The primitive protozoon Trichomonas vaginalis contains two methionine gamma-lyase genes that encode members of the gamma-family of pyridoxal 5'-phosphate-dependent enzymes

The potential of methioninase for cancer treatment

Cancer cells are addicted to L-methionine (L-Met) and have a much greater requirement for L-Met than normal cells due to excess transmethylation, termed the Hoffman effect. By targeting this vulnerability through dietary restriction of L-Met, researchers have been able to achieve promising results in inhibiting tumor growth and eradicating cancer cells. Methioninase (EC 4.4.1.11; METase) catalyzes the transformation of L-Met into α-ketobutyrate, ammonia, and methanethiol. The use of METase was initially limited due to its poor stability in vivo, high immunogenicity, and enzyme-induced inactivating antibodies. These issues could be partially resolved by PEGylation, encapsulation in erythrocytes, and various site-directed mutagenesis. The big breakthrough came when it was discovered that METase is effectively administered orally. The enzyme L-asparaginase is approved by the FDA for treatment of acute lymphoblastic leukemia. METase has more potential as a therapeutic since addiction to L-Met is a general and fundamental hallmark of cancer.

Sulfoxides of sulfur-containing amino acids are suicide substrates of Citrobacter freundii methionine γ-lyase. Structural bases of the enzyme inactivation

Interactions of Citrobacter freundii methionine γ-lyase (MGL) with sulfoxides of typical substrates were investigated. It was found that sulfoxides are suicide substrates of the enzyme. The products of the β- and γ-elimination reactions of sulfoxides, thiosulfinates, oxidize three cysteine residues of the enzyme. Three-dimensional structures of MGL inactivated by dimethyl thiosulfinate and diethyl thiosulfinate were determined at 1.46 Å and 1.59 Å resolution. Analysis of the structures identified SH groups oxidized by thiosulfinates and revealed the structural bases of MGL inactivation. The extent of inactivation of MGL in the catalysis of the β-elimination reaction depends on the length of the «tail» at oxidized Cys115. Oxidation of Cys115 results in MGL incapable to catalyze the stage of methyl mercaptan elimination of the physiological reaction.

Metabolomic profiling and stable isotope labelling of Trichomonas vaginalis and Tritrichomonas foetus reveal major differences in amino acid metabolism including the production of 2-hydroxyisocaproic acid, cystathionine and S-methylcysteine

Trichomonas vaginalis and Tritrichomonas foetus are pathogens that parasitise, respectively, human and bovine urogenital tracts causing disease. Using LC-MS, reference metabolomic profiles were obtained for both species and stable isotope labelling with D-[U-13C6] glucose was used to analyse central carbon metabolism. This facilitated a comparison of the metabolic pathways of T. vaginalis and T. foetus, extending earlier targeted biochemical studies. 43 metabolites, whose identities were confirmed by comparison of their retention times with authentic standards, occurred at more than 3-fold difference in peak intensity between T. vaginalis and T. foetus. 18 metabolites that were removed from or released into the medium during growth also showed more than 3-fold difference between the species. Major differences were observed in cysteine and methionine metabolism in which homocysteine, produced as a bi-product of trans-methylation, is catabolised by methionine γ-lyase in T. vaginalis but converted to cystathionine in T. foetus. Both species synthesise methylthioadenosine by an unusual mechanism, but it is not used as a substrate for methionine recycling. T. vaginalis also produces and exports high levels of S-methylcysteine, whereas only negligible levels were found in T. foetus which maintains significantly higher intracellular levels of cysteine. 13C-labeling confirmed that both cysteine and S-methylcysteine are synthesised by T. vaginalis; S-methylcysteine can be generated by recombinant T. vaginalis cysteine synthase using phosphoserine and methanethiol. T. foetus contained higher levels of ornithine and citrulline than T. vaginalis and exported increased levels of putrescine, suggesting greater flux through the arginine dihydrolase pathway. T. vaginalis produced and exported hydroxy acid derivatives of certain amino acids, particularly 2-hydroxyisocaproic acid derived from leucine, whereas negligible levels of these metabolites occurred in T. foetus.

Structure of methionine γ-lyase from Clostridium sporogenes

Methionine γ-lyase (MGL) is a pyridoxal 5'-phosphate-dependent enzyme that catalyzes the γ-elimination reaction of L-methionine. The enzyme is a promising target for therapeutic intervention in some anaerobic pathogens and has attracted interest as a potential cancer treatment. The crystal structure of MGL from Clostridium sporogenes has been determined at 2.37 Å resolution. The fold of the protein is similar to those of homologous enzymes from Citrobacter freundii, Entamoeba histolytica, Pseudomonas putida and Trichomonas vaginalis. A comparison of these structures revealed differences in the conformation of two flexible regions of the N- and C-terminal domains involved in the active-site architecture.

Phylogenetic analysis of methionine synthesis genes from Thalassiosira pseudonana

Diatoms are unicellular algae responsible for approximately 20% of global carbon fixation. Their evolution by secondary endocytobiosis resulted in a complex cellular structure and metabolism compared to algae with primary plastids. The sulfate assimilation and methionine synthesis pathways provide S-containing amino acids for the synthesis of proteins and a range of metabolites such as dimethylsulfoniopropionate. To obtain an insight into the localization and organization of the sulfur metabolism pathways we surveyed the genome of Thalassiosira pseudonana-a model organism for diatom research. We have identified and annotated genes for enzymes involved in respective pathways. Protein localization was predicted using similarities to known signal peptide motifs. We performed detailed phylogenetic analyses of enzymes involved in sulfate uptake/reduction and methionine metabolism. Moreover, we have found in up-stream sequences of studied diatoms methionine biosynthesis genes a conserved motif, which shows similarity to the Met31, a cis-motif regulating expression of methionine biosynthesis genes in yeast.

