Modulation of potassium channel function by methionine oxidation and reduction

Activation mechanism of Gi and Go by reactive oxygen species

Reactive oxygen species are proposed to work as intracellular mediators. One of their target proteins is the alpha subunit of heterotrimeric GTP-binding proteins (Galpha(i) and Galpha(o)), leading to activation. H(2)O(2) is one of the reactive oxygen species and activates purified Galpha(i2). However, the activation requires the presence of Fe(2+), suggesting that H(2)O(2) is converted to more reactive species such as c*OH. The analysis with mass spectrometry shows that seven cysteine residues (Cys(66), Cys(112), Cys(140), Cys(255), Cys(287), Cys(326), and Cys(352)) of Galpha(i2) are modified by the treatment with *OH. Among these cysteine residues, Cys(66), Cys(112), Cys(140), Cys(255), and Cys(352) are not involved in *OH-induced activation of Galpha(i2). Although the modification of Cys(287) but not Cys(326) is required for subunit dissociation, the modification of both Cys(287) and Cys(326) is necessary for the activation of Galpha(i2) as determined by pertussis toxin-catalyzed ADP-ribosylation, conformation-dependent change of trypsin digestion pattern or guanosine 5'-3-O-(thio)triphosphate binding. Wild type Galpha(i2) but not Cys(287)- or Cys(326)-substituted mutants are activated by UV light, singlet oxygen, superoxide anion, and nitric oxide, indicating that these oxidative stresses activate Galpha(i2) by the mechanism similar to *OH-induced activation. Because Cys(287) exists only in G(i) family, this study explains the selective activation of G(i)/G(o) by oxidative stresses.

High-quality life extension by the enzyme peptide methionine sulfoxide reductase

Cumulative oxidative damages to cell constituents are considered to contribute to aging and age-related diseases. The enzyme peptide methionine sulfoxide reductase A (MSRA) catalyzes the repair of oxidized methionine in proteins by reducing methionine sulfoxide back to methionine. However, whether MSRA plays a role in the aging process is poorly understood. Here we report that overexpression of the msrA gene predominantly in the nervous system markedly extends the lifespan of the fruit fly Drosophila. The MSRA transgenic animals are more resistant to paraquat-induced oxidative stress, and the onset of senescence-induced decline in the general activity level and reproductive capacity is delayed markedly. The results suggest that oxidative damage is an important determinant of lifespan, and MSRA may be important in increasing the lifespan in other organisms including humans.

Ca(2+)-activated K+ channel inhibition by reactive oxygen species

We studied the effect of H(2)O(2) on the gating behavior of large-conductance Ca(2+)-sensitive voltage-dependent K(+) (K(V,Ca)) channels. We recorded potassium currents from single skeletal muscle channels incorporated into bilayers or using macropatches of Xenopus laevis oocytes membranes expressing the human Slowpoke (hSlo) alpha-subunit. Exposure of the intracellular side of K(V,Ca) channels to H(2)O(2) (4-23 mM) leads to a time-dependent decrease of the open probability (P(o)) without affecting the unitary conductance. H(2)O(2) did not affect channel activity when added to the extracellular side. These results provide evidence for an intracellular site(s) of H(2)O(2) action. Desferrioxamine (60 microM) and cysteine (1 mM) completely inhibited the effect of H(2)O(2), indicating that the decrease in P(o) was mediated by hydroxyl radicals. The reducing agent dithiothreitol (DTT) could not fully reverse the effect of H(2)O(2). However, DTT did completely reverse the decrease in P(o) induced by the oxidizing agent 5,5'-dithio-bis-(2-nitrobenzoic acid). The incomplete recovery of K(V,Ca) channel activity promoted by DTT suggests that H(2)O(2) treatment must be modifying other amino acid residues, e.g., as methionine or tryptophan, besides cysteine. Noise analysis of macroscopic currents in Xenopus oocytes expressing hSlo channels showed that H(2)O(2) induced a decrease in current mediated by a decrease both in the number of active channels and P(o).

