Dihydrofolate Reductase/Thymidylate Synthase Fine-Tunes the Folate Status and Controls Redox Homeostasis in Plants.

Folates (B9 vitamins) are essential cofactors in one-carbon metabolism. Since C1 transfer reactions are involved in synthesis of nucleic acids, proteins, lipids, and other biomolecules, as well as in epigenetic control, folates are vital for all living organisms. This work presents a complete study of a plant DHFR-TS (dihydrofolate reductase-thymidylate synthase) gene family that implements the penultimate step in folate biosynthesis. We demonstrate that one of the DHFR-TS isoforms (DHFR-TS3) operates as an inhibitor of its two homologs, thus regulating DHFR and TS activities and, as a consequence, folate abundance. In addition, a novel function of folate metabolism in plants is proposed, i.e., maintenance of the redox balance by contributing to NADPH production through the reaction catalyzed by methylenetetrahydrofolate dehydrogenase, thus allowing plants to cope with oxidative stress.

Degradation and interconversion of plant pteridines during sample preparation and ultra-high performance liquid chromatography-tandem mass spectrometry.

The degradation and interconversion of a selected set of pterins (dihydroneopterin, hydroxymethyldihydropterin, dihydroxanthopterin, neopterin, hydroxymethylpterin, xanthopterin, 6-formylpterin, 6-carboxypterin and pterin), spiked to charcoal-treated potato and Arabidopsis thaliana matrix was investigated, together with their relative recovery in potato and A. thaliana. As a result, a matrix-specific procedure for the ultra-high performance liquid chromatography-tandem mass spectrometry based determination of 6 aromatic pterins (neopterin, hydroxymethylpterin, xanthopterin, 6-formylpterin, 6-carboxypterin and pterin) is proposed: 1.5ml of an N2-flushed, alkaline (pH=10) extraction solvent is added to 200mg of plant sample. After boiling and homogenization, the samples are incubated: Arabidopsis samples for 30min at room temperature, while shaking, and potato samples for 2h at 37°C (applying a dienzyme treatment with α-amylase and protease). After a final boiling step, the samples are ultrafiltrated and resulting extracts are analyzed by UHPLC-MS/MS.

Improving folate (vitamin B9) stability in biofortified rice through metabolic engineering.

Biofortification of staple crops could help to alleviate micronutrient deficiencies in humans. We show that folates in stored rice grains are unstable, which reduces the potential benefits of folate biofortification. We obtain folate concentrations that are up to 150 fold higher than those of wild-type rice by complexing folate to folate-binding proteins to improve folate stability, thereby enabling long-term storage of biofortified high-folate rice grains.

A validated ultra-high-performance liquid chromatography-tandem mass spectrometry method for the selective analysis of free and total folate in plasma and red blood cells.

A stable isotope dilution LC-MS/MS method is the method of choice for the selective quantitative determination of several folate species in clinical samples. By implementing an integrated approach to determine both the plasma and red blood cell (RBC) folate status, the use of consumables and time remains limited. Starting from a single 300µl whole blood sample, the folate status in plasma and RBCs can be determined after separating plasma and RBCs and sequential washing of the latter with isotonic buffer, followed by reproducible lysis using an ammonium-based buffer. Acidification combines both liberation of protein bound folates and protein precipitation. Sample cleanup is performed using a 96-well reversed-phase solid-phase extraction procedure, similar for both plasma and RBC samples. Analyses are performed by UHPLC-MS/MS. Method validation was successfully performed based on EMA-guidelines and encompassed selectivity, carry-over, linearity, accuracy, precision, recovery, matrix effect and stability. Plasma and RBC folates could be quantified in the range of 1-150nmol/l and 5-1500nmol/l, respectively. This method allows for the determination of 6 folate monoglutamates in both plasma and RBCs. It can be used to determine short and long term folate status in both normal and severely deficient subjects in a single analytical sequence.

Folates from metabolically engineered rice: a long-term study in rats.