L-methionase: a therapeutic enzyme to treat malignancies

Cancer is an increasing cause of mortality and morbidity throughout the world. L-methionase has potential application against many types of cancers. L-Methionase is an intracellular enzyme in bacterial species, an extracellular enzyme in fungi, and absent in mammals. L-Methionase producing bacterial strain(s) can be isolated by 5,5′-dithio-bis-(2-nitrobenzoic acid) as a screening dye. L-Methionine plays an important role in tumour cells. These cells become methionine dependent and eventually follow apoptosis due to methionine limitation in cancer cells. L-Methionine also plays an indispensable role in gene activation and inactivation due to hypermethylation and/or hypomethylation. Membrane transporters such as GLUT1 and ion channels like Na²⁺, Ca²⁺, K⁺, and Cl⁻ become overexpressed. Further, the α-subunit of ATP synthase plays a role in cancer cells growth and development by providing them enhanced nutritional requirements. Currently, selenomethionine is also used as a prodrug in cancer therapy along with enzyme methionase that converts prodrug into active toxic chemical(s) that causes death of cancerous cells/tissue. More recently, fusion protein (FP) consisting of L-methionase linked to annexin-V has been used in cancer therapy. The fusion proteins have advantage that they have specificity only for cancer cells and do not harm the normal cells.

Comparative proteomic analysis on wild type and nitric oxide-overproducing mutant (nox1) of Arabidopsis thaliana

Nitric oxide (NO) as a ubiquitous signal molecule plays an important role in plant development and growth. Here, we compared the proteomic changes between NO-overproducing mutant (nox1) and wild-type (WT) of Arabidopsis thaliana using two-dimensional electrophoresis coupled with MALDI-TOF MS. We successfully identified 59 differentially expressed proteins in nox1 mutant, which are predicted to play potential roles in specific cellular processes, such as post-translational modification, energy production and conversion, metabolism, transcription and signal transduction, cell rescue and defense, development and differentiation. Particularly, expression levels of five anti-oxidative enzymes were altered by the mutation; and assays of their respective enzymatic activities indicated an enhanced level of oxidative stress in nox1 mutant. Finally, some important proteins were further confirmed at transcriptional level using quantitative real-time PCR revealing the systemic changes between WT and nox1. The result suggests that obvious morphological changes in the nox1 mutant may be regulated by different mechanisms and factors, while excess endogenous NO maybe one of the possible reasons.

Improving the nutritive value of rice seeds: elevation of cysteine and methionine contents in rice plants by ectopic expression of a bacterial serine acetyltransferase

With the aim of increasing the cysteine level in rice (Oryza sativa L.) and thus improving its nutritional quality, transgenic rice plants were generated expressing an Escherichia coli serine acetyltransferase isoform (EcSAT), the enzyme synthesizing O-acetylserine, the precursor of cysteine. The gene was fused to the transit peptide of the Arabidopsis Rubisco and driven by a ubiquitin promoter to target the enzyme to plastids. Twenty-two transgenic plants were examined for transgene protein expression, and five lines with a high expression level and enzymatic activity, respectively, were selected for further analysis. In these lines, the contents of cysteine and glutathione increased 2.4-fold and 2-fold, respectively. More important is the increase in free methionine and methionine incorporated into the water-soluble protein fraction in seeds. Free methionine increased in leaves up to 2.7-fold, in seeds up to 1.4-fold, and bound to seed proteins up to 4.8-fold, respectively, while the bound methionine level remained constant or even decreased in leaves. Notably, the transgenic lines exhibited higher isoleucine, leucine, and valine contents (each up to 2-fold depending on tissue, free, or bound), indicating a potential conversion of methionine via methionine γ-lyase to isoleucine. As the transgenic rice plants overexpressing EcSAT had significantly higher levels of both soluble and protein-bound methionine, isoleucine, cysteine, and glutathione in rice they may represent a model and target system for improving the nutritional quality of cereal crops.

Transcriptomic identification of iron-regulated and iron-independent gene copies within the heavily duplicated Trichomonas vaginalis genome

Gene duplication is an important evolutionary mechanism and no eukaryote has more duplicated gene families than the parasitic protist Trichomonas vaginalis. Iron is an essential nutrient for Trichomonas and plays a pivotal role in the establishment of infection, proliferation, and virulence. To gain insight into the role of iron in T. vaginalis gene expression and genome evolution, we screened iron-regulated genes using an oligonucleotide microarray for T. vaginalis and by comparative EST (expressed sequence tag) sequencing of cDNA libraries derived from trichomonads cultivated under iron-rich (+Fe) and iron-restricted (−Fe) conditions. Among 19,000 ESTs from both libraries, we identified 336 iron-regulated genes, of which 165 were upregulated under +Fe conditions and 171 under −Fe conditions. The microarray analysis revealed that 195 of 4,950 unique genes were differentially expressed. Of these, 117 genes were upregulated under +Fe conditions and 78 were upregulated under −Fe conditions. The results of both methods were congruent concerning the regulatory trends and the representation of gene categories. Under +Fe conditions, the expression of proteins involved in carbohydrate metabolism, particularly in the energy metabolism of hydrogenosomes, and in methionine catabolism was increased. The iron–sulfur cluster assembly machinery and certain cysteine proteases are of particular importance among the proteins upregulated under −Fe conditions. A unique feature of the T. vaginalis genome is the retention during evolution of multiple paralogous copies for a majority of all genes. Although the origins and reasons for this gene expansion remain unclear, the retention of multiple gene copies could provide an opportunity to evolve differential expression during growth in variable environmental conditions. For genes whose expression was affected by iron, we found that iron influenced the expression of only some of the paralogous copies, whereas the expression of the other paralogs was iron independent. This finding indicates a very stringent regulation of the differentially expressed paralogous genes in response to changes in the availability of exogenous nutrients and provides insight into the evolutionary rationale underlying massive paralog retention in the Trichomonas genome.

The role of amino acid residues in the active site of L-methionine γ-lyase from Pseudomonas putida

Cys116, Lys240*, and Asp241* (asterisks indicate residues from the second subunit of the active dimer) at the active site of L-methionine γ-lyase of Pseudomonas putida (MGL_Pp) are highly conserved among heterologous MGLs. In a previous study, we found that substitution of Cys116 for His led to a drastic increase in activity toward L-cysteine and a decrease in that toward L-methionine. In this study, we examined some properties of the C116H mutant by kinetic analysis and 3D structural analysis. We assumed that substitution of Cys116 for His broke the original hydrogen-bond network and that this induced a significant effect of Tyr114 as a general acid catalyst, possibly due to the narrow space in the active site. The C116H mutant acquired a novel β-elimination activity and lead a drastic conformation change in the histidine residue at position 116 by binding the substrate, suggesting that this His residue affects the reaction specificity of C116H. Furthermore, we suggest that Lys240* is important for substrate recognition and structural stability and that Asp241* is also involved in substrate specificity in the elimination reaction. Based on this, we suggest that the hydrogen-bond network among Cys116, Lys240*, and Asp241* contributes to substrate specificity that is, to L-methionine recognition at the active site in MGL_Pp.

