l-Ascorbic Acid Metabolism in Vitaceae: Conversion to (+)-Tartaric Acid and Hexoses

The metabolic fate of l-ascorbic acid-1-¹⁴C and -6-¹⁴C has been investigated in two species in two genera of Vitaceae. Results suggest that ascorbic acid metabolism in the Vitaceae involves splitting the 6-carbon chain into 4- and 2-carbon fragments. The former, corresponding to C1 through C4 of ascorbic acid, is further oxidized to tartaric acid while the latter, corresponding to C5 and C6, is recycled into hexose phosphate metabolism. Comparison of these findings with previous observations on the conversion of ascorbic acid to (+)-tartaric acid in Pelargonium crispum clearly reveals two distinct processes of tartaric acid biosynthesis in those plants identified as tartaric acid accumulators.     

Biosynthesis and metabolism of l-ascorbic acid in virginia creeper (Parthenocissus quinquefolia L.)

Detached leaves of Parthenocissus quinquefolia L., Vitaceae convert d-glucose to l-ascorbic acid with conservation of the carbon chain sequence and with retention of the hydroxymethyl group at carbon 6. l-Ascorbic acid is cleaved between carbons 4 and 5. The C₄ fragment is converted to l-tartaric acid. The C₂ fragment, possibly glycolaldehyde, recycles into products of hexose phosphate metabolism. During the metabolic period a relatively high portion of carbon-1 of l-ascorbic acid, as compared with carbon-4, was released as CO₂. These studies demonstrate the usefulness of Virginia Creeper for yeararound research on ascorbic-acid metabolism and tartaric-acid biosynthesis in Vitaceae-type plants.Abbreviation AA l-Ascorbic Acid     

Biosynthesis of (+)-Tartaric Acid from l-[4-C]Ascorbic Acid in Grape and Geranium

The metabolic fate of l-[4-¹⁴C]ascorbic acid has been examined in the grape (Vitis labrusca L.) and lemon geranium (Pelargonium crispum L. L'Hér. cv. Prince Rupert) under conditions comparable to data from l-[1-¹⁴C]ascorbic acid and l-[6-¹⁴C]ascorbic acid experiments. In detached grape leaves and immature berries, l-[4-¹⁴C]ascorbic acid and l-[1-¹⁴C]ascorbic acid were equivalent precursors to carboxyl labeled (+)-tartaric acid. In geranium apices, l-[4-¹⁴C]ascorbic acid yielded internal labeled (+)-tartaric acid while l-[6-¹⁴C]ascorbic acid gave an equivalent conversion to carboxyl labeled (+)-tartaric acid. These findings clearly show that two distinct processes for the synthesis of (+)-tartaric acid from l-ascorbic acid exist in plants identified as (+)-tartaric acid accumulators. In grape leaves and immature berries, (+)-tartaric acid synthesis proceeds via preservation of a four-carbon fragment derived from carbons 1 through 4 of l-ascorbic acid while carbons 3 through 6 yield (+)-tartaric acid in geranium apices.     

Ascorbic acid metabolism in geranium and grape

l-Ascorbic acid-[UL-¹⁴C] has been used to follow the appearance of ¹⁴C-labeled oxalic acid and tartaric acid as metabolic products of oxidative cleavage of ascorbic acid in geranium apices (Pelargonium crispum). The enantiomeric specificity of ascorbic acid metabolism was established in geranium by comparing the incorporation of d- and l-ascorbic acid-[6-¹⁴C] in the presence of l-ascorbic acid-[4-³H]. l-Ascorbic acid-[4-³H] has been used to demonstrate the retention of ³H during biosynthesis of l-(+)-tartaric acid in the geranium and its exchange with water during biosynthesis of l-( +)-tartaric acid in the grape.     

