Methionine Adenosyltransferase Activity Research Articles

Engineering of Methionine Adenosyltransferase Reveals Key Roles of Electrostatic Interactions in Enhanced Catalytic Activity.

As an important dietary supplement, S-adenosylmethionine (SAM) is currently synthesized by methionine adenosyltransferase (MAT) using ATP and methionine as substrates. However, the activity of MAT is severely inhibited by product inhibition, which limits the industrial production of SAM. Here, MAT from Bacteroides fragilis (BfMAT), exhibiting relatively low product inhibition and moderate specific activity, was identified by gene mining. Based on molecular docking, residues within 5Å of ATP in BfMAT were subjected to mutagenesis for enhanced catalytic activity. Triple variants M3-1 (E42M/E55L/K290I), M3-2 (E42R/E55L/K290I), and M3-3 (E42C/E55L/K290I) with specific activities of 1.83, 1.81, and 1.94 U/mg were obtained, which were 110.5-125.6% higher than that of the wild type (WT). Furthermore, compared with WT, the Km values of M3-1 and M3-3 were decreased by 31.4% and 60.6%, leading to significant improvement in catalytic efficiency (kcat/Km) by 322.5% and 681.1%. All triple variants showed shifted optimal pH from 8.0 to 7.5. Moreover, interaction analysis suggests that the enhanced catalytic efficiency may be attributed to the decreased electrostatic interactions between ATP and the mutation sites (E42, E55, and K290). Based on MD simulation, coulomb energy and binding free energy analysis further reveal the importance of electrostatic interactions for catalytic activity of BfMAT, which could be an efficient strategy for improving catalytic performance of MATs.

Rumen-protected methionine during heat stress alters mTOR, insulin signaling, and 1-carbon metabolism protein abundance in liver, and whole-blood transsulfuration pathway genes in Holstein cows

We investigated effects of rumen-protected Met (RPM) during a heat stress (HS) challenge on (1) hepatic abundance of mTOR, insulin, and antioxidant signaling proteins, (2) enzymes in 1-carbon metabolism, and (3) innate immunity. Holstein cows (n = 32; mean ± standard deviation, 184 ± 59 d in milk) were randomly assigned to 1 of 2 environmental groups, and 1 of 2 diets [total mixed ration (TMR) with RPM (Smartamine M; 0.105% dry matter as top-dress) or TMR without (CON); n = 16/diet] in a split-plot crossover design. There were 2 periods with 2 phases. During phase 1 (9 d), all cows were in thermoneutral conditions (TN; temperature-humidity index = 60 ± 3) and fed ad libitum. During phase 2 (9 d), half the cows (n = 8/diet) were exposed to HS using electric heat blankets. The other half (n = 8/diet) remained in TN, but was pair-fed to HS counterparts. After a 14-d washout and 7-d adaptation period, the study was repeated (period 2) and environmental treatments were inverted relative to phase 2, but dietary treatments were the same. Blood was collected on d 6 of each phase 2 to measure immune function and isolate whole-blood RNA. Liver biopsies were performed at the end of each period for cystathione β-synthase (CBS) and methionine adenosyltransferase activity, glutathione concentration, and protein abundance. Data were analyzed using PROC MIXED in SAS. Abundance of CUL3, inhibitor of antioxidant responses, tended to be downregulated by HS suggesting increased oxidative stress. Heat-shock protein 70 abundance was upregulated by HS. Phosphorylated mTOR abundance was greater overall with RPM, suggesting an increase in pathway activity. An environment × diet (E × D) effect was observed for protein kinase B (AKT), whereas there was a tendency for an interaction for phosphorylated AKT. Abundance of AKT was upregulated in CON cows during HS versus TN, this was not observed in RPM cows. For phosphorylated AKT, tissue from HS cows fed CON had greater abundance compared with all other treatments. The same effect was observed for EIF2A (translation initiation) and SLC2A4 (insulin-induced glucose uptake). An E × D effect was observed for INSR due to upregulation in CON cows during HS versus TN cows fed CON or RPM. There was an E × D effect for CBS, with lower activity in RPM versus CON cows during HS. The CON cows tended to have greater CBS during HS versus TN. An E × D effect was observed for methionine adenosyltransferase, with lower activity in RPM versus CON during HS. Although activity increased in CON during HS versus TN, RPM cows tended to have greater activity during TN. Neutrophil and monocyte oxidative burst and monocyte phagocytosis decreased with HS. An (E × D) effect was observed for whole-blood mRNA abundance of CBS, SOD1 and CSAD; RPM led to upregulation during TN versus HS. Regardless of diet, CDO1, CTH, and SOD1 decreased with HS. Although HS increased hepatic HSP70 and seemed to alter antioxidant signaling, feeding RPM may help cows maintain homeostasis in mTOR, insulin signaling, and 1-carbon metabolism. Feeding RPM also may help maintain whole-blood antioxidant response during HS, which is an important aspect of innate immune function.