SCOPE: The biological impact of folates from folate rice, a metabolically engineered (biofortified) rice line, rich in folates, was investigated. Its consumption may be helpful to fight folate deficiency. Our objective was to investigate the potential of folate rice to supply the organism with folates and evaluate its biological effectiveness using a rat model. METHODS AND RESULTS: Five groups of 12 Wistar rats were monitored during a 7/12-wk depletion/repletion trial. Animals receiving folate-free diet (0 µg/rat/day) and those additionally receiving wild-type rice (on average 0.11 µg/rat/day) suffered from decreased hematocrit and lower folate concentrations in both plasma and RBCs. This resulted in serious morbidity and even lethality during the trial. In contrast, all animals receiving a daily supplement of folate rice or folic acid fortified rice (on average 3.00 µg/rat/day and 3.12 µg/rat/day, respectively) and those receiving a positive control diet (11.4 to 25.0 µg/rat/day), survived. In these groups, the hematocrit normalized, plasma and RBC folate concentrations increased and pronounced hyperhomocysteinemia was countered. CONCLUSION: Using an animal model, we demonstrated that biofortified folate rice is a valuable source of dietary folate, as evidenced by folate determination in plasma and RBCs, the alleviation of anemia and counteraction of pronounced hyperhomocysteinemia.

Folate Profiling in Potato (Solanum tuberosum) Tubers by Ultrahigh-Performance Liquid Chromatography-Tandem Mass Spectrometry.

An ultrahigh-performance liquid chromatography-tandem mass spectrometry method was developed and validated for the profiling of six folate species in potatoes. The calibration curves cover a wide, linear range (the lower and upper limits of quantitation range between 0.22-0.24 and 216.07-242.28 µg/100 g of fresh weight), allowing sensitive determination in small amounts of potato flesh. With a single exception, the acceptance criteria for intra- and interday precision and accuracy were met: for all quality controls, the percent relative standard deviation and the percent bias were lower than 15% (or 20% at the lower limit of quantitation). Application of the method on tubers at different stages of maturation demonstrated the large variability within a single variety: the folate content and polyglutamylation rate varied between 10.35 and 24.01 µg/100 g of fresh weight and between 4.96% and 60.49%, respectively. Additionally, the two-dimensional folate profiling of mature tubers demonstrated an increase in folate from center to peel, combined with a stable species distribution and polyglutamylation rate.

Enhancing pterin and para-aminobenzoate content is not sufficient to successfully biofortify potato tubers and Arabidopsis thaliana plants with folate.

Folates are important cofactors in one-carbon metabolism in all living organisms. Since only plants and micro- organisms are capable of biosynthesizing folates, humans depend entirely on their diet as a folate source. Given the low folate content of several staple crop products, folate deficiency affects regions all over the world. Folate biofortification of staple crops through enhancement of pterin and para-aminobenzoate levels, precursors of the folate biosynthesis pathway, was reported to be successful in tomato and rice. This study shows that the same strategy is not sufficient to enhance folate content in potato tubers and Arabidopsis thaliana plants and concludes that other steps in folate biosynthesis and/or metabolism need to be engineered to result in substantial folate accumulation. The findings provide a plausible explanation why, more than half a decade after the proof of concept in rice and tomato, successful folate biofortification of other food crops through enhancement of para-aminobenzoate and pterin content has not been reported thus far. A better understanding of the folate pathway is required in order to determine an engineering strategy that can be generalized to most staple crops.

Rice folate enhancement through metabolic engineering has an impact on rice seed metabolism, but does not affect the expression of the endogenous folate biosynthesis genes.

Folates are key-players in one-carbon metabolism in all organisms. However, only micro-organisms and plants are able to synthesize folates de novo and humans rely entirely on their diet as a sole folate source. As a consequence, folate deficiency is a global problem. Although different strategies are currently implemented to fight folate deficiency, up until now, all of them have their own drawbacks. As an alternative and complementary means to those classical strategies, folate biofortification of rice by metabolic engineering was successfully achieved a couple of years ago. To gain more insight into folate biosynthesis regulation and the effect of folate enhancement on general rice seed metabolism, a transcriptomic study was conducted in developing transgenic rice seeds, overexpressing 2 genes of the folate biosynthetic pathway. Upon folate enhancement, the expression of 235 genes was significantly altered. Here, we show that rice folate biofortification has an important effect on folate dependent, seed developmental and plant stress response/defense processes, but does not affect the expression of the endogenous folate biosynthesis genes.

