NADPH Regeneration by Glucose Dehydrogenase fromGluconobacter scleroidesforl-Leucovorin Synthesis

A new process for (6S)-tetrahydrofolate production from dihydrofolate was designed that used dihydrofolate reductase and an NADPH regeneration system. Glucose dehydrogenase from Gluconobacter scleroides KY3613 was used for recycling of the cofactor. The reaction mixture contained 200 mM dihydrofolate, 220 mM glucose, 2 mM NADP, 14.4 U/ml dihydrofolate reductase, and 14.4 U/ml Glucose dehydrogenase, and the reaction was complete after incubation at pH 8.0, and 40 degrees C for 2.5 hr. With (6S)-tetrahydrofolate as the starting material, l-leucovorin was synthesized via a methenyl derivative. The purity of the l-leucovorin was 100%, and its diastereomeric purity was greater than 99.5% d.e. as the (6S)-form.

5,10-methenyltetrahydrofolate cyclohydrolase, rat liver and chemically catalysed formation of 5-formyltetrahydrofolate

The 5,10-methenyltetrahydrofolate (5,10-CH=H4folate) synthetase catalyses the physiologically irreversible formation of 5,10-CH=H4folate from 5-formyltetrahydrofolate (5-HCO-H4folate) and ATP. It is not clear how (or if) 5-HCO-H4folate is formed in vivo. Using a spectrophotometric assay for 5-HCO-H4folate, human recombinant 5,10-CH=H4folate cyclohydrolase, which catalyses the hydrolysis of 5,10-CH=H4folate to 10-HCO-H4folate, was previously shown to catalyse inefficiently the formation of 5-HCO-H4folate at pH 7.3 [Pelletier and MacKenzie (1996) Bioorg. Chem. 24, 220-228]. In the present study, we report that (i) the human cyclohydrolase enzyme catalyses the conversion of 10-HCO-/5,10-CH=H4folate into 5-HCO-H4folate (it is also chemically formed) at pH 4.0-7.0; (ii) rat liver has a very low capacity to catalyse the formation of 5-HCO-H4folate when compared with the traditional activity of 5,10-CH=H4folate cyclohydrolase and the activity of the 5,10-CH=H4folate synthetase; and (iii) a substantial amount of 5-HCO-H4folate reported to be present in rat liver is chemically formed during analytical procedures. We conclude that (i) the cyclohydrolase represents some of the capacity of rat liver to catalyse the formation of 5-HCO-H4folate; (ii) the amount of 5-HCO-H4folate reported to be present in rat liver is overestimated (liver 5-HCO-H4folate content may be negligible); and (iii) there is little evidence that 5-HCO-H4folate inhibits one-carbon metabolism in mammals.

The GLOBULAR ARREST1 gene, which is involved in the biosynthesis of folates, is essential for embryogenesis in Arabidopsis thaliana

We identified a mutation in Arabidopsis that resulted in defective embryos, and we designated this mutation globular arrest1 (gla1). The predicted amino acid sequence encoded by the GLA1 gene is homologous to the amino acid sequences of folylpolyglutamate synthetase (FPGS) and dihydrofolate synthetase (DHFS), which participate in folate biosynthesis. The defect of gla1 in the formation of calli was rescued by the supplement of 5-formyl tetrahydrofolate. These results indicated that GLA1 is involved in the biosynthesis of tetrahydrofolate. The gla1 embryos developed normally in the early stage of development but did not undergo the transition to the heart stage. Thus, the function of the GLA1 gene in embryogenesis appears to be required after the globular stage. However, when the levels of GLA1 transcripts in transgenic plants were increased by introduction of several copies of a GLA1 transgene (GLA6.8), the gla1 embryos that grew on gla1/gla1 GLA6.8/- plants developed as far as the heart to bent-cotyledon stage. This result suggests that the GLA1 function is provided to embryos by maternal tissues until embryos reach the globular stage.