Structural basis of the dominant inheritance of hypermethioninemia associated with the Arg264His mutation in the MAT1A gene.

Methionine adenosyltransferase (MAT) deficiency, characterized by isolated persistent hypermethioninemia (IPH), is caused by mutations in the MAT1A gene encoding MATαl, one of the major hepatic enzymes. Most of the associated hypermethioninemic conditions are inherited as autosomal recessive traits; however, dominant inheritance of hypermethioninemia is caused by an Arg264His (R264H) mutation. This mutation has been confirmed in a screening programme of newborns as the most common mutation in babies with IPH. Arg264 makes an inter-subunit salt bridge located at the dimer interface where the active site assembles. Here, it is demonstrated that the R264H mutation results in greatly reduced MAT activity, while retaining its ability to dimerize, indicating that the lower activity arises from alteration at the active site. The first crystallographic structure of the apo form of the wild-type MATαl enzyme is provided, which shows a tetrameric assembly in which two compact dimers combine to form a catalytic tetramer. In contrast, the crystal structure of the MATαl R264H mutant reveals a weaker dimeric assembly, suggesting that the mutation lowers the affinity for dimer-dimer interaction. The formation of a hetero-oligomer with the regulatory MATβV1 subunit or incubation with a quinolone-based compound (SCR0911) results in the near-full recovery of the enzymatic activity of the pathogenic mutation R264H, opening a clear avenue for a therapeutic solution based on chemical interventions that help to correct the defect of the enzyme in its ability to metabolize methionine.

Hepatic betaine-homocysteine methyltransferase and methionine synthase activity and intermediates of the methionine cycle are altered by choline supply during negative energy balance in Holstein cows

Although choline requirements are unknown, enhanced postruminal supply may decrease liver triacylglycerol (TAG) storage and increase flux through the methionine cycle, helping cows during a negative energy balance (NEB). The objective was to investigate effects of postruminal choline supply during NEB on hepatic activity of betaine-homocysteine methyltransferase (BHMT), methionine synthase (MTR), methionine adenosyltransferase, transcription of enzymes, and metabolite concentrations in the methionine cycle. Ten primiparous rumen-cannulated Holstein cows (158 ± 24 d postpartum) were used in a replicated 5 × 5 Latin square design with 4-d treatment periods and 10 d of recovery (14 d/period). Treatments were unrestricted intake with abomasal infusion of water (A0), restricted intake (R; 60% of net energy for lactation requirements to induce NEB) with abomasal infusion of water (R0) or R plus abomasal infusion of 6.25, 12.5, or 25 g/d of choline ion. Liver tissue was collected on d 5 after the infusions ended, blood on d 1 to 5, and milk on d 1 to 4. Statistical contrasts were A0 versus R0 (CONT1) and tests of linear (L), quadratic (Q), and cubic (C) effects of choline dose. Plasma choline increased with R (CONT1) and choline (L). Although R decreased milk yield (CONT1), choline increased milk yield and liver phosphatidylcholine (PC), but decreased TAG (L). No differences were observed in plasma PC or very-low-density lipoprotein concentrations with R or choline. Activity and mRNA abundance of BHMT were greater with R (CONT1) and increased with choline (L). Although activity of MTR was lower with R (CONT1), it tended to increase with choline (L). No effect of R was detected for activity of methionine adenosyltransferase, but it changed cubically across dose of choline. Those responses were associated with linear increases in the concentrations of liver tissue (+13%) and plasma methionine concentrations. The mRNA abundance of CPT1A, SLC22A5, APOA5, and APOB, genes associated with fatty acid oxidation and lipoprotein metabolism, was upregulated by choline (Q). Overall, enhanced supply of choline during NEB increases hepatic activity of BHMT and MTR to regenerate methionine and PC, partly to help clear TAG. The relevance of these effects during the periparturient period merits further research.

