Novel GC/Py/GC/IRMS-Based Method for Isotope Measurements of Nitrate and Nitrite. I: Converting Nitrate to Benzyl Nitrate for δ18O Analysis.

The 18O/16O (and 15N/14N) ratio of natural nitrate (NO3-) and nitrite (NO2-) can be used to extract valuable information about their source and fate as environmental contaminants, their metabolism as macronutrients in plants and animals, and their behavior in the N biogeochemical cycle. We developed an accurate, precise, sensitive (minimum sample size: 0.2 µg NO3--equivalent), and reliable (minimal oxygen exchange, loss, or gain) method to selectively isolate and purify nitrate and nitrite from natural water, soil, air, and plant materials by strong anion exchange (SAX) for low- to normal-salinity samples or strong cation exchange (SCX) for high-salinity samples, followed by quantitative conversion to their respective benzyl esters, which can be separated and individually analyzed for δ18O (and potentially δ15N) by gas chromatography (GC)/pyrolysis/GC/isotope-ratio mass spectrometry (IRMS). The method compares favorably with the currently popular bacterial denitrification and chemical reduction methods, in terms of sensitivity and reliability, and has the potential to simultaneously measure δ15N and δ18O of nitrate and nitrite from natural samples of various origins.

GATA3 ameliorates melanocyte injuries in vitiligo through SIRT3-mediated HMGB1 deacetylation.

Vitiligo is a skin depigmentation disorder. GATA3 expression is downregulated in vitiligo patients, and its role and regulatory mechanism in vitiligo are unclear. GATA3 and HMGB1 levels were detected by qRT-PCR in peripheral blood cells of vitiligo patients and healthy controls, as well as H2 O2 -treated PIG1 cells. Their expression correlation was assessed by Pearson analysis. qRT-PCR, MTT assay, Ki67 immunostaining, flow cytometry, ELISA and Western blot were applied to determine GATA3 expression, cell survival, cell proliferation, cell apoptosis, melanin contents, and melanin-related protein expressions. The cellular distributions of HMGB1 and its deacetylation levels were detected by Western blot. The binding of GATA3 to SIRT3 promoter and effects on SIRT3 expression and HMGB1 deacetylation was determined by dual-luciferase assay, ChIP assay, and Western blot. GATA3 was decreased, and HMGB1 was increased in vitiligo. Pearson correlation assay showed that they were negatively correlated. H2 O2 significantly inhibited cell survival, proliferation, melanin secretion, and melanin-related protein expressions but remarkably increased cell apoptosis. GATA3 overexpression could distinctly reverse the effects of H2 O2 through decreasing HMGB1 expression and retained HMGB1 in nuclear due to the decreased HMGB1 acetylation. GATA3 bound to the SIRT3 and subsequently decreased H2 O2 -induced HMGB1 acetylation. Overexpressing HMGB1 or knockdown of SIRT3 could reverse the effects of GATA3 overexpression. GATA3 inhibited H2 O2 -induced injury in PIG1 cells and enhanced melanin secretion by SIRT3-regulated HMGB1 deacetylation, which might provide new evidence to treat vitiligo.

Probing stereoselectivity and pro-chirality of hydride transfer during short-chain alcohol dehydrogenase activity: a combined quantitative 2H NMR and computational approach.

Different members of the alcohol oxidoreductase family can transfer the hydride of NAD(P)H to either the re- or the si-face of the substrate. The enantioselectivity of transfer is very variable, even for a range of substrates reduced by the same enzyme. Exploiting quantitative isotopic (2)H NMR to measure the transfer of (2)H from NAD(P)(2)H to ethanol, a range of enantiomeric excess between 0.38 and 0.98, depending on the origin of the enzyme and the nature of the cofactor, has been determined. Critically, in no case was only (R)-[1-(2)H]ethanol or (S)-[1-(2)H]ethanol obtained. By calculating the relative energies of the active site models for hydride transfer to the re- or si-face of short-chain aldehydes by alcohol dehydrogenase from Saccharomyces cerevisiae and Lactobacillus brevis, it is shown that the differences in the energy of the systems when the substrate is positioned with the alkyl group in one or the other pocket of the active site could play a role in determining stereoselectivity. These experiments help to provide insight into structural features that influence the potential catalytic flexibility of different alcohol dehydrogenase activities.

