Interactions of pig liver serine hydroxymethyltransferase with methyltetrahydropteroylpolyglutamate inhibitors and with tetrahydropteroylpolyglutamate substrates

Regulatory mechanisms of one-carbon metabolism enzymes

One-carbon metabolism is a central metabolic pathway critical for the biosynthesis of several amino acids, methyl group donors, and nucleotides. The pathway mostly relies on the transfer of a carbon unit from the amino acid serine, through the cofactor folate (in its several forms), and to the ultimate carbon acceptors that include nucleotides and methyl groups used for methylation of proteins, RNA, and DNA. Nucleotides are required for DNA replication, DNA repair, gene expression, and protein translation, through ribosomal RNA. Therefore, the one-carbon metabolism pathway is essential for cell growth and function in all cells, but is specifically important for rapidly proliferating cells. The regulation of one-carbon metabolism is a critical aspect of the normal and pathological function of the pathway, such as in cancer, where hijacking these regulatory mechanisms feeds an increased need for nucleotides. One-carbon metabolism is regulated at several levels: via gene expression, posttranslational modification, subcellular compartmentalization, allosteric inhibition, and feedback regulation. In this review, we aim to inform the readers of relevant one-carbon metabolism regulation mechanisms and to bring forward the need to further study this aspect of one-carbon metabolism. The review aims to integrate two major aspects of cancer metabolism-signaling downstream of nutrient sensing and one-carbon metabolism, because while each of these is critical for the proliferation of cancerous cells, their integration is critical for comprehensive understating of cellular metabolism in transformed cells and can lead to clinically relevant insights.

Chloroplastic Serine Hydroxymethyltransferase From Medicago truncatula: A Structural Characterization

Serine hydroxymethyltransferase (SHMT, EC 2.1.2.1) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme which catalyzes the reversible serine-to-glycine conversion in either a tetrahydrofolate-dependent or -independent manner. The enzyme is also responsible for the tetrahydrofolate-independent cleavage of other β-hydroxy amino acids. In addition to being an essential player in the serine homeostasis, SHMT action is the main source of activated one-carbon units, which links SHMT activity with the control of cell proliferation. In plants, studies of SHMT enzymes are more complicated than of those of, e.g., bacterial or mammalian origins because plant genomes encode multiple SHMT isozymes that are targeted to different subcellular compartments: cytosol, mitochondria, plastids, and nucleus. Here we report crystal structures of chloroplast-targeted SHMT from Medicago truncatula (MtSHMT3). MtSHMT3 is a tetramer in solution, composed of two tight and obligate dimers. Our complexes with PLP internal aldimine, PLP-serine and PLP-glycine external aldimines, and PLP internal aldimine with a free glycine reveal structural details of the MtSHMT3-catalyzed reaction. Capturing the enzyme in different stages along the course of the slow tetrahydrofolate-independent serine-to-glycine conversion allowed to observe a unique conformation of the PLP-serine γ-hydroxyl group, and a concerted movement of two tyrosine residues in the active site.

Kinetic mechanism and the rate-limiting step of Plasmodium vivax serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT) is a pyridoxal 5'-phosphate (PLP)-dependent enzyme that catalyzes a hydroxymethyl group transfer from L-serine to tetrahydrofolate (H4folate) to yield glycine and 5,10-methylenetetrahydrofolate (CH2-H4folate). SHMT is crucial for deoxythymidylate biosynthesis and a target for antimalarial drug development. Our previous studies indicate that PvSHMT catalyzes the reaction via a ternary complex mechanism. To define the kinetic mechanism of this catalysis, we explored the PvSHMT reaction by employing various methodologies including ligand binding, transient, and steady-state kinetics as well as product analysis by rapid-quench and HPLC/MS techniques. The results indicate that PvSHMT can bind first to either L-serine or H4folate. The dissociation constants for the enzyme·L-serine and enzyme·H4folate complexes were determined as 0.18 ± 0.08 and 0.35 ± 0.06 mM, respectively. The amounts of glycine formed after single turnovers of different preformed binary complexes were similar, indicating that the reaction proceeds via a random-order binding mechanism. In addition, the rate constant of glycine formation measured by rapid-quench and HPLC/MS analysis is similar to the kcat value (1.09 ± 0.05 s(-1)) obtained from the steady-state kinetics, indicating that glycine formation is the rate-limiting step of SHMT catalysis. This information will serve as a basis for future investigation on species-specific inhibition of SHMT for antimalarial drug development.

