Localization and interconversion of tetrahydropteroylglutamates in isolated pea mitochondria

Single-taxon field measurements of bacterial gene regulation controlling DMSP fate

The 'bacterial switch' is a proposed regulatory point in the global sulfur cycle that routes dimethylsulfoniopropionate (DMSP) to two fundamentally different fates in seawater through genes encoding either the cleavage or demethylation pathway, and affects the flux of volatile sulfur from ocean surface waters to the atmosphere. Yet which ecological or physiological factors might control the bacterial switch remains a topic of considerable debate. Here we report the first field observations of dynamic changes in expression of DMSP pathway genes by a single marine bacterial species in its natural environment. Detection of taxon-specific gene expression in Roseobacter species HTCC2255 during a month-long deployment of an autonomous ocean sensor in Monterey Bay, CA captured in situ regulation of the first gene in each DMSP pathway (dddP and dmdA) that corresponded with shifts in the taxonomy of the phytoplankton community. Expression of the demethylation pathway was relatively greater during a high-DMSP-producing dinoflagellate bloom, and expression of the cleavage pathway was greater in the presence of a mixed diatom and dinoflagellate community [corrected].These field data fit the prevailing hypothesis for bacterial DMSP gene regulation based on bacterial sulfur demand, but also suggest a modification involving oxidative stress response, evidenced as upregulation of catalase via katG, when DMSP is demethylated.

Canadian Society of Plant Physiologists Gold Medal Review / Synthèse médaillée d'or de la Société canadienne physiologie végétaleThe fascinating world of folate and one-carbon metabolism

Folate was first isolated from spinach leaves in 1941 and characterized as pteroylglutamic acid. Although plants, fungi, and bacteria synthesize folate de novo, animal cells lack key enzymes of the folate biosynthetic pathway and a dietary source of folate is required for normal growth and development. Folates have importance in human nutrition, health, and disease, and antifolate drugs are commonly used in cancer chemotherapy. In the majority of living cells folates occur as one-carbon substituted tetrahydropteroylpolyglutamate derivatives. These folates donate one-carbon groups during the synthesis of purines, formylmethionyl-tRNA, thymidylate, serine, and methionine. In the last 30 years, research on the folate biochemistry of plant species has intensified and been aided by the development of improved methods for folate isolation and characterization. These studies have resulted in basic information on the nature of plant folylpolyglutamates, folate biosynthesis, the enzymology of several folate-dependent reactions, and the roles of chloroplasts, mitochondria, and the cytosol in the pathways of one-carbon metabolism.Key words: plants, folates, folate biosynthesis, folate-dependent enzymes, one-carbon metabolism.

Metabolism of methanol in plant cells. Carbon-13 nuclear magnetic resonance studies

Using (13)C-NMR, we demonstrate that [(13)C]methanol readily entered sycamore (Acer pseudoplatanus L.) cells to be slowly metabolized to [3-(13)C]serine, [(13)CH(3)]methionine, and [(13)CH(3)]phosphatidylcholine. We conclude that the assimilation of [(13)C]methanol occurs through the formation of (13)CH(3)H(4)Pte-glutamate (Glu)(n) and S-adenosyl-methionine, because feeding plant cells with [3-(13)CH(3)]serine, the direct precursor of (13)CH(2)H(4)Pte-Glu(n), can perfectly mimic [(13)CH(3)]methanol for folate-mediated single-carbon metabolism. On the other hand, the metabolism of [(13)C]methanol in plant cells revealed assimilation of label into a new cellular product that was identified as [(13)CH(3)]methyl-beta-D-glucopyranoside. The de novo synthesis of methyl-beta-D-glucopyranoside induced by methanol did not require the formation of (13)CH(3)H(4)Pte-Glu(n) and was very likely catalyzed by a "transglycosylation" process.

