Identification of the reactive sulfhydryl groups of S-adenosylmethionine synthetase

MetJ-Based Mutually Interfering SAM-ON/SAM-OFF Biosensors

SAM (S-adenosylmethionine) is an important metabolite that operates as a major donor of methyl groups and is a controller of various physiological processes. Its availability is also believed to be a major bottleneck in the biological production of numerous high-value metabolites. Here, we constructed SAM-sensing systems using MetJ, an SAM-dependent transcriptional regulator, as a core component. SAM is a corepressor of MetJ, which suppresses the MetJ promoter with an increasing cellular concentration of SAM (SAM-OFF sensor). The application of transcriptional interference and evolutionary tuning effectively inverted its response, yielding a SAM-ON sensor (signal increases with increasing SAM concentration). By linking two genes encoding fluorescent protein reporters in such a way that their transcription events interfere with each other's and by placing one of them under the control of MetJ, we could increase the effective signal-to-noise ratio of the SAM sensor while decreasing the batch-to-batch deviation in signal output, likely by canceling out the growth-associated fluctuation in translational resources. By taking the ratio of SAM-ON/SAM-OFF signals and by resetting the default pool size of SAM, we could rapidly identify SAM synthetase (MetK) mutants with increased cellular activity from a random library. The strategy described herein should be widely applicable for identifying activity mutants, which would be otherwise overlooked because of the strong homeostasis of metabolic networks.

Multiple inputs control sulfur-containing amino acid synthesis in Saccharomyces cerevisiae

Genes in Saccharomyces cerevisiae involved in sulfur-containing amino acid synthesis are transcriptionally induced by either cysteine or S-adenosyl-methionine deficiency, as well as defects in phosphatidylcholine synthesis. Met30p, a regulator of these genes, changes physically in inducing conditions, which may mediate its regulatory activity.

In Saccharomyces cerevisiae, transcription of the MET regulon, which encodes the proteins involved in the synthesis of the sulfur-containing amino acids methionine and cysteine, is repressed by the presence of either methionine or cysteine in the environment. This repression is accomplished by ubiquitination of the transcription factor Met4, which is carried out by the SCF(Met30) E3 ubiquitin ligase. Mutants defective in MET regulon repression reveal that loss of Cho2, which is required for the methylation of phosphatidylethanolamine to produce phosphatidylcholine, leads to induction of the MET regulon. This induction is due to reduced cysteine synthesis caused by the Cho2 defects, uncovering an important link between phospholipid synthesis and cysteine synthesis. Antimorphic mutants in S-adenosyl-methionine (SAM) synthetase genes also induce the MET regulon. This effect is due, at least in part, to SAM deficiency controlling the MET regulon independently of SAM's contribution to cysteine synthesis. Finally, the Met30 protein is found in two distinct forms whose relative abundance is controlled by the availability of sulfur-containing amino acids. This modification could be involved in the nutritional control of SCF(Met30) activity toward Met4.

NADP+ binding to the regulatory subunit of methionine adenosyltransferase II increases intersubunit binding affinity in the hetero-trimer

Mammalian methionine adenosyltransferase II (MAT II) is the only hetero-oligomer in this family of enzymes that synthesize S-adenosylmethionine using methionine and ATP as substrates. Binding of regulatory β subunits and catalytic α2 dimers is known to increase the affinity for methionine, although scarce additional information about this interaction is available. This work reports the use of recombinant α2 and β subunits to produce oligomers showing kinetic parameters comparable to MAT II purified from several tissues. According to isothermal titration calorimetry data and densitometric scanning of the stained hetero-oligomer bands on denatured gels, the composition of these oligomers is that of a hetero-trimer with α2 dimers associated to single β subunits. Additionally, the regulatory subunit is able to bind NADP(+) with a 1:1 stoichiometry, the cofactor enhancing β to α2-dimer binding affinity. Mutants lacking residues involved in NADP(+) binding and N-terminal truncations of the β subunit were able to oligomerize with α2-dimers, although the kinetic properties appeared altered. These data together suggest a role for both parts of the sequence in the regulatory role exerted by the β subunit on catalysis. Moreover, preparation of a structural model for the hetero-oligomer, using the available crystal data, allowed prediction of the regions involved in β to α2-dimer interaction. Finally, the implications that the presence of different N-terminals in the β subunit could have on MAT II behavior are discussed in light of the recent identification of several splicing forms of this subunit in hepatoma cells.