Plasmodium serine hydroxymethyltransferase as a potential anti-malarial target: inhibition studies using improved methods for enzyme production and assay

Background

There is an urgent need for the discovery of new anti-malarial drugs. Thus, it is essential to explore different potential new targets that are unique to the parasite or that are required for its viability in order to develop new interventions for treating the disease. Plasmodium serine hydroxymethyltransferase (SHMT), an enzyme in the dTMP synthesis cycle, is a potential target for such new drugs, but convenient methods for producing and assaying the enzyme are still lacking, hampering the ability to screen inhibitors.

Methods

Production of recombinant Plasmodium falciparum SHMT (PfSHMT) and Plasmodium vivax SHMT (PvSHMT), using auto-induction media, were compared to those using the conventional Luria Bertani medium with isopropyl thio-β-D-galactoside (LB-IPTG) induction media. Plasmodium SHMT activity, kinetic parameters, and response to inhibitors were measured spectrophotometrically by coupling the reaction to that of 5,10-methylenetetrahydrofolate dehydrogenase (MTHFD). The identity of the intermediate formed upon inactivation of Plasmodium SHMTs by thiosemicarbazide was investigated by spectrophotometry, high performance liquid chromatography (HPLC), and liquid chromatography-mass spectrometry (LC-MS). The active site environment of Plasmodium SHMT was probed based on changes in the fluorescence emission spectrum upon addition of amino acids and folate.

Results

Auto-induction media resulted in a two to three-fold higher yield of Pf- and PvSHMT (7.38 and 29.29 mg/L) compared to that produced in cells induced in LB-IPTG media. A convenient spectrophotometric activity assay coupling Plasmodium SHMT and MTHFD gave similar kinetic parameters to those previously obtained from the anaerobic assay coupling SHMT and 5,10-methylenetetrahydrofolate reductase (MTHFR); thus demonstrating the validity of the new assay procedure. The improved method was adopted to screen for Plasmodium SHMT inhibitors, of which some were originally designed as inhibitors of malarial dihydrofolate reductase. Plasmodium SHMT was slowly inactivated by thiosemicarbazide and formed a covalent intermediate, PLP-thiosemicarbazone.

Conclusions

Auto-induction media offers a cost-effective method for the production of Plasmodium SHMTs and should be applicable for other Plasmodium enzymes. The SHMT-MTHFD coupled assay is equivalent to the SHMT-MTHFR coupled assay, but is more convenient for inhibitor screening and other studies of the enzyme. In addition to inhibitors of malarial SHMT, the development of species-specific, anti-SHMT inhibitors is plausible due to the presence of differential active sites on the Plasmodium enzymes.

One-carbon metabolism in plants: characterization of a plastid serine hydroxymethyltransferase

SHMT (serine hydroxymethyltransferase; EC 2.1.2.1) catalyses reversible hydroxymethyl group transfer from serine to H4PteGlun (tetrahydrofolate), yielding glycine and 5,10-methylenetetrahydrofolate. In plastids, SHMTs are thought to catalytically direct the hydroxymethyl moiety of serine into the metabolic network of H4PteGlun-bound one-carbon units. Genes encoding putative plastid SHMTs were found in the genomes of various plant species. SHMT activity was detected in chloroplasts in pea (Pisum sativum) and barley (Hordeum vulgare), suggesting that plastid SHMTs exist in all flowering plants. The Arabidopsis thaliana genome encodes one putative plastid SHMT (AtSHMT3). Its cDNA was cloned by reverse transcription-PCR and the encoded recombinant protein was produced in Escherichia coli. Evidence that AtSHMT3 is targeted to plastids was found by confocal microscopy of A. thaliana protoplasts transformed with proteins fused to enhanced green fluorescent protein. Characterization of recombinant AtSHMT3 revealed that substrate affinity for and the catalytic efficiency of H4PteGlu1-8 increase with n, and that H4PteGlu1-8 inhibit AtSHMT3. 5-Methyltetrahydrofolate and 5-formyltetrahydrofolate with one and five glutamate residues inhibited AtSHMT3-catalysed hydroxymethyl group transfer from serine to H4PteGlu6, with the pentaglutamylated inhibitors being more effective. Calculations revealed inhibition with 5-methyltetrahydrofolate or 5-formyltetrahydrofolate resulting in little reduction in AtSHMT3 activity under folate concentrations estimated for plastids.