Isolation, characterization, and functional expression of cDNAs encoding NADH-dependent methylenetetrahydrofolate reductase from higher plants

Methylenetetrahydrofolate reductase (MTHFR) is the least understood enzyme of folate-mediated one-carbon metabolism in plants. Genomics-based approaches were used to identify one maize and two Arabidopsis cDNAs specifying proteins homologous to MTHFRs from other organisms. These cDNAs encode functional MTHFRs, as evidenced by their ability to complement a yeast met12 met13 mutant, and by the presence of MTHFR activity in extracts of complemented yeast cells. Deduced sequence analysis shows that the plant MTHFR polypeptides are of similar size (66 kDa) and domain structure to other eukaryotic MTHFRs, and lack obvious targeting sequences. Southern analyses and genomic evidence indicate that Arabidopsis has two MTHFR genes and that maize has at least two. A carboxyl-terminal polyhistidine tag was added to one Arabidopsis MTHFR, and used to purify the enzyme 640-fold to apparent homogeneity. Size exclusion chromatography and denaturing gel electrophoresis of the recombinant enzyme indicate that it exists as a dimer of approximately 66-kDa subunits. Unlike mammalian MTHFR, the plant enzymes strongly prefer NADH to NADPH, and are not inhibited by S-adenosylmethionine. An NADH-dependent MTHFR reaction could be reversible in plant cytosol, where the NADH/NAD ratio is 10(-3). Consistent with this, leaf tissues metabolized [methyl-(14)C]methyltetrahydrofolate to serine, sugars, and starch. A reversible MTHFR reaction would obviate the need for inhibition by S-adenosylmethionine to prevent excessive conversion of methylene- to methyltetrahydrofolate.

Distribution of Folate Derivatives and Enzymes for Synthesis of 10-Formyltetrahydrofolate in Cytosolic and Mitochondrial Fractions of Pea Leaves

Leaf extracts of 14-d-old pea (Pisum sativum L. cv Homesteader) seedlings were examined for folate derivatives and for 10-formyltetrahydrofolate synthetase (SYN), 5,10-methenyltetrahydrofolate cyclohydrolase (CYC), and 5,10-methylenetetrahydrofolate dehydrogenase (DHY) activities. Microbiological and enzyme assays showed that leaf folates SYN, CYC, and DHY were predominantly cytosolic. Extracts of Percoll gradient-purified mitochondria contained less than 1% of total leaf folate and less that 1% of each enzyme activity. Fractionation of whole-leaf homogenates resulted in the copurification of DHY and CYC (subunit 38 kD) and the isolation of a SYN protein (subunit 66 kD). Polyclonal antibodies were raised against purified cytosolic DHY-CYC (DHY-CYC-Ab) and cytosolic SYN (SYN-Ab), respectively. Immunoblots showed that DHY-CYC-Ab cross-reacted with a mitochondrial protein band (38 kD). Two mitochondrial protein bands (subunit Mr = 40,000 and 44,000) cross-reacted with SYN-Ab. Immunoaffinity chromatography (DHY-CYC-Ab as the immobile ligand) indicated that the bulk of mitochondrial SYN activity was not associated with mitochondrial DHY or CYC. When 9-d-old etiolated pea seedlings were exposed to light for up to 3 d, the specific enzyme activities of DHY-CYC in whole-leaf extracts rose 2-fold and more DHY-CYC-Ab cross-reacting protein was detected. In contrast, the specific activity of SYN fell from 5 to 1 [mu]mol min-1 mg-1 protein and less SYN-Ab cross-reacting protein was detected. The data suggest that in pea leaves, the bulk of one-carbon-substituted tetrahydrofolates and enzymes for the generation of 10-formyltetrahydrofolate are extra-mitochondrial.

Accumulation and distribution of free folic acid content in red beet (Beta vulgaris L.)

Among vegetable plants, red beet contains a relatively high level of the B vitamin folic acid. Although many leafy green vegetables contain high levels of folic acid, red beet is consumed primarily as a root vegetable. Folic acid levels have been quantified in various vegetable plants, but little information exists regarding the accumulation and distribution of this vitamin in plant tissues. The objective of this study was to characterize free folic acid content (FFAC) in shoot and root tissue during growth of two red beet inbreds. Experiments were conducted in a greenhouse during 1993, 1994 and 1995. Two inbreds, W384 and W357, were planted in randomized complete blocks and shoot and root tissues were separately harvested at 60, 80, and 100 days after planting (DAP). Significant differences between years, tissue portions, and among harvest dates were detected, however, similar patterns in FFAC accumulation and distribution were observed between inbreds and years. FFAC in shoot tissue was significantly greater than root tissue for both inbreds. Accumulation of FFAC was linear for both inbreds across harvest dates for root tissue but not for shoot tissue. FFAC accumulation in shoot tissue increased sharply from 60 to 80 DAP but decreased sharply from 80 to 100 DAP. These results demonstrate that FFAC accumulates differentially in root and shoot tissue in a red beet plant. Maximum folic acid levels in shoot tissue are achieved prior to those in root tissue.