PLP-dependent enzymes as potential drug targets for protozoan diseases

The chemical properties of the B(6) vitamers are uniquely suited for wide use as cofactors in essential reactions, such as decarboxylations and transaminations. This review addresses current efforts to explore vitamin B(6) dependent enzymatic reactions as drug targets. Several current targets are described that are found amongst these enzymes. The focus is set on diseases caused by protozoan parasites. Comparison across a range of these organisms allows insight into the distribution of potential targets, many of which may be of interest in the development of broad range anti-protozoan drugs. This article is part of a Special Issue entitled: Pyridoxal Phosphate Enzymology.

Mechanistic studies on the enzymatic processing of fluorinated methionine analogues by Trichomonas vaginalis methionine γ-lyase

TFM (L-trifluoromethionine), a potential prodrug, was reported to be toxic towards human pathogens that express MGL (L-methionine γ-lyase; EC 4.4.1.11), a pyridoxal phosphate-containing enzyme that converts L-methionine into α-oxobutyrate, ammonia and methyl mercaptan. It has been hypothesized that the extremely reactive thiocarbonyl difluoride is produced when the enzyme acts upon TFM, resulting in cellular toxicity. The potential application of the fluorinated thiomethyl group in other areas of biochemistry and medicinal chemistry requires additional studies. Therefore a detailed investigation of the theoretical and experimental chemistry and biochemistry of these fluorinated groups (CF3S− and CF2HS−) has been undertaken to trap and identify chemical intermediates produced by enzyme processing of molecules containing these fluorinated moieties. TvMGL (MGL from Trichomonas vaginalis) and a chemical model system of the reaction were utilized in order to investigate the cofactor-dependent activation of TFM and previously uninvestigated DFM (L-difluoromethionine). The differences in toxicity between TFM and DFM were evaluated against Escherichia coli expressing TvMGL1, as well as the intact human pathogen T. vaginalis. The relationship between the chemical structure of the reactive intermediates produced from the enzymatic processing of these analogues and their cellular toxicity are discussed.

Tea Catechin EGCg Suppresses the mgl Gene Associated with Halitosis

Epigallocatechin gallate (EGCg), the main antimicrobial tea catechin, has been reported to inhibit growth and virulence factors of oral pathogens in vitro. Although the mechanism is unclear, the potential of EGCg in reducing halitosis caused by volatile sulfur compounds (VSCs) has been suggested. This study tested the hypothesis that EGCg reduces VSCs by suppressing mgl, the gene encoding L-methionine-α-deamino-γ-mercaptomethane-lyase, responsible for methyl mercaptan (CH3SH) production by oral anaerobes. In this study, the effect of EGCg on in vitro growth, CH3SH production, and mgl gene expression in P. gingivalis W83 was investigated. EGCg inhibited growth of P. gingivalis W83 (MIC = 97.5 µg/mL) and was bactericidal (MBC = 187.5 µg/mL). At sub-MIC levels, EGCg inhibited CH3SH production, and mgl mRNA and protein expression (p < 0.05). We conclude that EGCg may represent a natural and alternative agent to the antimicrobial chemicals currently available for halitosis control.

Exploring methionine γ-lyase structure-function relationship via microspectrophotometry and X-ray crystallography

Pyridoxal 5'-phosphate (PLP) dependent methionine γ-lyase catalyzes the breakdown of L-methionine to α-ketobutyric acid, methanethiol and ammonia. This enzyme, present in anaerobic microorganisms, has biomedical interest both for its activity as antitumor agent, depleting methionine supply in methionine-dependent cancers, and as target in the treatment of human pathogen infections, activating the pro-drug trifluoromethionine. To validate the structure of the enzyme from Citrobacter freundii, crystallized from monomethyl ether polyethylene glycol 2000, for the development of lead compounds, the reactivity of the crystalline enzyme towards L-methionine, substrate analogs and inhibitors was determined by polarized absorption microspectrophotometry. Spectral data were also collected for enzyme crystals, grown in monomethyl ether polyethylene glycol 2000 in the presence of ammonium sulfate. The three-dimensional structure of these enzyme crystals, solved at 1.65Å resolution with R(free) 23.2%, revealed the surprising absence of the aldimine bond between the active site Lys210 and PLP. Different hypothesis are proposed and discussed in the light of spectral and structural data, pointing out to the relevance of the complementarity between X-ray crystallography and single crystal spectroscopy for the understanding of biological mechanisms at molecular level. This article is part of a Special Issue entitled: Protein Structure and Function in the Crystalline State.

The mercaptopyruvate sulfurtransferase of Trichomonas vaginalis links cysteine catabolism to the production of thioredoxin persulfide

Trichomonas vaginalis is a protozoan parasite of humans that is able to synthesize cysteine de novo using cysteine synthase but does not produce glutathione. In this study, high pressure liquid chromatography analysis confirmed that cysteine is the major intracellular redox buffer by showing that T. vaginalis contains high levels of cysteine ( approximately 600 mum) comprising more than 70% of the total thiols detected. To investigate possible mechanisms for the regulation of cysteine levels in T. vaginalis, we have characterized enzymes of the mercaptopyruvate pathway. This consists of an aspartate aminotransferase (TvAspAT1), which transaminates cysteine to form 3-mercaptopyruvate (3-MP), and mercaptopyruvate sulfurtransferase (TvMST), which transfers the sulfur of 3-MP to a nucleophilic acceptor, generating pyruvate. TvMST has high activity with 3-MP as a sulfur donor and can use several thiol compounds as sulfur acceptor substrates. Our analysis indicated that TvMST has a k(cat)/K(m) for reduced thioredoxin of 6.2 x 10(7) m(-1) s(-1), more than 100-fold higher than that observed for beta-mercaptoethanol and cysteine, suggesting that thioredoxin is a preferred substrate for TvMST. Thiol trapping and mass spectrometry provided direct evidence for the formation of thioredoxin persulfide as a product of this reaction. The thioredoxin persulfide could serve a biological function such as the transfer of the persulfide to a target protein or the sequestered release of sulfide for biosynthesis. Changes in MST activity of T. vaginalis in response to variation in the supply of exogenous cysteine are suggestive of a role for the mercaptopyruvate pathway in the removal of excess intracellular cysteine, redox homeostasis, and antioxidant defense.

