Cystathionine ß-synthase is involved in cysteine biosynthesis and H2S generation in Toxoplasma gondii.

Cystathionine ß-synthase (CBS) catalyzes the condensation of serine and homocysteine to water and cystathionine, which is then hydrolyzed to cysteine, α-ketobutyrate and ammonia by cystathionine γ-lyase (CGL) in the reverse transsulfuration pathway. The protozoan parasite Toxoplasma gondii, the causative agent of toxoplasmosis, includes both CBS and CGL enzymes. We have recently reported that the putative T. gondii CGL gene encodes a functional enzyme. Herein, we cloned and biochemically characterized cDNA encoding CBS from T. gondii (TgCBS), which represents a first example of protozoan CBS that does not bind heme but possesses two C-terminal CBS domains. We demonstrated that TgCBS can use both serine and O-acetylserine to produce cystathionine, converting these substrates to an aminoacrylate intermediate as part of a PLP-catalyzed ß-replacement reaction. Besides a role in cysteine biosynthesis, TgCBS can also efficiently produce hydrogen sulfide, preferentially via condensation of cysteine and homocysteine. Unlike the human counterpart and similar to CBS enzymes from lower organisms, the TgCBS activity is not stimulated by S-adenosylmethionine. This study establishes the presence of an intact functional reverse transsulfuration pathway in T. gondii and demonstrates the crucial role of TgCBS in biogenesis of H2S.

DNA damage in homocystinuria: 8-oxo-, 8-dihydro-2'-deoxyguanosine levels in cystathionine-ß-synthase deficient patients and the in vitro protective effect of N-acetyl­L­cysteine

Introduction: Homocysteine (Hcy) tissue accumulation occurs in a metabolic disease characterized biochemically by cystathionine ß-synthase (CBS) deficiency and clinically by mental retardation, vascular problems, and skeletal abnormalities. Previous studies indicate the occurrence of DNA damage secondary to hyperhomocysteinemia and it was observed that DNA damage occurs in leukocytes from CBS-deficient patients. This study aimed to investigate whether an oxidative mechanism could be involved in DNA damage previously found and investigated the in vitro effect of N-acety-L-cysteine (NAC) on DNA damage caused by high Hcy levels. Methods: We evaluated a biomarker of oxidative DNA damage in the urine of CBS­deficient patients, as well as the in vitro effect of NAC on DNA damage caused by high levels of Hcy. Moreover, a biomarker of lipid oxidative damage was also measured in urine of CBS deficient patients. Results: There was an increase in parameters of DNA (8-oxo-7,8-dihydro-2'- deoxyguanosine) and lipid (15-F2t-isoprostanes levels) oxidative damage in CBS-deficient patients when compared to controls. In addition, a significant positive correlation was found between 15-F2t-isoprostanes levels and total Hcy concentrations. Besides, an in vitro protective effect of NAC at concentrations of 1 and 5 mM was observed on DNA damage caused by Hcy 50 µM and 200 µM. Additionally, we showed a decrease in sulfhydryl content in plasma from CBS-deficient patients when compared to controls. Discussion: These results demonstrated that DNA damage occurs by an oxidative mechanism in CBS deficiency together with lipid oxidative damage, highlighting the NAC beneficial action upon DNA oxidative process, contributing with a new treatment perspective of the patients affected by classic homocystinuria.

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.

Pre-steady-state kinetic analysis of enzyme-monitored turnover during cystathionine ß-synthase-catalyzed H(2)S generation.

Cystathionine ß-synthase (CBS) catalyzes the first step in the transsulfuration pathway in mammals, i.e., the condensation of serine and homocysteine to produce cystathionine and water. Recently, we have reported a steady-state kinetic analysis of the three hydrogen sulfide (H(2)S)-generating reactions that are catalyzed by human and yeast CBS [Singh, S., et al. (2009) J. Biol. Chem. 284, 22457-22466]. In the study presented here, we report a pre-steady-state kinetic analysis of intermediates in the H(2)S-generating reactions catalyzed by yeast CBS (yCBS). Because yCBS does not have a heme cofactor, in contrast to human CBS, it is easier to observe reaction intermediates with yCBS. The most efficient route for H(2)S generation by yCBS is the ß-replacement of the cysteine thiol with homocysteine. In this reaction, yCBS first reacts with cysteine to release H(2)S and forms an aminoacrylate intermediate (k(obs) of 1.61 ± 0.04 mM(-1) s(-1) at low cysteine concentrations and 2.8 ± 0.1 mM(-1) s(-1) at high cysteine concentrations, at 20 °C), which has an absorption maximum at 465 nm. Homocysteine binds to the E·aminoacrylate intermediate with a bimolecular rate constant of 142 mM(-1) s(-1) and rapidly condenses to form the enzyme-bound external aldimine of cystathionine. The reactions could be partially rate limited by release of the products, cystathionine and H(2)S.