Thioredoxin and peptide methionine sulfoxide reductase: convergence of similar structure and function in distinct structural folds

Thioredoxin (Trx) and peptide methionine sulfoxide reductase (PMSR) are small thiol oxidoreductases implicated in antioxidant defense and redox regulation of cellular processes. Here we show that the structures of Trx and PMSR exhibit resemblance in their alphabeta core regions and that the active site cysteines in two proteins occupy equivalent positions downstream of a central beta-strand and at the N-terminus of an alpha-helix. Moreover, we identified a PMSR subfamily that contains an active site CxxC motif (two cysteines separated by two other amino acids) positioned similarly to the catalytic redox active CxxC motif in Trx. However, Trx and PMSR are characterized by distinct ancient folds that differ in both orientation of secondary structures and their patterns. Trx is a member of the Trx-fold superfamily, whereas PMSR has a unique fold not found in other proteins. The data suggest that similar structures and functions of Trx and PMSR were acquired independently during evolution and point to a general strategy of identifying new redox regulatory proteins.

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.

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 sulfoxide reductase (MsrA) is a regulator of antioxidant defense and lifespan in mammals

Oxidation of proteins by reactive oxygen species is associated with aging, oxidative stress, and many diseases. Although free and protein-bound methionine residues are particularly sensitive to oxidation to methionine sulfoxide derivatives, these oxidations are readily repaired by the action of methionine sulfoxide reductase (MsrA). To gain a better understanding of the biological roles of MsrA in metabolism, we have created a strain of mouse that lacks the MsrA gene. Compared with the wild type, this mutant: (i) exhibits enhanced sensitivity to oxidative stress (exposure to 100% oxygen); (ii) has a shorter lifespan under both normal and hyperoxic conditions; (iii) develops an atypical (tip-toe) walking pattern after 6 months of age; (iv) accumulates higher tissue levels of oxidized protein (carbonyl derivatives) under oxidative stress; and (v) is less able to up-regulate expression of thioredoxin reductase under oxidative stress. It thus seems that MsrA may play an important role in aging and neurological disorders.

Effects of hydrogen peroxide on the transient outward current in rabbit atrial myocytes

1. In the present study, we investigated the effects of hydrogen peroxide (H2O2) on the 4-aminopyridine-sensitive transient outward current (I(TO)) in rabbit atrial myocytes using the amphotericin B-perforated patch voltage-clamp method. 2. Superfusion of myocytes with H2O2 at 100 micromol/L gradually slowed the time-course of inactivation of I(TO) and increased the peak by 9% (n = 9). The H2O2-induced slowing of I(TO) inactivation was concentration dependent (over the concentration range 10 micromol/L to 1 mmol/L). These effects were hardly reversed by washout of H2O2, but were quickly abolished by dithiothreitol (2 mmol/L). 3. Bisindolylmaleimide (100 nmol/L), an inhibitor of protein kinase C, significantly attenuated the H2O2-induced effects on I(TO). 4. These results suggest that rabbit atrial I(TO) is susceptible to oxidation by H2O2 at concentrations relevant to those encountered during ischaemia/reperfusion and that protein kinase C modulates the effects of H2O2.

Suppression of Ca2+ influx through L-type voltage-dependent calcium channels by hydroxyl radical in mouse cerebral cortical neurons

In the present study, we investigated the effect of hydroxyl radical (.OH) produced by the Fenton reaction with FeSO(4) to H(2)O(2) on Ca2+ influx by measuring [(45)Ca2+] influx into mouse cerebral cortical neurons in primary culture.OH formed from 3 microM FeSO(4) and 0.01 microM H(2)O(2) significantly reduced 30 mM KCl-induced [(45)Ca2+] influx and this reduction was abolished by .OH scavengers such as N,N'-dimethylthiourea and mannitol. Nifedipine (1 microM), an inhibitor for L-type voltage-dependent Ca2+ channels (VDCCs) showed no additive effect on the reduction of the 30 mM KCl-induced [(45)Ca2+] influx, while the inhibitors for P/Q- and N-type VDCCs showed further suppression of the KCl-induced [(45)Ca2+] influx even in the presence of .OH. Bay k 8644, an activator of L-type VDCCs, dose-dependently stimulated [(45)Ca2+] influx, and this stimulation disappeared in the presence of nifedipine. Similarly, .OH also suppressed significantly [(45)Ca2+] influx induced by Bay k 8644. These inhibitory actions of .OH on the KCl- and Bay k 8644-induced [(45)Ca2+] influx were completely abolished by .OH scavengers. These results indicate that .OH has the activity to suppress Ca2+ influx through L-type VDCCs.