On the contributions of photorespiration and compartmentation to the contrasting intramolecular 2H profiles of C3 and C4 plant sugars.

Compartmentation of C4 photosynthetic biochemistry into bundle sheath (BS) and mesophyll (M) cells, and photorespiration in C3 plants is predicted to have hydrogen isotopic consequences for metabolites at both molecular and site-specific levels. Molecular-level evidence was recently reported (Zhou et al., 2016), but evidence at the site-specific level is still lacking. We propose that such evidence exists in the contrasting 2H distribution profiles of glucose samples from naturally grown C3, C4 and CAM plants: photorespiration contributes to the relative 2H enrichment in H5 and relative 2H depletion in H1 & H6 (the average of the two pro-chiral Hs and in particular H6,pro-R) in C3 glucose, while 2H-enriched C3 mesophyll cellular (chloroplastic) water most likely contributes to the enrichment at H4; export of (transferable hydrogen atoms of) NADPH from C4 mesophyll cells to bundle sheath cells (via the malate shuttle) and incorporation of 2H-relatively unenriched BS cellular water contribute to the relative depletion of H4 & H5 respectively; shuttling of triose-phosphates (PGA: phosphoglycerate dand DHAP: dihydroacetone phosphate) between C4 bundle sheath and mesophyll cells contributes to the relative enrichment in H1 & H6 (in particular H6,pro-R) in C4 glucose.

Study of the influence of alcoholic fermentation and distillation on the oxygen-18/oxygen-16 isotope ratio of ethanol.

A laboratory procedure for the analysis of the oxygen-18/oxygen-16 isotope ratios of ethanol derived from sugars and fruit juices by pyrolysis-isotope ratio mass spectrometry (IRMS) has been applied to the study of isotopic fractionation induced by the isotope effects of fermentation and distillation. For both processes, an experimental model has been established to describe and explain the observed fractionation phenomena. It is shown that reproducible results can be obtained when appropriate analytical conditions are used. Moreover, the ability of ethanol to act as a reliable indicator of the (18)O/(16)O ratio of sugars in orange juice (and therefore to be used as an internal reference for detecting water addition) is demonstrated both in theory and in practice.

Natural abundance hydrogen isotope affiliation between the reactants and the products in glucose fermentation with yeast.

In glucose fermentation, the hydrogen source of products such as ethanol and glycerol is the medium and the sugar. The site-specific natural isotope ratios of the products, (D/H)(i), and that of the medium and sugar, (D/H)(k), may be related by a matrix, A, of redistribution coefficients, a(ik), that characterizes the specific genealogies of the hydrogen atoms. (D/H)(i) = [A](D/H)(k), where (D/H)(i) and (D/H)(k) are the column vectors of the isotope ratios of sites i and k that can be measured by (2)H NMR. The complete redistribution matrix was determined in a set of isotope labeling experiments. Thus, we obtained a mathematical model representing the hydrogen isotope affiliation during alcoholic fermentation. It not only provides information about the biochemical reaction mechanism but also can be used to estimate the isotopic data of the products, based on those of the substrate and the medium. The results prove, in a quantitative way, that the metabolites contain isotopic information about the precursor in a biotransformation and can be used to identify its origin. The method established for the study of the hydrogen-transfer mechanism can be applied to other chemical and biochemical reactions.

Hydrogen isotopic profile in the characterization of sugars. Influence of the metabolic pathway.