Refolding and characterization of methionine adenosyltransferase from Euglena gracilis

Methionine adenosyltransferase from Euglena gracilis (MATX) is a recently discovered member of the MAT family of proteins that synthesize S-adenosylmethionine. Heterologous overexpression of MATX in Escherichia coli rendered the protein mostly in inclusion bodies under all conditions tested. Therefore, a refolding and purification procedure from these aggregates was developed to characterize the enzyme. Maximal recovery was obtained using inclusion bodies devoid of extraneous proteins by washing under mild urea (2M) and detergent (5%) concentrations. Refolding was achieved in two steps following solubilization in the presence of Mg(2+); chaotrope dilution to <1M and dialysis under reducing conditions. Purified MATX is a homodimer that exhibits Michaelis kinetics with a V(max) of 1.46 μmol/min/mg and K(m) values of approximately 85 and 260 μM for methionine and ATP, respectively. The activity is dependent on Mg(2+) and K(+) ions, but is not stimulated by dimethylsulfoxide. MATX exhibits tripolyphosphatase activity that is stimulated in the presence of S-adenosylmethionine. Far-UV circular dichroism revealed β-sheet and random coil as the main secondary structure elements of the protein. The high level of sequence conservation allowed construction of a structural model that preserved the main features of the MAT family, the major changes involving the N-terminal domain.

GAP promoter library for fine-tuning of gene expression in Pichia pastoris

A library of engineered promoters of various strengths is a useful genetic tool that enables the fine-tuning and precise control of gene expression across a continuum of broad expression levels. The methylotrophic yeast Pichia pastoris is a well-established expression host with a large academic and industrial user base. To facilitate manipulation of gene expression spanning a wide dynamic range in P. pastoris, we created a functional promoter library through mutagenesis of the constitutive GAP promoter. Using yeast-enhanced green fluorescent protein (yEGFP) as the reporter, 33 mutants were chosen to form the functional promoter library. The 33 mutants spanned an activity range between ∼0.6% and 19.6-fold of the wild-type promoter activity with an almost linear fluorescence intensity distribution. After an extensive characterization of the library, the broader applicability of the results obtained with the yEGFP reporter was confirmed using two additional reporters (β-galactosidase and methionine adenosyltransferase [MAT]) at the transcription and enzyme activity levels. Furthermore, the utility of the promoter library was tested by investigating the influence of heterologous MAT gene expression levels on cell growth and S-adenosylmethionine (SAM) production. The extensive characterization of the promoter strength enabled identification of the optimal MAT activity (around 1.05 U/mg of protein) to obtain maximal volumetric SAM production. The promoter library permits precise control of gene expression and quantitative assessment that correlates gene expression level with physiologic parameters. Thus, it is a useful toolbox for both basic and applied research in P. pastoris.

