Methionine Biosynthesis in Lemna: STUDIES ON THE REGULATION OF CYSTATHIONINE gamma-SYNTHASE, O-PHOSPHOHOMOSERINE SULFHYDRYLASE, AND O-ACETYLSERINE SULFHYDRYLASE

Regulation of enzymes of methionine biosynthesis was investigated by measuring the specific activities of O-phosphohomoserine-dependent cystathionine gamma-synthase, O-phosphohomoserine sulfhydrylase, and O-acetylserine sulfhydrylase in Lemna paucicostata Hegelm. 6746 grown under various conditions. For cystathionine gamma-synthase, it was observed that (a) adding external methionine (2 mum) decreased specific activity to 15% of control, (b) blocking methionine synthesis with 0.05 muml-aminoethoxyvinylglycine or with 36 mum lysine plus 4 mum threonine (Datko, Mudd 1981 Plant Physiol 69: 1070-1076) caused a 2- to 3-fold increase in specific activity, and (c) blocking methionine synthesis and adding external methionine led to the decreased specific activity characteristic of methionine addition alone. Activity in extracts from control cultures was unaffected by addition of methionine, lysine, threonine, lysine plus threonine, S-adenosylmethionine, or S-methylmethionine sulfonium to the assay mixture. Parallel studies of O-phosphohomoserine sulfhydrylase and O-acetylserine sulfhydrylase showed that O-phosphohomoserine sulfhydrylase activity responded to growth conditions identically to cystathionine gamma-synthase activity, whereas O-acetylserine sulfhydrylase activity remained unaffected. Lemna extracts did not catalyze lanthionine formation from O-acetylserine and cysteine. Estimates of kinetic constants for the three enzyme activities indicate that O-acetylserine sulfhydrylase has much higher activity and affinity for sulfide than O-phosphohomoserine sulfhydrylase.The results suggest that (a) methionine, or one of its products, regulates the amount of active cystathionine gamma-synthase in Lemna, (b) O-phosphohomoserine sulfhydrylase and cystathionine gamma-synthase are probably activities of one enzyme that has low specificity for its sulfur-containing substrate, and (c) O-acetylserine sulfhydrylase is a separate enzyme. The relatively high activity and affinity for sulfide of O-acetylserine sulfhydrylase provides an explanation in molecular terms for transsulfuration, and not direct sulfhydration, being the dominant pathway for homocysteine biosynthesis.     

A kinetic model of the branch-point between the methionine and threonine biosynthesis pathways in Arabidopsis thaliana.

This work proposes a model of the metabolic branch-point between the methionine and threonine biosynthesis pathways in Arabidopsis thaliana which involves kinetic competition for phosphohomoserine between the allosteric enzyme threonine synthase and the two-substrate enzyme cystathionine gamma-synthase. Threonine synthase is activated by S-adenosylmethionine and inhibited by AMP. Cystathionine gamma-synthase condenses phosphohomoserine to cysteine via a ping-pong mechanism. Reactions are irreversible and inhibited by inorganic phosphate. The modelling procedure included an examination of the kinetic links, the determination of the operating conditions in chloroplasts and the establishment of a computer model using the enzyme rate equations. To test the model, the branch-point was reconstituted with purified enzymes. The computer model showed a partial agreement with the in vitro results. The model was subsequently improved and was then found consistent with flux partition in vitro and in vivo. Under near physiological conditions, S-adenosylmethionine, but not AMP, modulates the partition of a steady-state flux of phosphohomoserine. The computer model indicates a high sensitivity of cystathionine flux to enzyme and S-adenosylmethionine concentrations. Cystathionine flux is sensitive to modulation of threonine flux whereas the reverse is not true. The cystathionine gamma-synthase kinetic mechanism favours a low sensitivity of the fluxes to cysteine. Though sensitivity to inorganic phosphate is low, its concentration conditions the dynamics of the system. Threonine synthase and cystathionine gamma-synthase display similar kinetic efficiencies in the metabolic context considered and are first-order for the phosphohomoserine substrate. Under these conditions outflows are coordinated.     

