Isolation, characterization, and structural organization of 10-formyltetrahydrofolate synthetase from spinach leaves

One-carbon metabolism mediated by folate coenzymes plays an essential role in several major cellular processes. In the prokaryotes studied, three folate-dependent enzymes, 10-formyltetrahydrofolate synthetase (EC 6.3.4.3), 5,10-methenyltetrahydrofolate cyclohydrolase (EC 3.5.4.9), and 5,10-methylenetetrahydrofolate dehydrogenase (EC 1.5.1.5) generally exist as monofunctional or bifunctional proteins, whereas in eukaryotes the three activities are present on one polypeptide. The structural organization of these enzymes in plants had not previously been examined. We have purified the 10-formyltetrahydrofolate synthetase activity from spinach leaves to homogeneity and raised antibodies to it. The protein was a dimer with a subunit molecular weight of Mr = 67,000. The Km values for the three substrates, (6R)-tetrahydrofolate, ATP, and formate were 0.94, 0.043, and 21.9 mM, respectively. The enzyme required both monovalent and divalent cations for maximum activity. The 5,10-methylenetetrahydrofolate dehydrogenase and 5,10-methenyltetrahydrofolate cyclohydrolase activities of spinach coeluted separately from the 10-formyltetrahydrofolate synthetase activity on a Matrex Green-A column. On the same column, the activities of the yeast trifunctional C1-tetrahydrofolate synthase coeluted. In addition, antibodies raised to the purified spinach protein immunoinactivated and immunoprecipitated only the 10-formyltetrahydrofolate synthetase activity in a crude extract of spinach leaves. These results suggest that unlike the trifunctional form of C1-tetrahydrofolate synthase in the other eukaryotes examined, 10-formyltetrahydrofolate synthetase in spinach leaves is monofunctional and 5,10-methyl-enetetrahydrofolate dehydrogenase and 5,10-methenyltetrahydrofolate cyclohydrolase appear to be bifunctional. Although structurally dissimilar to the other eukaryotic trifunctional enzymes, the 35 amino-terminal residues of spinach 10-formyltetrahydrofolate synthetase showed 35% identity with six other tetrahydrofolate synthetases.     

ALDH1L2 Is the Mitochondrial Homolog of 10-Formyltetrahydrofolate Dehydrogenase

Cytosolic 10-formyltetrahydrofolate dehydrogenase (FDH, ALDH1L1) is an abundant enzyme of folate metabolism. It converts 10-formyltetrahydrofolate to tetrahydrofolate and CO₂ in an NADP⁺-dependent reaction. We have identified a gene at chromosome locus 12q24.11 of the human genome, the product of which has 74% sequence similarity with cytosolic FDH. This protein has an extra N-terminal sequence of 22 amino acid residues, predicted to be a mitochondrial translocation signal. Transfection of COS-7 or A549 cell lines with a construct in which green fluorescent protein was introduced between the leader sequence and the rest of the putative mitochondrial FDH (mtFDH) has demonstrated mitochondrial localization of the fusion protein, suggesting that the identified gene encodes a mitochondrial enzyme. Purified pig liver mtFDH displayed dehydrogenase/hydrolase activities similar to cytosolic FDH. Real-time PCR performed on an array of human tissues has shown that although cytosolic FDH mRNA is highest in liver, kidney, and pancreas, mtFDH mRNA is most highly expressed in pancreas, heart, and brain. In contrast to the cytosolic enzyme, which is not detectable in cancer cells, the presence of mtFDH was demonstrated in several human cancer cell lines by conventional and real-time PCR and by Western blot. Analysis of genomes of different species indicates that the mitochondrial enzyme is a later evolutionary product when compared with the cytosolic enzyme. We propose that this novel mitochondrial enzyme is a likely source of CO₂ production from 10-formyltetrahydrofolate in mitochondria and plays an essential role in the distribution of one-carbon groups between the cytosolic and mitochondrial compartments of the cell.     