Identification of a mutation in the Bacillus subtilis S-adenosylmethionine synthetase gene that results in derepression of S-box gene expression

Genes in the S-box family are regulated by binding of S-adenosylmethionine (SAM) to the 5' region of the mRNA of the regulated gene. SAM binding was previously shown to promote a rearrangement of the RNA structure that results in premature termination of transcription in vitro and repression of expression of the downstream coding sequence. The S-box RNA element therefore acts as a SAM-binding riboswitch in vitro. In an effort to identify factors other than SAM that could be involved in the S-box regulatory mechanism in vivo, we searched for trans-acting mutations in Bacillus subtilis that act to disrupt repression of S-box gene expression during growth under conditions where SAM pools are elevated. We identified a single mutant that proved to have one nucleotide substitution in the metK gene, encoding SAM synthetase. This mutation, designated metK10, resulted in a 15-fold decrease in SAM synthetase activity and a 4-fold decrease in SAM concentration in vivo. The metK10 mutation specifically affected S-box gene expression, and the increase in expression under repressing conditions was dependent on the presence of a functional transcriptional antiterminator element. The observation that the mutation identified in this search affects SAM production supports the model that the S-box RNAs directly monitor SAM in vivo, without a requirement for additional factors.

Conformational dynamics of the active site loop of S-adenosylmethionine synthetase illuminated by site-directed spin labeling

S-adenosylmethionine synthetase (ATP: L-methionine S-adenosyltransferase, methionine adenosyltransferase, a.k.a. MAT) is one of numerous enzymes that have a flexible polypeptide loop that moves to gate access to the active site in a motion that is closely coupled to catalysis. Crystallographic studies of this tetrameric enzyme have shown that the loop is closed in the absence of bound substrates. However, the loop must open to allow substrate binding and a variety of data indicate that the loop is closed during the catalytic steps. Previous kinetic studies indicate that during turnover loop motion occurs on a time scale of 10(-2)s, ca. 10-fold faster than chemical transformations and turnover. Site-directed spin labeling has been used to introduce nitroxide groups at two positions in the loop to illuminate how the motion of the loop is affected by substrate binding. The two loop mutants constructed, G105C and D107C, retain wild type levels of MAT activity; attachment of a methanethiosulfonate spin label to convert the cysteine to the "R1" residue reduced the k(cat) only for the labeled D107R1 form (7-fold). The K(m) value for methionine increased 2- to 4-fold for the cysteine mutants and 2- to 7-fold for the labeled proteins, whereas the K(m) for ATP was changed by at most 2-fold. EPR spectra for both labeled proteins are nearly identical and show the presence of two major spin label environments with rotational diffusion rates differing by approximately 10-fold; the slower rate is ca. 4-fold faster than the estimated protein rotational rate. The spectra are not altered by addition of substrates or products. At both positions the less mobile conformation constitutes ca. 65% of the total species, indicating an equilibrium that only slightly favors one form, that in which the label is more immobilized. The equilibrium constant that relates the two forms is comparable to the equilibrium constant of 1.5 for a conformational change that was previously deduced from the viscosity dependence of the rate of AdoMet formation. The results suggest that the motion of the loop may be an intrinsic property of the protein and not be strictly ligand modulated.

Role of an intrasubunit disulfide in the association state of the cytosolic homo-oligomer methionine adenosyltransferase

Recombinant rat liver methionine adenosyltransferase has been refolded into fully active tetramers (MAT I) and dimers (MAT III), using as a source chaotrope-solubilized aggregates resulting from specific washes of inclusion bodies. The conditions of refolding, dialysis in the presence of 10 mm dithiothreitol or 10 mm GSH with 1 mm GSSG, allowed the production of both isoforms, the nature of the redox agent determining the capacity of the final product (MAT I/III) to interconvert. Refolding in the presence of 10 mm dithiothreitol yielded mainly MAT III in a concentration-dependent equilibrium with the homotetramer MAT I. However, refolding in the presence of the redox pair GSH/GSSG resulted in a stable MAT I and III mixture. Blockage of dimer-tetramer interconversion has been found related to the production of a single intramolecular disulfide in methionine adenosyltransferase during the GSH/GSSG folding process. The residues involved in this disulfide have been identified by mass spectrometry and using a set of single cysteine mutants as cysteines 35 and 61. In addition, a kinetic intermediate in the MAT I dissociation to MAT III has been detected. The physiological importance of these results is discussed in light of the structural and regulatory data available.