Methionine biosynthesis in lemna: inhibitor studies

A search was made for compounds that would inhibit methionine biosynthesis in Lemna paucicostata Hegelm. 6746. dl-Propargylglycine (0.15 micromolar) produced growth inhibition and morphological changes which were prevented by exogenous methionine. Also, dl-propargylglycine inhibits cystathionine gamma-synthase activity. l-Aminoethoxyvinylglycine (0.05 micromolar) produced growth inhibition and morphological changes partially preventable by exogenous methionine. l-Aminoethoxyvinylglycine impairs the cleavage of cystathionine to homocysteine. Lysine and threonine, at concentrations which individually had little effect on growth or morphology of Lemna, together produced growth inhibition and morphological changes preventable by exogenous methionine. The resulting metabolic block prevented conversion of cysteine to cystathionine, presumably secondary to depletion of the supply of O-phosphohomoserine.Inhibition of Lemna growth resulted when the molybdate:sulfate ratio in the medium was increased to 20:1 or more. Such inhibition was prevented by lowering this ratio to 0.3 or less. A non-steady-state experiment (molybdate:sulfate, 20:1) showed that molybdate inhibited sulfate uptake, but it provided no evidence of a further impairment in the organification of sulfate. Molybdate-induced growth inhibition of Lemna was prevented by cystine but not by cystathionine or methionine. Cystathionine is not converted by Lemna to cysteine rapidly enough to sustain growth.     

The role of amino acids in the regulation of hydrogen sulfide production during ultradian respiratory oscillation of Saccharomyces cerevisiae

We previously demonstrated that periodic H2S production during aerobic continuous culture of Saccharomyces cerevisiae resulted in ultradian respiratory oscillation, and that H2S production was dependent on the activity of sulfate uptake and the level of sulfite. To investigate the mechanism of regulation of the sulfate assimilation pathway and of respiratory oscillation, several amino acids were pulse-injected into cultures during respiratory oscillation. Injection of sulfur amino acids or their derivatives perturbed respiratory oscillation, with changes in the H2S production profile. Four major regulators of H2S production in the sulfate assimilation pathway and respiratory oscillation were identified: (1) O-acetylhomoserine, not O-acetylserine, as a sulfide acceptor, (2) homoserine/threonine as a regulator of O-acetylhomoserine supply, (3) methionine/S-adenosyl methionine as a negative regulator of sulfate assimilation, and (4) cysteine (or its derivatives) as an essential regulator. The results obtained after the addition of DL-propargylglycine (5 microM and 100 microM) and cystathionine (50 microM) suggested that the intracellular cysteine level and cystathionine gamma-lyase, rather than methionine/S-adenosylmethionine, play an essential role in the regulation of sulfate assimilation and respiratory oscillation. Based on these results and those of our previous reports, we propose that periodic depletion of cysteine (or its derivatives), which is involved in the detoxification of toxic materials originating from respiration, causes periodic H2S production.     

In vivo analysis of various substrates utilized by cystathionine gamma-synthase and O-acetylhomoserine sulfhydrylase in methionine biosynthesis

To gain insight into the evolution of the methionine biosynthesis pathway, in vivo complementation tests were performed. The substrate specificity of three enzymes that intrinsically use different homoserine-esterified substrates and have different sulfur assimilation pathways was examined: two cystathionine gamma-synthases (the Escherichia coli enzyme that naturally utilizes O-succinylhomoserine [OSH]) and the Arabidopsis thaliana enzyme that naturally exploits O-phosphohomoserine [OPH]. Both of these act through the transsulfuration pathway. The third enzyme investigated was O-acetylhomoserine (OAH) sulfhydrylase of Leptospira meyeri, representing the enzyme that utilizes OAH and operates through the direct sulfhydrylation pathway. All the three enzymes were able to utilize OSH and OAH as substrates, with different degrees of efficiency, but only the plant enzyme was able to utilize OPH as a substrate. In addition to their inherent activity in the transsulfuration pathway, the two cystathionine gamma-synthases were also capable of acting in the direct sulfhydrylation pathway. Based on the phylogenic tree and the results of the complementation tests, we suggest that the ancestral gene was able to act as OAH or OSH sulfhydrylase. In some bacteria and plants, this ancient enzyme most probably evolved into a cystathionine gamma-synthase, thereby maintaining the ability to utilize various homoserine-esterified substrates, as well as various sulfur sources, and thus keeping the multisubstrate specificity of its ancestor. In some organisms, this ancestral gene probably underwent a duplication event, which resulted in a cystathionine gamma-synthase and a separate OAH or OSH sulfhydrylase. This led to the development of two parallel pathways of methionine biosynthesis, transsulfuration and direct sulfhydrylation, in these organisms. Although both pathways exist in several organisms, most seem to favor a single specific pathway for methionine biosynthesis in vivo.     