Suppression of methionine-induced hyperhomocysteinemia by glycine and serine in rats.

The hyperhomocysteinemia induced by a dietary addition of 1% methionine was significantly suppressed by the concurrent addition of 1% glycine or 1.4% serine to the same degree. The methionine-induced increase in the hepatic concentration of methionine metabolites was significantly suppressed by glycine and serine, but the hepatic cystathionine beta-synthase activity was not enhanced by these amino acids. When the methionine-supplemented diet was changed to the methionine plus glycine or serine diet, the plasma homocysteine concentration rapidly decreased during and after the first day. The hyperhomocysteinemia induced by an intraperitoneal injection with methionine was also suppressed by concurrent injection with glycine or serine, although the effect of serine was significantly greater than that of glycine. These results indicate that glycine and serine were effective for suppressing methionine-induced hyperhomocysteinemia: serine and its precursor glycine are considered to have elicited their effects mainly by stimulating cystathionine synthesis by supplying serine, another substrate for cystathionine synthesis.

The enzymology of cystathionine biosynthesis: strategies for the control of substrate and reaction specificity.

The ability of enzymes to catalyze specific reactions, while excluding others, is central to cellular metabolism. Control of reaction specificity is of particular importance for enzymes that employ catalytically versatile cofactors, of which pyridoxal 5'-phosphate is a prime example. Cystathionine gamma-synthase and cystathionine beta-synthase are the first enzymes in the transsulfuration and reverse transsulfuration pathways, respectively. Each of them occupies branch-point positions in amino acid metabolism and as such are subject to transcriptional and post-translational regulation. Both enzymes catalyze the pyridoxal 5'-phosphate-dependent formation of l-cystathionine; however, their substrate and reaction specificities are distinct. The mechanisms whereby these enzymes control the chemistry of the cofactor are the subject of this review.

Immobilization of Saccharomyces cerevisiae cystathionine gamma-lyase and application of the product to cystathionine synthesis.

Cystathionine gamma-lyase of Saccharomyces cerevisiae was immobilized to aminohexyl-Sepharose through the cofactor pyridoxal 5'-phosphate and was characterized with respect to its cystathionine gamma-synthase activity. The immobilized product was so stable that it repeatedly catalyzed as many as five cycles of the reaction without losing activity.

Kinetics of the yeast cystathionine beta-synthase forward and reverse reactions: continuous assays and the equilibrium constant for the reaction.

Cystathionine beta-synthase (CBS) is a pyridoxal-phosphate-dependent enzyme that catalyzes a beta-replacement reaction in which the hydroxyl group of serine (L-Ser) is displaced by the thiol of homocysteine (L-Hcys) to form cystathionine (L-Cth) in the first step of the trans-sulfuration pathway. A new continuous assay for the forward reaction, employing cystathionine beta-lyase and L-lactate dehydrogenase as coupling enzymes, is described. It alleviates product inhibition by L-Cth and revealed that the values for (1.2 mM) and for substrate inhibition by L-Hcys ( = 2.0 mM) are lower than those previously reported. A continuous, 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB)-based assay for the CBS-catalyzed hydrolysis of L-Cth to L-Ser and L-Hcys provides a tool for investigation of the reverse reaction (k(catR) = 0.56 s(-)(1), = 0.083 mM). The (catR)/ versus pH profile of ytCBS is bell-shaped with a pH optimum of 8.3, and the pK(a) values for the acidic and basic limbs are 8.05 and 8.63, respectively. The latter is assigned to the alpha-amino group of L-Cth (pK(a) = 8.54). The internal aldimine of ytCBS remains protonated at pH < 11; therefore, the acidic pK(a) is assigned to an enzyme functionality that is not associated with the internal aldimine. K(eq) was determined directly and from the kinetic parameters, and the values are 0.61 and 1.2 microM, respectively.

Yeast cystathionine beta-synthase reacts with L-allothreonine, a non-natural substrate, and L-homocysteine to form a new amino acid, 3-methyl-L-cystathionine.