Development of Drug Resistance in Trichomonas vaginalis and its Overcoming with Natural Products

Trichomoniasis is an infectious disease afflicting women worldwide. The protozoan parasite Trichomonas vaginalis is the causative agent of this sexually-transmitted disease, including also men in its infection cycle. The disease is usually not life-threatening, but has been associated with the development of cervical cancer and increased susceptibility to HIV. Approved drugs are 5-nitroimidazoles, with metronidazole being the drug of first choice. These drugs act via induction of oxidative stress and DNA-damage, leading to cell death in the parasite. Nevertheless, with the development of resistant T. vaginalis strains the treatment of the disease becomes exceedingly difficult. Mechanisms of drug resistance are characterized by reduced expression or even loss of proteins necessary for drug activation and a decreased reductive nature in the parasite. A promising strategy for research into new drugs and moreover, to overcome drug resistance, are compounds derived from natural sources. The present study provides a summary of all so far investigated small molecules with antitrichomonal activity; promisingly, some also show efficacy against resistant strains. Whereas the list of chemically characterized compounds derived from plants is rather short, literature provides immense applications of crude plant extracts tested against T. vaginalis. This demonstrates the absence of studies in this field aimed to identify and isolate single natural products exhibiting antitrichomonal features. Likewise, elucidating their mode of action on a molecular basis is of paramount importance

Arabidopsis methionine gamma-lyase is regulated according to isoleucine biosynthesis needs but plays a subordinate role to threonine deaminase

The canonical pathway for isoleucine biosynthesis in plants begins with the conversion of threonine to 2-ketobutyrate by threonine deaminase (OMR1). However, demonstration of methionine gamma-lyase (MGL) activity in Arabidopsis (Arabidopsis thaliana) suggested that production of 2-ketobutyrate from methionine can also lead to isoleucine biosynthesis. Rescue of the isoleucine deficit in a threonine deaminase mutant by MGL overexpression, as well as decreased transcription of endogenous Arabidopsis MGL in a feedback-insensitive threonine deaminase mutant background, shows that these two enzymes have overlapping functions in amino acid biosynthesis. In mgl mutant flowers and seeds, methionine levels are significantly increased and incorporation of [(13)C]Met into isoleucine is decreased, but isoleucine levels are unaffected. Accumulation of free isoleucine and other branched-chain amino acids is greatly elevated in response to drought stress in Arabidopsis. Gene expression analyses, amino acid phenotypes, and labeled precursor feeding experiments demonstrate that MGL activity is up-regulated by osmotic stress but likely plays a less prominent role in isoleucine biosynthesis than threonine deaminase. The observation that MGL makes a significant contribution to methionine degradation, particularly in reproductive tissue, suggests practical applications for silencing the expression of MGL in crop plants and thereby increasing the abundance of methionine, a limiting essential amino acid.

Current and future perspectives on the chemotherapy of the parasitic protozoa Trichomonas vaginalis and Entamoeba histolytica

Trichomonas vaginalis and Entamoeba histolytica are clinically important protozoa that affect humans. T. vaginalis produces sexually transmitted infections and E. histolytica is the causative agent of amebic dysentery. Metronidazole, a compound first used to treat T. vaginalis in 1959, is still the main drug used worldwide to treat these pathogens. It is essential to find new biochemical differences in these organisms that could be exploited to develop new antiprotozoal chemotherapeutics. Recent findings associated with T. vaginalis and E. histolytica biochemistry and host–pathogen interactions are surveyed. Knowledge concerning the biochemistry of these parasites is serving to form the foundation for the development of new approaches to control these important human pathogens.

Application of two-dimensional electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for proteomic analysis of the sexually transmitted parasite Trichomonas vaginalis

Trichomonas vaginalis is a sexually transmitted protozoan parasite that infects the human urogenital tract causing trichomoniasis, a worldwide disease. In this work, a fresh clinical isolate of T. vaginalis was used for study of the protein expression in this species. Two-dimensional gel electrophoresis (2-DE) and MALDI-TOF/TOF mass spectrometry (MS) were employed to create a reference map of soluble proteins in the pH range 4-7. A set of 116 proteins belonging to functional classes expressed in high and low abundance was identified by peptide mass fingerprinting and tandem MS. These identifications corresponded to 67 different proteins, suggesting that post-translational modifications are common phenomena in T. vaginalis. Identified proteins were classified into 16 groups according to biological processes. Among detected proteins we identified the major enzymes involved in both cytosolic and hydrogenosomal metabolic pathways, as well as putative protein targets for new drug design. In addition, this analysis allows validation of previous gene predictions confirming the expression of 15 hypothetical proteins. Finally, the findings here reported represent the first reference proteome map of T. vaginalis and the first steps towards the description of a comprehensive proteome map of this parasite.

Apoptosis induced by selenomethionine and methioninase is superoxide mediated and p53 dependent in human prostate cancer cells

Selenomethionine (SeMet) is the chemical form or major component of selenium used for cancer chemoprevention in several clinical trials. However, evidence from experimental studies indicates that SeMet has weaker anticancer effects than most other forms of selenium. Recent studies showed that the anticancer activity of SeMet can be enhanced by methioninase (METase), indicating that SeMet metabolites are responsible for its anticancer activity. In the present study, we showed that wild-type p53-expressing LNCaP human prostate cancer cells were more sensitive to cotreatment with SeMet and METase than p53-null PC3 human prostate cancer cells. SeMet and METase cotreatment significantly increased levels of superoxide and apoptosis in LNCaP cells. Cotreatment with SeMet and METase resulted in increased levels of phosphorylated p53 (Ser15), total p53, Bax, and p21(Waf1) proteins. LNCaP cells treated with SeMet and METase also showed p53 translocation to mitochondria, decreased mitochondrial membrane potential, cytochrome c release into the cytosol, and activation of caspase-9. The effects of SeMet and METase were suppressed by pretreatment with a synthetic superoxide dismutase mimic or by knockdown of p53 via RNA interference. Reexpression of wild-type p53 in PC3 cells resulted in increases in superoxide production, apoptosis, and caspase-9 activity and a decrease in mitochondrial membrane potential following cotreatment with SeMet and METase. Our study shows that apoptosis induced by SeMet plus METase is superoxide mediated and p53 dependent via mitochondrial pathway(s). These results suggest that superoxide and p53 may play a role in cancer chemoprevention by selenium.