Redox regulation and reaction mechanism of human cystathionine-beta-synthase: a PLP-dependent hemesensor protein

Cystathionine beta-synthase in mammals lies at a pivotal crossroad in methionine metabolism directing flux toward cysteine synthesis and catabolism. The enzyme exhibits a modular organization and complex regulation. It catalyzes the beta-replacement of the hydroxyl group of serine with the thiolate of homocysteine and is unique in being the only known pyridoxal phosphate-dependent enzyme that also contains heme b as a cofactor. The heme functions as a sensor and modulates enzyme activity in response to redox change and to CO binding. Mutations in this enzyme are the single most common cause of hereditary hyperhomocysteinemia. Elucidation of the crystal structure of a truncated and highly active form of the human enzyme containing the heme- and pyridoxal phosphate binding domains has afforded a structural perspective on mechanistic and mutation analysis studies. The C-terminal regulatory domain containing two CBS motifs exerts intrasteric regulation and binds the allosteric activator, S-adenosylmethionine. Studies with mammalian cells in culture as well as with animal models have unraveled multiple layers of regulation of cystathionine beta-synthase in response to redox perturbations and reveal the important role of this enzyme in glutathione-dependent redox homestasis. This review discusses the recent advances in our understanding of the structure, mechanism, and regulation of cystathionine beta-synthase from the perspective of its physiological function, focusing on the clinically relevant human enzyme.     

Cysteine regulates expression of cysteine dioxygenase and gamma-glutamylcysteine synthetase in cultured rat hepatocytes

Rat hepatocytes cultured for 3 days in basal medium expressed low levels of cysteine dioxygenase (CDO) and high levels of gamma-glutamylcysteine synthetase (GCS). When the medium was supplemented with 2 mmol/l methionine or cysteine, CDO activity and CDO protein increased by >10-fold and CDO mRNA increased by 1.5- or 3.2-fold. In contrast, GCS activity decreased to 51 or 29% of basal, GCS heavy subunit (GCS-HS) protein decreased to 89 or 58% of basal, and GCS mRNA decreased to 79 or 37% of basal for methionine or cysteine supplementation, respectively. Supplementation with cysteine consistently yielded responses of greater magnitude than did supplementation with an equimolar amount of methionine. Addition of propargylglycine to inhibit cystathionine gamma-lyase activity and, hence, cysteine formation from methionine prevented the effects of methionine, but not those of cysteine, on CDO and GCS expression. Addition of buthionine sulfoximine to inhibit GCS, and thus block glutathione synthesis from cysteine, did not alter the ability of methionine or cysteine to increase CDO. GSH concentration was not correlated with changes in either CDO or GCS-HS expression. The effectiveness of cysteine was equivalent to or greater than that of its precursors (S-adenosylmethionine, cystathionine, homocysteine) or metabolites (taurine, sulfate). Taken together, these results suggest that cysteine itself is an important cellular signal for upregulation of CDO and downregulation of GCS.     

The transsulfuration pathway in Tetrahymena pyriformis.

Four enzymes necessary for the metabolism of methionine by the trans-sulfuration pathway, methionine adenosyltransferase (EC 2.5.1.6), adenosylhomocysteinase (EC 3.3.1.1), cystathionine beta-synthase (EC 4.2.1.22) and cystathionine gamma-lyase (EC 4.4.1.1) were identified in Tetrahymean pyriformis. The ability of these cells to transfer 35S from E135S]methionine to form [35S] cysteine was also observed and taken as direct evidence for the functional existence of this pathway in Tetrahymena. An intermediate in the pathway and an active methyl donor, S-adenosylmethionine, was qualitatively identified in Tetrahymena and its concentration was found to be greater in late stationary phase cells than in early stationary phase cells.     