Characterization of Plasmodium falciparum serine hydroxymethyltransferase-A potential antimalarial target

Serine hydroxymethyltransferase (SHMT) is a ubiquitous enzyme required for folate recycling and dTMP synthesis. A cDNA encoding Plasmodium falciparum (Pf) SHMT was expressed as a hexa-histidine tagged protein in Escherichia coli BL21-CodonPlus (DE3)-RIL. The protein was purified and the process yielded 3.6 mg protein/l cell culture. Recombinant His(6)-tagged PfSHMT exhibits a visible spectrum characteristic of pyridoxal-5'-phosphate enzyme and catalyzes the reversible conversion of l-serine and tetrahydrofolate (H(4)folate) to glycine and 5,10-methylenetetrahydrofolate (CH(2)-H(4)folate). Steady-state kinetics study indicates that His(6)-tagged PfSHMT catalyzes the reaction by a ternary-complex mechanism. The sequence of substrate binding to the enzyme was also examined by glycine product inhibition. A striking property that is unique for His(6)-tagged PfSHMT is the ability to use D-serine as a substrate in the folate-dependent serine-glycine conversion. Kinetic data in combination with expression result support the proposal of SHMT reaction being a regulatory step for dTMP cycle. This finding suggests that PfSHMT can be a potential target for antimalarial chemotherapy.

Serine hydroxymethyltransferase from Plasmodium vivax is different in substrate specificity from its homologues

The putative gene of Plasmodium vivax serine hydroxymethyltransferase (PvSHMT; EC 2.1.2.1) was cloned and expressed in Escherichia coli. The purified enzyme was shown to be a dimeric protein with a monomeric molecular mass of 49 kDa. PvSHMT has a maximum absorption peak at 422 nm with a molar absorption coefficient of 6370 M(-1) x cm(-1). The K(d) for binding of the enzyme and pyridoxal-5-phosphate was 0.14 +/- 0.01 microM. An alternative assay for measuring the tetrahydrofolate-dependent SHMT activity based on the coupled reaction with 5,10-methylenetetrahydrofolate reductase (EC 1.5.1.20) from E. coli was developed. PvSHMT uses a ternary-complex mechanism with a k(cat) value of 0.98 +/- 0.06 s(-1) and K(m) values of 0.18 +/- 0.03 and 0.14 +/- 0.02 mM for L-serine and tetrahydrofolate, respectively. The optimum pH of the SHMT reaction was 8.0 and an Arrhenius's plot showed a transition temperature of 19 degrees C. Besides L-serine, PvSHMT forms an external aldimine complex with D-serine, L-alanine, L-threonine and glycine. PvSHMT also catalyzes the tetrahydrofolate-independent retro-aldol cleavage of 3-hydroxy amino acids. Although L-serine is a physiological substrate for SHMT in the tetrahydrofolate-dependent reaction, PvSHMT can also use D-serine. In the absence of tetrahydrofolate at high pH, PvSHMT forms an enzyme-quinonoid complex with D-serine, but not with L-serine, whereas SHMT from rabbit liver was reported to form an enzyme-quinonoid complex with L-serine. The substrate specificity difference between PvSHMT and the mammalian enzyme indicates the dissimilarity between their active sites, which could be exploited for the development of specific inhibitors against PvSHMT.

A mathematical model gives insights into the effects of vitamin B-6 deficiency on 1-carbon and glutathione metabolism

We experimented with a mathematical model for 1-carbon metabolism and glutathione (GSH) synthesis to investigate the effects of vitamin B-6 deficiency on the reaction velocities and metabolite concentrations in this metabolic network. The mathematical model enabled us to independently alter the activities of each of the 5 vitamin B-6-dependent enzymes and thus determine which inhibitions were responsible for the experimentally observed consequences of a vitamin B-6 deficiency. The effect of vitamin B-6 deficiency on serine and glycine concentrations in tissues and plasma was almost entirely due to its effects on the activity of glycine decarboxylase. The effect of vitamin B-6 restriction on GSH concentrations appeared to be indirect, arising from the fact that vitamin B-6 restriction increases oxidative stress, which, in turn, affects several enzymes in 1-carbon metabolism as well as the GSH transporter. Vitamin B-6 restriction causes an abnormally high and prolonged homocysteine response to a methionine load test. This effect appeared to be mediated solely by its effects on cystathionine beta-synthase. Reduction of the enzymatic activity of serine hydroxymethyltransferase (SHMT) had negligible effects on most metabolite concentrations and reaction velocities. Reduction or total elimination of cytoplasmic SHMT had a surprisingly moderate effect on metabolite concentrations and reaction velocities. This corresponds to the experimental findings that a reduction in the enzymatic activity of SHMT has little effect on 1-carbon metabolism. Our simulations showed that the primary function of SHMT was to increase the rate by which the glycine-serine balance was reequilibrated after a perturbation.