Synthesis of L(+) Tartaric Acid from 5-Keto-D-gluconic Acid in Pelargonium

5-Keto-D-[1-¹⁴C]gluconic acid, the most effective precursorof L(+)tartaric acid among all labeled compounds which haveever been tested in grapes, was found to be a good precursorof L(+)tartaric acid in a species of Pelargonium. The synthesisof labeled L(+)tartaric acid from D-[1-¹⁴C]glucose in Pelargoniumwas remarkably depressed when a 0.5% solution of D-gluconateor 5-keto-D-gluconate was administered continuously to leavestogether with D-[1-¹⁴C]glucose. Our results provide strong evidence that D-[1-¹⁴C]glucose ismetabolized in Pelargonium to give labeled L(+)tartaric acidvia (probably D-gluconic acid and) 5-keto-D-gluconic acid withoutpassing through L-ascorbic acid. Labeled L-idonic acid was found in young leaves of Pelargoniumwhich had been labeled with L-[U-¹⁴C]ascorbic acid. The synthesisof the labeled L-idonic acid increased when a 0.1% solutionof L-threonate was administered continuously to leaves togetherwith L-[U-¹⁴C]ascorbic acid. Specifically labeled compounds, recognized as the members ofthe synthetic pathway for L(+)tartaric acid from L-ascorbicacid via L-idonic acid in grapes, were administered to youngleaves of Pelargonium. Each compound (2-keto-L-[U-¹⁴C]idonicacid, L-[U-¹⁴C]idonic acid, 5-keto-D-[1-¹⁴C]gluconic acid and5-keto-D-[6-¹⁴C]gluconic acid) was partly metabolized, as ingrapes. The metabolic pathway starting from L-ascorbic acidto L(+)tartaric acid via L-idonic acid, however, did not actuallycontribute to the synthesis of L(+)tartaric acid in Pelargoniumprobably because the activity of each metabolic step was muchlower than that observed in grapes. (Received May 28, 1984; Accepted July 30, 1984)     

Precursors of Asparagine in Lupins

Fumaric acid-1,4-¹⁴C, L-aspartic acid-4-¹⁴C, and glycine-2-¹⁴Cwere supplied for 1 to 3 h to etiolated lupin shoots that wererapidly synthesizing asparagine. The distribution of ¹⁴C inaspartate and asparagine isolated from this material was determined.With aspartic acid-4-¹⁴C adminstration radioactivity was mostlyin carbons 1 and 4 with carbon 4 having the greater amount;with fumaric acid 1,4-¹⁴C administration, radioactivity wasagain mostly in carbons 1 and 4. Aspartate labelled by fumaratecontained relatively more radioactivity in carbons 2 and 3.Little radioactivity from glycine-2-¹⁴C entered either aspartateor asparagine and this was mainly in carbon 2 and 3. It is concludedthat asparagine synthesis form aspartate is a major pathwayof asparagine biosynthesis in lupins.     

Effect of exogenous ascorbic acid intake on biosynthesis of ascorbic acid in mice

The effect of exogenous ascorbic acid intake on biosynthesis of ascorbic acid in mice has been studied. After the mice were on diets containing added ascorbic acid for two months, the activities of ascorbic acid synthesizing enzymes in the mouse liver homogenates were measured using L-gulono-gamma-lactone as a substrate. Exogenous ascorbic acid intake (0.5, 1 or 5% in the diet) was able to increase the concentration of ascorbic acid in the blood and to decrease the activities of ascorbic acid synthesizing enzymes in mouse liver. The results suggest that ascorbic acid synthesis was controlled by local regulatory mechanism or by the concentration of ascorbic acid in the hepatic portal blood. Ingestion of dietary erythorbic acid, a stereoisomer of ascorbic acid, had no effect on the activities of ascorbic acid synthesizing enzymes.     