Alterations in hepatic metabolism of sulfur-containing amino acids induced by ethanol in rats

Summary.  Alterations in hepatic metabolism of S-amino acids were monitored over one week in male rats treated with a single dose of ethanol (3 g/kg, ip). Methionine and S-adenosylhomocysteine concentrations were increased rapidly, but S-adenosylmethionine, cysteine, and glutathione (GSH) decreased following ethanol administration. Activities of methionine adenosyltransferase, cystathionine γ-lyase and cystathionine β-synthase were all inhibited. γ-Glutamylcysteine synthetase activity was increased from t = 8 hr, but GSH level did not return to control for 24 hr. Hepatic hypotaurine and taurine levels were elevated immediately, but reduced below control in 18 hr. Changes in serum and urinary taurine levels were consistent with results observed in liver. Cysteine dioxygenase activity was increased rapidly, but declined from t = 24 hr. The results show that a single dose of ethanol induces profound changes in hepatic S-amino acid metabolism, some of which persist for several days. Ethanol not only inhibits the cysteine synthesis but suppresses the cysteine availability further by enhancing its irreversible catabolism to taurine, which would play a significant role in the depletion of hepatic GSH. Received April 26, 2002 Accepted June 12, 2002 Published online October 14, 2002 Authors' address: Young C. Kim, Ph.D., Professor of Toxicology, College of Pharmacy, Seoul National University, San 56-1 Shinrim-Dong, Kwanak-Ku, Seoul, Korea, Fax: +82-2-872-1795, E-mail: youckim@snu.ac.kr Abbreviations: CβS, cystathionine β-synthase; CDC, cysteine sulfinate decarboxylase; CDO, cysteine dioxygenase; CγL, cystathionine γ-lyase; GCS, γ-Glutamylcysteine synthetase; GSH, glutathione; MAT, methionine adenosyltransferase; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine.     

The biosynthesis of cysteine and homocysteine in Methanococcus jannaschii

The pathway for the biosynthesis of cysteine and homocysteine in Methanococcus jannaschii has been examined using a gas chromatography-mass spectrometry (GC-MS) stable isotope dilution method to identify and quantitate the intermediates in the pathways. The first step in the pathway, and the one responsible for incorporation of sulfur into both cysteine and methionine, is the reaction between O-phosphohomoserine and a presently unidentified sulfur source present in cell extracts, to produce L-homocysteine. This sulfur source was shown not to be sulfide. The resulting L-homocysteine then reacts with O-phosphoserine to form L-cystathionine, which is cleaved to L-cysteine. The pathway has elements of both the plant and mammalian pathways in that the sulfur is first incorporated into homocysteine using O-phosphohomoserine as the acceptor and the resulting homocysteine, via transsulfuration, supplies the sulfur for cysteine formation. The pathway leading to these two amino acids represents an example of metabolic thrift where the preexisting cellular metabolites O-phosphohomoserine and O-phosphoserine are used as the ultimate source of the carbon framework for the biosynthesis of these amino acids. These findings explain the absence of identifiable genes in the genome of this organism for the biosynthesis of cysteine and homocysteine.     

Characterization of transsulfuration and cysteine biosynthetic pathways in the protozoan hemoflagellate, Trypanosoma cruzi. Isolation and molecular characterization of cystathionine beta-synthase and serine acetyltransferase from Trypanosoma

Sulfur-containing amino acids play an important role in a variety of cellular functions such as protein synthesis, methylation, and polyamine and glutathione synthesis. We cloned and characterized cDNA encoding cystathionine beta-synthase (CBS), which is a key enzyme of transsulfuration pathway, from a hemoflagellate protozoan parasite Trypanosoma cruzi. T. cruzi CBS, unlike mammalian CBS, lacks the regulatory carboxyl terminus, does not contain heme, and is not activated by S-adenosylmethionine. T. cruzi CBS mRNA is expressed as at least six independent isotypes with sequence microheterogeneity from tandemly linked multicopy genes. The enzyme forms a homotetramer and, in addition to CBS activity, the enzyme has serine sulfhydrylase and cysteine synthase (CS) activities in vitro. Expression of the T. cruzi CBS in Saccharomyces cerevisiae and Escherichia coli demonstrates that the CBS and CS activities are functional in vivo. Enzymatic studies on T. cruzi extracts indicate that there is an additional CS enzyme and stage-specific control of CBS and CS expression. We also cloned and characterized cDNA encoding serine acetyltransferase (SAT), a key enzyme in the sulfate assimilatory cysteine biosynthetic pathway. Dissimilar to bacterial and plant SAT, a recombinant T. cruzi SAT showed allosteric inhibition by l-cysteine, l-cystine, and, to a lesser extent, glutathione. Together, these studies demonstrate the T. cruzi is a unique protist in possessing both transsulfuration and sulfur assimilatory pathways.     