A mathematical model of the folate cycle: new insights into folate homeostasis

A mathematical model is developed for the folate cycle based on standard biochemical kinetics. We use the model to provide new insights into several different mechanisms of folate homeostasis. The model reproduces the known pool sizes of folate substrates and the fluxes through each of the loops of the folate cycle and has the qualitative behavior observed in a variety of experimental studies. Vitamin B(12) deficiency, modeled as a reduction in the V(max) of the methionine synthase reaction, results in a secondary folate deficiency via the accumulation of folate as 5-methyltetrahydrofolate (the "methyl trap"). One form of homeostasis is revealed by the fact that a 100-fold up-regulation of thymidylate synthase and dihydrofolate reductase (known to occur at the G(1)/S transition) dramatically increases pyrimidine production without affecting the other reactions of the folate cycle. The model also predicts that an almost total inhibition of dihydrofolate reductase is required to significantly inhibit the thymidylate synthase reaction, consistent with experimental and clinical studies on the effects of methotrexate. Sensitivity to variation in enzymatic parameters tends to be local in the cycle and inversely proportional to the number of reactions that interconvert two folate substrates. Another form of homeostasis is a consequence of the nonenzymatic binding of folate substrates to folate enzymes. Without folate binding, the velocities of the reactions decrease approximately linearly as total folate is decreased. In the presence of folate binding and allosteric inhibition, the velocities show a remarkable constancy as total folate is decreased.

Location of the pteroylpolyglutamate-binding site on rabbit cytosolic serine hydroxymethyltransferase

Serine hydroxymethyltransferase (SHMT; EC 2.1.2.1) catalyzes the reversible interconversion of serine and glycine with transfer of the serine side chain one-carbon group to tetrahydropteroylglutamate (H(4)PteGlu), and also the conversion of 5,10-methenyl-H(4)PteGlu to 5-formyl-H(4)PteGlu. In the cell, H(4)PteGlu carries a poly-gamma-glutamyl tail of at least 3 glutamyl residues that is required for physiological activity. This study combines solution binding and mutagenesis studies with crystallographic structure determination to identify the extended binding site for tetrahydropteroylpolyglutamate on rabbit cytosolic SHMT. Equilibrium binding and kinetic measurements of H(4)PteGlu(3) and H(4)PteGlu(5) with wild-type and Lys --> Gln or Glu site mutant homotetrameric rabbit cytosolic SHMTs identified lysine residues that contribute to the binding of the polyglutamate tail. The crystal structure of the enzyme in complex with 5-formyl-H(4)PteGlu(3) confirms the solution data and indicates that the conformation of the pteridine ring and its interactions with the enzyme differ slightly from those observed in complexes of the monoglutamate cofactor. The polyglutamate chain, which does not contribute to catalysis, exists in multiple conformations in each of the two occupied binding sites and appears to be bound by the electrostatic field created by the cationic residues, with only limited interactions with specific individual residues.

Folic acid: nutritional biochemistry, molecular biology, and role in disease processes

This paper reviews the chemistry, metabolism, and molecular biology of folic acid, with a particular emphasis on how it is, or may be, involved in many disease processes. Folic acid prevents neural tube defects like spina bifida, while its ability to lower homocysteine suggests it might have a positive influence on cardiovascular disease. A role for this B vitamin in maintaining good health may, in fact, extend beyond these clinical conditions to encompass other birth defects, several types of cancer, dementia, affective disorders, Down's syndrome, and serious conditions affecting pregnancy outcome. The effect of folate in these conditions can be explained largely within the context of folate-dependent pathways leading to methionine and nucleotide biosynthesis, and genetic variability resulting from a number of common polymorphisms of folate-dependent enzymes involved in the homocysteine remethylation cycle. Allelic variants of folate genes that have a high frequency in the population, and that may play a role in disease formation include 677C --> T-MTHFR, 1298A --> C-MTHFR, 2756A --> G-MetSyn, and 66A --> G-MSR. Future work will probably uncover further polymorphisms of folate metabolism, and lead to a wider understanding of the interaction between this essential nutrient and the many genes which underpin its enzymatic utilization in a plethora of critical biosynthetic reactions, and which, under adverse nutritional conditions, may promote disease.

Altered folate metabolism and disposition in mothers affected by a spina bifida pregnancy: influence of 677c --> t methylenetetrahydrofolate reductase and 2756a --> g methionine synthase genotypes