Synthesis of glutamine, glycine and 10-formyl tetrahydrofolate is coregulated with purine biosynthesis in Saccharomyces cerevisiae

Glutamine, glycine and 10-formyl tetrahydrofolate are consumed during de novo purine biosynthesis. We have found that, in Saccharomyces cerevisiae, synthesis of these cosubstrates is coregulated with synthesis of enzymes of the purine biosynthetic pathway. Analysis of three genes required for synthesis of glutamine, glycine and 10-formyl tetrahydrofolate (GLN1, SHM2 and MTD1, respectively) shows that their expression is repressed by adenine and requires the transcription factors Baslp and Bas2p. Northern analysis reveals that regulation of SHM2 and MTD1 expression by adenine takes place at the transcriptional level. We also show that Bas1p and Bas2p bind in vitro to the promoters of the SHM2 and MTD1 genes, and that mutations in the consensus Bas1p binding sequences strongly affect expression of these genes in vivo. Finally, we have found that a SHM2-lacZ fusion is expressed at a significantly higher level in a bas2-2 disrupted strain than in bas1-2 or bas1-2 bas2-2 mutant strains. The BAS1-dependent, BAS2-independent expression of SHM2-lacZ suggests that, in the absence of Bas2p, Bas1p can interact with another protein partner to activate SHM2 expression.

Purification, crystallization, and preliminary X-ray studies of 10-formyltetrahydrofolate synthetase from Clostridia acidici-urici

The monofunctional enzyme 10-formyltetrahydrofolate synthetase (THFS), which is responsible for the recruitment of single carbon units from the formate pool into a variety of folate-dependent biosynthetic pathways, has been subcloned, purified, and crystallized. The crystals belong to space group P2(1), with unit cell dimensions a = 102.4 A, b = 116.5 A, c = 115.8 A, and beta = 103.5. The crystal unit cell and diffraction is consistent with an asymmetric unit consisting of the enzyme tetramer, and a specific volume of the unit cell of 2.7 A3/ Da. The crystals diffract to at least 2.3 A resolution after flash-cooling, when using a rotating anode x-ray source and an RAXIS image plate detector.

Production of dihydrofolate reductase by cloned Escherichia coli and its application to asymmetric synthesis of l-leucovorin

We have investigated culture conditions for production of dihydrofolate reductase by Escherichia coli harboring a high expression plasmid, pTP64-1. Sorbitol addition and pH control were effective for the production of the enzyme in a jar fermentor. The enzyme was purified from a cell-free extract by column chromatographies on DEAE-Cellulofine and Superose Prep12 and showed a single band on SDS-polyacrylamide gel electrophoresis. The reduction of 200 mM dihydrofolate to 6(S)-tetrahydrofolate, an intermediate for l-leucovorin synthesis, was complete in 2 hr under anaerobic conditions, using 1.5 units/ml of the purified enzyme.

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.

Experimental production of elevated serum folate in dogs with intestinal blind loops. II. Nature of bacterially produced folate coenzymes in blind loop fluid

Jejunal and ileal blind loops were constructed in separate groups of dogs and the folate coenzymes present in these loops were investigated. Although only those dogs with high blind loops had appreciably elevated folate levels active for Lactobacillus casei but not Streptococcus faecalis, every blind loop appeared to contain four folate compounds--5-formyltetrahydrofolic acid, 5-methyltetrahydrofolic acid, tetrahydrofolic acid and a compound tentatively identified as 5-formyltetrahydrofolic triglutamate. Why increased levels of 5-methyltetrahydrofolate alone are present in the serum of dogs with a high blind loop is discussed.

Antiplasmodial efficacy of 2,4--diaminopyrimidine0sylfonamide combinations, especially against chloroquine-resistant malaria

This presentation deals with the historical development of the antifolate pyrimidines and related compounds, first as antimalarial substances and later as potent antibacterial agents. It describes the first quantitation of the combined action, through sequential blockade, of the substances with sulfonamides, and outlines the usefulness of the combinations in the therapy of normally sensitive and multiresistant strains of Plasmodium falciparum.