Functional characterization of a methionine gamma-lyase in Arabidopsis and its implication in an alternative to the reverse trans-sulfuration pathway

Methionine gamma-lyase (MGL) catalyzes the degradation of L-methionine to alpha-ketobutyrate, methanethiol and ammonia. The Arabidopsis (Arabidopsis thaliana) genome includes a single gene (At1g64660) encoding a protein (AtMGL) with approximately 35% identity to bacterial and protozoan MGLs. When overexpressed in Escherichia coli, AtMGL allowed growth on L-methionine as sole nitrogen source and conferred a high rate of methanethiol emission. The purified recombinant protein exhibited a spectrum typical of pyridoxal 5'-phosphate enzymes, and had high activity toward l-methionine, L-ethionine, L-homocysteine and seleno-L-methionine, but not L-cysteine. Quantitation of mRNA showed that the AtMGL gene is expressed in aerial organs and roots, and that its expression in leaves was increased 2.5-fold by growth on low sulfate medium. Emission of methanethiol from Arabidopsis plants supplied with 10 mM L-methionine was undetectable (<0.5 nmol min(-1) g(-1) FW), suggesting that AtMGL is not an important source of volatile methanethiol. Knocking out the AtMGL gene significantly increased leaf methionine content (9.2-fold) and leaf and root S-methylmethionine content (4.7- and 7-fold, respectively) under conditions of sulfate starvation, indicating that AtMGL carries a significant flux in vivo. In Arabidopsis plantlets fed L-[(35)S]methionine on a low sulfate medium, label was incorporated into protein-bound cysteine as well as methionine, but incorporation into cysteine was significantly (30%) less in the knockout mutant. These data indicate that plants possess an alternative to the reverse trans-sulfuration pathway (methionine-->homocysteine-->cystathionine-->cysteine) in which methanethiol is an intermediate.

Current therapeutics, their problems, and sulfur-containing-amino-acid metabolism as a novel target against infections by "amitochondriate" protozoan parasites

The "amitochondriate" protozoan parasites of humans Entamoeba histolytica, Giardia intestinalis, and Trichomonas vaginalis share many biochemical features, e.g., energy and amino acid metabolism, a spectrum of drugs for their treatment, and the occurrence of drug resistance. These parasites possess metabolic pathways that are divergent from those of their mammalian hosts and are often considered to be good targets for drug development. Sulfur-containing-amino-acid metabolism represents one such divergent metabolic pathway, namely, the cysteine biosynthetic pathway and methionine gamma-lyase-mediated catabolism of sulfur-containing amino acids, which are present in T. vaginalis and E. histolytica but absent in G. intestinalis. These pathways are potentially exploitable for development of drugs against amoebiasis and trichomoniasis. For instance, L-trifluoromethionine, which is catalyzed by methionine gamma-lyase and produces a toxic product, is effective against T. vaginalis and E. histolytica parasites in vitro and in vivo and may represent a good lead compound. In this review, we summarize the biology of these microaerophilic parasites, their clinical manifestation and epidemiology of disease, chemotherapeutics, the modes of action of representative drugs, and problems related to these drugs, including drug resistance. We further discuss our approach to exploit unique sulfur-containing-amino-acid metabolism, focusing on development of drugs against E. histolytica.

Methionine catabolism in Arabidopsis cells is initiated by a gamma-cleavage process and leads to S-methylcysteine and isoleucine syntheses

Despite recent progress in elucidating the regulation of methionine (Met) synthesis, little is known about the catabolism of this amino acid in plants. In this article, we present several lines of evidence indicating that the cleavage of Met catalyzed by Met gamma-lyase is the first step in this process. First, we cloned an Arabidopsis cDNA coding a functional Met gamma-lyase (AtMGL), a cytosolic enzyme catalyzing the conversion of Met into methanethiol, alpha-ketobutyrate, and ammonia. AtMGL is present in all of the Arabidopsis organs and tissues analyzed, except in quiescent dry mature seeds, thus suggesting that AtMGL is involved in the regulation of Met homeostasis in various situations. Also, we demonstrated that the expression of AtMGL was induced in Arabidopsis cells in response to high Met levels, probably to bypass the elevated Km of the enzyme for Met. Second, [13C]-NMR profiling of Arabidopsis cells fed with [13C]Met allowed us to identify labeled S-adenosylmethionine, S-methylmethionine, S-methylcysteine (SMC), and isoleucine (Ile). The unexpected production of SMC and Ile was directly associated to the function of Met gamma-lyase. Indeed, we showed that part of the methanethiol produced during Met cleavage could react with an activated form of serine to produce SMC. The second product of Met cleavage, alpha-ketobutyrate, entered the pathway of Ile synthesis in plastids. Together, these data indicate that Met catabolism in Arabidopsis cells is initiated by a gamma-cleavage process and can result in the formation of the essential amino acid Ile and a potential storage form for sulfide or methyl groups, SMC.

Cysteine biosynthesis in Trichomonas vaginalis involves cysteine synthase utilizing O-phosphoserine

Trichomonas vaginalis is an early divergent eukaryote with many unusual biochemical features. It is an anaerobic protozoan parasite of humans that is thought to rely heavily on cysteine as a major redox buffer, because it lacks glutathione. We report here that for synthesis of cysteine from sulfide, T. vaginalis relies upon cysteine synthase. The enzyme (TvCS1) can use either O-acetylserine or O-phosphoserine as substrates. The K(m) values of the enzyme for sulfide are very low (0.02 mm), suggesting that the enzyme may be a means of ensuring that sulfide in the parasite is maintained at a low level. T. vaginalis appears to lack serine acetyltransferase, the source of O-acetylserine in many cells, but has a functional 3-phosphoglycerate dehydrogenase and an O-phosphoserine aminotransferase that together result in the production of O-phosphoserine, suggesting that this is the physiological substrate. TvCS1 can also use thiosulfate as substrate. Overall, TvCS1 has substrate specificities similar to those reported for cysteine synthases of Aeropyrum pernix and Escherichia coli, and this is reflected by sequence similarities around the active site. We suggest that these enzymes are classified together as type B cysteine synthases, and we hypothesize that the use of O-phosphoserine is a common characteristic of these cysteine synthases. The level of cysteine synthase in T. vaginalis is regulated according to need, such that parasites growing in an environment rich in cysteine have low activity, whereas exposure to propargylglycine results in elevated cysteine synthase activity. Humans lack cysteine synthase; therefore, this parasite enzyme could be an exploitable drug target.