Periconceptional folate prevents spina bifida although the mechanisms involved are unclear. We present the genotype frequency for the 677 ct methylenetetrahydrofolate reductase (MTHFR) and 2756ag methionine synthase (MetSyn) polymorphisms. Calculated odds ratios (OR) show that neither the homozygous recessive genotype, carriage of the mutant allele, nor frequency of the mutant allele represent significantly increased risk for neural tube defect (NTD). This is true for both polymorphisms. Simultaneous carriage of t and g alleles is also not a significantly increased risk for NTD. OR and 95% CI for carriage of (i) t allele, (ii) g allele, and (iii) simultaneous carriage of t and g alleles in NTD are 0.89 (0.28-2.82), 0.97 (0.28-3.30), and 0.61 (0.11-3.52), respectively. OR and 95% CI for frequency of t and g alleles are 0.94 (0.42-2.13) and 0.88 (0. 29-2.67), respectively. Unlike some previous studies, we could not detect a significantly increased risk for NTD conferred by the 677ct MTHFR tt genotype; OR 0.98 (0.19-6.49). Differences were found to exist in the circulating whole blood folate profile: total formyl-H(4)PteGlu was significantly higher than total 5-methyl-H(4)PteGlu in control (P = 0.036) but not NTD blood. When broken down into the various 677 ct MTHFR and 2756ag MetSyn genotypes, carriage of the 677ct MTHFR allele appears to affect formyl-H(4)PteGlu metabolism in non-NTD mothers. In addition, NTD mothers exhibited noticeably lower formyl-H(4)PteGlu levels compared to controls; these effects, however, were not significant. 2756ag MetSyn is similarly associated with an altered formyl-H(4)PteGlu disposition. The ag genotype had significantly more formyl-H(4)PteGlu relative to 5-methyl-H(4)PteGlu than wildtype 2756ag MetSyn (P = 0.024). This heterozygous increase in the relative formyl-H(4)PteGlu level holds true for controls only; no such relationship occurred in NTD samples. Folyl hexaglutamates are the active cellular coenzyme forms. We showed that where 5-methyl-H(4)PteGlu(6) predominates, Hcy levels are highest. As the relative abundance of formyl-H(4)PteGlu(6) increased, so Hcy decreased, presumably due to increased Hcy remethylation, a process in which 5-methyl-H(4)PteGlu(6) is demethylated and downstream folates like formyl-H(4)PteGlu(6) are produced. The negative linear association between the hexaglutamate ratio (formyl-H(4)PteGlu(6)/5-methyl-H(4)PteGlu(6)) and Hcy is significant for control (r = -0.64, P = 0.003) but not NTD samples. This effect, centering on Hcy remethylation, is supported by a statistically elevated formyl-H(4)PteGlu(6) to 5-methyl-H(4)PteGlu(6) level in controls relative to NTDs (P = 0.047). The overall (polymorphism independent) effect of exogenous 5,10-methenyl-H(4)PteGlu(1) substrate on the cellular folate profile was to preferentially increase formyl-H(4)PteGlu, while exogenous 5-methyl-H(4)PteGlu(1) substrate dramatically increased metabolic production of 5, 10-methylene-H(4)PteGlu. The following differences were observed between NTD and control samples: (i) a reduced expansion of the formyl-H(4)PteGlu(6) pool in NTD with exogenous 5, 10-methenyl-H(4)PteGlu(1) (P = 0.0005 for control expansion, NS for NTD increase); (ii) a reduced initial expansion of the 5, 10-methylene-H(4)PteGlu pool in NTD following treatment with exogenous 5-methyl-H(4)PteGlu(1) substrate (difference between subject groups; P = 0.031). In addition, taking polymorphisms into account, lysate from NTD-MTHFR wildtypes utilized less exogenous 5-methyl-H(4)PteGlu(1) substrate than control-MTHFR wildtypes in the short (P = 0.011) and long term (P = 0.036). Commensurate with this latter effect, the initial production of 5,10-methylene-H(4)PteGlu due to exogenous 5-methyl-H(4)PteGlu(1) substrate was significantly reduced in the NTD-MTHFR wildtype (P = 0.037). These two MTHFR wildtype effects imply that the 677 ct polymorphism is not the only mutation affecting folate metabolism in NTD mothers. (ABSTRACT TRUNCATED)

Serine hydroxymethyltransferase and threonine aldolase: are they identical?

Serine hydroxymethyltransferase, a pyridoxal phosphate-dependent enzyme, catalyses the interconversion of serine and glycine, both of which are major sources of one-carbon units necessary for the synthesis of purine, thymidylate, methionine, and so on. Threonine aldolase catalyzes the pyridoxal phosphate-dependent, reversible reaction between threonine and acetaldehyde plus glycine. No extensive studies have been carried out on threonine aldolase in animal tissues, and it has long been believed that serine hydroxymethyltransferase and threonine aldolase are the same, i.e. one entity. This is based on the finding that rabbit liver serine hydroxymethyltransferase possesses some threonine aldolase activity. Recently, however, many kinds of threonine aldolase and corresponding genes were isolated from micro-organisms, and these enzymes were shown to be distinct from serine hydroxymethyltransferase. The experiments with isolated hepatocytes and cell-free extracts from various animals revealed that threonine is degraded mainly through the pathway initiated by threonine 3-dehydrogenase, and there is little or no contribution by threonine aldolase. Thus, although serine hydroxymethyltransferase from some mammalian livers exhibits a low threonine aldolase activity, the two enzymes are distinct from each other and mammals lack the "genuine" threonine aldolase.