Metal-nucleobase hybrid nanoparticles for enhancing the activity and stability of metal-activated enzymes.

A novel strategy for enhancing the activity and stability of metal-activated enzymes by allosteric control and confinement of metal-nulceobase hybrid coordination was reported in this study. Despite the lack of active Mg2+ ions, methionine adenosyltransferase (MAT) immobilized by Zn2(adenine) still exhibited robust catalytic activity and stability at pH 3 and 80 °C.

S-Adenosyl Methionine Synthesis Impacts Maintenance of Acute Myeloid Leukemia Cells with Regulation of Transcriptional Elongation

Epigenetic dysregulation is at the heart of Acute Myeloid Leukemia (AML), and its mechanistic understanding holds promise in improving AML dismal prognosis. Modification of chromatin accessibility through post-translational modification of histones alters gene expression activity and is directly dependent on cell metabolism for generation of the modifying groups. Methionine metabolism generates S-adenosyl methionine (SAM), the universal methyl donor in the cell, through the activity of multimeric methionine adenosyl transferase (MAT) enzymes. MAT2A is an obligatory subunit in hematopoietic cells, and we have recently tabled it in a CRISPR drop-out screen [1] as a candidate vulnerability in AML.We have validated the anti-AML effects of MAT2A ablation using shRNA-mediated gene expression knockdown, and could further phenocopy cell line selectivity observed in our CRISPR screen through the use of the MAT2A selective inhibitor FIDAS-5 [2] (IC50 MOLM-13 1.0μM, OCI-AML3 0.7μM, MV4.11 11μM).MAT2A inhibition significantly depleted SAM levels (fold change to DMSO, FC=0.20±0.07; p<0.05, MOLM-13 cells), and resulted in decreased growth and enhanced apoptosis (MOLM-13 FC=1.34±0.21;p<0.05, OCI-AML3 FC=3.0±0.34; p<0.05, MV4.11 FC=1.09±0.19;p=0.52) of cultured AML cells, without an obvious impact on differentiation. Similar results were observed in AML patient samples (n=11), which displayed reduced capacity to initiate colonies in semi-solid medium (FC=0.47±0.21; p<0.0005) and had hindered expansion in medium-term co-cultures on MS-5 stroma, with accumulation of cells with intermediate, but not high, levels of primitive marker CD34. MLL-AF9 transformed primary mouse bone marrow cells were equally affected (FC colony efficiency=0.05±0.03; p<0.005). Significantly, MAT2A inhibition did not affect colony-formation or multi-lineage differentiation of cord blood-derived CD34+ cells in colony-forming progenitor assays (FC=1.01±0.25p=0.97), indicating selective vulnerability of AML cells.We analysed the consequences of MAT2A inhibition on chromatin methyl modifications, and observed a specific reduction in the active elongation mark H3K36me3. The associated transcriptional programme is currently being investigated. Interestingly, contrary to reports in colon cancer cells upon methionine restriction and reduced SAM levels [3], we did not observe loss of H3K4me3 at active promoters, suggesting a selective AML dependence on transcription elongation.In conclusion, we identify SAM synthesis as a novel vulnerability in AML through regulation of transcriptional elongation. Our findings highlight the therapeutic potential of modulating metabolite availability for epigenetic and transcriptional re-programming in leukemia.

339 - Comparison of two parenteral lipid emulsions and their effects on hepatic DNA methylation in newborns guinea pigs

