Redox processes of methionine relevant to beta-amyloid oxidation and Alzheimer's disease.

This minireview gives an overview over the oxidation mechanisms of methionine (Met) relevant for analogous processes which may lead to the oxidation of beta-amyloid (betaA) peptides. The Cu(II)-catalyzed oxidation of a C-terminal Met(35) residue in betaA peptides may be a key to the known propensities of these peptides to form H2O2 and free radicals. Though the reduction potentials of Cu(II) and Met would seem unfavorable, there are several structural features of betaA, which may promote a one-electron oxidation of Met. The potentially close association of the Met sulfur with the C=O group C-terminal of Ile(31) in the C-terminus of betaA may support the formation of an S-O bonded radical cation intermediate. Evidence for such S-O bond formation has recently been obtained for a model, N-acetylmethionine amide. Additional support for a potential catalytic role of an oxygen-containing functional group comes from numerous studies with organic model sulfides.     

Cyclic oxidation and reduction of protein methionine residues is an important antioxidant mechanism

Almost all forms of reactive oxygen species (ROS) oxidize methionine residues of proteins to a mixture of the R- and S-isomers of methionine sulfoxide. Because organisms contain methionine sulfoxide reductases (Msr's) that can catalyze the thioredoxin-dependent reduction of the sulfoxides back to methionine, it was proposed that the cyclic oxidation/reduction of methionine residues might serve as antioxidants to scavenge ROS, and also to facilitate the regulation of critical enzyme activities. We summarize here results of studies showing that organisms possess two different forms of Msr – namely, MsrA that catalyzes reduction of the S-isomer and MsrB that catalyzes the reduction of the R-isomer. Deletion of the msrA gene in mice leads to increased sensitivity to oxidative stress and to a decrease (40%) in the maximum lifespan. This suggests that elimination of both Msr's would have more serious consequences.     

Purification and characterization of methionine sulfoxide reductases from mouse and Staphylococcus aureus and their substrate stereospecificity.

Many organisms have been shown to possess a methionine sulfoxide reductase (MsrA), exhibiting high specificity for reduction the S form of free and protein-bound methionine sulfoxide to methionine. Recently, a different form of the reductase (referred to as MsrB) has been detected in several organisms. We show here that MsrB is a selenoprotein that exhibits high specificity for reduction of the R forms of free and protein-bound methionine sulfoxide. The enzyme was partially purified from mouse liver and a derivative of the mouse MsrB gene, in which the codon specifying selenocystein incorporation was replaced by the cystein codon, was prepared, cloned, and overexpressed in Escherichia coli. The properties of the modified MsrB protein were compared directly with those of MsrA. Also, we have shown that in Staphylococcus aureus there are two MsrA and one nonselenoprotein MsrB, which demonstrates the same substrate stereospecificity as the mouse MsrB.     

Methionine in proteins: The Cinderella of the proteinogenic amino acids

Methionine in proteins, apart from its role in the initiation of translation, is assumed to play a simple structural role in the hydrophobic core, in a similar way to other hydrophobic amino acids such as leucine, isoleucine, and valine. However, research from a number of laboratories supports the concept that methionine serves as an important cellular antioxidant, stabilizes the structure of proteins, participates in the sequence‐independent recognition of protein surfaces, and can act as a regulatory switch through reversible oxidation and reduction. Despite all these evidences, the role of methionine in protein structure and function is largely overlooked by most biochemists. Thus, the main aim of the current article is not so much to carry out an exhaustive review of the many and diverse processes in which methionine residues are involved, but to review some illustrative examples that may help the nonspecialized reader to form a richer and more precise insight regarding the role‐played by methionine residues in such processes.     

Peptide methionine sulfoxide reductase: structure, mechanism of action, and biological function.