Peptide methionine sulfoxide reductase from Escherichia coli and Mycobacterium tuberculosis protects bacteria against oxidative damage from reactive nitrogen intermediates

Inducible nitric oxide synthase (iNOS) plays an important role in host defense. Macrophages expressing iNOS release the reactive nitrogen intermediates (RNI) nitrite and S-nitrosoglutathione (GSNO), which are bactericidal in vitro at a pH characteristic of the phagosome of activated macrophages. We sought to characterize the active intrabacterial forms of these RNI and their molecular targets. Peptide methionine sulfoxide reductase (MsrA; EC ) catalyzes the reduction of methionine sulfoxide (Met-O) in proteins to methionine (Met). E. coli lacking MsrA were hypersensitive to killing not only by hydrogen peroxide, but also by nitrite and GSNO. The wild-type phenotype was restored by transformation with plasmids encoding msrA from E. coli or M. tuberculosis, but not by an enzymatically inactive mutant msrA, indicating that Met oxidation was involved in the death of these cells. It seemed paradoxical that nitrite and GSNO kill bacteria by oxidizing Met residues when these RNI cannot themselves oxidize Met. However, under anaerobic conditions, neither nitrite nor GSNO was bactericidal. Nitrite and GSNO can both give rise to NO, which may react with superoxide produced by bacteria during aerobic metabolism, forming peroxynitrite, a known oxidant of Met to Met-O. Thus, the findings are consistent with the hypotheses that nitrite and GSNO kill E. coli by intracellular conversion to peroxynitrite, that intracellular Met residues in proteins constitute a critical target for peroxynitrite, and that MsrA can be essential for the repair of peroxynitrite-mediated intracellular damage.

Peptide methionine sulfoxide reductase: biochemistry and physiological role

The oxidation of methionine to methionine sulfoxide both in vivo and in vitro can lead to the loss of biological activity in a variety of proteins. This loss of activity can be reversed by an enzyme called methionine sulfoxide reductase. The gene for this enzyme has been cloned and sequenced from a variety of prokaryotic and eukaryotic cells, and the deduced amino acid sequence is very highly conserved. The mechanism of action of the bovine enzyme has been shown to involve a critical cysteine residue located at position 72 of the protein. In addition to its role as a "repair" enzyme, other evidence suggests that the enzyme may be involved in bacterial adherence and regulation of protein activity.

Rat peptide methionine sulphoxide reductase: cloning of the cDNA, and down-regulation of gene expression and enzyme activity during aging

Peptide methionine sulphoxide reductase (PMSR, EC 1.8.4.6), the msrA or pmsR gene product, is a ubiquitous enzyme catalysing the reduction of methionine sulphoxide to methionine in proteins. Decreased expression and/or activity of the PMSR with age could explain, at least in part, the accumulation of oxidized protein observed upon aging. To test this hypothesis, the rat pmsR cDNA was cloned and sequenced. The recombinant protein was expressed, its catalytic activity checked with a synthetic substrate and polyclonal antibodies were raised against recombinant PMSR. The expression of the pmsR gene and protein as well as its catalytic activity were then analysed as a function of age in the rat brain and in two organs that express the most PMSR, liver and kidney. It appears that pmsR gene expression decreases with age in liver and kidney as early as 18 months, whereas protein level and protein activity are reduced in the three organs at the very end of the life of the rat (26 months). These results suggest that the down-regulation of PMSR can contribute to the accumulation of oxidized protein that has been associated with the aging process.

A missense mutation in a novel gene encoding a putative cation channel is associated with catatonic schizophrenia in a large pedigree

Schizophrenia is a common and etiologically heterogeneous disorder. Although inheritance of schizophrenic syndromes is complex with genetic and environmental factors contributing to the clinical phenotype, periodic catatonia, a familial subtype of catatonic schizophrenia, appears to be transmitted in an autosomal dominant manner. We report here that a Leu309Met mutation in WKL1, a positional candidate gene on chromosome 22q13.33 encoding a putative non-selective cation channel expressed exclusively in brain, co-segregates with periodic catatonia in an extended pedigree. Structural analyses revealed that this missense mutation results in conformational changes of the mutant protein. Our results not only underscore the importance of genetic mechanisms in the etiology of schizophrenic syndromes, but also provide a better understanding of the pathogenesis and incapacitating course of catatonic schizophrenia and related disorders.

