cells with varied energy metabolism and antioxidant status. The wild-type cells of varied genetic backgrounds and their isogenic mutants with impaired antioxidant defences (Δsod mutants) or response to environmental stress (ESR) (Δmsn2, Δmsn4 and double Δmsn2msn4 mutants) were used. Short-term exposure to DMSO even at a wide range of concentrations (2-20%) had little effect on the metabolic activity of the yeast cells and the stability of their cell membranes, but induced free radicals production and clearly altered their proliferative activity. Cells of the Δsod1 mutant showed greater sensitivity to DMSO in these conditions. DMSO at concentrations from 4 to 10-14% (depending on the strain and genetic background) activated the ESR programme. The effects of long-term exposure to DMSO were mainly depended on the type of energy metabolism and antioxidant system efficiency. Yeast cells with reduced antioxidant system efficiency and/or aerobic respiration were more susceptible to the toxic effects of DMSO than cells with a wild-type phenotype and respiro-fermentative or fully fermentative metabolism. These studies suggest a key role of stress response programs in both the processes of cell adaptation to small doses of this xenobiotic and the processes related to its toxicity resulting from large doses or chronic exposure to DMSO.

Chloroplast phosphate transporter CrPHT4-7 regulates phosphate homeostasis and photosynthesis in Chlamydomonas

In eukaryotic cells, phosphorus is assimilated and utilized primarily as phosphate (Pi). Pi homeostasis is mediated by transporters that have not yet been adequately characterized in green algae. This study reports on PHOSPHATE TRANSPORTER 4-7 (CrPHT4-7) from Chlamydomonas reinhardtii, a member of the PHT4 transporter family, which exhibits remarkable similarity to AtPHT4;4 from Arabidopsis (Arabidopsis thaliana), a chloroplastic ascorbate transporter. Using fluorescent protein tagging, we show that CrPHT4-7 resides in the chloroplast envelope membrane. Crpht4-7 mutants, generated by the CRISPR/Cas12a-mediated single-strand templated repair, show retarded growth, especially in high light, reduced ATP level, strong ascorbate accumulation, and diminished non-photochemical quenching in high light. On the other hand, total cellular phosphorous content was unaffected, and the phenotype of the Crpht4-7 mutants could not be alleviated by ample Pi supply. CrPHT4-7-overexpressing lines exhibit enhanced biomass accumulation under high light conditions in comparison with the wild-type strain. Expressing CrPHT4-7 in a yeast (Saccharomyces cerevisiae) strain lacking Pi transporters substantially recovered its slow growth phenotype, demonstrating that CrPHT4-7 transports Pi. Even though CrPHT4-7 shows a high degree of similarity to AtPHT4;4, it does not display any substantial ascorbate transport activity in yeast or intact algal cells. Thus, the results demonstrate that CrPHT4-7 functions as a chloroplastic Pi transporter essential for maintaining Pi homeostasis and photosynthesis in C. reinhardtii.

A CYBDOM protein impacts iron homeostasis and primary root growth under phosphate deficiency in Arabidopsis

Arabidopsis primary root growth response to phosphate (Pi) deficiency is mainly controlled by changes in apoplastic iron (Fe). Upon Pi deficiency, apoplastic Fe deposition in the root apical meristem activates pathways leading to the arrest of meristem maintenance and inhibition of cell elongation. Here, we report that a member of the uncharacterized cytochrome b561 and DOMON domain (CYBDOM) protein family, named CRR, promotes iron reduction in an ascorbate-dependent manner and controls apoplastic iron deposition. Under low Pi, the crr mutant shows an enhanced reduction of primary root growth associated with increased apoplastic Fe in the root meristem and a reduction in meristematic cell division. Conversely, CRR overexpression abolishes apoplastic Fe deposition rendering primary root growth insensitive to low Pi. The crr single mutant and crr hyp1 double mutant, harboring a null allele in another member of the CYDOM family, shows increased tolerance to high-Fe stress upon germination and seedling growth. Conversely, CRR overexpression is associated with increased uptake and translocation of Fe to the shoot and results in plants highly sensitive to Fe excess. Our results identify a ferric reductase implicated in Fe homeostasis and developmental responses to abiotic stress, and reveal a biological role for CYBDOM proteins in plants.

Advances in Novel Animal Vitamin C Biosynthesis Pathways and the Role of Prokaryote-Based Inferences to Understand Their Origin

Vitamin C (VC) is an essential nutrient required for the optimal function and development of many organisms. VC has been studied for many decades, and still today, the characterization of its functions is a dynamic scientific field, mainly because of its commercial and therapeutic applications. In this review, we discuss, in a comparative way, the increasing evidence for alternative VC synthesis pathways in insects and nematodes, and the potential of myo-inositol as a possible substrate for this metabolic process in metazoans. Methodological approaches that may be useful for the future characterization of the VC synthesis pathways of Caenorhabditis elegans and Drosophila melanogaster are here discussed. We also summarize the current distribution of the eukaryote aldonolactone oxidoreductases gene lineages, while highlighting the added value of studies on prokaryote species that are likely able to synthesize VC for both the characterization of novel VC synthesis pathways and inferences on the complex evolutionary history of such pathways. Such work may help improve the industrial production of VC.