Reactive oxygen and nitrogen intermediates can cause damage to many cellular components and have been implicated in a number of diseases. Cells have developed a variety of mechanisms to destroy these reactive molecules or repair the damage once it occurs. In proteins one of the amino acids most easily oxidized is methionine, which is converted to methionine sulfoxide. An enzyme, peptide methionine sulfoxide reductase (MsrA), catalyzes the reduction of methionine sulfoxide in proteins back to methionine. There is growing evidence that MsrA plays an important role in protecting cells against oxidative damage. This paper reviews the biochemical properties and biological role of MsrA.     

High levels of methionine and methionine sulfoxide: Impact on adenine nucleotide hydrolysis and redox status in platelets and serum of young rats

We investigated acute and chronic effects administration of methionine (Met) and/or methionine sulfoxide (MetO) on ectonucleotidases and oxidative stress in platelets and serum of young rats. Wistar rats were divided into four groups: control, Met, MetO, and Met + MetO. In acute treatment, the animals received a single subcutaneous injection of amino acid(s) and were euthanized after 1 and 3 hours. In chronic protocol, Met and/or MetO were administered twice a day with an 8-hour interval from the 6th to the 28th day of life. Nucleoside triphosphate phosphohydrolase and 5′-nucleotidase activities were reduced in platelets and serum by Met, MetO, and Met + MetO after 3 hours and 21 days. Adenosine deaminase activity reduced in platelets at 3 hours after MetO and Met + MetO administration and increased after 21 days in animals treated with Met + MetO. Superoxide dismutase and catalase activities decreased in platelets in MetO and Met + MetO groups after 3 hours, while reactive oxygen species (ROS) levels increased in same groups. Catalase activity in platelets decreased in all experimental groups after chronic treatment. Met, MetO, and Met + MetO administration increased plasmatic ROS levels in acute and chronic protocols; glutathione S-transferase activity increased by MetO and Met + MetO administration at 3 hours, and ascorbic acid decreased in all experimental groups in acute and chronic protocols. Thiobarbituric acid reactive substances increased, superoxide dismutase and catalase activities reduced in the Met and/or MetO groups at 3 hours and in chronic treatment. Our data demonstrated that Met and/or MetO induced changes in adenine nucleotide hydrolysis and redox status of platelets and serum, which can be associated with platelet dysfunction in hypermethioninemia.     

Kinetics and mechanism of the chemiluminescence associated with the free radical-mediated oxidation of amino acids

When amino acids are incubated in the presence of a free radical source [2,2′-azobis(2-amidinopropane) dihydrocloride], only tyrosine (Tyr) and tryptophan (Trp) produce significant chemiluminescence. The relationship between the observed light intensity, the rate of the oxidation process and the substrate concentration is complex and can not be explained in terms of the formation of excited states in termination processes involving two peroxyl radicals (Russell's mechanism). The observed increase in light emission with the incubation time, for both Trp and Tyr, would indicate the participation of more than one reaction product as intermediates in the pathways leading to the production of excited molecules. However, the fact that after product accumulation a high proportion of the observed luminescence is quenched by Trolox addition, implies that the main chemiluminescent process must involve the interaction of product(s) and free radicals. From the effect of added Ebselen, it is proposed that hydroperoxides and peroxides, formed along the reaction path, are the intermediates whose accumulation leads to the observed increase in chemiluminescence with elapsed time. The observed time profiles and the proposed mechanism strongly resemble those associated with the oxidation of complex biological systems, suggesting that protein oxidation could be one of the main sources of chemiluminescence in biological oxidations. Copyright © 2000 John Wiley & Sons, Ltd.     

Impact of methionine oxidation as an initial event on the pathway of human prion protein conversion