Volatile sulfur compounds produced by Helicobacter pylori

GOALS

To assess the volatile sulfur compounds produced by three strains of Helicobacter pylori in broth cultures mixed with sulfur-containing amino acids.

BACKGROUND

Halitosis has been reported in H. pylori-positive patients, and volatile sulfur compounds such as hydrogen sulfide and methyl mercaptan are known to be responsible for inducing oral malodor. Whether H. pylori produces these volatile sulfur compounds has yet to be established.

STUDY

Three strains of H. pylori (ATCC 43504, SS 1, DSM 4867) were cultured with 5 mM cysteine and methionine. After 72 hours of incubation, the headspace air was aspirated and injected directly into a gas chromatograph. The concentrations of hydrogen sulfide and methyl mercaptan were analyzed and compared between experimental and control cultures

RESULTS

In broth containing 5 mM cysteine, hydrogen sulfide was increased by ATCC 43504 (P < 0.01) and SS 1 (P < 0.05), while methyl mercaptan was elevated only by SS 1 (P < 0.05). In broth containing 5 mM methionine, methyl mercaptan increases were significant for SS 1 (P < 0.05) and DSM 4867 (P < 0.05). In broth containing 5 mM cysteine and 5 mM methionine, the concentration of hydrogen sulfide was higher than in controls for all three strains (P < 0.01); that of methyl mercaptan was higher only for SS 1 (P < 0.01). Cysteine addition to cultures containing methionine increased hydrogen sulfide and methyl mercaptan for ATCC 43504 (P < 0.05) and SS 1 (P < 0.05). Conversely, addition of methionine to cultures containing cysteine increased methyl mercaptan only for DSM 4867 (P < 0.01).

CONCLUSIONS

The production of volatile sulfur compounds by H. pylori is not only very complicated but also strain-specific. Nevertheless, H. pylori was shown to produce hydrogen sulfide and methyl mercaptan, which suggests that this microorganism can contribute to the development of halitosis.

L-methionine γ-lyase from Citrobacter freundii: Cloning of the gene and kinetic parameters of the enzyme

It is shown for the first time for the Enterobacteriaceae family that a gene encoding L-methionine gamma-lyase (MGL) is present in the genome of Citrobacter freundii. Homogeneous enzyme has been purified from C. freundii cells and its N-terminal sequence has been determined. The hybrid plasmid pUCmgl obtained from the C. freundii genomic library contains an EcoRI insert of about 3000 bp, which ensures the appearance of MGL activity when expressed in Escherichia coli TG1 cells. The nucleotide sequence of the EcoRI fragment contains two open reading frames. The first frame (the megL gene) encodes a protein of 398 amino acid residues that has sequence homology with MGLs from different sources. The second frame encodes a protein with sequence homology with proteins belonging to the family of permeases. To overexpress the megL gene it was cloned into pET-15b vector. Recombinant enzyme has been purified and its kinetic parameters have been determined. It is demonstrated that a presence of a hybrid plasmid pUCmgl, containing the megL gene in the E. coli K12 cells, leads to a decrease in efficiency of EcoKI-restriction. It seems likely that decomposition of L-methionine under the action of MGL leads to a decrease in the intracellular content of S-adenosylmethionine. Expression of the megL gene in the C. freundii genome occurs only upon induction by a significant amount of L-methionine.

Biochemical and functional characterization of phosphoserine aminotransferase from Entamoeba histolytica, which possesses both phosphorylated and non-phosphorylated serine metabolic pathways

The enteric protozoan parasite Entamoeba histolytica is a unicellular eukaryote that possesses both phosphorylated and non-phosphorylated serine metabolic pathways. In the present study, we described enzymological and functional characterization of phosphoserine aminotransferase (PSAT) from E. histolytica. E. histolytica PSAT (EhPSAT) showed maximum activity for the forward reaction at basic pH, dissimilar to mammalian PSAT, which showed sharp neutral optimum pH. EhPSAT activity was significantly inhibited by substrate analogs, O-phospho-d-serine, O-phospho-l-threonine, and O-acetylserine, suggesting possible regulation of the amoebic PSAT by these metabolic intermediates. Fractionation of the whole parasite lysate and rEhPSAT by anion exchange chromatography verified that EhPSAT represents a dominant PSAT activity. EhPSAT showed a close kinship to PSAT from bacteroides based on amino acid alignment and phylogenetic analyses, suggesting that E. histolytica gained this gene from bacteroides by lateral gene transfer. Comparisons of kinetic properties of recombinant PSAT from E. histolytica and Arabidopsis thaliana showed that EhPSAT possesses significantly higher affinity toward glutamate than the A. thaliana counterpart, which may be explained by significant differences in the isoelectric point and the substitution of arginine, which is involved the binding to the gamma-carboxylate moiety of glutamate, in Escherichia coli PSAT, to serine or threonine in E. histolytica or A. thaliana PSAT, respectively. Heterologous expression of EhPSAT successfully rescued growth defect of a serine-auxotrophic E. coli strain KL282, where serC was deleted, confirming its in vivo role in serine biosynthesis. Together with our previous demonstration of phosphoglycerate dehydrogenase, the present study reinforces physiological significance of the phosphorylated pathway in amoeba.

Evolutionary plasticity of methionine biosynthesis

Methionine is an essential cellular constituent, the initiator of protein synthesis and a precursor in many metabolic activities, such as methylation and formylation. Here we investigate the genomic distribution of the methionine biosynthetic pathway and analyze its evolutionary history by reconstructing the phylogeny of its enzymatic components. We demonstrate the evolutionary complexity of methionine synthesis and describe the various mechanisms that have shaped this biosynthetic pathway: gene duplication, functional reassignment, lateral acquisition and gene loss. Lateral gene transfer within and between domains and gene recruitment have played an important role in the evolution of this pathway, especially in its first and third enzymatic steps--homoserine activation and homocysteine methylation. These analyses are also the basis of predictions regarding methionine synthesis in Archaea, where the pathway is yet to be characterized. This study illustrates how diverse molecular solutions can fulfill a conserved function in living beings.