Methylglyoxal-Scavenging Enzyme Activities Trigger Erythroascorbate Peroxidase and Cytochrome c Peroxidase in Glutathione-Depleted Candida albicans

γ-Glutamylcysteine synthetase (Gcs1) and glutathione reductase (Glr1) activity maintains minimal levels of cellular methylglyoxal in Candida albicans. In glutathione-depleted Δgcs1, we previously saw that NAD(H)-linked methylglyoxal oxidoreductase (Mgd1) and alcohol dehydrogenase (Adh1) are the most active methylglyoxal scavengers. With methylglyoxal accumulation, disruptants lacking MGD1 or ADH1 exhibit a poor redox state. However, there is little convincing evidence for a reciprocal relationship between methylglyoxal scavenger genes-disrupted mutants and changes in glutathione-(in)dependent redox regulation. Herein, we attempt to demonstrate a functional role for methylglyoxal scavengers, modeled on a triple disruptant (Δmgd1/Δadh1/Δgcs1), to link between antioxidative enzyme activities and their metabolites in glutathione-depleted conditions. Despite seeing elevated methylglyoxal in all of the disruptants, the result saw a decrease in pyruvate content in Δmgd1/Δadh1/Δgcs1 which was not observed in double gene-disrupted strains such as Δmgd1/Δgcs1 and Δadh1/Δgcs1. Interestingly, Δmgd1/Δadh1/Δgcs1 exhibited a significantly decrease in H2O2 and superoxide which was also unobserved in Δmgd1/Δgcs1 and Δadh1/Δgcs1. The activities of the antioxidative enzymes erythroascorbate peroxidase and cytochrome c peroxidase were noticeably higher in Δmgd1/Δadh1/Δgcs1 than in the other disruptants. Meanwhile, Glr1 activity severely diminished in Δmgd1/Δadh1/Δgcs1. Monitoring complementary gene transcripts between double gene-disrupted Δmgd1/Δgcs1 and Δadh1/Δgcs1 supported the concept of an unbalanced redox state independent of the Glr1 activity for Δmgd1/Δadh1/Δgcs1. Our data demonstrate the reciprocal use of Eapx1 and Ccp1 in the absence of both methylglyoxal scavengers; that being pivotal for viability in non-filamentous budding yeast.

Pentose metabolism and conversion to biofuels and high-value chemicals in yeasts

Pentose sugars are widespread in nature and two of them, D-xylose and L-arabinose belong to the most abundant sugars being the second and third by abundance sugars in dry plant biomass (lignocellulose) and in general on planet. Therefore, it is not surprising that metabolism and bioconversion of these pentoses attract much attention. Several different pathways of D-xylose and L-arabinose catabolism in bacteria and yeasts are known. There are even more common and really ubiquitous though not so abundant pentoses, D-ribose and 2-deoxy-D-ribose, the constituents of all living cells. Thus, ribose metabolism is example of endogenous metabolism whereas metabolism of other pentoses, including xylose and L-arabinose, represents examples of the metabolism of foreign exogenous compounds which normally are not constituents of yeast cells. As a rule, pentose degradation by the wild-type strains of microorganisms does not lead to accumulation of high amounts of valuable substances; however, productive strains have been obtained by random selection and metabolic engineering. There are numerous reviews on xylose and (less) L-arabinose metabolism and conversion to high value substances; however, they mostly are devoted to bacteria or the yeast Saccharomyces cerevisiae. This review is devoted to reviewing pentose metabolism and bioconversion mostly in non-conventional yeasts, which naturally metabolize xylose. Pentose metabolism in the recombinant strains of S. cerevisiae is also considered for comparison. The available data on ribose, xylose, L-arabinose transport, metabolism, regulation of these processes, interaction with glucose catabolism and construction of the productive strains of high-value chemicals or pentose (ribose) itself are described. In addition, genome studies of the natural xylose metabolizing yeasts and available tools for their molecular research are reviewed. Metabolism of other pentoses (2-deoxyribose, D-arabinose, lyxose) is briefly reviewed.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe²⁺. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe²⁺ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H₂O₂ produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

Gigaspora margarita with and without its endobacterium shows adaptive responses to oxidative stress

Arbuscular mycorrhizal (AM) fungi experience oxidative stress during the plant-fungal interaction, due to endogenous reactive oxygen species (ROS) produced by fungal metabolism and exogenous ROS produced by plant cells. Here, we examine the responses to H₂O₂ in Gigaspora margarita, an AM fungus containing the endobacterial symbiont Candidatus Glomeribacter gigasporarum (CaGg). Previous studies revealed that G. margarita with its endobacterium produces more ATP and has higher respiratory activity than a cured line that lacks the endobacterium. This higher bioenergetic potential leads to higher production of ROS and to a higher ROS-detoxifying capacity, suggesting a direct or indirect role of the endobacterium in modulating fungal antioxidant responses. To test the hypothesis that the fungal-endobacterial symbiosis may enhance the fitness of the AM fungus in the presence of oxidative stress, we treated the fungus with a sublethal concentration of H₂O₂ and performed RNA-seq analysis. Our results demonstrate that (i) irrespective of the endobacterium presence, G. margarita faces oxidative stress by activating multiple metabolic processes (methionine oxidation, sulfur uptake, the pentose phosphate pathway, activation of ROS-scavenger genes); (ii) in the presence of its endobacterium, G. margarita upregulates some metabolic pathways, like chromatin status modifications and iron metabolism; and (iii) contrary to our hypothesis, the cured line responds to H₂O₂ by activating the transcription of specific ROS scavengers. We confirmed the RNA-seq findings by measuring the glutathione and ascorbate concentration, which was the same in both lines after H₂O₂ treatment. We conclude that both fungal lines may face oxidative stress, but they activate alternative strategies.