Serine hydroxymethyltransferase catalyzes the hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate

The combined activities of rabbit liver cytosolic serine hydroxymethyltransferase and C1-tetrahydrofolate synthase convert tetrahydrofolate and formate to 5-formyltetrahydrofolate. In this reaction C1-tetrahydrofolate synthase converts tetrahydrofolate and formate to 5,10-methenyltetrahydrofolate, which is hydrolyzed to 5-formyltetrahydrofolate by a serine hydroxymethyltransferase-glycine complex. Serine hydroxymethyltransferase, in the presence of glycine, catalyzes the conversion of chemically synthesized 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate with biphasic kinetics. There is a rapid burst of product that has a half-life of formation of 0.4 s followed by a slower phase with a completion time of about 1 h. The substrate for the burst phase of the reaction was shown not to be 5,10-methenyltetrahydrofolate but rather a one-carbon derivative of tetrahydrofolate which exists in the presence of 5,10-methenyltetrahydrofolate. This derivative is stable at pH 7 and is not an intermediate in the hydrolysis of 5,10-methenyltetrahydrofolate to 10-formyltetrahydrofolate by C1-tetrahydrofolate synthase. Cytosolic serine hydroxymethyltransferase catalyzes the hydrolysis of 5,10-methenyltetrahydrofolate pentaglutamate to 5-formyltetrahydrofolate pentaglutamate 15-fold faster than the hydrolysis of the monoglutamate derivative. The pentaglutamate derivative of 5-formyltetrahydrofolate binds tightly to serine hydroxymethyltransferase and dissociates slowly with a half-life of 16 s. Both rabbit liver mitochondrial and Escherichia coli serine hydroxymethyltransferase catalyze the conversion of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate at rates similar to those observed for the cytosolic enzyme. Evidence that this reaction accounts for the in vivo presence of 5-formyltetrahydrofolate is suggested by the observation that mutant strains of E. coli, which lack serine hydroxymethyltransferase activity, do not contain 5-formyltetrahydrofolate, but both these cells, containing an overproducing plasmid of serine hydroxymethyltransferase, and wild-type cells do have measurable amounts of this form of the coenzyme.     

Isolation and characterization of the Saccharomyces cerevisiae MIS1 gene encoding mitochondrial C1-tetrahydrofolate synthase

C1-Tetrahydrofolate synthase is a trifunctional polypeptide found in eukaryotic organisms that catalyzes 10-formyltetrahydrofolate synthetase (EC 6.3.4.3), 5,10-methenyltetrahydrofolate cyclohydrolase (EC 3.5.4.9), and 5,10-methylenetetrahydrofolate dehydrogenase (EC 1.5.1.5) activities. In Saccharomyces cerevisiae, C1-tetrahydrofolate synthase is found in both the cytoplasm and the mitochondria. The gene encoding yeast mitochondrial C1-tetrahydrofolate synthase was isolated using synthetic oligonucleotide probes based on the amino-terminal sequence of the purified protein. Hybridization analysis shows that the gene (designated MIS1) has a single copy in the yeast genome. The predicted amino acid sequence of mitochondrial C1-tetrahydrofolate synthase shares 71% identity with yeast C1-tetrahydrofolate synthase and shares 39% identity with clostridial 10-formyltetrahydrofolate synthetase. Chromosomal deletions of the mitochondrial C1-tetrahydrofolate synthase gene were generated using the cloned MIS1 gene. Mutant strains which lack a functional MIS1 gene are viable and can grow in medium containing a nonfermentable carbon source. In fact, deletion of the MIS1 locus has no detectable effect on cell growth.     