Prion diseases comprise a group of fatal neurodegenerative disorders characterized by the autocatalytic conversion of the cellular prion protein PrP^C into the infectious misfolded isoform PrP^Sc. Increasing evidence supports a specific role of oxidative stress in the onset of pathogenesis. Although the associated molecular mechanisms remain to be elucidated in detail, several studies currently suggest that methionine oxidation already detected in misfolded PrP^Sc destabilizes the native PrP fold as an early event in the conversion pathway. To obtain more insights about the specific impact of surface-exposed methionine residues on the oxidative-induced conversion of human PrP we designed, produced, and comparatively investigated two new pseudosulfoxidation mutants of human PrP 121–231 that comprises the well-folded C-terminal domain. Applying circular dichroism spectroscopy and dynamic light scattering techniques we showed that pseudosulfoxidation of all surface exposed Met residues formed a monomeric molten globule-like species with striking similarities to misfolding intermediates recently reported by other groups. However, individual pseudosulfoxidation at the polymorphic M129 site did not significantly contribute to the structural destabilization. Further metal-induced oxidation of the partly unfolded pseudosulfoxidation mutant resulted in the formation of an oligomeric state that shares a comparable size and stability with PrP oligomers detected after the application of different other triggers for structural conversion, indicating a generic misfolding pathway of PrP. The obtained results highlight the specific importance of methionine oxidation at surface exposed residues for PrP misfolding, strongly supporting the hypothesis that increased oxidative stress could be one causative event for sporadic prion diseases and other neurodegenerative disorders.     

氨基酸对炎症性肠病的调控作用北大核心CSCD

炎症性肠病是胃肠道的一种慢性复发性炎症,包括克罗恩病和溃疡性结肠炎,患者众多,而且目前很难实现彻底治愈。由于患者消化功能受损,食物不容易吸收,很容易出现营养不良的情况,临床经常使用营养治疗来克服营养不足、改变炎症状态。氨基酸作为辅助营养治疗,可能有助于维持炎症性肠病患者的肠道完整性,减少炎症、氧化应激和肠道细胞死亡,对炎症性肠病的治疗起到积极的作用。最近在动物方面的研究已经证明氨基酸在炎症性肠病治疗中存在着巨大的潜力,氨基酸的供应和代谢可能是一种有前景的辅助治疗方法。本文就谷氨酰胺、精氨酸、甘氨酸等特定氨基酸的免疫调节作用进行综述,以期提供一种新的治疗炎症性肠病的思路。     

Neuronal expression of myeloperoxidase is increased in Alzheimer's disease

Myeloperoxidase, a heme protein expressed by professional phagocytic cells, generates an array of oxidants which are proposed to contribute to tissue damage during inflammation. We now report that enzymatically active myeloperoxidase and its characteristic amino acid oxidation products are present in human brain. Further, expression of myeloperoxidase is increased in brain tissue showing Alzheimer's neuropathology. Consistent with expression in phagocytic cells, myeloperoxidase immunoreactivity was present in some activated microglia in Alzheimer brains. However, the majority of immunoreactive material in brain localized with amyloid plaques and, surprisingly, neurons including granule and pyramidal neurons of the hippocampus. Confirming neuronal localization of the enzyme, several neuronal cell lines as well as primary neuronal cultures expressed myeloperoxidase protein. Myeloperoxidase mRNA was also detected in neuronal cell lines. These results reveal the unexpected presence of myeloperoxidase in neurons. The increase in neuronal myeloperoxidase expression we observed in Alzheimer disease brains raises the possibility that the enzyme contributes to the oxidative stress implicated in the pathogenesis of the neurodegenerative disorder.     

Screening and development of enzymes for determination and transformation of amino acids

ABSTRACT

The high stereo- and substrate specificities of enzymes have been utilized for micro-determination of amino acids. Here, I review the discovery of l-Phe dehydrogenase and its practical use in the diagnosis of phenylketonuria in more than 5,400,000 neonates over two decades in Japan. Screening and uses of other selective enzymes for micro-determination of amino acids have also been discussed. In addition, novel enzymatic assays with the systematic use of known enzymes, including assays based on a pyrophosphate detection system using pyrophosphate dikinase for a variety of l-amino acids with amino-acyl-tRNA synthetase have been reviewed. Finally, I review the substrate specificities of a few amino acid-metabolizing enzymes that have been altered, using protein engineering techniques, mainly for production of useful chemicals, thus enabling the wider use of natural enzymes.     