Alleviation of intrasteric inhibition by the pathogenic activation domain mutation,D444N,in human cystathionine beta-synthase

Human cystathionine beta-synthase is a heme protein that catalyzes the condensation of serine and homocysteine to form cystathionine in a pyridoxal phosphate-dependent reaction. Mutations in this enzyme are the leading cause of hereditary hyperhomocysteinemia with attendant cardiovascular and other complications. The enzyme is activated approximately 2-fold by the allosteric regulator S-adenosylmethionine (AdoMet), which is presumed to bind to the C-terminal regulatory domain. The regulatory domain exerts an inhibitory effect on the enzyme, and its deletion is correlated with a 2-fold increase in catalytic activity and loss of responsiveness to AdoMet. A mutation in the C-terminal regulatory domain, D444N, displays high levels of enzyme activity, yet is pathogenic. In this study, we have characterized the biochemical penalties associated with this mutation and demonstrate that it is associated with a 4-fold lower steady-state level of cystathionine beta-synthase in a fibroblast cell line that is homozygous for the D444N mutation. The activity of the recombinant D444N enzyme mimics the activity of the wild-type enzyme seen in the presence of AdoMet and can be further activated approximately 2-fold in the presence of supraphysiolgical concentrations of the allosteric regulator. The mutation increases the K(act) for AdoMet from 7.4 +/- 0.2 to 460 +/- 130 microM, thus rendering the enzyme functionally unresponsive to AdoMet under physiological concentrations. These results indicate that the D444N mutation partially abrogates the intrasteric inhibition imposed by the C-terminal domain. We propose a model that takes into account the three kinetically distinguishable states that are observed with human cystathionine beta-synthase: "basal" (i.e., wild-type enzyme as isolated), "activated" (wild-type enzyme + AdoMet or the D444N mutant as isolated), and superactivated (D444N mutant + AdoMet or wild-type enzyme lacking the C-terminal regulatory domain).     

Functional demonstration of reverse transsulfuration in the Mycobacterium tuberculosis complex reveals that methionine is the preferred sulfur source for pathogenic Mycobacteria

Methionine can be used as the sole sulfur source by the Mycobacterium tuberculosis complex although it is not obvious from examination of the genome annotation how these bacteria utilize methionine. Given that genome annotation is a largely predictive process, key challenges are to validate these predictions and to fill in gaps for known functions for which genes have not been annotated. We have addressed these issues by functional analysis of methionine metabolism. Transport, followed by metabolism of (35)S methionine into the cysteine adduct mycothiol, demonstrated the conversion of exogenous methionine to cysteine. Mutational analysis and cloning of the Rv1079 gene showed it to encode the key enzyme required for this conversion, cystathionine gamma-lyase (CGL). Rv1079, annotated metB, was predicted to encode cystathionine gamma-synthase (CGS), but demonstration of a gamma-elimination reaction with cystathionine as well as the gamma-replacement reaction yielding cystathionine showed it encodes a bifunctional CGL/CGS enzyme. Consistent with this, a Rv1079 mutant could not incorporate sulfur from methionine into cysteine, while a cysA mutant lacking sulfate transport and a methionine auxotroph was hypersensitive to the CGL inhibitor propargylglycine. Thus, reverse transsulfuration alone, without any sulfur recycling reactions, allows M. tuberculosis to use methionine as the sole sulfur source. Intracellular cysteine was undetectable so only the CGL reaction occurs in intact mycobacteria. Cysteine desulfhydrase, an activity we showed to be separable from CGL/CGS, may have a role in removing excess cysteine and could explain the ability of M. tuberculosis to recycle sulfur from cysteine, but not methionine.     