Cytochrome c peroxidase regulates intracellular reactive oxygen species and methylglyoxal via enzyme activities of erythroascorbate peroxidase and glutathione-related enzymes in Candida albicans

D-erythroascorbate peroxidase (EAPX1) deficiency causes glutathione deprivation, leading to the accumulation of methylglyoxal and reactive oxygen species (ROS), and especially, induction of cytochrome c peroxidase (Ccp1) in Candida albicans. Nevertheless, reciprocal effects between changes in Ccp1 activity and the antioxidative D-erythroascorbic acid- and glutathione-dependent redox status, which reflects methylglyoxal biosynthesis altering pathophysiology are unclear in eukaryotes. To elucidate the effect of CCP1 expression on EAPX1 and glutathione reductase (Glr1) activity-mediated D-erythroascorbic acid biosynthesis and redox homeostasis, the CCP1 gene was disrupted and overexpressed. First, we demonstrated both glutathione-independent and-dependent metabolite contents and their corresponding gene transcripts and enzyme activities (i.e., Ccp1, catalase-peroxidase [KatG], superoxide dismutase [Sod], Eapx1, and Glr1) in CCP1 mutants. Second, methylglyoxal-oxidizing alcohol dehydrogenase (Adh1) and methylglyoxal-reducing oxidoreductase activity on glycolytic methylglyoxal and pyruvate production and NAD(P)H content were determined in these mutants. Contrary to our expectation, CCP1 disruption (42.19±3.22nmolO₂h^-1mgwetcell^-1) failed to affect cell respiration compared to the wild-type strain (41.62±7.11nmolO₂h^-1mgwetcell^-1) under cyanide treatment, and in contrast to hydrogen peroxide (H₂O₂) treatment (21.74±1.03nmol O₂h^-1mgwetcell^-1). Additionally, Ccp1 predominantly detoxified H₂O₂ rather than negligible scavenging activities towards methylglyoxal and other oxidants. CCP1 deficiency stimulated Sod and Adh1 activity but downregulated Glr1, Eapx1, catalase, and peroxidase activity while enhancing KatG, EAPX1, and GLR1 transcription by decreasing glutathione and D-erythroascorbic acid and increasing pyruvate. Noticeably, the ROS-accumulating CCP1-deficient mutant maintained steady-state levels of methylglyoxal, which was revealed to be regulated by methylglyoxal-oxidizing and -reducing activity with drastic changes in NAD(P)H. We confirmed and clarified our results by showing that CCP1/EAPX1 double disruptants underwent severe growth defects due to the D-erythroascorbic acid and glutathione depletion because of pyruvate overaccumulation. These observations were made in both budding and hyphal-growing CCP1 mutants. The revealed metabolic network involving Ccp1 and other redox regulators affected ROS and methylglyoxal through D-erythroascorbic acid and glutathione-dependent metabolites, thereby influencing dimorphism. This is the first report of the Ccp1-mediated D-erythroascorbic acid and glutathione biosynthesis accompanying methylglyoxal scavengers for full fungal virulence.

Methylglyoxal upregulates Dictyostelium discoideum slug migration by triggering glutathione reductase and methylglyoxal reductase activity

Glutathione (GSH)-deprived Dictyostelium discoideum accumulates methylglyoxal (MG) and reactive oxygen species (ROS) during vegetative growth. However, the reciprocal effects of the production and regulation of these metabolites on differentiation and cell motility are unclear. Based on the inhibitory effects of γ-glutamylcysteine synthetase (gcsA) disruption and GSH reductase (gsr) overexpression on aggregation and culmination, respectively, we overexpressed GSH-related genes encoding superoxide dismutase (Sod2), catalase (CatA), and Gcs, in D. discoideum. Wild-type KAx3 and gcsA-overexpressing (gcsA^OE) slugs maintained GSH levels at levels of approximately 2.1-fold less than the reference GSH synthetase-overexpressing mutant; their GSH levels did not correlate with slug migration ability. Through prolonged KAx3 migration by treatment with MG and H₂O₂, we found that MG increased after the mound stage in this strain, with a 2.6-fold increase compared to early developmental stages; in contrast, ROS were maintained at high levels throughout development. While the migration-defective sod2- and catA-overexpressing mutant slugs (sod2^OE and catA^OE) decreased ROS levels by 50% and 53%, respectively, these slugs showed moderately decreased MG levels (36.2±5.8 and 40.7±1.6nmolg^-1 cells wet weight, P<0.05) compared to the parental strain (54.2±3.5nmolg^-1). Importantly, defects in the migration of gcsA^OE slugs decreased MG considerably (13.8±4.2nmolg^-1, P<0.01) along with a slight decrease in ROS. In contrast to the increase observed in migrating sod2^OE and catA^OE slugs by treatment with MG and H₂O₂, the migration of gcsA^OE slugs appeared unaffected. This behavior was caused by MG-triggered Gsr and NADPH-linked aldolase reductase activity, suggesting that GSH biosynthesis in gcsA^OE slugs is specifically used for MG-scavenging activity. This is the first report showing that MG upregulates slug migration via MG-scavenging-mediated differentiation.