The stimulation of yeast mitochondrial protein synthesis by low molecular weight cytoplasmic factors: requirement for intact mitochondria and lack of effect by folate derivatives

Protein synthesis by GTP-supplemented yeast mitochondria is stimulated by a fraction of molecular weight less than 2,000 isolated from yeast high-speed supernatant (S-150). The low molecular weight fraction works independently of the respiratory chain as the stimulation effect is not cyanide-sensitive. Stimulation of mitochondrial protein synthesis by cytoplasmic factors is dependent upon the method of mitochondrial isolation. The low molecular weight stimulatory factor(s) are not reduced folate derivatives which supply formyl groups required for initiation of mitochondrial protein synthesis.

Folylpolyglutamate synthetase activities of Neurospora crassa: nature of products formed by soluble and particulate enzymes in the wild type and polyglutamate-deficient mutants

The folylpolyglutamate synthetase activities of Neurospora crassa wild type (FGSC 853) and two polyglutamate-deficient mutants (met-6, 35809, FGSC 1330 and mac, 65108, FGSC 3609) were examined using dialyzed extracts prepared during exponential mycelial growth. Enzyme assay was based on incorporation of [U-3H]glutamate in folylpolyglutamates that were separated by gradient elution from DEAE-cellulose. Extracts of the wild type produced H4PteGlu2 (15%), H4PteGlu3 (35%) and H4PteGlu6 (50%) when anaerobically incubated with glutamate, ATP, and H4PteGlu. Under these conditions, the met-6 produced only H4PteGlu2 and higher polyglutamates (H4PteGlu4 and H4PteGlu5) were not utilized. The mac mutant failed to catalyze addition of glutamate to H4PteGlu. However, H4PteGlu2 was effectively converted to the tri-, and hexaglutamates. Mixing wild type and met-6 protein stimulated the formation of tri-, and hexaglutamates. Mixing mac and met-6 extracts resulted in H4PteGlu3 and H4PteGlu6 labeling when glutamate and H4PteGlu were provided. Fractionation of wild type extracts by addition of (NH4)2SO4 or by differential centrifugation provided evidence for different synthetase activities. Protein of the 0-35% (NH4)2SO4 fraction and that associated with the mitochondrial pellet, catalyzed an H4PteGlu2 leads to H4PteGlu3 reaction. These fractions failed to utilize H4PteGlu or the corresponding tetra-, and pentagluatmates. This triglutamate-forming activity was lacking in mac and met-6. The 45-60% (NH4)2SO4 fraction of the wild type catalyzed formation of di-, tri-, and hexaglutamate from H4PteGlu and glutamate. Hexaglutamate was also formed when the folate substrate wa H4PteGlu2, H2PteGlu4, or H4PteGlu5. These activities were associated with the cytosolic fraction when crude isotonic extracts were centrifuged to remove mitochondria. The characteristic synthetase activities of met-6 and mac were associated with protein of the 45-60% (NH4)2SO4 and cytosolic fractions. It is suggested that folypolyglutamate synthesis in N. crassa involves more than one synthetase-catalyzed reaction. Production of cellular folylhexaglutamate appears to involve two steps, catalyzed by cytosolic enzymes; viz: H4PteGlu leads to H2PteGlu2 followed by H4PteGlu2 leads to H4PteGlu6. These partial reactions are lacking in mac and met-6 respectively. The mitochondrial synthetase of the wild type may not represent a mandatory step in the biosynthesis of folylhexaglutamate but could have significance in generation of compartmented folylpolyglutamates.

Pteroylpolyglutamates

Reduced derivatives of the vitamin pteroylglutamic acid (folic acid) are essential coenzymes for the biosynthesis of purine nucleotides, methionine, thymidylate and for many other enzyme catalyzed reactions involving the transfer, oxidation and reduction of single carbon units. Pteroylglutamic acid is found in tissues in the form of poly-gamma-glutamyl derivatives of varying chain length. The present review covers the detection, distribution, synthesis, degradation, coenzyme function and inhibitory activities of pteroyl-gamma-glutamates. The biosynthesis and inhibitory activities of poly-gamma-glutamyl derivatives of methotrexate, an analog of pteroylglutamic acid having antitumor activity, are also considered. An hypothesis on the coenzymatic role of pteroylpoly-gamma-glutamates in the coordination of sequential enzymatic steps in the metabolism of single carbon units is presented.