Our studies of the reaction mechanism of cystathionine beta-synthase from yeast (Saccharomyces cerevisiae) are facilitated by the spectroscopic properties of the pyridoxal phosphate coenzyme. The enzyme catalyzes the reaction of L-serine with L-homocysteine to form L-cystathionine through a series of pyridoxal phosphate intermediates. In this work, we explore the substrate specificity of the enzyme by use of substrate analogues combined with kinetic measurements under pre-steady-state conditions and with circular dichroism and fluorescence spectroscopy under steady-state conditions. Our results show that L-allothreonine, but not L-threonine, serves as an effective substrate. L-Allothreonine reacts with the pyridoxal phosphate cofactor to form a stable 3-methyl aminoacrylate intermediate that absorbs maximally at 446 nm. The rapid-scanning stopped-flow results show that the binding of L-allothreonine as the external aldimine is faster than formation of the 3-methyl aminoacrylate intermediate. The 3-methyl aminoacrylate intermediate reacts with L-homocysteine to form a new amino acid, 3-methyl-L-cystathionine, which was characterized by nuclear magnetic resonance spectroscopy. This new amino acid may be a useful analogue of L-cystathionine.

The reaction of yeast cystathionine beta-synthase is rate-limited by the conversion of aminoacrylate to cystathionine.

Our studies of the reaction mechanism of cystathionine beta-synthase from Saccharomyces cerevisiae (yeast) are facilitated by the spectroscopic properties of the pyridoxal phosphate coenzyme that forms a series of intermediates in the reaction of L-serine and L-homocysteine to form L-cystathionine. To characterize these reaction intermediates, we have carried out rapid-scanning stopped-flow and single-wavelength stopped-flow kinetic measurements under pre-steady-state conditions, as well as circular dichroism and fluorescence spectroscopy under steady-state conditions. We find that the gem-diamine and external aldimine of aminoacrylate are the primary intermediates in the forward half-reaction with L-serine and that the external aldimine of aminoacrylate or its complex with L-homocysteine is the primary intermediate in the reverse half-reaction with L-cystathionine. The second forward half-reaction of aminoacrylate with L-homocysteine is rapid. No primary kinetic isotope effect was obtained in the forward half-reaction with L-serine. The results provide evidence (1) that the formation of the external aldimine of L-serine is faster than the formation of the aminoacrylate intermediate, (2) that aminoacrylate is formed by the concerted removal of the alpha-proton and the hydroxyl group of L-serine, and (3) that the rate of the overall reaction is rate-limited by the conversion of aminoacrylate to L-cystathionine. We compare our results with cystathionine beta-synthase with those of related investigations of tryptophan synthase and O-acetylserine sulfhydrylase.

Occurrence of transsulfuration in synthesis of L-homocysteine in an extremely thermophilic bacterium, Thermus thermophilus HB8.

A cell extract of an extremely thermophilic bacterium, Thermus thermophilus HB8, cultured in a synthetic medium catalyzed cystathionine gamma-synthesis with O-acetyl-L-homoserine and L-cysteine as substrates but not beta-synthesis with DL-homocysteine and L-serine (or O-acetyl-L-serine). The amounts of synthesized enzymes metabolizing sulfur-containing amino acids were estimated by determining their catalytic activities in cell extracts. The syntheses of cystathionine beta-lyase (EC 4.4.1.8) and O-acetyl-L-serine sulfhydrylase (EC 4.2.99.8) were markedly repressed by L-methionine supplemented to the medium. L-Cysteine and glutathione, both at 0.5 mM, added to the medium as the sole sulfur source repressed the synthesis of O-acetylserine sulfhydrylase by 55 and 73%, respectively, confirming that this enzyme functions as a cysteine synthase. Methionine employed at 1 to 5 mM in the same way derepressed the synthesis of O-acetylserine sulfhydrylase 2.1- to 2.5-fold. A method for assaying a low concentration of sulfide (0.01 to 0.05 mM) liberated from homocysteine by determining cysteine synthesized with it in the presence of excess amounts of O-acetylserine and a purified preparation of the sulfhydrylase was established. The extract of cells catalyzed the homocysteine gamma-lyase reaction, with a specific activity of 5 to 7 nmol/min/mg of protein, but not the methionine gamma-lyase reaction. These results suggested that cysteine was also synthesized under the conditions employed by the catalysis of O-acetylserine sulfhydrylase using sulfur of homocysteine derived from methionine. Methionine inhibited O-acetylserine sulfhydrylase markedly. The effects of sulfur sources added to the medium on the synthesis of O-acetylhomoserine sulfhydrylase and the inhibition of the enzyme activity by methionine were mostly understood by assuming that the organism has two proteins having O-acetylhomoserine sulfhydrylase activity, one of which is cystathionine gamma-synthase. Although it has been reported that homocysteine is directly synthesized in T. thermophilus HB27 by the catalysis of O-acetylhomoserine sulfhydrylase on the basis of genetic studies (T. Kosuge, D. Gao, and T. Hoshino, J. Biosci. Bioeng. 90:271-279, 2000), the results obtained in this study for the behaviors of related enzymes indicate that sulfur is first incorporated into cysteine and then transferred to homocysteine via cystathionine in T. thermophilus HB8.