Jugular-infused methionine,lysine and branched-chain amino acids does not improve milk production in Holstein cows experiencing heat stress

Poor utilization of amino acids contributes to losses of milk protein yield in dairy cows exposed to heat stress (HS). Our objective was to test the effect of essential amino acids on milk production in lactating dairy cows exposed to short-term HS conditions. To achieve this objective, 12 multiparous, lactating Holstein cows were assigned to two environments (thermoneutral (THN) or HS) from days 1 to 14 in a split-plot type cross-over design. All cows received 0 g/day of essential amino acids from days 1 to 7 (negative control (NC)) followed by an intravenous infusion of l-methionine (12 g/day), l-lysine (21 g/day), l-leucine (35 g/day), l-isoleucine (15 g/day) and l-valine (15 g/day, methionine, lysine and branched-chain amino acids (ML+BCAA)) from days 8 to 14. The basal diet was composed of ryegrass silage and hay, and a concentrate mix. This diet supplied 44 g of methionine, 125 g of lysine, 167 g of leucine, 98 g of isoleucine and 109 g of valine per day to the small intestine of THN cows. Temperature–humidity index was maintained below 66 for the THN environment, whereas the index was maintained above 68, peaking at 76, for 14 continuous h/day for the HS environment. Heat stress conditioning increased the udder temperature from 37.0°C to 39.6°C. Cows that received the ML+BCAA treatment had greater p.m. rectal and vaginal temperatures (0.50°C and 0.40°C, respectively), and respiration rate (8 breaths/min) compared with those on the NC treatment and exposed to a HS environment. However, neither NC nor ML+BCAA affected rectal or vaginal temperatures and respiration rates in the THN environment. Compared with THN, the HS environment reduced dry matter intake (1.48 kg/day), milk yield (2.82 kg/day) and milk protein yield (0.11 kg/day). However, compared with NC, the ML+BCAA treatment increased milk protein percent by 0.07 points. For the THN environment, the ML+BCAA treatment increased concentrations of milk urea nitrogen. For the HS environment, the ML+BCAA treatment decreased plasma concentrations of arginine, ornithine and citrulline; however, differences were not observed for the THN environment. In summary, HS elicited expected changes in production; however, infusions of ML+BCAA failed to increase milk protein yield. Lower dry matter intake and greater heat load in response to ML+BCAA contributed to the lack of response in milk production in HS cows. The ML+BCAA treatment may have reduced the breakdown of muscle protein in heat-stressed cows.     

Synthesis of symmetric and asymmetric diamides of citric acid and amino acids

Summary A convenient method for the synthesis of symmetric and asymmetric diamides of amino acids including DOPA and citric acid from 2-tert-butyl-1,3-di(N-hydroxysuccinimidyl)citrate and 1-tert-butyl-2,3-di(N-hydroxysuccinimidyl)citrate is described.Abbreviations AcOtBu tert-butyl acetate - i-Bu iso-butyl - tBu tert-butyl - Bzl benzyl - p-OH-Bzl p-hydroxybenzyl - m,p-(OH)₂-Bzl m,p-dihydroxybenzyl - DCCI dicyclohexylcarbodiimide - Et ethyl - Me methyl - Su succinimidyl - SuOH N-hydroxysuccinimide - Ph phenyl     

Follow the leader: preference for specific amino acids directly following the initial methionine in proteins of different organisms

It is well established that the vast majority of proteins of all taxonomical groups and species are initiated by an AUG codon, translated into the amino acid methionine (Met). Many attempts were made to evaluate the importance of the sequences surrounding the initiation codon, mostly focusing on the RNA sequence. However, the role and importance of the amino acids following the initiating Met residue were rarely investigated, mostly in bacteria and fungi. Herein, we computationally examined the protein sequences of all major taxonomical groups represented in the Swiss-Prot database, and evaluated the preference of each group to specific amino acids at the positions directly following the initial Met. The results indicate that there is a species-specific preference for the second amino acid of the majority of protein sequences. Interestingly, the preference for a certain amino acid at the second position changes throughout evolution from lysine in prokaryotes, through serine in lower eukaryotes, to alanine in higher plants and animals.     