Crystal structure at 2.4 A resolution of E. coli serine hydroxymethyltransferase in complex with glycine substrate and 5-formyl tetrahydrofolate

Serine hydroxymethyltransferase (EC 2.1.2.1), a member of the alpha-class of pyridoxal phosphate enzymes, catalyzes the reversible interconversion of serine and glycine, changing the chemical bonding at the C(alpha)-C(beta) bond of the serine side-chain mediated by the pyridoxal phosphate cofactor. Scission of the C(alpha)-C(beta) bond of serine substrate produces a glycine product and most likely formaldehyde, which reacts without dissociation with tetrahydropteroylglutamate cofactor. Crystal structures of the human and rabbit cytosolic serine hydroxymethyltransferases (SHMT) confirmed their close similarity in tertiary and dimeric subunit structure to each other and to aspartate aminotransferase, the archetypal alpha-class pyridoxal 5'-phosphate enzyme. We describe here the structure at 2.4 A resolution of Escherichia coli serine hydroxymethyltransferase in ternary complex with glycine and 5-formyl tetrahydropteroylglutamate, refined to an R-factor value of 17.4 % and R(free) value of 19.6 %. This structure reveals the interactions of both cofactors and glycine substrate with the enzyme. Comparison with the E. coli aspartate aminotransferase structure shows the distinctions in sequence and structure which define the folate cofactor binding site in serine hydroxymethyltransferase and the differences in orientation of the amino terminal arm, the evolution of which was necessary for elaboration of the folate binding site. Comparison with the unliganded rabbit cytosolic serine hydroxymethyltransferase structure identifies changes in the conformation of the enzyme, similar to those observed in aspartate aminotransferase, that probably accompany the binding of substrate. The tetrameric quaternary structure of liganded E. coli serine hydroxymethyltransferase also differs in symmetry and relative disposition of the functional tight dimers from that of the unliganded eukaryotic enzymes. SHMT tetramers have surface charge distributions which suggest distinctions in folate binding between eukaryotic and E. coli enzymes. The structure of the E. coli ternary complex provides the basis for a thorough investigation of its mechanism through characterization and structure determination of site mutants.

Mechanisms of acquired resistance to modulation of 5-fluorouracil by leucovorin in HCT-8 human ileocecal carcinoma cells

Repeated (10x) exposure of HCT-8 human ileocecal carcinoma cells to 5-fluorouracil (5-FU) for 2 or 72 hr, both incubations in the continuous presence of 20 microM leucovorin (LV), yielded two stable modulation-resistant sublines, FL2h and FL72h. Although LV potentiated growth inhibition by 5-FU 2-fold in parental HCT-8 cells, it did not potentiate the effect of 5-FU in the FL2h or FL72h sublines. LV modulation of 5-fluorodeoxyuridine (5-FdUrd) was also reduced (FL72h) or eliminated (FL2h). In the FL2h and FL72h sublines, the level of thymidylate synthase (TS) protein and TS activity in cell extracts, TS activity in situ, the rate of cellular uptake and metabolism of LV, and the level of 5-FU incorporation into total cellular RNA were similar to those in parental HCT-8 cells. However, LV significantly (P < 0.01) potentiated the inhibition of TS activity in situ in HCT-8 cells at 24 hr after a 2-hr treatment with either 5-FU or 5-FdUrd, but had no such activity in the FL2h and FL72h sublines (P > 0.1). Resistance to modulation of 5-FU by LV was associated with the inability of LV to increase the formation of intracellular TS-FdUMP-methylenetetrahydrofolate ternary complexes, and these complexes dissociated more rapidly (T1/2 > 1.5- to 3-fold faster) in the presence of different concentrations of 5,10-methylenetetrahydropteroylpentaglutamate. Thus, decreased stability of ternary complexes appears to be the mechanism of acquired resistance to the LV modulation of fluoropyrimidine cytotoxicity, possibly due to mutation(s) of TS in these two modulation-resistant HCT-8 sublines.