The specific features of methionine biosynthesis and metabolism in plants.

Plants, unlike other higher eukaryotes, possess all the necessary enzymatic equipment for de novo synthesis of methionine, an amino acid that supports additional roles than simply serving as a building block for protein synthesis. This is because methionine is the immediate precursor of S-adenosylmethionine (AdoMet), which plays numerous roles of being the major methyl-group donor in transmethylation reactions and an intermediate in the biosynthesis of polyamines and of the phytohormone ethylene. In addition, AdoMet has regulatory function in plants behaving as an allosteric activator of threonine synthase. Among the AdoMet-dependent reactions occurring in plants, methylation of cytosine residues in DNA has raised recent interest because impediment of this function alters plant morphology and induces homeotic alterations in flower organs. Also, AdoMet metabolism seems somehow implicated in plant growth via an as yet fully understood link with plant-growth hormones such as cytokinins and auxin and in plant pathogen interactions. Because of this central role in cellular metabolism, a precise knowledge of the biosynthetic pathways that are responsible for homeostatic regulation of methionine and AdoMet in plants has practical implications, particularly in herbicide design.

Liver DNA and RNA metabolism in rats fed diets lacking methionine plus cysteine with or without restricted energy.

Ponderal parameters, soluble protein content and nucleic acid (DNA and RNA) metabolism were studied in liver of growing male Wistar rats fed with different diets: control diet (Control group) containing 10% protein (casein 9.8% plus DL-methionine 0.2%); diet 2 (Group 2) containing 10% protein, lacking Met plus Cys; and diet 3 (Group 3), containing 20% protein, lacking Met plus Cys and with 50% of energy restriction (restricted food intake by experimental design). Diets 2 and 3 were compared with the control diet to know the effects produced by the lack of Met plus Cys and the energy restriction, for an experimental period of 14 days, the animals being slaughtered on the 4th, 8th and 14th days. Food intake, body and liver weights, relative liver weight, cellular size and RNA content per organ and per mg protein decreased in groups 2 and 3, group 3 being affected more than group 2. Diet 2 produced a decrease in DNA content, due to lack of Met+Cys. Acid DNAse activity per organ diminished in group 2 on days 8 and 14, and in group 3 on the 8th day. RNA/DNA ratio diminished in group 2 and 3 due to a proportional RNA reduction with respect to the DNA content. Acid RNAse activity per organ diminished in group 2 on the 8th and 14th days, RNAse per mg of protein increased in group 3 at the end of the treatment, therefore the RNA content decreased. The content of DNA in liver is lower than RNA content in rats fed diet 2 and the opposite occurs with diet 3.

Transsulfuration depends on heme in addition to pyridoxal 5'-phosphate. Cystathionine beta-synthase is a heme protein.

The first committed step of transsulfuration is catalyzed by cystathionine beta-synthase (CBS), a known pyridoxal 5'-phosphate (PLP) enzyme. The inferred amino acid sequences of rat liver CBS and rat liver hemoprotein H-450 are identical. We now confirm the presence of heme b in rat and human liver CBS. Heme almost entirely accounts for the visible spectrum of CBS rather than PLP. Human CBS, expressed in Escherichia coli, acquires heme b from the host bacteria. delta-Aminolevulinate supplementation during bacterial growth increases both the heme saturation and the specific activity of the homogeneous enzyme more than 3-fold. 1 mol of the 63-kDa CBS subunit binds 1 mol of each (heme and PLP). The presence of heme is required for PLP binding, and the amount of PLP bound is limited by the heme content. Removal of PLP, but not heme, from CBS is reversible. These findings suggest that heme is functionally incorporated into CBS only during protein folding. This report describes the first instance of an enzyme that depends upon both heme and PLP for its function.