*NO, RSNO, ONOO-, NO+, *NOO, NOx--dynamic regulation of oxidant scavenging, nitric oxide stores, and cyclic GMP-independent cell signaling

Following its release from nitric oxide synthase, nitric oxide seldom perfuses the cytosol; rather this reactive mediator quickly interacts with available target molecules proximate to its site of release. Within the cell, virtually every component, low-molecular-weight oxidants and reductants, proteins, lipids, sugars, and nucleic acids can be modified by nitrogen oxides thus acting as potential targets for reactive nitrogen oxides. Adducts formed by nitrogen oxides often modulate the cellular activities of the target molecules, and these modified molecules may be differentially metabolized or localized. The formation of nitrogen oxide adducts can be a reversible process, and the reactive nitrogen species released may be specifically oxidized or reduced during the process. Recently, numerous studies have demonstrated that reversible nitration of cellular proteins acts to transduce molecular signals regulating such diverse processes as muscle contraction, neurotransmission, protein metabolism, and apoptosis. The vast numbers of molecules that undergo biologically relevant interactions with nitrogen oxides imply that the cellular concentration of nitrosated and nitrated species may effectively comprise a reserve or cellular store. Potentially, these nitroso reserves function as critical components of the overall redox status of the intracellular environs. Understanding the dynamic regulation of nitric oxide/nitrogen oxides release from these stores is likely to provide clues important in resolving the complex pathophysiology of poorly understood multifactorial disorders, including neurodegeneration, multiorgan failure, cardiomyopathy, and septic shock.

Regulation of sodium currents through oxidation and reduction of thiol residues

Changes in redox state are involved in several physiological and pathophysiological processes. Previous experiments have demonstrated that nitric oxide can function as a reactive oxygen species, inhibiting neuronal sodium currents by nitrosylation of thiol residues. We hypothesized that nitric oxide and thiol oxidizers similarly modulate voltage-dependent sodium currents. Voltage-dependent sodium currents were studied with the whole-cell patch-clamp technique in NB41A3 neuroblastoma cells. The nitric oxide donor 3-(2-hydroxy-2-nitroso-1-propylhydrazino)-1-propanamine did not affect sodium currents. In contrast, the thiol oxidizers thimerosal and 4,4'-dithiopyridine significantly inhibited sodium currents. The effect of thimerosal persisted after washout, but could be fully reversed by the reducing agent dithiothreitol. Reduced glutathione did not restore the sodium current amplitude when given extracellularly, while intracellular glutathione prevented the inhibitory effect of thimerosal. Pretreatment with the alkylating agent N-ethylmaleimide blocked the inhibitory action of thimerosal. Thiol oxidation caused a shift in the voltage dependence of fast and slow inactivation to more hyperpolarized potentials without concomitant effects on the voltage dependence of activation. Mercaptoethanol and reduced glutathione enhanced sodium currents by shifting the voltage dependence of inactivation to depolarized potentials. These results demonstrate that the oxidation and reduction of thiol residues alters the properties of voltage-sensitive sodium channels and may play an important role in the regulation of membrane excitability.