4-Chlorothreonine is substrate, mechanistic probe, and mechanism-based inactivator of serine hydroxymethyltransferase

Serine hydroxymethyltransferase catalyzes the cleavage of a variety of beta-hydroxy-L-amino acids to form glycine and aldehyde products. 4-chloro-L-threonine has been synthesized and shown to be both a substrate and a mechanism-based inactivator of serine hydroxymethyltransferase. kcat values for the formation of glycine in the absence of tetrahydrofolate were determined for 4-chloro-L-threonine and other beta-hydroxyamino acid substrates; an inverse relationship between the rate of cleavage of the amino acid and the electrophilicity of the product aldehyde was demonstrated. 4-Chloro-L-threonine inactivates serine hydroxymethyltransferase in a time- and concentration-dependent manner and exhibits saturation of the rate of inactivation at high concentrations. Our evidence suggests that 4-chlorothreonine undergoes aldol cleavage, and generation of chloroacetaldehyde at the active site of the enzyme results in inactivation. Serine or glycine protect the enzyme against inactivation by chlorothreonine, while tetrahydrofolate does not. The enzyme is also protected from inactivation by 2-mercaptoethanol or by alcohol dehydrogenase and NADH. These studies suggest that halothreonine derivatives that generate electrophilic aldehyde products will be effective inhibitors of serine hydroxymethyltransferase and might be potentially useful chemotherapeutic agents.

Characterization of the folate-dependent mitochondrial oxidation of carbon 3 of serine

The folate-dependent catabolism of serine was studied in intact rat liver mitochondria and soluble extracts from sonicated mitochondria. Formate and CO2 are both known to be products of the mitochondrial oxidation of carbon 3 of serine. The present work tests the proposal [Barlowe, C. K., & Appling, D. R. (1988) Biofactors 1, 171-176] that carbon 3 of serine is first oxidized to 10-formyltetrahydrofolate, which can be either oxidized to CO2 or converted to formate. Oxidation of carbon 3 of serine to formate and CO2 was shown to be dependent on the respiratory state of the mitochondria. Formate production was greatest in state-3 (actively respiring) mitochondria and lowest in uncoupled mitochondria. In contrast, CO2 production was greatest in uncoupled mitochondria and lowest in respiratory-inhibited mitochondria. Formate production appeared to be favored when high concentrations of NADP+ and ADP were present, but there was no clear correlation between the NADP+:NADPH redox state and CO2 production. In soluble mitochondrial extracts, CO2 production depended on NADP+ and tetrahydrofolate, whereas formate production required ADP in addition to NADP+ and the reduced folate cofactor. Unlike CO2 production, however, formate production showed a complete dependence on a polyglutamylated form of the folate cofactor. These experiments support the proposed folate-mediated serine oxidation as a major pathway for the flux of one-carbon units through mitochondria.

Interaction of folylpolyglutamates with enzymes in one-carbon metabolism

Of all the coenzymes, tetrahydrofolate exhibits the most structural diversity. The relationship of these structural forms to physiological function is under intense study by numerous research groups. In textbooks, tetrahydrofolate (tetrahydropteroylmonoglutamate) is shown as the coenzyme of one-carbon metabolism, but it has been known for several decades that the physiologically active forms of the coenzyme contain from 4 to 7 glutamyl residues linked by amide bonds through the gamma-carboxyl group. These glutamyl residues do not serve a direct function in transferring the one-carbon group. The tetrahydrofolylpolyglutamates were originally thought to be simply storage forms of the coenzyme, but studies now show that the polyglutamate chain of the coenzyme affects the transport properties of the coenzyme, alters the kinetic properties of many enzymes in one-carbon metabolism, and results in channeling of the coenzyme between several enzymes. In general, the dissociation constants of this group of enzymes for the tetrahydrofolylpolyglutamates are very low, in the 0.1 to 1 microM range. The concentration of the coenzyme in the cell appears to be similar to the concentration of folate-utilizing enzymes, suggesting that the concentration of unbound coenzyme in the cell may be very low. Several of the enzymes in one-carbon metabolism are either multifunctional proteins or multienzyme complexes. An active area of research is to determine if there is a functional relationship between these multifunctional enzymes and the polyglutamate portion of the coenzyme.

Folylpolyglutamate synthesis and role in the regulation of one-carbon metabolism

The physiological importance of folylpolyglutamates is now well established. These derivatives are the intracellular substrates and regulators of one-carbon metabolism, and their synthesis is required for normal folate retention by tissues. Over the last few years, a considerable amount of information has been obtained on the mechanism by which these compounds are synthesized, on how this synthesis is regulated, and on the effects of the polyglutamate chain on the interaction of folate substrates and inhibitors with folate-dependent enzymes. Many regulatory implications have been suggested by these studies, but the physiological relevance of some of these observations remains to be explored. Folates in mammalian tissues are metabolized to polyglutamates of chain lengths considerably longer than that required for folate retention, but the metabolic advantages of this are not entirely clear. Several in vivo model systems have been developed to explore the functioning of specific folylpolyglutamate chain lengths in metabolic cycles of one-carbon metabolism, and these are likely to shed further light on this point. The role of folate-binding proteins in folate transport, the metabolic role of glutamylhydrolases, and the role of folylpolyglutamates in putative multifunctional protein complexes are also areas that are being actively pursued at present and are likely to produce new insights in the future. Recent studies on the retention of antifolates by cells and on their substrate efficacy for folylpolyglutamate synthetases have also suggested mechanisms for the differential cytotoxicity of these agents for different tissues.