Human red blood cell-mediated metabolism of leucovorin [(R,S)5-formyltetrahydrofolate

The ability of human blood in vitro, and partially purified red blood cells, to metabolize leucovorin, or 5-formyltetrahydrofolate, has been examined. A radioenzymatic assay based upon entrapment of 5,10-methylenetetrahydrofolate, and other reduced folates after cycling to this form, into a stable ternary complex with thymidylate synthase and tritiated 5-fluoro-2'-deoxyuridine-5'-monophosphate was used to estimate reduced folate metabolites. Incubation of whole blood samples with (R,S)5-formyltetrahydrofolate resulted in a time- and concentration-dependent extracellular accumulation of the reduced folates, 5-methyltetrahydrofolate, tetrahydrofolate, 10-formyltetrahydrofolate, and 5,10-methylenetetrahydrofolate. While accumulation with time was nonlinear, the tetrahydrofolate pool showed the greatest overall increase in concentration. 5-Methyltetrahydrofolate, which was the only reduced folate detected in plasma prior to introduction of (R,S)5-formyltetrahydrofolate, accumulated more slowly than tetrahydrofolate. 10-Formyltetrahydrofolate and 5,10-methylenetetrahydrofolate accumulated even more slowly but exhibited nonlinear kinetic patterns similar to those of tetrahydrofolate and 5-methyltetrahydrofolate. When blood cells were removed by centrifugation, a complete loss of metabolic activity was observed. Exposure of purified red blood cells to (R,S)5-formyltetrahydrofolate resulted in accumulation of extracellular reduced folates that was similar to that in whole blood samples while partially purified white blood cells exhibited little activity. Metabolism of the (S) diastereomer of 5-formyltetrahydrofolate accounted for essentially all of the observed extracellular accumulation of reduced folates. We propose that red blood cell-mediated metabolism of 5-formyltetrahydrofolate could, in part at least, account for reduced folate accumulation in plasma when leucovorin is administered to humans.     

Oxidation of 10-formyltetrahydrofolate to 10-formyldihydrofolate by complex IV of rat mitochondria

We hypothesized that the unanticipated bioactivity of orally administered unnatural carbon-6 isomers, (6R)-5-formyltetrahydrofolate (5-HCO-THF) and (6S)-5,10-methenyltetrahydrofolate (5,10-CH-THF), in humans [Baggott, J. E., and Tamura, T. (1999) Biochim. Biophys. Acta 1472, 323-32] is explained by the rapid oxidation of (6S)-10-formyltetrahydrofolate (10-HCO-THF), which is produced by in vivo chemical processes from the above folates. An oxidation of 10-HCO-THF produces 10-formyldihydrofolate (10-HCO-DHF), which no longer has the asymmetric center at carbon-6 and is metabolized by aminoimidazole carboxamide ribotide (AICAR) transformylase forming bioactive dihydrofolate. Since cytochrome c (Fe(3+)) rapidly oxidizes both (6R)- and (6S)-10-HCO-THF [Baggott et al. (2001) Biochem. J. 354, 115-22], we investigated the metabolism of 10-HCO-THF by isolated rat liver mitochondria. We found that 10-HCO-THF supported the respiration of mitochondria without uncoupling ATP synthesis. The site of electron donation was identified as complex IV, which contains cytochrome c; the folate product was 10-HCO-DHF, and the reaction was saturable with respect to 10-HCO-THF. Both (6S)- (unnatural) and (6R)-10-HCO-THF supported the respiration of mitochondria, whereas (6S)-5-formyltetrahydrofolate (5-HCO-THF) was inactive. To our knowledge, this cytochrome c oxidation of 10-HCO-THF to 10-HCO-DHF in the mitochondrial intermembrane space represents a possible folate metabolic pathway previously unidentified and would explain the bioactivity of unnatural carbon-6 isomers, (6R)-5-HCO-THF and (6S)-5,10-CH-THF, in humans.     