Quantitative and qualitative transcriptome analysis of four industrial strains of Claviceps purpurea with respect to ergot alkaloid production

The fungus Claviceps purpurea is a biotrophic phytopathogen widely used in the pharmaceutical industry for its ability to produce ergot alkaloids (EAs). The fungus attacks unfertilized ovaries of grasses and forms sclerotia, which represent the only type of tissue where the synthesis of EAs occurs. The biosynthetic pathway of EAs has been extensively studied; however, little is known concerning its regulation. Here, we present the quantitative transcriptome analysis of the sclerotial and mycelial tissues providing a comprehensive view of transcriptional differences between the tissues that produce EAs and those that do not produce EAs and the pathogenic and non-pathogenic lifestyle. The results indicate metabolic changes coupled with sclerotial differentiation, which are likely needed as initiation factors for EA biosynthesis. One of the promising factors seems to be oxidative stress. Here, we focus on the identification of putative transcription factors and regulators involved in sclerotial differentiation, which might be involved in EA biosynthesis. To shed more light on the regulation of EA composition, whole transcriptome analysis of four industrial strains differing in their alkaloid spectra was performed. The results support the hypothesis proposing the composition of the amino acid pool in sclerotia to be an important factor regulating the final structure of the ergopeptines produced by Claviceps purpurea.

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.

The invertebrate Caenorhabditis elegans biosynthesizes ascorbate

L-ascorbate, commonly known as vitamin C, serves as an antioxidant and cofactor essential for many biological processes. Distinct ascorbate biosynthetic pathways have been established for animals and plants, but little is known about the presence or synthesis of this molecule in invertebrate species. We have investigated ascorbate metabolism in the nematode Caenorhabditis elegans, where this molecule would be expected to play roles in oxidative stress resistance and as cofactor in collagen and neurotransmitter synthesis. Using high-performance liquid chromatography and gas-chromatography mass spectrometry, we determined that ascorbate is present at low amounts in the egg stage, L1 larvae, and mixed animal populations, with the egg stage containing the highest concentrations. Incubating C. elegans with precursor molecules necessary for ascorbate synthesis in plants and animals did not significantly alter ascorbate levels. Furthermore, bioinformatic analyses did not support the presence in C. elegans of either the plant or the animal biosynthetic pathway. However, we observed the complete ¹³C-labeling of ascorbate when C. elegans was grown with ¹³C-labeled Escherichia coli as a food source. These results support the hypothesis that ascorbate biosynthesis in invertebrates may proceed by a novel pathway and lay the foundation for a broader understanding of its biological role.

Antioxidant activity of L-ascorbic acid in wild-type and superoxide dismutase deficient strains ofSaccharomyces cerevisiae

Much has been published on the non-enzymatic antioxidant L-ascorbic acid (vitamin C), but even so its interaction with endogenous cellular defense systems has not yet been fully elucidated. Our study investigated the antioxidant activity of L-ascorbic acid in wild-type strain EG103 (SOD) Saccharomyces cerevisiae and isogenic mutant strains deficient in cytosolic superoxide dismutase (sod1delta), mitochondrial superoxide dismutase (sod2delta) or both (sod1delta sod2delta), metabolizing aerobically or anaerobically with and without the stressing agent paraquat. The results show that during both aerobic and anaerobic metabolism there was a significant increase in the survival of both wild-type S. cerevisiae cells and the mutant cells (sod1delta, sod2delta and sod1delta sod2delta) when pretreated with L-ascorbic acid before exposure to paraquat. Exposure to paraquat resulted in higher catalase activity but this significantly decreased when the cells were pre-treated with L-ascorbic acid. These results demonstrate that due to the damage caused by paraquat, the antioxidant protection of L-ascorbic acid seems to be mediated by catalase levels in yeast cells.

Metabolic engineering of Kluyveromyces lactis for L-ascorbic acid (vitamin C) biosynthesis

BACKGROUND

L-ascorbic acid (L-AA) is naturally synthesized in plants from D-glucose by 10 steps pathway. The pathway branch to synthesize L-galactose, the key intermediate for L-ascorbic acid biosynthesis, has been recently elucidated. Budding yeast produces an 5-carbon ascorbic acid analogue Dehydro-D-arabinono 1,4-lactone (D-DAL), which is synthesized from D-arabinose. Yeast is able to synthesize L-ascorbic acid only if it is cultivated in the presence of one of its precursors: L-galactose, L-galactono 1,4-lactone, or L-gulono 1,4-lactone extracted from plants or animals. To avoid feeding the yeast culture with this "L" enantiomer, we engineered Kluyveromyces lactis with L-galactose biosynthesis pathway genes: GDP-mannose 3,5-epimerase (GME), GDP-L-galactose phosphorylase (VTC2) and L-galactose-1-phosphate phosphatase (VTC4) isolated from Arabidopsis thaliana.

RESULTS

Plasmids were constructed and modified such that the cloned plant genes were targeted to the K. lactis LAC4 Locus by homologous recombination and that the expression was associated to the growth on D-galactose or lactose. Upon K. lactis transformation, GME was under the control of the native LAC4 promoter whereas VTC2 and VTC4 were expressed from the S. cerevisiae promoters GPD1 and ADH1 respectively. The expression in K. lactis, of the L-galactose biosynthesis genes was determined by Reverse Transcriptase-PCR and western blotting. The recombinant yeasts were capable to produce about 30 mg.L(-1) of L-ascorbic acid in 48 hours of cultivation when cultured on rich medium with 2% (w/v) D-galactose. We also evaluated the L-AA production culturing recombinant recombinant strains in cheese whey, a waste product during cheese production, as an alternative source of lactose.