Phytoremediation of Cr(VI) by Spirodela polyrrhiza (L.) Schleiden employing reducing and chelating agents

Phytoremediation of Cr(VI) by Spirodela polyrrhiza in binary combinations with low molecular weight organic compounds (LMWOCs) with a reducing or chelating potential, viz., ascorbic acid, citric acid, tartaric acid, oxalic acid, lactic acid, and glycerol was studied in Cr(VI) containing hydroponic media. Significant increase in the relative dry weight of plants with respect to Cr(VI) treated controls was observed with ascorbic acid and glycerol. The uptake of chromium by S. polyrrhiza followed Michaelis-Menten kinetics of active ion uptake. Interaction between Cr and ascorbic acid, oxalic acid, and lactic acid decreased Cr uptake, whereas citric acid, glycerol, and tartaric acid increased it. Supplementation of LMWOCs to Cr(VI) containing media decreased the MDA content of the plants. Multiple regression models revealed that LMWOCs decrease lipid peroxidation independently, as well as that induced by Cr(VI). It was found that superoxide dismutase (SOD), guaiacol peroxidase (GPX), and catalase (CAT) activities were increased significantly in plants growing in media containing Cr(VI). The study established that lactic acid, citric acid, ascorbic acid, and glycerol were most effective in increasing the Cr(VI) phytoremediating potential of S. polyrrhiza and LMWOCs with reducing or chelating properties decrease Cr(VI) stress in S. polyrrhiza.     

The stimulatory effect of mannitol on levan biosynthesis: Lessons from metabolic systems analysis of Halomonas smyrnensis AAD6T

Halomonas smyrnensis AAD^T is a halophilic, gram‐negative bacterium that can efficiently produce levan from sucrose as carbon source via levansucrase activity. However, systems‐based approaches are required to further enhance its metabolic performance for industrial application. As an important step toward this goal, the genome‐scale metabolic network of Chromohalobacter salexigens DSM3043, which is considered a model organism for halophilic bacteria, has been reconstructed based on its genome annotation, physiological information, and biochemical information. In the present work, the genome‐scale metabolic network of C. salexigens was recruited, and refined via integration of the available biochemical, physiological, and phenotypic features of H. smyrnensis AAD6^T. The generic metabolic model, which comprises 1,393 metabolites and 1,108 reactions, was then systematically analyzed in silico using constraints‐based simulations. To elucidate the relationship between levan biosynthesis and other metabolic processes, an enzyme‐graph representation of the metabolic network and a graph decomposition technique were employed. Using the concept of control effective fluxes, significant links between several metabolic processes and levan biosynthesis were estimated. The major finding was the elucidation of the stimulatory effect of mannitol on levan biosynthesis, which was further verified experimentally via supplementation of mannitol to the fermentation medium. The optimal concentration of 30 g/L mannitol supplemented to the 50 g/L sucrose‐based medium resulted in a twofold increase in levan production in parallel with increased sucrose hydrolysis rate, accumulated extracellular glucose, and decreased fructose uptake rate. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1386–1397, 2013     

Thiol-dependent degradation of protoporphyrin IX by plant peroxidases

Protoporphyrin IX (PP) is the last porphyrin intermediate in common between heme and chlorophyll biosynthesis. This pigment normally does not accumulate in plants because its highly photodynamic nature makes it toxic. While the steps leading to heme and chlorophylls are well characterized, relatively little is known of the metabolic fate of excess PP in plants. We have discovered that plant peroxidases can rapidly degrade this pigment in the presence of thiol-containing substrates such as glutathione and cysteine. This thiol-dependent degradation of PP by horseradish peroxidase consumes oxygen and is inhibited by ascorbic acid.     