Evaluation of pteroyl-S-alkylhomocysteine sulfoximines as inhibitors of mammalian folylpolyglutamate synthetase

The similarity between the reactions catalyzed by folylpoly-gamma-glutamate synthetase (FPGS), gamma-glutamylcysteine synthetase, and glutamine synthetase, as well as the susceptibility of the latter two enzymes to inhibition by methionine sulfoximine, suggest that folic acid derivatives with methionine sulfoximine or its alkyl homologs in place of the glutamate side chain of folate are good candidates to act as enzyme-generated transition state analog inhibitors of the FPGS reaction. Thus, pteroylmethionine sulfoximine, and the homologous S-ethyl-, S-propyl-, and S-butylhomocysteine sulfoximine derivatives were evaluated as inhibitors of FPGS that was partially purified from mouse liver and from mouse L1210 cells. The related compound, pteroyl-S-methylhomocysteine sulfone, which cannot undergo enzyme-mediated activation, was also investigated. Unexpectedly, none of these compounds showed significant inhibition of FPGS from these sources under a variety of conditions. These results, taken together with previously established structure-activity correlations, imply that a negative charge at the gamma-position of folate analogs may be required for initial binding to FPGS and thus constitutes a prerequisite for activity of potential mechanism-based inhibitors of this enzyme.

Folylpolyglutamates as substrates and inhibitors of folate-dependent enzymes

The true intracellular substrates for folate-dependent enzymes are folylpolyglutamates. We have used measurements of the Ki values of folylpolyglutamate dead end inhibitors to assess the relative affinities of folate-dependent enzymes for folate derivatives of different polyglutamate chain lengths. Studies of four enzymes from pig liver, methylenetetrahydrofolate reductase, serine hydroxymethyltransferase, methylenetetrahydrofolate dehydrogenase and thymidylate synthase, have indicated that folylpolyglutamate inhibitors are bound 3-500 fold more tightly than the corresponding monoglutamates. The individual enzymes differ in their selectivity for polyglutamate vs. monoglutamate inhibitors, and in the chain length associated with the greatest affinity of enzyme for inhibitor. We have also examined the effect of polyglutamate chain length on the catalytic parameters associated with folate substrates. Two enzymes, methylenetetrahydrofolate reductase and serine hydroxymethyltransferase, show decreases in Km values for folylpolyglutamate substrates. Methylenetetrahydrofolate dehydrogenase shows no detectable differences in the catalytic parameters of polyglutamate vs. monoglutamate substrates and no change in the order of substrate addition or product release. Thymidylate synthase shows small effects of Km and Vmax values, but the order of addition of substrates and of release of products is reversed with polyglutamate as compared with monoglutamate substrates. Our studies with thymidylate synthase from L. casei have shown that the bacterial enzyme also exhibits a greatly increased affinity for polyglutamate vs. monoglutamate derivatives of folic acid, and that reversal in the order of substrate addition and product release also occurs with polyglutamate as compared with monoglutamate substrates. We have also studied the polyglutamate specificity of methionine synthase, which is responsible for the conversion of CH3-H4PteGlu1 into H4PteGlu1. This reaction is required for the incorporation of plasma folate into the cellular folate pool, because methyltetrahydrofolate is a poor substrate for folylpolyglutamate synthetase. Our studies demonstrate that CH3-H4PteGlu6, and suggest that incorporation of plasma CH3-H4PteGlu1 will only occur when methylenetetrahydrofolate reductase is inhibited by adenosylmethionine and cellular pools of CH3-H4PteGlu6 are at very low levels.

Preparation of tritium-labeled tetrahydropteroylpolyglutamates of high specific radioactivity

Tritium-labeled [6S]-tetrahydropteroylpolyglutamates of high radiospecific activity were prepared from the corresponding pteroylpolyglutamates. Malic enzyme and D,L-[2-3H]malate were used as a generating system to produce [4A-3H]NADPH which was coupled to the dihydrofolate reductase-catalyzed reduction of chemically prepared dihydropteroylpolyglutamate derivatives. Passage of the reaction mixtures through a column of immobilized boronate effectively removed NADPH, and the tetrahydropteroylpolyglutamates were subsequently purified by chromatography on DEAE-cellulose. Overall yields of the [6S]-tetrahydro derivatives were 18-48% and the radiospecific activities were 3-4.5 mCi X mumol-1.