Oxidative regulation of large conductance calcium-activated potassium channels

Reactive oxygen/nitrogen species are readily generated in vivo, playing roles in many physiological and pathological conditions, such as Alzheimer's disease and Parkinson's disease, by oxidatively modifying various proteins. Previous studies indicate that large conductance Ca(2+)-activated K(+) channels (BK(Ca) or Slo) are subject to redox regulation. However, conflicting results exist whether oxidation increases or decreases the channel activity. We used chloramine-T, which preferentially oxidizes methionine, to examine the functional consequences of methionine oxidation in the cloned human Slo (hSlo) channel expressed in mammalian cells. In the virtual absence of Ca(2+), the oxidant shifted the steady-state macroscopic conductance to a more negative direction and slowed deactivation. The results obtained suggest that oxidation enhances specific voltage-dependent opening transitions and slows the rate-limiting closing transition. Enhancement of the hSlo activity was partially reversed by the enzyme peptide methionine sulfoxide reductase, suggesting that the upregulation is mediated by methionine oxidation. In contrast, hydrogen peroxide and cysteine-specific reagents, DTNB, MTSEA, and PCMB, decreased the channel activity. Chloramine-T was much less effective when concurrently applied with the K(+) channel blocker TEA, which is consistent with the possibility that the target methionine lies within the channel pore. Regulation of the Slo channel by methionine oxidation may represent an important link between cellular electrical excitability and metabolism.

Regulation of cell function by methionine oxidation and reduction

Reactive oxygen species (ROS) are generated during normal cellular activity and may exist in excess in some pathophysiological conditions, such as inflammation or reperfusion injury. These molecules oxidize a variety of cellular constituents, but sulfur-containing amino acid residues are especially susceptible. While reversible cysteine oxidation and reduction is part of well-established signalling systems, the oxidation and the enzymatically catalysed reduction of methionine is just emerging as a novel molecular mechanism for cellular regulation. Here we discuss how the oxidation of methionine to methionine sulfoxide in signalling proteins such as ion channels affects the function of these target proteins. Methionine sulfoxide reductase, which reduces methionine sulfoxide to methionine in a thioredoxin-dependent manner, is therefore not only an enzyme important for the repair of age- or degenerative disease-related protein modifications. It is also a potential missing link in the post-translational modification cycle involved in the specific oxidation and reduction of methionine residues in cellular signalling proteins, which may give rise to activity-dependent plastic changes in cellular excitability.

The chaperone-like activity of a small heat shock protein is lost after sulfoxidation of conserved methionines in a surface-exposed amphipathic alpha-helix

The small heat shock proteins (sHsps) possess a chaperone-like activity which prevents aggregation of other proteins during transient heat or oxidative stress. The sHsps bind, onto their surface, molten globule forms of other proteins, thereby keeping them in a refolding competent state. In Hsp21, a chloroplast-located sHsp in all higher plants, there is a highly conserved region forming an amphipathic alpha-helix with several methionines on the hydrophobic side according to secondary structure prediction. This paper describes how sulfoxidation of the methionines in this amphipathic alpha-helix caused conformational changes and a reduction in the Hsp21 oligomer size, and a complete loss of the chaperone-like activity. Concomitantly, there was a loss of an outer-surface located alpha-helix as determined by limited proteolysis and circular dichroism spectroscopy. The present data indicate that the methionine-rich amphipathic alpha-helix, a motif of unknown physiological significance which evolved during the land plant evolution, is crucial for binding of substrate proteins and has rendered the chaperone-like activity of Hsp21 very dependent on the chloroplast redox state.

Crystal structure of the Escherichia coli peptide methionine sulphoxide reductase at 1.9 A resolution

BACKGROUND

Peptide methionine sulphoxide reductases catalyze the reduction of oxidized methionine residues in proteins. They are implicated in the defense of organisms against oxidative stress and in the regulation of processes involving peptide methionine oxidation/reduction. These enzymes are found in numerous organisms, from bacteria to mammals and plants. Their primary structure shows no significant similarity to any other known protein.

RESULTS

The X-ray structure of the peptide methionine sulphoxide reductase from Escherichia coli was determined at 3 A resolution by the multiple wavelength anomalous dispersion method for the selenomethionine-substituted enzyme, and it was refined to 1.9 A resolution for the native enzyme. The 23 kDa protein is folded into an alpha/beta roll and contains a large proportion of coils. Among the three cysteine residues involved in the catalytic mechanism, Cys-51 is positioned at the N terminus of an alpha helix, in a solvent-exposed area composed of highly conserved amino acids. The two others, Cys-198 and Cys-206, are located in the C-terminal coil.