C1-Tetrahydrofolate synthase from rabbit liver. Structural and kinetic properties of the enzyme and its two domains

C1-Tetrahydrofolate synthase is a multifunctional enzyme which catalyzes three reactions in 1-carbon metabolism: 10-formyltetrahydrofolate synthetase; 5,10-methenyltetrahydrofolate cyclohydrolase; 5,10-methylenetetrahydrofolate dehydrogenase. A rapid 1-day purification procedure has been developed which gives 40 mg of pure enzyme from 10 rabbit livers. The 10-formyltetrahydrofolate synthetase activity of this trifunctional enzyme has a specific activity that is 4-fold higher than the enzyme previously purified from rabbit liver. Conditions have been developed for the rapid isolation of a tryptic fragment of the enzyme which contains the methylenetetrahydrofolate dehydrogenase and methenyltetrahydrofolate cyclohydrolase activities. This fragment is a monomer exhibiting a subunit and native molecular weight of 36,000 in most buffers. However, in phosphate buffers the native molecular weight suggests that the fragment is a dimer. Conditions are also given whereby chymotryptic digestion allows the simultaneous isolation from the native enzyme of a large fragment containing the 10-formyltetrahydrofolate synthetase activity and a smaller fragment containing the dehydrogenase and cyclohydrolase activities. The large fragment is a dimer with a subunit molecular weight of 66,000. The small fragment retains all of the dehydrogenase and cyclohydrolase activities of the native enzyme. The large fragment is unstable but retains most of the 10-formyltetrahydrofolate synthetase activity. Km values of substrates for the two fragments are the same as the values for the native enzyme. The 10-formyltetrahydrofolate synthetase activity of the native enzyme requires ammonium or potassium ions for expression of full catalytic activity. The effect of these two ions on the catalytic activity of the large chymotryptic fragment is the same as with the native enzyme. We have shown by differential scanning calorimetry that the native enzyme contains two protein domains which show thermal transitions at 47 and 60 degrees C. Evidence is presented that the two domains are related to the two protein fragments generated by proteolysis of the native enzyme. The larger of the two domains contains the active site for the 10-formyltetrahydrofolate synthetase activity while the smaller domain contains the active site which catalyzes the dehydrogenase and cyclohydrolase reactions. Replacement of sodium ion buffers with either ammonium or potassium ions results in an increase in stability of the large domain of the native enzyme. This change in stability is not accompanied by a change in the quaternary structure of the enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)     

Monoclonal Antibodies to the [alpha]- and [beta]-Subunits of the Plant Mitochondrial F1-ATPase

We have generated nine monoclonal antibodies against subunits of the maize (Zea mays L.) mitochondrial F1-ATPase. These monoclonal antibodies were generated by immunizing mice against maize mitochondrial fractions and randomly collecting useful hybridomas. To prove that these monoclonal antibodies were directed against ATPase subunits, we tested their cross-reactivity with purified F1-ATPase from pea cotyledon mitochondria. One of the antibodies ([alpha]-ATPaseD) cross-reacted with the pea F1-ATPase [alpha]-subunit and two ([beta]-ATPaseD and [beta]-ATPaseE) cross-reacted with the pea F1-ATPase [beta]-subunit. This established that, of the nine antibodies, four react with the maize [alpha]-ATPase subunit and the other five react with the maize [beta]-ATPase subunit. Most of the monoclonal antibodies cross-react with the F1-ATPase from a wide range of plant species. Each of the four monoclonal antibodies raised against the [alpha]-subunit recognizes a different epitope. Of the five [beta]-subunit antibodies, at least three different epitopes are recognized. Direct incubation of the monoclonal antibodies with the F1-ATPase failed to inhibit the ATPase activity. The monoclonal antibodies [alpha]-ATPaseD and [beta]-ATPaseD were bound to epoxide-glass QuantAffinity beads and incubated with a purified preparation of pea F1-ATPase. The ATPase activity was not inhibited when the antibodies bound the ATPase. The antibodies were used to help map the pea F1-ATPase subunits on a two-dimensional map of whole pea cotyledon mitochondrial protein. In addition, the antibodies have revealed antigenic similarities between various isoforms observed for the [alpha]- and [beta]-subunits of the purified F1-ATPase. The specificity of these monoclonal antibodies, along with their cross-species recognition and their ability to bind the F1-ATPase without inhibiting enzymic function, makes these antibodies useful and invaluable tools for the further purification and characterization of plant mitochondrial F1-ATPases.     

Hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate at pH 2.5 to 4.5

At pH 4.0 to 4.5, 5,10-methenyltetrahydrofolate is hydrolyzed to only 5-formyltetrahydrofolate if reducing agents are present or iron-redox cycling is suppressed. At pH 4.0, the equilibrium position for this hydrolysis is approximately equal concentrations of both folates. If no reducing agents are used or iron-redox cycling is promoted, considerable amounts of 10-formyldihydrofolate are also formed. It is likely that 10-formyldihydrofolate has been misidentified as 5,10-hydroxymethylenetetrahydrofolate, which was reported to accumulate during the hydrolysis of 5, 10-methenyltetrahydrofolate to 5-formyltetrahydrofolate [Stover, P. and Schirch, V. (1992) Biochemistry 31, 2148-2155 and 2155-2164; (1990) J. Biol. Chem. 265, 14227-14233]. Since 5, 10-hydroxymethylenetetrahydrofolate is reported to be the viable in vivo substrate for serine hydroxymethyltransferase-catalyzed formation of 5-formyltetrahydrofolate, and 5, 10-hydroxymethylenetetrahydrofolate probably does not accumulate, the above folate metabolism is now doubtful. It is hypothesized that mildly acidic subcellular organelles provide an environment for the hydrolysis of 5,10-methenyltetrahydrofolate to 5-formyltetrahydrofolate in vivo, and there is no requirement for enzyme catalysis. Finally, 10-formyltetrahydrofolate is susceptible to iron-catalyzed oxidation to 10-formyldihydrofolate at pH 4 to 4.5.     

Effect of nitrous oxide inactivation of vitamin B12-dependent methionine synthetase on the subcellular distribution of folate coenzymes in rat liver

The effects of nitrous oxide inactivation of the vitamin B12-dependent enzyme, methionine synthetase (EC 2.1.1.13), on the subcellular distribution of hepatic folate coenzymes was determined. In controls, cytosolic folates were 5-methyltetrahydrofolate (45%), 5- and 10-formyltetrahydrofolate (9 and 19%, respectively), and tetrahydrofolate (27%). Exposure of rats to an atmosphere containing 80% nitrous oxide for 18 h resulted in a marked shift in this distribution pattern to 5-methyltetrahydrofolate, 84%; 5- and 10-formyltetrahydrofolate, 2.1 and 9.1%, respectively; and tetrahydrofolate, 4.7%. Activity of the cytosolic enzyme, methionine synthetase, was reduced by about 84% as compared to that of air breathing controls. In controls, mitochondrial folates were 5-methyltetrahydrofolate (7.3%), 5- and 10-formyltetrahydrofolate (11.5 and 33.1%, respectively), and tetrahydrofolate (48.1%). This distribution did not change after exposure to nitrous oxide. These results show that the effects of nitrous oxide inactivation of vitamin B12 are confined to the cytosol, at least in the short term, and suggest that there is little, if any, transport of free folates between the cytosolic and mitochondrial compartments.     

Co-purification of three folate enzymes from porcine liver

Three folate enzymes, 5,10-methylenetetrahydrofolate dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase, and 10-formyltetrahydrofolate synthetase have been purified 100-fold from porcine liver. The three activities co-purify through fractionation with (NH₄)₂SO₄, polyethylene glycol-6000, and chromatography on DEAE-Sephadex and phosphocellulose columns. In addition, the observation that NADP, a substrate for the dehydrogenase, protects all three enzymes from heat inactivation suggests that the enzymes are present as a protein complex.     