The site-specific natural hydrogen isotope ratios of plant metabolites determined by 2H nuclear magnetic resonance (SNIF-NMR method) can provide powerful criteria for inferring mechanistic and environmental effects on biosynthetic pathways. This work examines the potential of isotopic profiles for the main constituents of carbohydrates, glucose and fructose, to distinguish different photosynthetic pathways. An appropriate analytical strategy, involving three suitable isotopic probes, has been elaborated with a view to measuring simultaneously, in conditions devoid of isotopic perturbations, all (or nearly all) of the carbon-bound hydrogen isotope ratios. It is shown that the type of photosynthetic metabolism, either C3 (sugar beet, orange, and grape), C4 (maize and sugar cane), or CAM (pineapple), and the physiological status of the precursor plant exert strong influences on the deuterium distribution in the sugar molecules. Consequently, this isotopic fingerprint may be a rich source of information for the comparison of mechanisms in metabolic pathways. In addition, it can provide complementary criteria to ethanol as a probe for the origin of sugars.

A test to identify cyanide origin by isotope ratio mass spectrometry for forensic investigation.

Cyanide is one of the common poisons in murders. When cyanide has been used, to identify the origin of cyanide may be necessary in the forensic investigation. We have examined the possibility of distinguishing different commercial cyanide samples through the δ(13)C and δ(15)N values and developed a protocol for the isotope analysis of cyanide extracted from several matrices as food and medicine. Several cyanide precipitates were tested for the isotope analysis. The results show that cupric ferrocyanide Cu(2)[Fe(CN)(6)] is the most appropriate precipitate for the analysis. Thirteen batches of KCN and nine batches of NaCN chemicals were randomly chosen from different suppliers. The cyanides were converted to cupric ferrocyanide and then analysed by isotope ratio mass spectrometry coupled to elemental analysis (EA-IRMS). The isotopic signature of the commercial samples varied from -51.96 to -25.77 ‰ for δ(13)C and from -4.51 to +3.81 ‰ for δ(15)N, highlighting the potential of applying EA-IRMS technique to identify cyanide from different batches and sources. The influence of the cyanide extraction and isolation from spiked matrix on the isotopic analysis was also studied. Three matrices: orange juice, yogurt drink and a medicine were tested. In many cases, the isotopic analysis results obtained from the original cyanides precipitates and those isolated from the matrices showed a good accordance, especially for δ(15)N. In some matrices, the (13)C analysis was interfered by co-precipitates. With carefully elaborated working protocol, determining the isotope ratio of N and C in cyanide by EA-IRMS is a promising method for forensic investigations.

Non-equivalence of hydrogen transfer from glucose to the pro-R and pro-S methylene positions of ethanol during fermentation by Leuconostoc mesenteroides quantified by 2H NMR at natural abundance.

The anaerobic fermentation of glucose by Leuconostoc mesenteroides via the reductive pentose phosphate pathway leads to the accumulation of lactic acid and ethanol. The isotope redistribution coefficients (a(ij)) that characterize the specific derivation of each hydrogen atom in ethanol in relation to the non-exchangeable hydrogen atoms in glucose and the medium water have been determined using quantitative (2)H NMR. First, it is confirmed that the hydrogens of the methylene group are related only to the 1 and 3 positions of glucose via the NAD(P)H pool and not to the 4 position, in contrast to ethanol produced by Saccharomyces cerevisiae. Second, it is found that the conversion factors (C(f)) for the transfer of hydrogen to the pro-S and pro-R positions of the methylene group are not equivalent: the C(f)-1-R:C(f)-1-S ratio is 2.1, whereas the C(f)-3-R:C(f)-3-S ratio is 0.8. It is shown that this non-equivalence is not determined by the stereochemistry of the terminal NADH- and NADPH-dependent alcohol dehydrogenases, but is dependent on the cofactor selectivities of the reductive and oxidative steps of the reduced nucleotide cycle.

Cofactor recycling mechanism in asymmetric biocatalytic reduction of carbonyl compounds mediated by yeast: which is the efficient electron donor?