CONCLUSIONS

Sequence alignments show that the overall fold of the peptide methionine sulphoxide reductase from E. coli is likely to be conserved in many species. The characteristics observed in the Cys-51 environment are in agreement with the expected accessibility of the active site of an enzyme that reduces methionine sulphoxides in various proteins. Cys-51 could be activated by the influence of an alpha helix dipole. The involvement of the two other cysteine residues in the catalytic mechanism requires a movement of the C-terminal coil. Several conserved amino acids and water molecules are discussed as potential participants in the reaction.

Structure and mechanism of peptide methionine sulfoxide reductase, an "anti-oxidation" enzyme

Peptide methionine sulfoxide reductase (MsrA) reverses oxidative damage to both free methionine and methionine within proteins. As such, it helps protect the host organism against stochastic damage that can contribute to cell death. The structure of bovine MsrA has been determined in two different modifications, both of which provide different insights into the biology of the protein. There are three cysteine residues located in the vicinity of the active site. Conformational changes in a glycine-rich C-terminal tail appear to allow all three thiols to come together and to participate in catalysis. The structures support a unique, thiol-disulfide exchange mechanism that relies upon an essential cysteine as a nucleophile and additional conserved residues that interact with the oxygen atom of the sulfoxide moiety.

Modulation of potassium channels in the hearts of transgenic and mutant mice with altered polyamine biosynthesis

Inward rectification of cardiac I(K1)channels was modulated by genetic manipulation of the naturally occurring polyamines. Ornithine decarboxylase (ODC) was overexpressed in mouse heart under control of the cardiac alpha -myosin heavy chain promoter (alpha MHC). In ODC transgenic hearts, putrescine and cadaverine levels were highly elevated ( identical with 35-fold for putrescine), spermidine was increased 3.6-fold, but spermine was essentially unchanged. I(K1)density was reduced by identical with 38%, although the voltage-dependence of rectification was essentially unchanged. Interestingly, the fast component of transient outward (I(to,f)) current was increased, but the total outward current amplitude was unchanged. I(K1)and I(to)currents were also studied in myocytes from mutant Gyro (Gy) mice in which the spermine synthase gene is disrupted, leading to a complete loss of spermine. I(K1)current densities were not altered in Gy myocytes, but the steepness of rectification was reduced indicating a role for spermine in controlling rectification. Intracellular dialysis of myocytes with putrescine, spermidine and spermine caused reduction, no change and increase of the steepness of rectification, respectively. Taken together with kinetic analysis of I(K1)activation these results are consistent with spermine being a major rectifying factor at potentials positive to E(K), spermidine dominating at potentials around and negative to E(K), and putrescine playing no significant role in rectification in the mouse heart.

Regulation of protein function by S-glutathiolation in response to oxidative and nitrosative stress

Protein S-glutathiolation, the reversible covalent addition of glutathione to cysteine residues on target proteins, is emerging as a candidate mechanism by which both changes in the intracellular redox state and the generation of reactive oxygen and nitrogen species may be transduced into a functional response. This review will provide an introduction to the concepts of oxidative and nitrosative stress and outline the molecular mechanisms of protein regulation by oxidative and nitrosative thiol-group modifications. Special attention will be paid to recently published work supporting a role for S-glutathiolation in stress signalling pathways and in the adaptive cellular response to oxidative and nitrosative stress. Finally, novel insights into the molecular mechanisms of S-glutathiolation as well as methodological problems related to the interpretation of the biological relevance of this post-translational protein modification will be discussed.

Thiol-disulfide exchange is involved in the catalytic mechanism of peptide methionine sulfoxide reductase

Peptide methionine sulfoxide reductase (MsrA; EC ) reverses the inactivation of many proteins due to the oxidation of critical methionine residues by reducing methionine sulfoxide, Met(O), to methionine. MsrA activity is independent of bound metal and cofactors but does require reducing equivalents from either DTT or a thioredoxin-regenerating system. In an effort to understand these observations, the four cysteine residues of bovine MsrA were mutated to serine in a series of permutations. An analysis of the enzymatic activity of the variants and their free sulfhydryl states by mass spectrometry revealed that thiol-disulfide exchange occurs during catalysis. In particular, the strictly conserved Cys-72 was found to be essential for activity and could form disulfide bonds, only upon incubation with substrate, with either Cys-218 or Cys-227, located at the C terminus. The significantly decreased activity of the Cys-218 and Cys-227 variants in the presence of thioredoxin suggested that these residues shuttle reducing equivalents from thioredoxin to the active site. A reaction mechanism based on the known reactivities of thiols with sulfoxides and the available data for MsrA was formulated. In this scheme, Cys-72 acts as a nucleophile and attacks the sulfur atom of the sulfoxide moiety, leading to the formation of a covalent, tetracoordinate intermediate. Collapse of the intermediate is facilitated by proton transfer and the concomitant attack of Cys-218 on Cys-72, leading to the formation of a disulfide bond. The active site is returned to the reduced state for another round of catalysis by a series of thiol-disulfide exchange reactions via Cys-227, DTT, or thioredoxin.