Cloning, expression, and purification of 5,10-methenyltetrahydrofolate synthetase from Mus musculus

Folate metabolism is necessary for the biosyntheses of purine nucleotides and thymidylate and for the synthesis of S-adenosylmethionine, a cofactor required for cellular methylation reactions and a precursor of spermidine and spermine syntheses. Disruption of folate metabolism is associated with several pathologies and developmental anomalies including cancer and neural tube defects. The enzyme 5,10-methenyltetrahydrofolate synthetase (MTHFS, EC 6.3.3.2) catalyzes the ATP-dependent conversion of 5-formyltetrahydrofolate to 5,10-methenyltetrahydrofolate, and has been shown to affect intracellular folate concentrations by accelerating folate degradation. Mammalian MTHFS proteins described to date are not stable and no recombinant mammalian MTHFS protein has been successfully expressed in Escherichia coli. The three-dimensional structure of MTHFS has not been solved. The cDNA coding for Mus musculus MTHFS was isolated and expressed in E. coli with a hexa-histidine tag. Milligram quantities of recombinant mouse MTHFS were purified using metal affinity chromatography and the protein was stabilized with Tween 20. Mouse MTHFS has a molecular mass of 23 kDa and is 84% identical in amino acid sequence to the human enzyme. Activity assays confirmed the functionality of the recombinant protein, with K_m=5 μM for (6S)-5-formyltetrahydrofolate and K_m=769 μM for Mg–ATP. This is the first example of a mammalian form of MTHFS expressed in E. coli that yielded sufficient quantities of stable purified protein to allow for detailed characterization of its three-dimensional structure and kinetic properties.     

Role of enzyme-bound 5,10-methenyltetrahydropteroylpolyglutamate in catalysis by Escherichia coli DNA photolyase

DNA photolyase catalyzes the photoreversal of pyrimidine dimers. The enzymes from Escherichia coli and yeast contain a flavin chromophore and a folate cofactor, 5,10-methenyltetrahydropteroylpolyglutamate. E. coli DNA photolyase contains about 0.3 mol of folate/mol flavin, whereas the yeast photolyase contains the full complement of folate. E. coli DNA photolyase is reconstituted to a full complement of the folate by addition of 5,10-methenyltetrahydrofolate to cell lysates or purified enzyme samples. The reconstituted enzyme displays a higher photolytic cross section under limiting light. Treatment of photolyase with sodium borohydride or repeated camera flashing results in the disappearance of the absorption band at 384 nm and is correlated with the formation of modified products from the enzyme-bound 5,10-methenyltetrahydrofolate. Photolyase modified in this manner has a decreased photolytic cross section under limiting light. Borohydride reduction results in the formation of 5,10-methylenetetrahydrofolate and 5-methyltetrahydrofolate, both of which are released from the enzyme. Repeated camera flashing results in photodecomposition of the enzyme-bound 5,10-methenyltetrahydrofolate and release of the decomposition products. Finally, it is observed that photolyase binds 10-formyltetrahydrofolate and appears to cyclize it to form the 5,10-methenyltetrahydrofolate chromophore.     

Bioactivity of orally administered unnatural isomers, [6R]-5-formyltetrahydrofolate and [6S]-5,10-methenyltetrahydrofolate, in humans

It has been assumed that humans cannot utilize 5,6,7,8-tetrahydrofolates with the unnatural configuration at carbon 6, since these folates are enzymatically and microbiologically inactive. We hypothesized that orally administered unnatural [6R]-5-formyltetrahydrofolate or [6S]-5,10-methenyltetrahydrofolate is bioactive in humans. Subjects were given independent oral doses of these unnatural folates and of a natural [6S]-5-formyltetrahydrofolate. Plasma, before and after the dose for 4 h, and 2 h urine were collected. Areas under the curve for the change in plasma folate concentrations were measured microbiologically and urinary folates were measured using HPLC. Based on findings of plasma and urinary folates, the unnatural folates were estimated to be 14-50% active as compared to [6S]-5-formyltetrahydrofolate. The major plasma and urinary folate was [6S]-5-methyltetrahydrofolate in all experiments. In urine, a [6S]-5-formyltetrahydrofolate peak was observed only after a [6S]-5-HCO-H4folate dose and peaks of unnatural [6S]-10-formyltetrahydrofolate and 5-formyltetrahydrofolate were identified after a [6R]-5-formyltetrahydrofolate dose. A possible pathway that explains our findings is discussed. This pathway includes the oxidation of the unnatural [6S]-10-formyltetrahydrofolate to 10-formyl-7,8-dihydrofolate which can be further metabolized by 5-amino-4-imidazolecarboxamide-ribotide transformylase producing dihydrofolate. Dihydrofolate can then be metabolized to [6S]-5-methyltetrahydrofolate by well-established metabolism.     