CONCLUSIONS

This work is the first attempt to engineer K. lactis cells for L-ascorbic acid biosynthesis by a fermentation process without any trace of "L" isomers precursors in the culture medium. We have engineered K. lactis strains capable of converting lactose and D-galactose into L-galactose, by the integration of the genes from the A. thaliana L-galactose pathway. L-galactose is a rare sugar, which is one of the main precursors for L-AA production.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

A common need for microbial cells is the ability to respond to potentially toxic environmental insults. Here we review the progress in understanding the response of the yeast Saccharomyces cerevisiae to two important environmental stresses: heat shock and oxidative stress. Both of these stresses are fundamental challenges that microbes of all types will experience. The study of these environmental stress responses in S. cerevisiae has illuminated many of the features now viewed as central to our understanding of eukaryotic cell biology. Transcriptional activation plays an important role in driving the multifaceted reaction to elevated temperature and levels of reactive oxygen species. Advances provided by the development of whole genome analyses have led to an appreciation of the global reorganization of gene expression and its integration between different stress regimens. While the precise nature of the signal eliciting the heat shock response remains elusive, recent progress in the understanding of induction of the oxidative stress response is summarized here. Although these stress conditions represent ancient challenges to S. cerevisiae and other microbes, much remains to be learned about the mechanisms dedicated to dealing with these environmental parameters.

Cyclophilin A Cpr1 Protein Modulates the Response of Antioxidant Molecules to Menadione-induced Oxidative Stress in Saccharomyces cerevisiae KNU5377Y

Objectives

Methods

Results

Conclusion

A cytochrome b561 with ferric reductase activity from the parasitic blood fluke, Schistosoma japonicum

BACKGROUND

Iron has an integral role in numerous cellular reactions and is required by virtually all organisms. In physiological conditions, iron is abundant in a largely insoluble ferric state. Ferric reductases are an essential component of iron uptake by cells, reducing iron to the soluble ferrous form. Cytochromes b561 (cyts-b561) are a family of ascorbate reducing transmembrane proteins found in most eukaryotic cells. The identification of the ferric reductase duodenal cytochrome b (dcytb) and recent observations that other cyts-b561 may be involved in iron metabolism have opened novel perspectives for elucidating their physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have identified a new member of the cytochrome b561 (Sjcytb561) family in the pathogenic blood fluke Schistosoma japonicum that localises to the outer surface of this parasitic trematode. Heterologous expression of recombinant Sjcyt-b561 in a Saccharomyces cerevisiae mutant strain that lacks plasma membrane ferrireductase activity demonstrated that the molecule could rescue ferric reductase activity in the yeast.

SIGNIFICANCE/CONCLUSIONS

This finding of a new member of the cytochrome b561 family further supports the notion that a ferric reductase function is likely for other members of this protein family. Additionally, the localisation of Sjcytb561 in the surface epithelium of these blood-dwelling schistosomes contributes further to our knowledge concerning nutrient acquisition in these parasites and may provide novel targets for therapeutic intervention.

Production, stability, and antioxidative and antimicrobial activities of two L-ascorbate analogues from phycomyces blakesleeanus: D-erythroascorbate and D-erythroascorbate glucoside

D-erythroascorbate (D-EAA), a five-carbon analogue of L-ascorbate (L-AA), and D-erythroascorbate monoglucoside (D-EAAG) are accumulated in Phycomyces blakesleeanus grown on glucose (99.5 and 1084 μg/g mycelial dry weight, respectively) and also excreted into the culture medium. Both compounds showed UV spectral properties and ionization constants similar to those of L-AA. D-EAAG was much more stable to aerobic oxidation than D-EAA and L-AA at acidic pH. D-EAAG is synthesized from D-erythroascorbate by a mycelial glucosyltransferase activity that uses UDP-glucose as glucose substrate donor with K(m) = 2.5 mM and 41.3 μM for D-EAA. This glucosyltransferase activity was maximal in the stationary growth phase in parallel with maximal production of D-EAAG. The presence of D-arabinose or D-arabinono-1,4-lactone in the culture medium produces the maximal accumulation of D-EAA and D-EAAG (about 30- and 4-fold with respect to that obtained in glucose culture). Both compounds showed greater antioxidant activity than L-AA and other standard antioxidants, with a capacity similar to that of L-AA to inhibit the growth of Escherichia coli.

Expression of a glutathione reductase from Brassica rapa subsp. pekinensis enhanced cellular redox homeostasis by modulating antioxidant proteins in Escherichia coli

Glutathione reductase (GR) is an enzyme that recycles a key cellular antioxidant molecule glutathione (GSH) from its oxidized form (GSSG) thus maintaining cellular redox homeostasis. A recombinant plasmid to overexpress a GR of Brassica rapa subsp. pekinensis (BrGR) in E. coli BL21 (DE3) was constructed using an expression vector pKM260. Expression of the introduced gene was confirmed by semiquantitative RT-PCR, immunoblotting and enzyme assays. Purification of the BrGR protein was performed by IMAC method and indicated that the BrGR was a dimmer. The BrGR required NADPH as a cofactor and specific activity was approximately 458 U. The BrGR-expressing E. coli cells showed increased GR activity and tolerance to H(2)O(2), menadione, and heavy metal (CdCl(2), ZnCl(2) and AlCl(2))-mediated growth inhibition. The ectopic expression of BrGR provoked the co-regulation of a variety of antioxidant enzymes including catalase, superoxide dismutase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase. Consequently, the transformed cells showed decreased hydroperoxide levels when exposed to stressful conditions. A proteomic analysis demonstrated the higher level of induction of proteins involved in glycolysis, detoxification/oxidative stress response, protein folding, transport/binding proteins, cell envelope/porins, and protein translation and modification when exposed to H(2)O(2) stress. Taken together, these results indicate that the plant GR protein is functional in a cooperative way in the E. coli system to protect cells against oxidative stress.