Leishmania donovani methionine adenosyltransferase. Role of cysteine residues in the recombinant enzyme

Methionine adenosyltransferase (MAT, EC 2.5.1.6)-mediated synthesis of S-adenosylmethionine (AdoMet) is a two-step process consisting of the formation of AdoMet and the subsequent cleavage of the tripolyphosphate (PPPi) molecule, a reaction induced, in turn, by AdoMet. The fact that the two activities, AdoMet synthesis and tripolyphosphate hydrolysis, can be measured separately is particularly useful when the site-directed mutagenesis approach is used to determine the functional role of the amino acid residues involved in each. The present report describes the cloning and subsequent functional refolding, using a bacterial expression system, of the MAT gene (GenBank accession number AF179714) from Leishmania donovani, the etiological agent of visceral leishmaniasis. The absolute need to include a sulfhydryl-protection reagent in the refolding buffer for this protein, in conjunction with the rapid inactivation of the functionally refolded protein by N-ethylmaleimide, suggests the presence of crucial cysteine residues in the primary structure of the MAT protein. The seven cysteines in L. donovani MAT were mutated to their isosterical amino acid, serine. The C22S, C44S, C92S and C305S mutants showed a drastic loss of AdoMet synthesis activity compared to the wild type, and the C33S and C47S mutants retained a mere 12% of wild-type MAT activity. C106S mutant activity and kinetics remained unchanged with respect to the wild-type. Cysteine substitutions also modified PPPi cleavage and AdoMet induction. The C22S, C44S and C305S mutants lacked in tripolyphosphatase activity altogether, whereas C33S, C47S and C92S retained low but detectable activity. The behavior of the C92S mutant was notable: its inability to synthesize AdoMet combined with its retention of tripolyphosphatase activity appear to be indicative of the specific involvement of the respective residue in the first step of the MAT reaction.

Acetaminophen-induced suppression of hepatic AdoMet synthetase activity is attenuated by prodrugs of L-cysteine

Administration of acetaminophen (ACP, 400 mg/kg, i.p.) to fasted, male Swiss-Webster mice caused a rapid 90% decrease in total hepatic glutathione (GSH) and a 58% decrease in mitochondrial GSH by 2 h post ACP. This was followed by a time-dependent decrease (72%) in hepatic AdoMet synthetase activity and rise in plasma ALT levels (>10000 U/l) at 24 h post ACP treatment. AdoMet synthetase activity was maintained at 82, 78 and 60% of controls, respectively, by the cysteine prodrugs PTCA, CySSME and NAC. Total hepatic and mitochondrial GSH levels were also protected from severe ACP-induced depletion by CySSME and MTCA. These results suggest that the maintenance of GSH homeostasis by cysteine prodrugs can protect mouse hepatic AdoMet synthetase, a sulfhydryl enzyme whose integrity is dependent on GSH, as well as the liver itself from the consequences of oxidative stress elicited by toxic metabolites of xenobiotics.

The active-site arginine of S-adenosylmethionine synthetase orients the reaction intermediate