In asymmetric reduction of carbonyl compounds mediated by microorganisms, the cofactors that transfer hydride should be regenerated by using a recycling system. In most cases, this recycling system consists of carbohydrate molecules, especially glucose or sucrose. Other molecules such as ethanol and acetate have been used as electron donors too. The reduction can even be conducted without added electron donors. To improve biocatalytic synthesis, it is important to understand the cofactor recycling mechanism. In this work, the hydride-transfer mechanism in cofactor regeneration, which takes place in bioreduction mediated by yeast, was studied by means of an isotope tracing technique. The results show that, when glucose was used, the NADH involved in the glycolysis was consumed directly in the formation of ethanol and was not used in the bioreduction. Hence, the regeneration of cofactors in the reduction is not coupled with glycolysis. Nevertheless, glucose is an efficient electron donor that transfers hydride through the hexose monophosphate (HMP) pathway in which the main hydrogen source is C-1 and C-3 hydrogen of glucose. Ethanol is not a good electron donor, since, when it was used, only a small quantity of hydrogen was transferred from this molecule, and the main hydrogen source was water. Therefore, the ethanol oxidation pathway may not be efficient. In the absence of added auxiliary substrates, the yeast cells may use electron donors stored in its cellules. However, in this case we observed that the main hydrogen source for cofactor recycling was water, while only very few hydrogen atoms were from unexchangeable sites. This is similar to the case in which ethanol is used, and is in contradiction with the HMP pathway if stored glucose was the electron donor. The question that remains to be investigated is "what is the efficient electron donor recycling mechanism in the yeast cellules?"

Application of 2H NMR to the study of natural site-specific hydrogen isotope transfer among substrate, medium, and glycerol in glucose fermentation with yeast.

2H NMR is a very useful tool in isotope tracing studies. This technique was applied to a quantitative study of a site-specific deuterium affiliation among the substrate, the medium, and a product (glycerol), in glucose fermentation with yeast. The quality of the results depends on the quantitative 2H NMR analysis of glycerol. After comparing several potential analysis probe molecules, the derivative of glycerol, 2,2-dimethyl-1,3-dioxolane-4-methanol, was chosen as the most advantageous. Using this probe in a set of isotope-labeling experiments, we describe how a complete quantitative site-specific hydrogen isotope transfer model, which connects the site-specific isotopic ratios of the substrate, the medium, and the products, can be established. This model can provide information on complex hydrogen transfer mechanisms during biochemical reactions and can be useful for the prediction of site-specific hydrogen isotopic ratios at natural abundance of the products, based on that of the substrate or reactants and the medium.

Comparison of isotopic fractionation in lactic acid and ethanol fermentations.

Pure D(-) and L(+) enantiomers of lactic acid were prepared by fermentation reactions with specific bacteria. In addition, naturally deuterated ethanol was prepared and converted into diastereoisomers using mandelic acid. Various sugars and nutrients were fermented into lactic acid in water having different deuterium contents and ethanol samples were obtained from yeast fermentation of sugars from different botanical origins. The methine and methylene groups in lactic acid and ethanol respectively show similar deuterium contents which are related to that found in the fermentation water. However, the methyl groups of both molecules are significantly different whatever the botanical origin of the carbon source in the fermentation medium.

Quantitative 2H NMR at natural abundance can distinguish the pathway used for glucose fermentation by lactic acid bacteria.

For any given metabolic pathway, isotope redistribution coefficients (a(ij)) that characterize the specific derivation of each hydrogen atom can be defined. By using quantitative deuterium NMR, the redistribution of deuterium at natural abundance in lactic acid produced by the bacterial fermentation of glucose has been determined for each non-labile hydrogen atom of glucose or water and the hydrogen atoms of lactic acid. Distinct differences are observed in the lactic acid isolated from Lactococcus lactis and Leuconostoc mesenteroides that can be interpreted in terms of the different fermentative pathways used. Specifically, the affiliations observed between the H1, H3, and H4 positions of glucose with methyl and hydroxymethylene of lactic acid can give quantitative information on whether the glycolytic or the reductive pentose-phosphate pathway was involved in glucose catabolism.