Cigarette smoke extract increases S-adenosylmethionine and cystathionine in human lung epithelial-like (A549) cells

The effect of cigarette smoke extract (CSE) on S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), and sulfur amino acid metabolism was examined in human lung epithelial-like (A549) cells exposed to various CSE concentrations (2.5-100%) for 24 or 48 h. Intracellular SAM and SAM/SAH ratio were elevated after exposure to CSE for 48 h. Cell SAH content decreased, but the effect was not consistent. Cellular cystathionine, cysteine, and methionine levels were increased after CSE exposure for 48h. Sub-acute exposure to CSE induced increases in cellular SAM and SAM/SAH ratio. The transsulfuration pathway was likely activated by CSE since cystathionine increased, potentially contributing to the increased total intracellular GSH content.     

Inhibition of cystathionine-gamma-lyase leads to loss of glutathione and aggravation of mitochondrial dysfunction mediated by excitatory amino acid in the CNS

Oxidative stress has been implicated in the pathogenesis and progression of neurodegenerative disorders and antioxidants potentially have a major role in neuroprotection. Optimum levels of glutathione (gamma-glutamylcysteinyl glycine), an endogenous thiol antioxidant are required for the maintenance of the redox status of cells. Cystathionine gamma-lyase is the rate-limiting enzyme for the synthesis of cysteine from methionine and availability of cysteine is a critical factor in glutathione synthesis. In the present study, we have examined the role of cystathionine gamma-lyase in maintaining the redox homeostasis in brain, particularly with reference to mitochondrial function since the complex I of the electron transport chain is sensitive to redox perturbation. Inhibition of cystathionine gamma-lyase by l-propargylglycine caused loss of glutathione and decrease in complex I activity in the brain although the enzyme activity in mouse brain was 1% of the corresponding hepatic activity. We then examined the effect of this inhibition on the neurotoxicity mediated by the excitatory amino acid, l-beta-oxalyl amino-l-alanine, which is the causative factor of a type of motor neuron disease, neurolathyrism. l-beta-Oxalyl amino-l-alanine toxicity was exacerbated by l-propargylglycine measured as loss of complex I activity indicating the importance of cystathionine gamma-lyase in maintaining glutathione levels and in turn the mitochondrial function during excitotoxicity. Oxidative stress generated by l-beta-oxalyl amino-l-alanine itself inhibited cystathionine gamma-lyase, which could be prevented by prior treatment with thiol antioxidant. Thus, cystathionine gamma-lyase itself is susceptible to inactivation by oxidative stress and this can potentially exacerbate oxidant-induced damage. Cystathionine gamma-lyase is present in neuronal cells in human brain and its activity is several-fold higher compared to mouse brain. It could potentially play an important role in maintaining glutathione and protein thiol homeostasis in brain and hence afford neuroprotection.     

Production of the neuromodulator H2S by cystathionine beta-synthase via the condensation of cysteine and homocysteine

Hydrogen sulfide (H2S) has been observed in relatively high concentrations in the mammalian brain and has been shown to act as a neuromodulator. However, there is confusion in the literature regarding the actual source of H2S production. Reactions catalyzed by the cystathionine beta-synthase enzyme (CBS) are one possible source for the production of H2S. Here we show that the CBS enzyme can efficiently produce H2S via a beta-replacement reaction in which cysteine is condensed with homocysteine to form cystathionine and H2S. The production of H2S by this reaction is at least 50 times more efficient than that produced by hydrolysis of cysteine alone via beta-elimination. Kinetic studies demonstrate that the Km and Kcat for cysteine is 3-fold higher and 2-fold lower, respectively, than that for serine. Consistent with these data, in vitro reconstitution studies show that at physiologically relevant concentrations of serine, homocysteine, and cysteine, about 5% of the cystathionine formed is from cysteine. We also show that AdoMet stimulates this H2S producing reaction but that there is no evidence for stimulation by calcium and calmodulin as reported previously. In summary, these results confirm the ability of CBS to produce H2S, but show in contrast to prior reports that the major mechanism is via beta-replacement and not cysteine hydrolysis. In addition, these studies provide a biochemical explanation for the previously inexplicable homocysteine-lowering effects of N-acetylcysteine treatments in humans.