Background Parenteral nutrition (PN) is necessary for preterm infants unable to receive enteral nutrition. However, PN is contaminated by peroxides inducing oxidative stress, which is known to modify the enzymes involved in DNA methylation. Therefore, alteration of DNA methylation is suspected in infants on PN. Among PN ingredients, lipid emulsion is an important source of oxidants. Because of differences in their content – higher vitamin E and lower omega-6 PUFA in SMOF- we hypothesize that DNA methylation differs whether PN is compounded with Intralipid (IL) or SMOFLipid (SMOF). Objective Assess the impact of PN containing IL or SMOF, on redox potential of glutathione, S-adenosylmethionine (SAM), methylcytosine (5-MedCyd), methionine adenosyltransferase (MAT) and DNA methyltransferases (DNMT) in newborn animals. Methods 3-day-old guinea pigs were randomized into 3 groups: IL: infused with total PN containing IL as lipid emulsion; SMOF: infused with total PN containing SMOF as lipid emulsion; and Control: orally fed. Redox potential, SAM, 5-MedCyd, MAT and DNMTs activity were quantified in the liver after 4 days of experiment. Statistics: ANOVA, p Results PN groups were more oxidized (-210±2 > -228±1 mV) and presented higher rates of 5-MedCyd (8.2±0.5 >5.6±0.9 pmol/μg DNA), SAM (22.2±1.4 > 14±2) and increased activity of DNMTs (524.6±69 > 274.3±50) compared to control. 5-MedCyd rates was more increased in SMOF than IL. Redox potential was associated with global DNA hypermethylation (R2 = 0.55 p 19±1 nmol/g tissue). Conclusion PN changes the hepatic redox potential leading to modifications in DNA methylation metabolism. SMOF exhibits more pronounced effects compare to IL, suggesting a cautious before using SMOF as an alternative to IL in PN. Further analyses are required to explore the molecular mechanisms caused by the different components such as fatty acids on DNA methylation.

Lentivirus-mediated downregulation of MAT2B inhibits cell proliferation and induces apoptosis in melanoma.

Malignant melanoma is the most lethal of skin cancers and its pathogenesis is complex and heterogeneous. The efficacy of conventional therapeutic regimens for melanoma remains limited. Thus, it is important to explore novel effective therapeutic targets in the treatment of melanoma. The MAT2B gene encodes for the regulatory subunit of methionine adenosyltransferase (MAT). Recent studies have suggested that MAT2B may have functional roles other than modulating catalytic activity of MAT. In order to identify the roles of MAT2B in the tumorigenesis of malignant melanoma, we compared MAT2B expression profile in melanoma tissues with that in benign nevus samples. We employed lentivirus-mediated RNAi to downregulate the expression of MAT2B in malignant melanoma cell lines (A375 and Mel-RM), and investigated the effects of MAT2B on cell growth, colony-formation ability and apoptosis invitro, as well as tumor growth of a xenograft model invivo. The expression levels of BCL2 and XAF1 proteins, which were closely related to tumor cell apoptosis, were analyzed by western blot analysis. Our data showed that MAT2B was elevated in both primary and metastatic melanoma tissues compared with benign nevus samples. Lentivirus-mediated downregulation of MAT2B suppressed cell growth, colony formation and induced apoptosis in A375 and Mel-RM cell lines invitro, affected protein expression of BCL2 and XAF1, extended the transplanted tumor growth invivo. These results indicated that MAT2B was critical in the proliferation of melanoma cells and tumorigenicity. It may be considered as a potential anti-melanoma therapeutic target.

The Improvement of SAM Accumulation by Integrating the Endogenous Methionine Adenosyltransferase Gene SAM2 in Genome of the Industrial Saccharomyces cerevisiae Strain.

S-Adenosyl-L-methionine (SAM) plays important roles in trans-methylation, trans-sulfuration, and polyamine synthesis in all living cells, and it is also an effective cure for liver disease, depressive syndromes, and osteoarthritis. The increased demands of SAM in pharmaceuticals industry have aroused lots of attempts to improve its production. In this study, a multiple-copy integrative plasmid pYMIKP-SAM2 was introduced into the chromosome of wild-type Saccharomyces cerevisiae strain ZJU001 to construct the recombined strain R1-ZJU001. Further studies showed that the recombinant yeast exhibited higher enzymatic activity of methionine adenosyltransferase and improved its SAM biosynthesis. With a three-phase fed-batch strategy in 15-liter bench-top fermentor, 8.81 g/L SAM was achieved after 52 h cultivation of R1-ZJU001, about 27.1 % increase over its parent strain ZJU001, whereas the SAM content was also improved from 64.6 mg/g DCW to 91.0 mg/g DCW. Our results shall provide insights into the metabolic engineering of SAM pathway in yeast for improved productivity of SAM and subsequent industrial applications.