Inhibition of p-aminobenzoate and folate syntheses in plants and apicomplexan parasites by natural product rubreserine.

Glutamine amidotransferase/aminodeoxychorismate synthase (GAT-ADCS) is a bifunctional enzyme involved in the synthesis of p-aminobenzoate, a central component part of folate cofactors. GAT-ADCS is found in eukaryotic organisms autonomous for folate biosynthesis, such as plants or parasites of the phylum Apicomplexa. Based on an automated screening to search for new inhibitors of folate biosynthesis, we found that rubreserine was able to inhibit the glutamine amidotransferase activity of the plant GAT-ADCS with an apparent IC(50) of about 8 µM. The growth rates of Arabidopsis thaliana, Toxoplasma gondii, and Plasmodium falciparum were inhibited by rubreserine with respective IC(50) values of 65, 20, and 1 µM. The correlation between folate biosynthesis and growth inhibition was studied with Arabidopsis and Toxoplasma. In both organisms, the folate content was decreased by 40-50% in the presence of rubreserine. In both organisms, the addition of p-aminobenzoate or 5-formyltetrahydrofolate in the external medium restored the growth for inhibitor concentrations up to the IC(50) value, indicating that, within this range of concentrations, rubreserine was specific for folate biosynthesis. Rubreserine appeared to be more efficient than sulfonamides, antifolate drugs known to inhibit the invasion and proliferation of T. gondii in human fibroblasts. Altogether, these results validate the use of the bifunctional GAT-ADCS as an efficient drug target in eukaryotic cells and indicate that the chemical structure of rubreserine presents interesting anti-parasitic (toxoplasmosis, malaria) potential.

A folate independent role for cytosolic HPPK/DHPS upon stress in Arabidopsis thaliana.

Cytosolic HPPK/DHPS (cytHPPK/DHPS) in Arabidopsis is a functional enzyme with activity similar to its mitochondrial isoform. Genomic complementation of the cytHPPK/DHPS knockout mutant with the wild type gene led to a complete rescue of the stress sensitive mutant phenotype in seed germination tests under abiotic stress conditions. Moreover, over-expression of the gene resulted in higher germination rate under stress as compared to the wild-type, confirming its role in stress resistance. Analysis of folates in seedlings, inflorescence and dry seeds showed unchanged levels in the wild-type, mutant and over-expressor line, upon stress and normal conditions, suggesting a role for cytHPPK/DHPS distinct from folate biosynthesis and a folate-independent stress resistance mechanism. This apparently folate-independent mechanism of stress resistance points towards a possible role of pterins, since the product of HPPK/DHPS is dihydropteroate.

Ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) for the sensitive determination of folates in rice.

High-performance liquid chromatography, coupled to tandem mass spectrometry (HPLC-MS/MS) has been established as the method of choice for the sensitive and simultaneous determination of different folates in a particular matrix, especially when only minute quantities of material are available. Using a previously developed and validated HPLC-MS/MS method as a starting point, we here report on the development and validation of an ultra-performance liquid chromatography (UPLC-MS/MS) method for analysis of folates in rice, which allows higher throughput and better resolution. UPLC was performed under gradient conditions on an Acquity HSS T3 column, followed by tandem mass spectrometry detection. The method was validated based on linearity, sensitivity, precision, accuracy and matrix effects. The limits of detection and the lower limits of quantification varied between 0.06 and 0.45 microg/100 g and 0.12 and 0.91 microg/100 g, respectively. Two linear calibration curves were established, one for the low and the other for the high concentration range. Analysis of the distribution and levels of folates in wild-type and folate-biofortified rice showed up to 50-fold enrichment in biofortified rice, with total folate levels of up to 900 microg/100 g rice. This is the first successful implementation of a UPLC method for the rapid and sensitive quantitative determination of folates in plant material.