Properties and intramitochondrial localization of serine hydroxymethyltransferase in leaves of higher plants

The activity of serine hydroxymethyltransferase in mitochondria isolated from spinach leaves was absolutely dependent on tetrahydrofolate; pyridoxal phosphate has no effect on the activity. The stability of this activity in the isolated mitochondria was dependent on the presence of sulfhydryl compounds. It was apparently more stable at pH 7.0 to 7.5 than at higher pH even though the pH optimum of serine hydroxymethyltransferase was 8.5 for both the mitochondrial and cytoplasmic fractions. Distribution studies have indicated that serine hydroxymethyltransferase was predominantly located in the mitochondria. The activity of serine hydroxymethyltransferase was observed to be co-compartmented with glycine decarboxylation and malate dehydrogenase behind the mitochondrial inner membrane. This activity could be solubilized by KCl from osmotically ruptured mitochondrial membrane fractions but substantial activity (35 to 40%) was still retained with the membrane fractions at 0.3 m KCl. This suggests that the glycine decarboxylation-serine hydroxymethyltransferase complex may be closely bound to the internal surface of the mitochondrial inner membrane.The relationship of this integrated enzyme complex to CO(2) evolution and serine synthesis during photorespiration and the physiological role of the dicarboxylate shuttle were discussed.

One-carbon metabolism in Neurospora crassa wild-type and in mutants partially deficient in serine hydroxymethyltransferase

1. The concentrations of folate-dependent enzymes in Neurospora crassa Lindegren A wild type (FGSC no. 853), Ser-l mutant, strain H605a (FGSC no. 118), and for mutant, strain C-24 (FGSC no. 9), were compared during exponential growth on defined minimal media. Both mutants were partially lacking in serine hydroxymethyltransferase, but contained higher concentrations of 10-formyltetrahydrofolate synthetase than did the wild type. Mycelia of the mutants contained higher concentrations of these enzymes when growth media were supplemented with 1mM-glycine. In the wild-type, this glycine supplement also increased the specific activities of 5,10-methylenetetrahydrofolate dehydrogenase and 5,10-methylenetetrahydrofolate reductase. 5. During growth, total folate and polyglutamyl folate concentrations were greatest in the wild-type. Methylfolates were not detected in mutant Ser-l, and were only present in the for mutant after growth in glycine-supplemented media. Exogenous glycine increased folate concentration threefold in the wild type, mainly owing to increases in unsubstituted polyglutamyl derivatives. 3. Feeding experiments using 14C-labelled substrates showed that C1 units were generated from formate, glycine and serine in the wild type. Greater incorporation of 14C occurred when mycelia were cultured in glycine-supplemented media. Formate and serine were precursors of C1 units in the mutants, but the ability to cleave glycine was slight or lacking.

Partial purification and properties of an enzyme from Escherichia coli that catalyzes the conversion of glutamic acid and 10-formyltetrahydropteroylglutamic acid to 10-formyltetrahydropterol-gamma-glutamyglutamic acid

An enzyme that catalyzes the conversion of L-glutamic acid and 10-formyl-H4folic acid (also known as 10-formyl-H4pteroylglutamic acid) to 10-formyl-H4pteroyl-gamma-glutamylglutamic acid has been purified by 74-fold from extracts of Escherichia coli. ATP, Mg-2+, and a monovalent cation (K+ or NH-4, but not Na+) are required for the enzyme to function. Radioactive and bioautographic analyses revealed the formation of a single product. This product was identified as 10-formyl-H-4pteroyl-gamma-glutamylglutamic acid from its spectral characteristics, its ability to be used effectively as a growth faster for Lactobacillus casei 7469, and from radioactive analysis that indicated the incorporation into the product of 1 mol glutamate/mol of 10-formyl-H-4pteroylglutamic acid utilized. The enzyme functions optimally at pH 9.0-9.8 and at 50 degrees. Its molecular weight is estimated at 42,000-43,000. The Km values are 180 muM for L-glutamic acid and less than 2 muM for (-) 10-formyl-H-4pteroylglutamic acid. The only other naturally occurring folate compounds with significant activity as substrate are H-4pteroylglutamic acid and 5,10-methylene-H-4pteroylglutamic acid; however, these compounds are not used as effectively (K-m values are 10-12 mu-M) as 10-formyl-H-4pteroylglutamic acid.