Impact of glutathione supplementation of parenteral nutrition on hepatic methionine adenosyltransferase activity

BackgroundThe oxidation of the methionine adenosyltransferase (MAT) by the combined impact of peroxides contaminating parenteral nutrition (PN) and oxidized redox potential of glutathione is suspected to explain its inhibition observed in animals. A modification of MAT activity is suspected to be at origin of the PN-associated liver disease as observed in newborns. We hypothesized that the correction of redox potential of glutathione by adding glutathione in PN protects the MAT activity. AimTo investigate whether the addition of glutathione to PN can reverse the inhibition of MAT observed in animal on PN. MethodsThree days old guinea pigs received through a jugular vein catheter 2 series of solutions. First with methionine supplement, (1) Sham (no infusion); (2) PN: amino acids, dextrose, lipids and vitamins; (3) PN-GSSG: PN+10μM GSSG. Second without methionine, (4) D: dextrose; (5) D+180μM ascorbylperoxide; (6) D+350μM H2O2. Four days later, liver was sampled for determination of redox potential of glutathione and MAT activity in the presence or absence of 1mM DTT. Data were compared by ANOVA, p<0.05. ResultsMAT activity was 45±4% lower in animal infused with PN and 23±7% with peroxides generated in PN. The inhibition by peroxides was associated with oxidized redox potential and was reversible by DTT. Correction of redox potential (PN+GSSG) or DTT was without effect on the inhibition of MAT by PN. The slope of the linear relation between MAT activity and redox potential was two fold lower in animal infused with PN than in others groups. ConclusionThe present study suggests that prevention of peroxide generation in PN and/or correction of the redox potential by adding glutathione in PN are not sufficient, at least in newborn guinea pigs, to restore normal MAT activity.

Overexpression of methionine adenosyltransferase in Corynebacterium glutamicum for production of S-adenosyl-l-methionine.

Two genes encoding methionine adenosyltransferase, SAM2 from Saccharomyces cerevisiae and metK from Corynebacterium glutamicum, were individually cloned into pDXW-8, the shuttle vector between Escherichia coli and C. glutamicum, and overexpressed in E. coli DH5α and C. glutamicum ATCC13032. In DH5α, both genes were overexpressed and their protein products showed the activity of methionine adenosyltransferase. In ATCC13032, metK was overexpressed, its product MetK showed the enzyme activity and could convert l-methionine to S-adenosyl-l-methionine (SAM). However, when SAM2 was overexpressed in ATCC13032, neither the enzyme activity nor the conversion of SAM from l-methionine was observed. Reverse transcription PCR analysis and SDS-PAGE showed that SAM2 was transcribed but not translated in C. glutamicum. Therefore, SAM2-C, a mutant SAM2, was constructed by codon optimization, and overexpressed in ATCC13032; it was well transcribed and translated, and could convert l-methionine to SAM. Finally, SAM2-C and metK were individually overexpressed in E. coli BL21(DE3), and their products SAM2-C and MetK were purified and characterized. The optimum activity for both enzymes was found at pH 8.5 and 35°C; SAM2-C and MetK have similar Km for ATP, but quite different Km for l-methionine. These results suggest that SAM2-C and MetK can be useful for developing C. glutamicum to produce SAM.

An alkaline pH control strategy for methionine adenosyltransferase production in Pichia pastoris fermentation

Pichia pastoris is a successful system for expressing heterologous proteins and its fermentation pH is always maintained below 7.0. However, particular proteins are unstable under acidic conditions, such as methionine adenosyltransferase (MAT), and thus fermentation under acidic pH conditions is unsuitable because protein activity is lost owing to denaturation. Here, a strategy employing alkaline pH in the late fermentation period was developed to improve MAT production. Initially, P. pastoris KM71 was transformed with the mat gene to overexpress MAT. After 72 h of in vitro incubation at different pH values, the expressed MAT displayed highest stability at pH 8.0; however, pH 8.0 inhibited cell growth and induced cell rupture, thus affecting protein production. To balance MAT stability and Pichia cell viability, different pH control strategies were compared. In strategy A (reference), the induction pH was maintained at 6.0, whereas in strategy B, it was gradually elevated to 8.0 through a 25 h transition period (80 ∼ 105 h). MAT activity was 0.86 U/mg (twofold higher than the control). However, MAT content was reduced by 50% when compared with strategy A, because of proteases released upon cell lysis. To improve cell viability under alkaline conditions, glycerol was added in addition to methanol (strategy C). When compared with strategy B, the MAT-specific activity remained nearly constant, whereas the expression level increased to 1.27 g/L. The alkaline pH control strategy presented herein for MAT production represents an excellent alternative for expressing proteins that are stable only under alkaline conditions.