Permeability of membranes to amino acids and modified amino acids: Mechanisms involved in translocation

Summary The amino acid permeability of membranes is of interest because they are one of the key solutes involved in cell function. Membrane permeability coefficients (P) for amino acid classes, including neutral, polar, hydrophobic, and charged species, have been measured and compared using a variety of techniques. Decreasing lipid chain length increased permeability slightly (5-fold), while variations in pH had only minor effects on the permeability coefficients of the amino acids tested in liposomes. Increasing the membrane surface charge increased the permeability of amino acids of the opposite charge, while increasing the cholesterol content decreased membrane permeability. The permeability coefficients for most amino acids tested were surprisingly similar to those previously measured for monovalent cations such as sodium and potassium (approximately 10^–12–10^–13 cm · s^–1). This observation suggests that the permeation rates for the neutral, polar and charged amino acids are controlled by bilayer fluctuations and transient defects, rather than partition coefficients and Born energy barriers. Hydrophobic amino acids were 10² more permeable than the hydrophilic forms, reflecting their increased partition coefficient values.External pH had dramatic effects on the permeation rates for the modified amino acid lysine methyl ester in response to transmembrane pH gradients. It was established that lysine methyl ester and other modified short peptides permeate rapidly (P = 10^–2 cm · s^–1) as neutral (deprotonated) molecules. It was also shown that charge distributions dramatically alter permeation rates for modified di-peptides. These results may relate to the movement of peptides through membranes during protein translocation and to the origin of cellular membrane transport on the early Earth.Abbreviations DCP dicetylphosphate - DMPC dimyristoyl phosphatidylcholine - EPC egg phosphatidylcholine - LUV large unilamellar vesicle - MLV multilamellar vesicle - PLM planar lipid membrane - SUV small unilamellar vesicle - pH transmembrane pH gradient     

Synthesis of the sulfur amino acids: cysteine and methionine

This review will assess new features reported for the molecular and biochemical aspects of cysteine and methionine biosynthesis in Arabidopsis thaliana with regards to early published data from other taxa including crop plants and bacteria (Escherichia coli as a model). By contrast to bacteria and fungi, plant cells present a complex organization, in which the sulfur network takes place in multiple sites. Particularly, the impact of sulfur amino-acid biosynthesis compartmentalization will be addressed in respect to localization of sulfur reduction. To this end, the review will focus on regulation of sulfate reduction by synthesis of cysteine through the cysteine synthase complex and the synthesis of methionine and its derivatives. Finally, regulatory aspects of sulfur amino-acid biosynthesis will be explored with regards to interlacing processes such as photosynthesis, carbon and nitrogen assimilation.     

A sulfur amino acid deficiency changes the amino acid composition of body protein in piglets

Experiments carried out to determine the amino acid requirement in growing animals are often based on the premise that the amino acid composition of body protein is constant. However, there are indications that this assumption may not be correct. The objective of this study was to test the effect of feeding piglets a diet deficient or not in total sulfur amino acids (TSAA; Met + Cys) on nitrogen retention and amino acid composition of proteins in different body compartments. Six blocks of three pigs each were used in a combined comparative slaughter and nitrogen balance study. One piglet in each block was slaughtered at 42 days of age, whereas the other piglets received a diet deficient or not in TSAA for 19 days and were slaughtered thereafter. Two diets were formulated to provide either 0.20% Met and 0.45% TSAA (on a standardized ileal digestible basis) or 0.46% Met and 0.70% TSAA. Diets were offered approximately 25% below ad libitum intake. At slaughter, the whole animal was divided into carcass, blood, intestines, liver, and the combined head, tail, feet and other organs (HFTO), which were analyzed for nitrogen and amino acid contents. Samples of the longissimus muscle (LM) were analyzed for myosin heavy chain (MyHC) and actin contents. Nitrogen retention was 20% lower in piglets receiving the TSAA-deficient diet (P < 0.01). In these piglets, the nitrogen content in tissue gain was lower in the empty body, carcass, LM and blood (P < 0.05) or tended to be lower in HFTO (P < 0.10), but was not different in the intestines and liver. The Met content in retained protein was lower in the empty body, LM and blood (P < 0.05), and tended to be lower in the carcass (P < 0.10). The Cys content was lower in LM, but higher in blood of piglets receiving the TSAA-deficient diet (P < 0.05). Skeletal muscle appeared to be affected most by the TSAA deficiency. In LM, the Met content in retained protein was reduced by 12% and total Met retention by more than 60%. The MyHC and actin contents in LM were not affected by the TSAA content of the diet. These results show that a deficient TSAA supply affects the amino acid composition of different body proteins. This questions the use of a constant ideal amino acid profile to express dietary amino acid requirements, but also illustrates the plasticity of the animal to cope with nutritional challenges.     