Efficiency of lignin biosynthesis: a quantitative analysis

Lignin is derived mainly from three alcohol monomers: p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol. Biochemical reactions probably responsible for synthesizing these three monomers from sucrose, and then polymerizing the monomers into lignin, were analysed to estimate the amount of sucrose required to produce a unit of lignin. Included in the calculations were amounts of respiration required to provide NADPH (from NADP(+)) and ATP (from ADP) for lignin biosynthesis. Two pathways in the middle stage of monomer biosynthesis were considered: one via tyrosine (found in monocots) and the other via phenylalanine (found in all plants). If lignin biosynthesis proceeds with high efficiency via tyrosine, 76.9, 70.4 and 64.3 % of the carbon in sucrose can be retained in the fraction of lignin derived from p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, respectively. The corresponding carbon retention values for lignin biosynthesis via phenylalanine are less, at 73.2, 65.7 and 60.7 %, respectively. Energy (i.e. heat of combustion) retention during lignin biosynthesis via tyrosine could be as high as 81.6, 74.5 and 67.8 % for lignin derived from p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, respectively, with the corresponding potential energy retention values for lignin biosynthesis via phenylalanine being less, at 77.7, 69.5 and 63.9 %, respectively. Whether maximum efficiency occurs in situ is unclear, but these values are targets that can be considered in: (1) plant breeding programmes aimed at maximizing carbon or energy retention from photosynthate; (2) analyses of (minimum) metabolic costs of responding to environmental change or pest attack involving increased lignin biosynthesis; (3) understanding costs of lignification in older tissues; and (4) interpreting carbon balance measurements of organs and plants with large lignin concentrations.     

Metabolic and proteomic adaptation of Lactobacillus rhamnosus strains during growth under cheese‐like environmental conditions compared to de Man,Rogosa, and Sharpe medium

The aim of this study was to demonstrate the metabolic and proteomic adaptation of Lactobacillus rhamnosus strains, which were isolated at different stages of Parmigiano Reggiano cheese ripening. Compared to de Man, Rogosa, and Sharpe (MRS) broth, cultivation under cheese‐like conditions (cheese broth, CB) increased the number of free amino acids used as carbon sources. Compared with growth on MRS or pasteurized and microfiltrated milk, all strains cultivated in CB showed a low synthesis of d,l ‐lactic acid and elevated levels of acetic acid. The proteomic maps of the five representative strains, showing different metabolic traits, were comparatively determined after growth on MRS and CB media. The amount of intracellular and cell‐associated proteins was affected by culture conditions and diversity between strains, depending on their time of isolation. Protein spots showing decreased (62 spots) or increased (59 spot) amounts during growth on CB were identified using MALDI‐TOF‐MS/MS or LC‐nano‐ESI‐MS/MS. Compared with cultivation on MRS broth, the L. rhamnosus strains cultivated under cheese‐like conditions had modified amounts of some proteins responsible for protein biosynthesis, nucleotide, and carbohydrate metabolisms, the glycolysis pathway, proteolytic activity, cell wall, and exopolysaccharide biosynthesis, cell regulation, amino acid, and citrate metabolism, oxidation/reduction processes, and stress responses.     

ATP citrate lyase activity is post‐translationally regulated by sink strength and impacts the wax,cutin and rubber biosynthetic pathways

Cytosolic acetyl‐CoA is involved in the synthesis of a variety of compounds, including waxes, sterols and rubber, and is generated by the ATP citrate lyase (ACL). Plants over‐expressing ACL were generated in an effort to understand the contribution of ACL activity to the carbon flux of acetyl‐CoA to metabolic pathways occurring in the cytosol. Transgenic Arabidopsis plants synthesizing the polyester polyhydroxybutyrate (PHB) from cytosolic acetyl‐CoA have reduced growth and wax content, consistent with a reduction in the availability of cytosolic acetyl‐CoA to endogenous pathways. Increasing the ACL activity via the over‐expression of the ACLA and ACLB subunits reversed the phenotypes associated with PHB synthesis while maintaining polymer synthesis. PHB production by itself was associated with an increase in ACL activity that occurred in the absence of changes in steady‐state mRNA or protein level, indicating a post‐translational regulation of ACL activity in response to sink strength. Over‐expression of ACL in Arabidopsis was associated with a 30% increase in wax on stems, while over‐expression of a chimeric homomeric ACL in the laticifer of roots of dandelion led to a four‐ and two‐fold increase in rubber and triterpene content, respectively. Synthesis of PHB and over‐expression of ACL also changed the amount of the cutin monomer octadecadien‐1,18‐dioic acid, revealing an unsuspected link between cytosolic acetyl‐CoA and cutin biosynthesis. Together, these results reveal the complexity of ACL regulation and its central role in influencing the carbon flux to metabolic pathways using cytosolic acetyl‐CoA, including wax and polyisoprenoids.     