Alterations in the metabolomics of sulfur-containing substances in rat kidney by betaine

Earlier studies have shown that betaine administration may modulate the metabolism of sulfur amino acids in the liver. In this study, we determined the changes in the metabolomics of sulfur-containing substances induced by betaine in the kidney, the other major organ actively involved in the transsulfuration reactions. Male rats received betaine (1%) in drinking water for 2 weeks before killing. Betaine intake did not affect betaine-homocysteine methyltransferase activity or its protein expression in the renal tissue. Expression of methionine synthase was also unchanged. However, methionine levels were increased significantly both in plasma and kidney. Renal methionine adenosyltransferase activity and S-adenosylmethionine concentrations were increased, but there were no changes in S-adenosylhomocysteine, homocysteine, cysteine levels or cystathionine β-synthase expression. γ-Glutamylcysteine synthetase expression or glutathione levels were not altered, but cysteine dioxygenase and taurine levels were decreased significantly. In contrast, betaine administration induced cysteine sulfinate decarboxylase and its metabolic product, hypotaurine. These results indicate that the metabolomics of sulfur-containing substances in the kidney is altered extensively by betaine, although the renal capacity for methionine synthesis is unresponsive to this substance unlike that of the liver. It is suggested that the increased methionine availability due to an enhancement of its uptake from plasma may account for the alterations in the metabolomics of sulfur-containing substances in the kidney. Further studies need to be conducted to clarify the physiological/pharmacological significance of these findings.

Acute Liver Injury Induces Nucleocytoplasmic Redistribution of Hepatic Methionine Metabolism Enzymes

The discovery of methionine metabolism enzymes in the cell nucleus, together with their association with key nuclear processes, suggested a putative relationship between alterations in their subcellular distribution and disease. Using the rat model of d-galactosamine intoxication, severe changes in hepatic steady-state mRNA levels were found; the largest decreases corresponded to enzymes exhibiting the highest expression in normal tissue. Cytoplasmic protein levels, activities, and metabolite concentrations suffered more moderate changes following a similar trend. Interestingly, galactosamine treatment induced hepatic nuclear accumulation of methionine adenosyltransferase (MAT) α1 and S-adenosylhomocysteine hydrolase tetramers, their active assemblies. In fact, galactosamine-treated livers showed enhanced nuclear MAT activity. Acetaminophen (APAP) intoxication mimicked most galactosamine effects on hepatic MATα1, including accumulation of nuclear tetramers. H35 cells that overexpress tagged-MATα1 reproduced the subcellular distribution observed in liver, and the changes induced by galactosamine and APAP that were also observed upon glutathione depletion by buthionine sulfoximine. The H35 nuclear accumulation of tagged-MATα1 induced by these agents correlated with decreased glutathione reduced form/glutathione oxidized form ratios and was prevented by N-acetylcysteine (NAC) and glutathione ethyl ester. However, the changes in epigenetic modifications associated with tagged-MATα1 nuclear accumulation were only prevented by NAC in galactosamine-treated cells. Cytoplasmic and nuclear changes in proteins that regulate the methylation index follow opposite trends in acute liver injury, their nuclear accumulation showing potential as disease marker. Altogether these results demonstrate galactosamine- and APAP-induced nuclear accumulation of methionine metabolism enzymes as active oligomers and unveil the implication of redox-dependent mechanisms in the control of MATα1 subcellular distribution.

Alterations in Sulfur Amino Acid Metabolism in Mice Treated with Silymarin: A Novel Mechanism of Its Action Involved in Enhancement of the Antioxidant Defense in Liver