The accumulation and metabolism of amino acids by Biomphalaria glabrata, a freshwater pulmonate snail

Freshwater pulmonate snails (Biomphalaria glabrata), pre-treated under bacteriostatic conditions, were incubated in 10 ml of standard medium containing various U-¹⁴C-labelled amino acids at concentrations of 10 μM. Measurements of mass-specific accumulation rates (MSARs) based on HPLC and the accumulation of U-¹⁴C-labelled amino acids into snail tissues have shown unequivocally for the first time that freshwater snails achieved a net accumulation of all the amino acids tested, including aminoisobutyrate (AIB), aspartate, alanine and a mixture of 13 amino acids. There were no significant differences between the MSAR values determined by HPLC from those based on the use of radiolabelled amino acids, whereas MSAR values for control snails were negligible and significantly less. Incubation of snails in media containing radiolabelled aspartate and a mixture of amino acids showed that the accumulated amino acids were readily distributed through the snail’s tissues and then metabolized. The ecological and biochemical questions arising from the fact that freshwater snails are capable of net accumulation of exogenous amino acids at naturally occurring concentrations and subsequent metabolic conversion, contrary to widely held views, are addressed.     

Effect of N-chloroamino acids on the erythrocyte

Amino acids present in blood plasma may be targets for oxidation and chlorination by HOCl/OCl^?. N-Chloroamino acids have been reported to be less reactive, but more selective than HOCl/OCl^? in their reactions; therefore, they may act as secondary mediators of HOCl/OCl^?-induced injury. This study compared the effects of five N-chloroamino acids (AlaCl, LysCl, SerCl, AspCl and PheCl) on erythrocytes with the action of HOCl/OCl^?. The N-chloroamino acids differed in stability and reactivity. They had a weaker haemolytic action than HOCl/OCl^?; HOCl/OCl^?, AlaCl and PheCl increased osmotic fragility of erythrocytes at a concentration of 1 mm. Oxidation of glutathione, formation of protein-glutathione mixed disulphides and efflux of GSSG from erythrocytes were observed for erythrocytes treated with all the employed chloroderivatives, while increased oxidation of 2′,7′-dichlorofluorescin was detected only after treatment of the cells with 1 mm HOCl/OCl^?, AlaCl and PheCl. Generally, the reactivity of at least some N-chloroamino acids may be not much lower with respect to HOCl/OCl^?.     

Simultaneous analysis of enantiomeric composition of amino acids and N-acetyl-amino acids by enantioselective chromatography

Among the three chiral columns, CHIROBIOTIC T, CHIRLPAK WH, and CHIRALCEL OD-R, tested for the separation of racemic amino acids and N-acetyl-amino acids, only CHIROBIOTIC T chiral column which is based on covalently bonded amphoteric glycopeptide, teicoplanin, as the stationary phase ligand could be successfully developed to enantiomerically separate racemic amino acids and N-acetyl amino acids simultaneously. This method can be used to determine the enantiomeric composition of amino acids and N-acetyl-amino acids in the catalysis of D-aminoacylase or L-aminoacylase and the conversion rate of N-acylamino acid racemases.