Relationships between flavour, lipid composition and antioxidants in organic, free-range and conventional chicken breasts from modelling

Consumers expect organic, free-range and corn-fed chicken to be nutritionally wholesome and have premium flavour characters. Interrelationships between flavour, fatty acids and antioxidants of retailed breasts were explored using simple correlations and chemometrics. Saturated fatty acid C16:0, and n-6 polyunsaturated C20:4 and C22:4 contents were correlated with lipid oxidation products (thiobarbituric acid reactive substances) and in partial least-squares regression (PLS1) with 32 high-resonance gas chromatography (flame ionization) flavour components (r2>0.90), and also linked (r2>0.80) to antioxidants (alpha-tocopherol, glutathione and catalase). A further 10 high-resonance gas chromatography nitrogen phosphorus detector flavour components were correlated (r2>0.85) with C18:3(n-3) content. Chicken character was correlated with C18:3(n-3), and C18:3(n-6) inversely with oily, off-flavour and lipid oxidation. Sweet, fruity and oily aromas were linked in PLS1 with 13 specific fatty acids (r2>0.6), and bland taste with total summed (six) fatty acid fractions (r2>0.81). Specific antioxidants were correlated with sweet, fruity and chicken aromas, and alpha-tocopherol inversely with lipid oxidation. PLS2 confirmed relationships between fatty acid composition, antioxidants and the subsets of 32 and 10 flavour components. Clear relationships were thus observed between lipid and antioxidant compositions and flavour in chicken breast meat.

Antioxidant activity of the yeast mitochondrial one-Cys peroxiredoxin is dependent on thioredoxin reductase and glutathione in vivo

Peroxiredoxins are ubiquitous enzymes which protect cells against oxidative stress. The first step of catalysis is common to all peroxiredoxins and results in oxidation of a conserved peroxidatic cysteine residue to sulfenic acid. This forms an intermolecular disulfide bridge in the case of 2-Cys peroxiredoxins, which is a substrate for the thioredoxin system. 1-Cys Prx's contain a peroxidatic cysteine but do not contain a second conserved cysteine residue, and hence the identity of the in vivo reduction system has been unclear. Here, we show that the yeast mitochondrial 1-Cys Prx1 is reactivated by glutathionylation of the catalytic cysteine residue and subsequent reduction by thioredoxin reductase (Trr2) coupled with glutathione (GSH). This novel mechanism does not require the usual thioredoxin (Trx3) redox partner of Trr2 for antioxidant activity, although in vitro assays show that the Trr2/Trx3 and Trr2/GSH systems exhibit similar capacities for supporting Prx1 catalysis. Our data also indicate that mitochondria are a main target of cadmium-induced oxidative stress and that Prx1 is particularly required to protect against mitochondrial oxidation. This study demonstrates a physiological reaction mechanism for 1-Cys peroxiredoxins and reveals a new role in protection against mitochondrial heavy metal toxicity.

Antioxidant small molecules confer variable protection against oxidative damage in yeast mutants

To assess the capacity of small molecules to function as antioxidants in pathologic conditions, a set of yeast assays utilizing strains deficient in the antioxidant machinery was applied with measurements of reactive oxygen species (ROS), glutathione (GSH/GSSG), and induction of the stress responsive proteins oye2 and oye3. Yeast strains deficient in superoxide dismutase (Delta sod1), catalase A (Delta cta1), and double-deficient in Old Yellow enzyme 2 and glutathione reductase 1 (Delta oye2 glr1) were supplemented with ascorbic acid, beta-carotene, caffeic acid, or quercetin, subjected to pro-oxidant insult, and monitored for growth recovery. Ascorbic acid and caffeic acid protected cells under most circumstances, whereas beta-carotene and quercetin protection was highly context dependent, exhibiting protection in some cases and inhibition in others. Beta-carotene and quercetin elevated substantially endogenous levels of ROS in some yeast mutants. Quercetin supplementation increased significantly GSH and GSSG levels but could not maintain GSH levels in H(2)O(2)-exposed cells. Induction of the stress response machinery was manifested by the strong up-regulation of a chromosomally encoded OYE2-GFP fusion. In the case of quercetin, there was simultaneous induction of OYE3-GFP, which was previously shown to sensitize cells to H(2)O(2)-induced programmed cell death (PCD). Taken together, the results show that mutations in the antioxidant machinery affect significantly the capacity of dietary antioxidants to protect cells.

Filamentous fungi for production of food additives and processing aids

Filamentous fungi are metabolically versatile organisms with a very wide distribution in nature. They exist in association with other species, e.g. as lichens or mycorrhiza, as pathogens of animals and plants or as free-living species. Many are regarded as nature's primary degraders because they secrete a wide variety of hydrolytic enzymes that degrade waste organic materials. Many species produce secondary metabolites such as polyketides or peptides and an increasing range of fungal species is exploited commercially as sources of enzymes and metabolites for food or pharmaceutical applications. The recent availability of fungal genome sequences has provided a major opportunity to explore and further exploit fungi as sources of enzymes and metabolites. In this review chapter we focus on the use of fungi in the production of food additives but take a largely pre-genomic, albeit a mainly molecular, view of the topic.