It has been known that silymarin exhibits protective activity against oxidative liver injury induced by various hepatotoxicants, but the underlying mechanism of its beneficial action remains unclear. We determined the alterations in sulfur-containing amino acid metabolism induced by silymarin in association with its effects on the antioxidant capacity of liver. Male mice were treated with silymarin (100 or 200 mg/kg, p. o.) every 12 h for a total of 3 doses, and sacrificed 6 h after the final dosing. The hepatic methionine level was increased, but the activity and protein expression of methionine adenosyltransferase were decreased by silymarin in a dose-dependent manner. S-Adenosylmethionine or homocysteine concentration was not changed, whereas the sulfur-containing metabolites generated from homocysteine in the transsulfuration pathway including cystathionine, cysteine, and glutathione were increased significantly. Cystathionine β-synthase was induced, but cysteine dioxygenase was downregulated, both of which would contribute to the elevation of cysteine and its product, glutathione, in liver. Oxygen radical scavenging capacity of liver cytosol against peroxyl radical and peroxynitrite was increased, and also hepatic lipid peroxidation was diminished in the silymarin-treated mice. Taken together, the results demonstrate that silymarin enhances hepatic glutathione generation by elevating cysteine availability via an increment in cysteine synthesis and an inhibition of its catabolism to taurine, which may subsequently contribute to the antioxidant defense of liver.

Significance of alterations in the metabolomics of sulfur-containing amino acids during liver regeneration

It has been known that liver regeneration is accompanied with a profound change in the metabolomics of sulfur-containing substances in liver. However, its physiological significance in the liver regenerative process is still unclear. Our previous work showed that buthioninesulfoximine and phorone, both widely used to deplete intracellular glutathione (GSH) in biological experiments, induced contrasting changes in the sulfur-containing amino acid metabolism in liver. In this study we employed these GSH-depleting agents to evaluate the role of sulfur-containing substances in the early phase of liver regeneration. Male rats treated with buthioninesulfoximine or phorone were subjected to two-thirds partial hepatectomy (PHx). At the doses used, the magnitude of GSH depletion after PHx was comparable, but buthioninesulfoximine administration inhibited the progression of liver regeneration as determined by liver weight increase, elevation of serum alanine aminotransferase activity, and cyclin D1 and proliferating cell nuclear antigen (PCNA) protein expressions, whereas liver recovery was significantly accelerated in the phorone-treated rats, suggesting that the role of GSH in this process is minimal. Hepatic concentrations of methionine, S-adenosylmethionine, cysteine, taurine and GSH were all elevated by PHx. Methionine adenosyltransferase activity was also induced in the remnant liver. Buthioninesulfoximine administration depressed the elevation of S-adenosylmethionine, but increased the catabolism of cysteine to taurine. In contrast, S-adenosylmethionine elevation was augmented whereas cysteine, hypotaurine and taurine were decreased in the phorone-treated rats. PHx elevated hepatic putrescine and spermidine, but lowered spermine concentrations. Buthioninesulfoximine administration increased putrescine further, but decreased spermidine and spermine concentrations. On the contrary, both spermidine and spermine concentrations were elevated in the rats treated with phorone. The results suggest that the availability of S-adenosylmethionine plays a critical role in the progression of liver regeneration via enhancement of polyamine synthesis. These findings raise the possibility that regulating hepatic transsulfuration reactions may be capable of modifying the recovery process after liver injury.

Alterations in hepatic metabolism of sulfur amino acids in non-obese type-2 diabetic Goto-Kakizaki rats

Elevated plasma homocysteine has been identified as a risk factor for cardiovascular disease and non-alcoholic liver disease, which are major complications of diabetes. Hence, hepatic homocysteine metabolism has become a major focus of diabetes research. However, little information is available regarding plasma homocysteine levels in non-obese diabetic animals. Therefore, we investigated the hepatic metabolism of sulfur-amino acids in non-obese type-2 diabetic Goto-Kakizaki rats. The experiments were performed using 9-week-old Goto-Kakizaki rats and age-matched Wistar rats. The major finding of this study is that homocysteine levels in the liver and plasma are maintained by a balance between the up-regulation of betaine homocysteine methyltransferase and the inhibition of cystathionine β-synthase in non-obese type-2 diabetic rats. Hepatic levels of cysteine and its metabolites, such as hypotaurine, taurine, and glutathione, were increased despite inhibition of the transsulfuration of homocysteine to cysteine. The elevated hepatic taurine and glutathione levels may be attributed to the up-regulation of cysteine dioxygenase expression and increased cysteine availability for glutathione synthesis. Inhibition of hepatic methionine adenosyltransferase activity in Goto-Kakizaki rats was associated with a decrease in hepatic S-adenosylmethionine, which serves as an allosteric activator of cystathionine β-synthase. The non-obese type-2 diabetic condition results in profound changes in hepatic sulfur-amino acid metabolism and raises the possibility that sulfur-amino acid metabolism may be regulated by obesity- as well as diabetes-associated factors. Further study to elucidate the pathological significance of sulfur-amino acid metabolism in chronic liver disease in type-2 diabetic animals is underway in this laboratory.