S-Adenosylmethionine (AdoMet) synthetase catalyzes the formation of AdoMet and tripolyphosphate (PPPi) from ATP and L-methionine and the subsequent hydrolysis of the PPPi to PPi and Pi before product release. Little is known about the roles of active-site residues involved in catalysis of the two sequential reactions that occur at opposite ends of the polyphosphate chain. Crystallographic studies of Escherichia coli AdoMet synthetase showed that arginine-244 is the only arginine near the polyphosphate-binding site. Arginine-244 is embedded as the seventh residue in the conserved sequence DxGxTxxKxI which is also found at the active site of inorganic pyrophosphatases, suggesting a potential pyrophosphate-binding motif. Chemical modification of AdoMet synthetase by the arginine-specific reagents phenylglyoxal or p-hydroxyphenylglyoxal inactivates the enzyme. ATP and PPPi protect the enzyme from inactivation, consistent with the presence of an important arginine residue in the vicinity of the polyphosphate-binding site. Site-specific mutagenesis has been used to change the conserved arginine-244 to either leucine (R244L) or histidine (R244H). In the overall reaction, the R244L mutant has the kcat reduced approximately 10(3)-fold, with a 7 to 10-fold increase in substrate Km values; the R244H mutant has an approximately 10(5)-fold decrease in kcat. In contrast, the kcat values for hydrolysis of added PPPi by the R244L and R244H mutants have been reduced by less than 2 orders of magnitude. In contrast to the wild-type enzyme in which 98% of the Pi formed originates as the gamma-phosphoryl group of ATP, in the R244L mutant the orientation of the PPPi intermediate equilibrates at the active site yielding equal amounts of Pi from the alpha- and gamma-phosphoryl groups of ATP. Thus, the active-site arginine has a profound role in the cleavage of PPPi from ATP during AdoMet formation and in maintaining the orientation of PPPi in the active site, while playing a lesser role in the subsequent PPPi hydrolytic reaction.

Recombinant rat liver S-adenosyl-L-methionine synthetase tetramers and dimers are in equilibrium

Rat liver S-adenosyl-L-methionine synthetase is present in two oligomeric forms, tetramers and dimers, with different substrate kinetics and regulation. In vivo the relative amounts of both forms may change in some instances. The basis of this regulatory mechanism is not known. When rat liver cDNA was used to express the protein in Escherichia coli the two oligomeric forms were found. Gel filtration chromatography of the purified recombinant enzyme suggested that these two isoforms might be in equilibrium. This was confirmed by kinetic experiments which showed that the specific activity of the enzyme was dependent on the protein concentration. From these experiments, apparent equilibrium constants of (5.6 +/- 0.4) x 10(5) M-1 and (3.5 +/- 0.9) x 10(5) M-1 were obtained at 2mM and 60 microM methionine concentrations, respectively. Using hydrophobic chromatography on phenyl-Sepharose to separate the tetrameric and dimeric forms, an equilibrium constant of (4.9 +/- 0.7) x 10(5) M-1 was calculated. A rate constant for the dissociation of the tetramer of k-1 = (8.1 +/- 0.4) x 10(-4) s-1 at 4 degrees C was also calculated using the same approach. In summary, we have shown that the rat liver S-adenosyl-L-methionine synthetase produced in bacterial cells is present in two oligomeric forms, tetramers and dimers, which are in equilibrium. This system might be useful for studying the dynamics and the regulation of the distribution of oligomeric forms in the mammalian liver.

Structure and function of S-adenosylmethionine synthetase: crystal structures of S-adenosylmethionine synthetase with ADP, BrADP, and PPi at 28 angstroms resolution

S-Adenosylmethionine synthetase (MAT,ATP:L-methionine S-adenosltransferase, EC 2.5.1.6) plays a central metabolic role in all organisms. MAT catalyzes the two-step reaction which synthesizes S-adenosylmethionine (AdoMet), pyrophosphate (PPi), and orthophosphate (Pi) from ATP and L-methionine. AdoMet is the primary methyl group donor in biological systems. The first crystal structure of MAT from Escherichia coli has recently been determined [Takusagawa et al. (1995) J. Biol. Chem. 271, 136-147]. In order to elucidate the active site and possible catalytic reaction mechanism, the MAT structures in the crystals grown with the substrate ATP (and BrATP) and the product PPi have been determined (space group P6(2)22; unit cell a = b = 128.9 Angstroms, c= 139.8 Angstroms, resolution limit 2.8 Angstroms; R O.19; Rfree 0.26). The enzyme consists of four identical subunits; two subunits form a spherical dimer, and pairs of these tightly bound dimers form a tetrameric enzyme. Each dimer has two active sites which are located between the subunits. Each subunit consists of three domains related to each other by a pseudo 3-fold symmetry. The crystal structures showed that the ATP molecules were hydrolyzed to ADP and Pi by the enzyme. Those products were found at the active site along with the essential metal ions (K+ and Mg2+). This rather unexpected finding was first confirmed by the structure of the complex with PPi and later by an HPLC analysis. The enzyme hydrolyzed ATP to ADP and Pi in 72 h under the same conditions as the crystallization of the enzyme. In the active site, the diphosphate moiety of ADP and Pi interacts extensively with amino acid residues from the two subunits of the enzyme, whereas the adenine and ribose moieties have little interaction with the enzyme. The enzyme structure is little changed upon binding ADP. All amino acid residues involved in the active site are found to be conserved in the 14 reported sequences of MAT from a wide range of organisms. Thus the structure determined in this study can be utilized as a model for other members of the MAT family. On the basis of the crystal structures, the catalytic reaction mechanisms of AdoMet formation and hydrolysis of tripolyphosphate are proposed.