Heterologous production of secondary metabolites as pharmaceuticals in Saccharomyces cerevisiae

Heterologous expression of genes involved in the biosynthesis of various products is of increasing interest in biotechnology and in drug research and development. Microbial cells are most appropriate for this purpose. Availability of more microbial genomic sequences in recent years has greatly facilitated the elucidation of metabolic and regulatory networks and helped gain overproduction of desired metabolites or create novel production of commercially important compounds. Saccharomyces cerevisiae, as one of the most intensely studied eukaryotic model organisms with a rich density of knowledge detailing its genetics, biochemistry, physiology, and large-scale fermentation performance, can be capitalized upon to enable a substantial increase in the industrial application of this yeast. In this review, we describe recent efforts made to produce commercial secondary metabolites in Saccharomyces cerevisiae as pharmaceuticals. As natural products are increasingly becoming the center of attention of the pharmaceutical and nutraceutical industries, such as naringenin, coumarate, artemisinin, taxol, amorphadiene and vitamin C, the use of S. cerevisiae for their production is only expected to expand in the future, further allowing the biosynthesis of novel molecular structures with unique properties.

Biosynthesis of vitamin C by yeast leads to increased stress resistance

Background

In industrial large scale bio-reactions micro-organisms are generally exposed to a variety of environmental stresses, which might be detrimental for growth and productivity. Reactive oxygen species (ROS) play a key role among the common stress factors–directly-through incomplete reduction of O₂ during respiration, or indirectly-caused by other stressing factors. Vitamin C or L-ascorbic acid acts as a scavenger of ROS, thereby potentially protecting cells from harmful oxidative products. While most eukaryotes synthesize ascorbic acid, yeast cells produce erythro-ascorbic acid instead. The actual importance of this antioxidant substance for the yeast is still a subject of scientific debate.

Methodology/Principal Findings

We set out to enable Saccharomyces cerevisiae cells to produce ascorbic acid intracellularly to protect the cells from detrimental effects of environmental stresses. We report for the first time the biosynthesis of L-ascorbic acid from D-glucose by metabolically engineered yeast cells. The amount of L-ascorbic acid produced leads to an improved robustness of the recombinant cells when they are subjected to stress conditions as often met during industrial fermentations. Not only resistance against oxidative agents as H₂O₂ is increased, but also the tolerance to low pH and weak organic acids at low pH is increased.

Conclusions/Significance

This platform provides a new tool whose commercial applications may have a substantial impact on bio-industrial production of Vitamin C. Furthermore, we propose S. cerevisiae cells endogenously producing vitamin C as a cellular model to study the genesis/protection of ROS as well as genotoxicity.

Sclerotial biomass and carotenoid yield of Penicillium sp. PT95 under oxidative growth conditions and in the presence of antioxidant ascorbic acid

AIMS

To determine the effect of oxidative stress and exogenous ascorbic acid on sclerotial biomass and carotenoid yield of Penicillium sp. PT95.

METHODS

In this experiment, high oxidative stress was applied by the inclusion of FeSO(4) in the growth medium and exposure to light. Low oxidative stress was applied by omitting iron from the growth medium and by incubation in the dark. Supplementation of exogenous ascorbic acid (as antioxidant) to the basal medium caused a concentration-dependent delay of sclerotial differentiation (up to 48 h), decrease of sclerotial biomass (up to 40%) and reduction of carotenoid yield (up to 91%). On the contrary, the exogenous ascorbic acid also caused a concentration-dependent decrease of lipid peroxidation in colonies of this fungus.

CONCLUSIONS

Under high oxidative stress growth condition, the sclerotial biomass and carotenoid yield of PT95 strain in each plate culture reached 305 mg and 32.94 microg, which were 1.23 and 3.71 times higher, respectively, than those at low oxidative stress growth condition. These data prompted us to consider that in order to attain higher sclerotial biomass and pigment yield, the strain PT95 should be grown under high oxidative stress and in the absence of antioxidants.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results suggest that strain PT95 may be used for solid-state fermentation of carotenoid production under high oxidative stress growth conditions.

NAD+-specific D-arabinose dehydrogenase and its contribution to erythroascorbic acid production in Saccharomyces cerevisiae

Erythroascorbic acid (eAsA) is a five-carbon analog of ascorbic acid, and it is synthesized from D-arabinose by D-arabinose dehydrogenase (ARA) and D-arabinono-gamma-lactone oxidase. We found an NAD+-specific ARA activity which is operative under submillimolar level of d-arabinose in the extracts of Saccharomyces cerevisiae. The hypothetical protein encoded by YMR041c showed a significant homology to a l-galactose dehydrogenase which plays in plant ascorbic acid biosynthesis, and we named it as Ara2p. Recombinant Ara2p showed NAD+-specific ARA activity with Km=0.78 mM to d-arabinose, which is 200-fold lower than that for the conventional NADP+-specific ARA, Ara1p. Gene disruptant of ARA2 lost entire NAD+-specific ARA activity and the conspicuous increase in intracellular eAsA by exogenous d-arabinose feeding, while the double knockout mutant of ARA1 and ARA2 still retained measurable amount of eAsA. It demonstrates that Ara2p, not Ara1p, mainly contributes to the production of eAsA from d-arabinose in S. cerevisiae.