Bacterial conversion of folinic acid is required for antifolate resistance

Antifolates, which are among the first antimicrobial agents invented, inhibit cell growth by creating an intracellular state of folate deficiency. Clinical resistance to antifolates has been mainly attributed to mutations that alter structure or expression of enzymes involved in de novo folate synthesis. We identified a Mycobacterium smegmatis mutant, named FUEL (which stands for folate utilization enzyme for leucovorin), that is hypersusceptible to antifolates. Chemical complementation indicated that FUEL is unable to metabolize folinic acid (also known as leucovorin or 5-formyltetrahydrofolate), whose metabolic function remains unknown. Targeted mutagenesis, genetic complementation, and biochemical studies showed that FUEL lacks 5,10-methenyltetrahydrofolate synthase (MTHFS; also called 5-formyltetrahydrofolate cyclo-ligase; EC 6.3.3.2) activity responsible for the only ATP-dependent, irreversible conversion of folinic acid to 5,10-methenyltetrahydrofolate. In trans expression of active MTHFS proteins from bacteria or human restored both antifolate resistance and folinic acid utilization to FUEL. Absence of MTHFS resulted in marked cellular accumulation of polyglutamylated species of folinic acid. Importantly, MTHFS also affected M. smegmatis utilization of monoglutamylated 5-methyltetrahydrofolate exogenously added to the medium. Likewise, Escherichia coli mutants lacking MTHFS became susceptible to antifolates. These results indicate that folinic acid conversion by MTHFS is required for bacterial intrinsic antifolate resistance and folate homeostatic control. This novel mechanism of antimicrobial antifolate resistance might be targeted to sensitize bacterial pathogens to classical antifolates.     

Tissue and Cellular Distribution of Glutamine Synthetase in Roots of Pea (Pisum sativum) Seedlings

The effect of nitrate application on glutamine synthetase activity in roots of pea (Pisum sativum L.) seedlings (2 weeks old) was studied. Separation of organelles from root fragments by sucrose density-gradient centrifugation revealed that both nitrite reductase and glutamine synthetase activities increased in root plastids as a response to nitrate application and that no such response was induced by ammonium application. Glutamine synthetase activity was also found to increase in plastids with distance from apex in nitrate-treated plants, the highest specific activity being located in the fourth 1-centimeter segment. Separation by SDS-PAGE and characterization by Western blotting showed that cytosolic glutamine synthetase contains one subunit polypeptide (28 kilodaltons) and that plastid glutamine synthetase contains both the 38-kilodalton subunit and a heavier subunit. When nitrate was present in the nutrient solution, the heavier subunit increased in abundance in protein fractions obtained from purified root plastids.     

A low-molecular-weight protein cross-reacting with human liver N-acetyl-β-d-glucosaminidase

Antisera were raised to preparations of hexosaminidase isoenzymes A and B purified from human liver. Protein that cross-reacted with the liver hexosaminidase was detected by an antibody-consumption method. A cross-reacting protein with a low molecular weight (20000) was partially characterized and purified from control human liver. This protein is also present in the liver of patients with Tay-Sachs disease or with Sandhoff's disease. Hexosaminidases A and B gave an immunological reaction of partial identity with the low-molecular-weight protein. The possible identity of the low-molecular-weight cross-reacting protein as a subunit of hexosaminidase is discussed.