Structural and functional roles of cysteine 90 and cysteine 240 in S-adenosylmethionine synthetase

Site-specific mutagenesis was performed on the structural gene for Escherichia coli S-adenosylmethionine (AdoMet) synthetase to introduce mutations at cysteines 90 and 240, residues previously implicated by chemical modification studies to be catalytically and/or structurally important. The AdoMet synthetase mutants (i.e. MetK/C90A, MetK/C90S, and MetK/C240A) retained up to approximately 10% of wild type activity, demonstrating that neither sulfhydryl is required for catalytic activity. Mutations at Cys-90 produced a mixture of noninterconverting dimeric and tetrameric proteins, suggesting a structural significance for Cys-90. Dimeric Cys-90 mutants retained approximately 1% of wild type activity, indicating a structural influence on enzyme activity. Both dimeric and tetrameric MetK/C90A had up to a approximately 70-fold increase in Km for ATP, while both dimeric and tetrameric MetK/C90S had Km values for ATP similar to the wild type enzyme, suggesting a linkage between Cys-90 and the ATP binding site. MetK/C240A was isolated solely as a tetramer and differed from wild type enzyme only in its 10-fold reduction in specific activity, suggesting that the mutation affects the rate-limiting step of the reaction, which for the wild type enzyme is the joining of ATP and L-methionine to yield AdoMet and tripolyphosphate. Remarkably all of the mutants are much more thermally stable than the wild type enzyme.

Methionine adenosyltransferase: structure and function

Methionine adenosyltransferase (MAT), a key enzyme in metabolism, catalyzes the synthesis of one of the most important and pivotal biological molecules, S-adenosyl-methionine. In every organism studied thus far, MAT exists in multiple forms; most are encoded by related, but distinct genes. Molecular and immunological studies revealed the presence of considerable conservation in the structure of MAT from different species; however, the various MAT isozymes differ in their physical and kinetic properties in ways that allow them to be regulated differently. Recent studies suggest that human MAT is composed of nonidentical subunits that can assume multiple states of aggregation, each with different kinetic characteristics. The tissue distribution of MAT isozymes and the ability of cells within the same tissue to switch between the different forms of MAT suggest that this mode of regulation is important for cellular function and differentiation. Therefore, understanding the regulation and structure-function relationship of this fascinating enzyme should help us clarify its role in biology and may provide us with tools to effectively manipulate its activity in clinical situations such as cancer, autoimmunity and organ transplantation.

How is rat liver S-adenosylmethionine synthetase regulated?

The in vivo regulation of S-adenosylmethionine synthetase, a key enzyme in methionine metabolism, is so far unknown. The enzyme activity has been shown to be modulated by glutathione and the oxidation state of its sulfhydryl groups. Analysis of the protein sequence has revealed the presence of putative phosphorylation sites. A mixed regulatory mechanism combining phosphorylation and the oxido/reduction of sulfhydryl groups is proposed. The role of glutathione in this mechanism is also discussed.