Three mammalian cytochromes b561 are ascorbate-dependent ferrireductases

Cytochromes b(561) are a family of transmembrane proteins found in most eukaryotic cells. Three evolutionarily closely related mammalian cytochromes b(561) (chromaffin granule cytochrome b, duodenal cytochrome b, and lysosomal cytochrome b) were expressed in a Saccharomyces cerevisiaeDeltafre1Deltafre2 mutant, which lacks almost all of its plasma membrane ferrireductase activity, to study their ability to reduce ferric iron (Fe(3+)). The expression of each of these cytochromes b(561) was able to rescue the growth defect of the Deltafre1Deltafre2 mutant cells in iron-deficient conditions, suggesting their involvement in iron metabolism. Plasma membrane ferrireductase activities were measured using intact yeast cells. Each cytochrome b(561) showed significant FeCN and Fe(3+)-EDTA reductase activities that were dependent on the presence of intracellular ascorbate. Site-directed mutagenesis of lysosomal cytochrome b was conducted to identify amino acids that are indispensable for its activity. Among more than 20 conserved or partially conserved amino acids that were investigated, mutations of four His residues (H47, H83, H117 and H156), one Tyr (Y66) and one Arg (R67) completely abrogated the FeCN reductase activity, whereas mutations of Arg (R149), Phe (F44), Ser (S115), Trp (W119), Glu (E196), and Gln (Q131) affected the ferrireductase activity to some degree. These mutations may affect the heme coordination, ascorbate binding, and/or ferric substrate binding. Possible roles of these residues in lysosomal cytochrome b are discussed. This study demonstrates the ascorbate-dependent transmembrane ferrireductase activities of members of the mammalian cytochrome b(561) family of proteins.

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.

Role of the non-respiratory pathways in the utilization of molecular oxygen by Saccharomyces cerevisiae

Saccharomyces cerevisiae is a facultative anaerobe devoid of mitochondrial alternative oxidase. In this yeast, the structure and biogenesis of the respiratory chain, on the one hand, and the functional interactions of oxidative phosphorylation with the cellular energetic metabolism, on the other, are well documented. However, to our knowledge, the molecular aspects and the physiological roles of the non-respiratory pathways that utilize molecular oxygen have not yet been reviewed. In this paper, we review the various non-respiratory pathways in a global context of utilization of molecular oxygen in S. cerevisiae. The roles of these pathways are examined as a function of environmental conditions, using either physiological, biochemical or molecular data. Special attention is paid to the characterization of the so-called 'cyanide-resistant respiration' that is induced by respiratory deficiency, catabolic repression and oxygen limitation during growth. Finally, several aspects of oxygen sensing are discussed.

The interaction of sodium chlorite with phospholipids and glutathione: a comparison of effects in vitro, in mammalian and in microbial cells

In this study the interaction of the preservative sodium chlorite with unsaturated lipids and glutathione was investigated, in comparison with peroxides, sodium hypochlorite, and benzalkonium chloride. The aim was to determine whether the action of sodium chlorite could involve membrane lipid damage or antioxidant depletion, and how this related to toxicity in both mammalian and microbial cells. The treatment of phospholipids with chlorite yielded low levels of hydroperoxides, but sodium chlorite oxidized the thiol-containing antioxidant glutathione to its disulfide form very readily in vitro, with a 1:4 oxidant:GSH stoichiometry. In cultured cells, sodium chlorite also caused a substantial depletion of intracellular glutathione, whereas lipid oxidation was not very prominent. Sodium chlorite had a lower toxicity to ocular mammalian cells than benzalkonium chloride, which could be responsible for the different effects of long-term application in the eye. The fungal cells, which were most resistant to sodium chlorite, maintained higher percentage levels of intracellular glutathione during treatment than the mammalian cells. The results show that sodium chlorite can cause oxidative stress in cells, and suggest that cell damage is more likely to be due to interaction with thiol compounds than with cell membrane lipids. The study also provides important information about the differential resistance of ocular cells and microbes to various preservatives and oxidants.

A phylogenetic study of cytochrome b561 proteins

BACKGROUND

As an antioxidant and cofactor to numerous metabolic enzymes, ascorbate has an essential role in plants and animals. Cytochromes b561 constitute a class of intrinsic membrane proteins involved in ascorbate regeneration. Despite their importance in ascorbate metabolism, no evolutionary analysis has been presented so far on this newly described protein family.

RESULTS

Cytochromes b561 have been identified in a large number of phylogenetically distant species, but are absent in fungi and prokaryotes. Most species contain three or four cytochrome b561 paralogous proteins, and the encoding genes usually have four or five exons. At the protein level, sequence similarities are rather low between cytochromes b561 within a single species (34-45% identity), and among phylogenetically distant species (around 30% identity). However, particular structural features characterizing this protein family are well conserved in members from all species investigated. These features comprise six transmembrane helices, four strictly conserved histidine residues, probably coordinating the two heme molecules, and putative ascorbate and monodehydro-ascorbate (MDHA) substrate-binding sites. Analysis of plant cytochromes b561 shows a separation between those from monocotyledonous and dicotyledonous species in a phylogenetic tree.

CONCLUSIONS

All cytochromes b561 have probably evolved from a common ancestral protein before the separation of plants and animals. Their phyletic distribution mirrors the use of ascorbate as primary antioxidant, indicating their role in ascorbate homeostasis and antioxidative defense. In plants, the differentiation into four cytochrome b561 isoforms probably occurred before the separation between monocots and dicots.

The role of ascorbic acid role in the differentiation of sclerotia in Sclerotinia minor

Sclerotinia minor in culture produces ascorbic acid in levels dependent on oxidative growth conditions and stage of development. During differentiation reduced/oxidized ascorbate ratio decreased by 12 and 6 fold at high and low oxidative stress, respectively. Exogenous ascorbate caused a concentration-dependent decrease of oxidative stress (lipid peroxidation), inhibition of sclerotial differentiation (up to 100%) and delay of differentiatlon (up to 10 days). Ascorbic acid may be produced to help the fungus reduce oxidative stress during growth. The data of this study support our theory proposing that oxidative stress is the inducing factor of sclerotial differentiation in fungi.

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.

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.