Effects of the disruption of transmethylation in the central nervous system: an animal model.

INTRODUCTION: Central nervous system (CNS) methyltransferases methylate a wide range of substrates including proteins, lipids, nucleic acids and hormones. In every instance the methyl donor is S-adenosylmethionine (SAMe) and the demethylated product is S-adenosylhomocysteine (SAH). Methylation can be disrupted when there is an inadequate supply of methionine synthase (following vitamin B12 deficiency or folate deficiency), SAMe synthetase (due to ethanol), or SAH hydrolase (for unknown reasons). MATERIAL AND METHODS: 5-week-old pigs were maintained in an environment of either air or nitrous oxide, which inhibits methionine synthase, and were fed either a methionine-unsupplemented or methionine-enriched diet. After 3 to 10 weeks, pigs were killed by pentobarbitone injection and the levels of methionine and SAMe in the pigs' brain, spinal cord, plasma, liver, and kidney assessed. RESULTS: Pigs maintained in nitrous oxide displayed a dramatic fall in methionine levels in plasma and brain tissues but maintained relatively normal SAMe levels in these tissues. Brain and spinal cord cystathionine levels were markedly elevated, especially in those animals receiving oral methionine, as in the absence of methionine synthase homocysteine can be metabolized only through the catabolic pathway to cystathionine and cysteine. CONCLUSION: Disorders such as vitamin B12 deficiency or folate deficiency inhibit methylation by limiting the availability of SAMe or by elevating levels of the inhibitor SAH. In either case, the disruption of a wide range of methylation reactions can cause clinical sequelae ranging from structural abnormalities such as myelopathy to functional abnormalities such as depression.

Directed evolution of biosynthetic pathways. Recruitment of cysteine thioethers for constructing the cell wall of Escherichia coli.

We report that expansion of thioether biosynthesis in Escherichia coli generates sulfur-containing amino acids that can replace meso-diaminopimelate, the essential amino acid used for cross-linking the cell wall. This was accomplished by jointly overexpressing the metB gene coding for L-cystathionine gamma-synthase and disrupting the metC gene, whose product, L-cystathionine beta-lyase, is responsible for the destruction of L-cystathionine and other L-cysteine thioethers. As a result, meso-lanthionine and L-allo-cystathionine were produced endogenously and incorporated in the peptidoglycan, thereby enabling E. coli strains auxotrophic for diaminopimelate to grow in its absence. Thus, current techniques of metabolic engineering can be applied to evolving the chemical constitution of living cells beyond its present state.

The pathogenesis of homocysteinemia: interruption of the coordinate regulation by S-adenosylmethionine of the remethylation and transsulfuration of homocysteine.

A unified, biochemical hypothesis is proposed to explain the pathogenesis of homocysteinemia. This hypothesis is based on the existence of coordinate regulation by S-adenosylmethionine (SAM) of the partitioning of homocysteine between de novo methionine synthesis and catabolism through cystathionine synthesis. This coordination, which serves to modulate the cellular concentration of homocysteine based on the requirements for methionine, is impaired in homocysteinemia. This hypothesis is evaluated in the context of the conditions known to be associated with homocysteinemia, including enzymatic defects and vitamin deficiencies. The novelty of the hypothesis is the assertion that impairment of one homocysteine metabolic pathway must lead to the impairment of the other homocysteine metabolic pathway to cause homocysteinemia. This extends the simplistic view that a block of only one of the pathways is sufficient to cause homocysteinemia.

Methionine metabolism in mammals. The methionine-sparing effect of cystine.

Cystine can replace approximately 70% of the dietary requirement for methionine. We used standard enzyme assays, determinations of the hepatic concentrations of metabolites and an in vitro system which simulates the regulatory site formed by the enzymes which utilize homocysteine in this study of the mechanism for this adaptation. A significant alteration in the pattern of hepatic homocysteine metabolism occurs following the substitution of cystine for methionine. The major change is a marked reduction in the synthesis of cystathionine. Decreases in both the level of cystathionine synthase and in the concentration of adenosyl-methionine, a positive effector of the enzyme, explain this finding. Despite significant increases in the hepatic levels of betaine-homocysteine methyltransferase and methyltetrahydrofolate-homocysteine methyltransferase, flow through these reactions remains relatively constant. The betaine enzyme may be essential for efficient methionine conservation. In the absence of choline, cystine cannot replace methionine in an adequate diet limited in the latter amino acid.