Antigenic conservation of primary structural regions of S-adenosylmethionine synthetase

Although the physical and kinetic properties of S-adenosylmethionine (AdoMet) synthetases from different sources are quite different, it appears that these enzymes have structurally or antigenically conserved regions as demonstrated by studies with AdoMet synthetase specific antibodies. Polyclonal anti-human lymphocyte AdoMet synthetase crossreacted with enzyme from rat liver (beta isozyme), Escherichia coli and yeast. In addition, polyclonal anti-E. coli enzyme and antibodies to synthetic peptides copying several regions of the yeast enzyme reacted with the human gamma and rat beta isozymes. Antibodies to yeast SAM1 encoded protein residues 6-21, 87-113 and 87-124 inhibited the activity of human lymphocyte AdoMet synthetase, while antibodies to residues 272-287 had no effect on the enzyme activity. Our results suggest that these conserved regions may be important in enzyme activity.

Phosphorylation of bovine platelet myosin by protein kinase C

Bovine platelet myosin is phosphorylated by protein kinase C at multiple sites. Most of the phosphate is incorporated in the 20,000-dalton light chain although some phosphate is incorporated in the heavy chain. Phosphorylation of the 20,000-dalton light chain of platelet myosin is 10 times faster than the phosphorylation of smooth muscle myosin. Platelet myosin light chain is first phosphorylated at a threonine residue followed by a serine residue. Dominant phosphorylation sites of the 20,000-dalton light chain are estimated as serine-1, serine-2, and threonine-9. Prolonged phosphorylation by protein kinase C resulted in an additional phosphorylation site which, on the basis of limited proteolysis, appears to be either serine-19 or threonine-18. Phosphorylation by protein kinase C causes an inhibition of actin-activated ATPase activity of platelet myosin prephosphorylated by myosin light chain kinase. Inhibition of ATPase activity is due to a decreased affinity of myosin for actin, and no change in Vmax is observed. It is shown that platelet myosin also exhibits the 6S to 10S conformation transition as judged by viscosity and gel filtration methods. Mg2(+)-ATPase activity of platelet myosin is paralleled with the 10S-6S transition. Phosphorylation by protein kinase C affects neither the 10S-6S transition nor the myosin filament formation. Therefore, the inhibition of actin-activated ATPase activity of platelet myosin is not due to the change in the myosin conformation.

Inactivation and dissociation of S-adenosylmethionine synthetase by modification of sulfhydryl groups and its possible occurrence in cirrhosis

Catalytically active human and rat liver S-adenosylmethionine synthetase exists mainly in tetramer and dimer form. In liver biopsy samples from cirrhotic patients a marked reduction in total S-adenosylmethionine synthetase activity and a specific loss of the tetrameric form of the enzyme exist. We have investigated the possible role of sulfhydryl groups in maintaining the structure and activity of S-adenosylmethionine synthetase. Both forms of S-adenosylmethionine synthetase are rapidly inactivated by N-ethylmaleimide, and the loss of enzyme activity correlates with the incorporation of approximately 2 moles N-ethylmaleimide per mole of subunit. In addition, reaction with N-ethylmaleimide resulted in displacement of the tetramer-dimer equilibrium of the enzyme toward the dimer, but no monomer was detected under these conditions. A catalytically active monomeric S-adenosylmethionine synthetase was detected in the cytosolic extract from a liver biopsy sample from a cirrhotic patient, supporting our model for the structure of S-adenosylmethionine synthetase. Because treatment of S-adenosylmethionine synthetase with N-ethylmaleimide resembles the situation of this enzyme in cirrhotic patients, it is proposed that impaired protection of the enzyme from oxidizing agents caused by a decreased synthesis of glutathione can explain the diminished synthesis of S-adenosylmethionine in liver cirrhosis.