Induction of hepatic glutathione synthesis via alterations in sulfur amino acid metabolism in mice treated with silymarin acutely

It has been known that silymarin exhibits protective effects against chemical‐induced liver injury, but the underlying mechanism of its beneficial action remains unclear. We examined the silymarin‐induced changes in the metabolism of sulfur‐containing amino acids in association with its effects on antioxidant capacity of liver. Male mice were treated with silymarin (100 or 200 mg/kg, po) every 12 hr for a total of 3 doses. Hepatic methionine level was increased, but the activity and protein expression of methionine adenosyltransferase were decreased dose‐dependently in liver of the mice treated with silymarin. S‐Adenosylmethionine or homocysteine was not altered whereas cystathionine, cysteine and glutathione (GSH) were increased significantly. Betaine‐homocysteine methyltransferase and cysteine dioxygenase were down‐regulated, but cystathionine β‐synthase was induced. γ‐Glutamylcysteine synthetase was unchanged by silymarin treatment. Oxygen radical scavenging capacity of liver cytosol against peroxyl radical and peroxynitrite was increased, and hepatic lipid peroxidation was diminished in the silymarin‐treated mice. It is concluded that silymarin enhances hepatic GSH biosynthesis by elevating cysteine availability via increment in cysteine synthesis and inhibition of its catabolism to taurine, which may subsequently contribute to the antioxidant defense of liver.

Hepatic metabolism of sulfur amino acids in db/db mice

To determine the effect of type-2 diabetes and obesity on the hepatic metabolism of sulfur amino acids, hepatic sulfur amino acid metabolism was determined in db/db mice. Hepatic methionine was markedly decreased in db/db mice, although the hepatic activity of betaine homocysteine methyltransferase was increased. The decrease in hepatic methionine was reflected by decreased sulfur-containing methionine metabolites, including S-adenosylmethionine, homocysteine, cysteine, and hypotaurine in liver and plasma. In contrast, S-adenosylhomocysteine, putrescine, and spermidine were increased in db/db mice. The hepatic level and activity of methionine adenosyltransferase I/III, an S-adenosylmethionine synthesizing enzyme, were significantly increased. These results suggest that increased polyamine synthesis, in conjunction with decreased hepatic methionine levels, is partly responsible for the reduction in hepatic S-adenosylmethionine. Decreased homocysteine in liver and plasma may be attributable to the decrease in hepatic methionine and upregulation of hepatic betaine homocysteine methyltransferase. Glutathione in liver and plasma did not change despite decreased γ-glutamylcysteine ligase activity. The decreased hepatic hypotaurine may be attributable to the downregulation of cysteine dioxygenase. The major finding of this study is that db/db mice exhibited decreases in hepatic methionine and its sulfurcontaining metabolites.

Inhibition of hepatic methionine adenosyltransferase by peroxides contaminating parenteral nutrition leads to a lower level of glutathione in newborn Guinea pigs

Premature newborn infants on total parenteral nutrition (TPN) are at risk of oxidative stress because of peroxides contaminating TPN and low glutathione level. Low cysteine availability limits glutathione synthesis. In this population, the main source of cysteine derives from the hepatic conversion of methionine. The first enzyme of this conversion, methionine adenosyltransferase (MAT), contains redox-sensitive cysteinyl residues. We hypothesize that inhibition of MAT by peroxides contaminating TPN leads to a lower availability of cysteine for glutathione synthesis. At 3 days of life, animals were fitted with a jugular catheter for intravenous infusion. Four groups were compared by ANOVA (P<0.05): (1) Control, without surgery, fed regular chow; (2) Sham, fitted with an obstructed catheter, fed orally regular chow; (3) TPN, fed exclusively TPN (dextrose, amino acids, fat, vitamins) containing 350 μM peroxides; (4) H2O2, fed regular chow orally and infused with 350 μM H2O2. Four days later, MAT activity and glutathione in liver and blood were lower in TPN and H2O2 groups. The redox potential was more oxidized in blood and liver of the TPN group. In conclusion, peroxides generated in TPN inhibit methionine adenosyltransferase activity with, among consequences, a low level of glutathione and a more oxidized redox potential.