Non-Enzymatic Reduction of Nitrite by Means of Ascorbic Acid in Citrus and Other Higher Plant Tissues

It has been proved that the nitrite reduction in the leaves and other plant tissues of citrus and other green plants is partly or mainly a non-enzymatic chemical process, and a heat-stable factor present in these tissues is responsible for this reduction. It is suggested that ascorbic acid plays a major role in this chemical reaction since the reduction is inhibited by ascorbic acid oxidase. A significant association was also found between the ascorbic acid content and the nitrite reduction capacity of citrus leaves. Evidence has been presented that this non-enzymatic chemical reduction of nitrite occurs also in vivo as undetached citrus leaves on branches placed in NaNO₂ solution have shown diminution of their ascorbic acid content along with the absorption of nitrite. Stronger accumulation of nitrite in these leaf tissues was observed under dark conditions, apparently due to the inhibition of the biosynthesis of the ascorbic acid.     

气体信号分子硫化氢在植物中的生理功能及作用机制

硫化氢（hydrogen sulfide,H2S）是继一氧化氮（nitric oxide,NO）和一氧化碳（carbon monoxide,CO）之后发现的第3种气体信号分子,它能参与生物体内的多种生理生化过程并发挥特定功能。在动物体内,H2S能够调节血管及神经系统功能。植物也能通过产生内源H2S来提高对环境的适应能力,缓解多种逆境胁迫造成的损伤和毒害,参与特定的生理代谢过程,诸如参与气孔运动和延缓衰老等。本文从H2S产生和代谢途径、已发现的生理功能和信号转导机制等方面综述H2S在植物中的最新研究进展,同时也探讨了H2S与其它信号分子的相互作用以及H2S对蛋白质的修饰机制。     

Biosynthesis of L-ascorbic acid and conversion of carbons 1 and 2 of L-ascorbic acid to oxalic acid occurs within individual calcium oxalate crystal idioblasts

L-Ascorbic acid (AsA) and its metabolic precursors give rise to oxalic acid (OxA) found in calcium oxalate crystals in specialized crystal idioblast cells in plants; however, it is not known if AsA and OxA are synthesized within the crystal idioblast cell or transported in from surrounding mesophyll cells. Isolated developing crystal idioblasts from Pistia stratiotes were used to study the pathway of OxA biosynthesis and to determine if idioblasts contain the entire path and are essentially independent in OxA synthesis. Idioblasts were supplied with various (14)C-labeled compounds and examined by micro-autoradiography for incorporation of (14)C into calcium oxalate crystals. [(14)C]OxA gave heavy labeling of crystals, indicating the isolated idioblasts are functional in crystal formation. Incubation with [1-(14)C]AsA also gave heavy labeling of crystals, whereas [6-(14)C]AsA gave no labeling. Labeled precursors of AsA (L-[1-(14)C]galactose; D-[1-(14)C]mannose) also resulted in crystal labeling, as did the ascorbic acid analog, D-[1-(14)C]erythorbic acid. Intensity of labeling of isolated idioblasts followed the pattern OxA > AsA (erythorbic acid) > L-galactose > D-mannose. Our results demonstrate that P. stratiotes crystal idioblasts synthesize the OxA used for crystal formation, the OxA is derived from the number 1 and 2 carbons of AsA, and the proposed pathway of ascorbic acid synthesis via D-mannose and L-galactose is operational in individual P. stratiotes crystal idioblasts. These results are discussed with respect to fine control of calcium oxalate precipitation and the concept of crystal idioblasts as independent physiological compartments.