High production of methyl mercaptan by l-methionine-α-deamino-γ-mercaptomethane lyase from Treponema denticola

Methyl mercaptan is derived from l-methionine by the action of l-methionine-alpha-deamino-gamma-mercaptomethane lyase (METase) and is a major component of oral malodor. This compound is highly toxic and is thought to play an important role in periodontal disease. We found that Treponema denticola, a member of the subgingival biofilm at periodontal disease sites, produced a large amount of methyl mercaptan even at low concentration of l-methionine. METase activity in a cell-free extract from T. denticola was detected by two-dimensional electrophoresis under non-denaturing conditions, and the protein spot that exhibited high METase activity was identified using a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer. The identified gene produced a METase with a K(m) value for l-methionine (0.55mM) that is much lower than those of METases previously identified in the other organisms. This result suggests that T. denticola is an important producer of methyl mercaptan in the subgingival biofilm.

Sulfur-Containing Amino Acid Metabolism in Parasitic Protozoa

Sulfur-containing amino acids play indispensable roles in a wide variety of biological activities including protein synthesis, methylation, and biosynthesis of polyamines and glutathione. Biosynthesis and catabolism of these amino acids need to be carefully regulated to achieve the requirement of the above-mentioned activities and also to eliminate toxicity attributable to the amino acids. Genome-wide analyses of enzymes involved in the metabolic pathways of sulfur-containing amino acids, including transsulfuration, sulfur assimilatory de novo cysteine biosynthesis, methionine cycle, and degradation, using genome databases available from a variety of parasitic protozoa, reveal remarkable diversity between protozoan parasites and their mammalian hosts. Thus, the sulfur-containing amino acid metabolic pathways are a rational target for the development of novel chemotherapeutic and prophylactic agents against diseases caused by protozoan parasites. These pathways also demonstrate notable heterogeneity among parasites, suggesting that the metabolism of sulfur-containing amino acids reflects the diversity of parasitism among parasite species, and probably influences their biology and pathophysiology such as virulence competence and stress defense.

The amitochondriate eukaryote Trichomonas vaginalis contains a divergent thioredoxin-linked peroxiredoxin antioxidant system

Trichomonas is an amitochondriate parasitic protozoon specialized for an anaerobic lifestyle. Nevertheless, it is exposed to oxygen and is able to cope with the resultant oxidative stress. In the absence of glutathione, cysteine has been thought to be the major antioxidant. We now report that the parasite contains thioredoxin reductase, which functions together with thioredoxin and thioredoxin peroxidase to detoxify potentially damaging oxidants. Thioredoxin reductase and thioredoxin also reduce cystine and so may play a role in maintaining the cellular cysteine levels. The importance of the thioredoxin system as one of the major antioxidant defense mechanisms in Trichomonas was confirmed by showing that the parasite responds to environmental changes resulting in increased oxidative stress by up-regulating thioredoxin and thioredoxin peroxidases levels. Sequence data indicate that the thioredoxin reductase of Trichomonas differs fundamentally in structure from that of its human host and thus may represent a useful drug target. The protein is generally similar to thioredoxin reductases present in other lower eukaryotes, all of which probably originated through horizontal gene transfer from a prokaryote. The phylogenetic signal in thioredoxin peroxidase is weak, but evidence from trees suggests that this gene has been subject to repeated horizontal gene transfers from different prokaryotes to different eukaryotes. The data are thus consistent with the complexity hypothesis that predicts that the evolution of simple pathways such as the thioredoxin cascade are likely to be affected by horizontal gene transfer between species.

Identification and characterization of two isoenzymes of methionine gamma-lyase from Entamoeba histolytica: a key enzyme of sulfur-amino acid degradation in an anaerobic parasitic protist that lacks forward and reverse trans-sulfuration pathways

To better understand the metabolism of sulfur-containing amino acids, which likely plays a key role in a variety of cell functions, in Entamoeba histolytica, we searched the genome data base for genes encoding putative orthologs of enzymes known to be involved in the metabolism. The search revealed that E. histolytica possesses only incomplete cysteine-methionine conversion pathways in both directions. Instead, this parasite possesses genes encoding two isoenzymes of methionine gamma-lyase (EC 4.4.1.11, EhMGL1/2), which has been implicated in the degradation of sulfur-containing amino acids. The two amebic MGL isoenzymes, showing 69% identity to each other, encode 389- and 392-amino acid polypeptides with predicted molecular masses of 42.3 and 42.7 kDa and pIs of 6.01 and 6.63, respectively. Amino acid comparison and phylogenetic analysis suggested that these amebic MGLs are likely to have been horizontally transferred from the Archaea, whereas an MGL from another anaerobic protist Trichomonas vaginalis has MGL isotypes that share a common ancestor with bacteria. Enzymological and immunoblot analyses of the partially purified native amebic MGL confirmed that both of the MGL isotypes are expressed in a comparable amount predominantly in the cytosol and form a homotetramer. Recombinant EhMGL1 and 2 proteins catalyzed degradation of L-methionine, DL-homocysteine, L-cysteine, and O-acetyl-L-serine to form alpha-keto acid, ammonia, and hydrogen sulfide or methanethiol, whereas activity toward cystathionine was negligible. These two isoenzymes showed notable differences in substrate specificity and pH optimum. In addition, we showed that EhMGL is an ideal target for the development of new chemotherapeutic agents against amebiasis by demonstrating an amebicidal effect of the methionine analog trifluoromethionine on trophozoites in culture (IC50 18 mum) and that this effect of trifluoromethionine was completely abolished by the addition of the MGL-specific inhibitor DL-propargylglycine.

Biochemical and genetic evidence for a family of heterotrimeric G-proteins in Trichomonas vaginalis

We have cloned a single copy gene from the human parasite Trichomonas vaginalis that encodes a putative protein of 402 amino acids with approximately 35% sequence identity to known alpha subunits of heterotrimeric G-proteins. It contains the characteristic GTP binding domains G-1 to G-5 with the key residues conserved. The new sequence has an unusual N-terminal extension of approximately 70 residues that cannot be aligned to reference G-proteins and which is characterised by proline-rich repeats. To investigate the expression and cellular localisation of the protein we produced specific antisera against a recombinant fusion protein. The antisera recognised a protein of an apparent molecular mass of 51 kDa in protein extracts from T. vaginalis and immunofluorescent microscopy established that the protein is localised to discrete endomembranes. Using a protocol designed to purify mammalian heterotrimeric G-proteins incorporating a GTPgammaS binding assay, we isolated two proteins from Trichomonas that are recognised by an heterologous GA/1 antisera raised to a peptide of the conserved G-1 domain of G-protein alpha subunits. These two proteins have an apparent molecular mass of 61 and 48 kDa, respectively, larger and smaller than the translation product of the cloned gene. Consistent with these results, the GA/1 antisera did not cross-react with the fusion protein produced from the gene we have cloned. These data suggest T. vaginalis possesses more than one heterotrimeric G-protein alpha subunit. Based on the sequence features of the cloned gene and the biochemical properties of the purified proteins, we suggest that these alpha subunits are likely to be part of classic heterotrimeric G-protein complexes.

Thiol-based redox metabolism of protozoan parasites

The review considers redox enzymes of Plasmodium spp., Trypanosomatida, Trichomonas, Entamoeba and Giardia, with special emphasis on their potential use as targets for drug development. Thiol-based redox systems play pivotal roles in the success and survival of these parasitic protozoa. The synthesis of cysteine, the key molecule of any thiol metabolism, has been elucidated in trypanosomatids and anaerobes. In trypanosomatids, trypanothione replaces the more common glutathione system. The enzymes of trypanothione synthesis have recently been identified. The role of trypanothione in the detoxification of reactive oxygen species is reflected in the multiplicity of trypanothione-dependent peroxidases. In Plasmodium falciparum, the crystal structures of glutathione reductase and glutamate dehydrogenase are now available; another drug target, thioredoxin reductase, has been demonstrated to be essential for the malarial parasite.

3-Chloro-DL-alanine resistance by L-methionine-alpha-deamino-gamma-mercaptomethane-lyase activity

The antibacterial agent 3-chloro-DL-alanine (3CA) is an inhibitor of peptidoglycan synthesis. Fusobacterium nucleatum and Porphyromonas gingivalis, the bacteria responsible for oral malodor, are shown to be resistant to 1 mM 3CA, whereas Streptococcus mutans and Escherichia coli are sensitive to this antibacterial agent at the same concentration. We isolated the 3CA resistance gene from F. nucleatum and showed that the gene encodes an L-methionine-alpha-deamino-gamma-mercaptomethane-lyase that catalyzes the alpha,gamma-elimination of L-methionine to produce methyl mercaptan. The enzyme also exhibits 3CA chloride-lyase (deaminating) activity. This antibacterial agent is expected to be useful for specific selection of malodorous oral bacteria producing high amounts of methyl mercaptan.

Methyl mercaptan production by periodontal bacteria

Oral malodour is principally caused by volatile sulphur compounds (VSC) such as hydrogen sulphide, methyl mercaptan and dimethyl sulphide. Methyl mercaptan is highly toxic, and its presence within a periodontal pocket suggests involvement in the induction and/or progression of periodontal disease. Methyl mercaptan is produced from L-methionine by L-methionine- alpha -deamino- gamma -mercaptomethane-lyase (METase). METase catalyses the alpha,gamma-eliminating reaction of L-methionine, which results in the release of alpha-ketobutyrate, methyl mercaptan and ammonia. Although methyl mercaptan is produced by a variety of microorganisms, Porphyromonas gingivalis is considered to be the most potent producer. METases of P. gingivalis have been characterised and the genes responsible for their production, the mg/genes, have been sequenced. To ascertain the role of METase in P. gingivalis pathogenicity, a METase-deficient mutant strain (M1217) from P. gingivalis strain W83 was engineered. Only 7.7% of the mice infected with W83 survived 4 days after subcutaneous injection, whereas 36% of the mice infected with M1217 survived over the same time period. Many papers have reported the periodontal pathogenesis of VSC. It has been argued that methyl mercaptan may play a significant role in the pathogenicity of P. gingivalis.

Separable putative polyadenylation and cleavage motifs in Trichomonas vaginalis mRNAs

3' Untranslated region processing and polyadenylation in Trichomonas vaginalis was analyzed by 3' rapid amplification of cDNA ends and sequence analysis of T. vaginalis mRNAs. A putative polyadenylation signal with the sequence UAAA was found 11-30 nucleotides upstream from the cleavage site. The motif pyrimidine( downward arrow)(A)(0-3)AAUU is proposed to be the cleavage site for polyadenylation of transcripts. This potential sequence defining the cleavage site for polyadenylation in eukaryotes is a novel finding. As in other eukaryotes, runs of several U's downstream from the cleavage site were identified. A working hypothesis is proposed which couples the UAA translation stop codon with the signaling for the 3'end processing of transcripts in this early divergent parasitic protozoa.

Correlation between oral malodor and periodontal bacteria

Volatile sulfur compounds (VSCs), including hydrogen sulfide, methyl mercaptan, and dimethyl sulfide, are primarily responsible for oral malodor. Recently, the mgl gene encoding L-methionine-alpha-deamino-gamma-mercaptomethane-lyase, which produces methyl mercaptan, was cloned from Porphyromonas gingivalis. This article discusses the mechanism and pathogenic role of the formation of VSCs by oral bacteria.

L-Methionine-gamma-lyase, as a target to inhibit malodorous bacterial growth by trifluoromethionine

Methyl mercaptan is a major component responsible for oral malodor. This compound arises from the bacterial metabolism of methionine. Here we show that the growth of Porphyromonas gingivalis, a periodontal microorganism that produces large amounts of methyl mercaptan, was strongly inhibited by l-trifluoromethionine (TFM), a fluorinated derivative of methionine. In contrast, TFM had no effect on the growth of bacteria which do not produce methyl mercaptan. In addition, the survival rate of P. gingivalis-infected mice was remarkably increased by the co-injection of TFM. These results suggest that TFM is a promising antibacterial agent specific to the malodorous oral bacteria.

Trifluoromethionine, a prodrug designed against methionine gamma-lyase-containing pathogens, has efficacy in vitro and in vivo against Trichomonas vaginalis

Methionine gamma-lyase, the enzyme which catalyzes the single-step conversion of methionine to alpha-ketobutyrate, ammonia, and methanethiol, is highly active in many anaerobic pathogenic microorganisms but has no counterpart in mammals. This study tested the hypothesis that this pathogen-specific enzyme can be exploited as a drug target by prodrugs that are exclusively activated by it. Trifluoromethionine was confirmed as such a prodrug and shown to be highly toxic in vitro to the anaerobic protozoan parasite Trichomonas vaginalis, to anaerobic bacteria containing methionine gamma-lyase, and to Escherichia coli expressing the trichomonad gene. The compound also has exceptional activity against the parasite growing in vivo, with a single dose preventing lesion formation in five of the six mice challenged. These findings suggest that trifluoromethionine represents a lead compound for a novel class of anti-infective drugs with potential as chemotherapeutic agents against a range of prokaryotic and eukaryotic anaerobic pathogens.