HIV-2 protease is inactivated after oxidation at the dimer interface and activity can be partly restored with methionine sulphoxide reductase

Human immunodeficiency viruses encode a homodimeric protease that is essential for the production of infectious virus. Previous studies have shown that HIV-1 protease is susceptible to oxidative inactivation at the dimer interface at Cys-95, a process that can be reversed both chemically and enzymically. Here we demonstrate a related yet distinct mechanism of reversible inactivation of the HIV-2 protease. Exposure of the HIV-2 protease to H(2)O(2) resulted in conversion of the two methionine residues (Met-76 and Met-95) to methionine sulphoxide as determined by amino acid analysis and mass spectrometry. This oxidation completely inactivated protease activity. However, the activity could be restored (up to 40%) after exposure of the oxidized protease to methionine sulphoxide reductase. This treatment resulted in the reduction of methionine sulphoxide 95 but not methionine sulphoxide 76 to methionine, as determined by peptide mapping/mass spectrometry. We also found that exposure of immature HIV-2 particles to H(2)O(2) led to the inhibition of polyprotein processing in maturing virus particles comparable to that demonstrated for HIV-1 particles. Thus oxidative inactivation of the HIV protease in vitro and in maturing viral particles is not restricted to the type 1 proteases. These studies indicate that two distinct retroviral proteases are susceptible to inactivation after a very minor modification at residue 95 of the dimer interface and suggest that the dimer interface might be a viable target for the development of novel protease inhibitors.

Reactive oxygen species and nitric oxide mediate plasticity of neuronal calcium signaling

Reactive oxygen species (ROS) and nitric oxide (NO) are important participants in signal transduction that could provide the cellular basis for activity-dependent regulation of neuronal excitability. In young rat cortical brain slices and undifferentiated PC12 cells, paired application of depolarization/agonist stimulation and oxidation induces long-lasting potentiation of subsequent Ca(2+) signaling that is reversed by hypoxia. This potentiation critically depends on NO production and involves cellular ROS utilization. The ability to develop the Ca(2+) signal potentiation is regulated by the developmental stage of nerve tissue, decreasing markedly in adult rat cortical neurons and differentiated PC12 cells.

Acceleration of P/C-type inactivation in voltage-gated K(+) channels by methionine oxidation

Oxidation of amino acid residues causes noticeable changes in gating of many ion channels. We found that P/C-type inactivation of Shaker potassium channels expressed in Xenopus oocytes is irreversibly accelerated by patch excision and that this effect was mimicked by application of the oxidant H(2)O(2), which is normally produced in cells by the dismutase action on the superoxide anion. The inactivation time course was also accelerated by high concentration of O(2). Substitution of a methionine residue located in the P-segment of the channel with a leucine largely eliminated the channel's sensitivity to patch excision, H(2)O(2), and high O(2). The results demonstrate that oxidation of methionine is an important regulator of P/C-type inactivation and that it may play a role in mediating the cellular responses to hypoxia/hyperoxia.

Thymosin beta 4 sulfoxide is an anti-inflammatory agent generated by monocytes in the presence of glucocorticoids

The possibility that glucocorticoids upregulate the expression of anti-inflammatory mediators is an exciting prospect for therapy in inflammatory diseases, because these molecules could give the therapeutic benefits of steroids without toxic side effects. Supernatants from monocytes and macrophages cultured in the presence of glucocorticoids increase the dispersion of neutrophils from a cell pellet in the capillary tube migration assay. This supernatant factor, unlike other neutrophil agonists, promotes dispersive locomotion of neutrophils at uniform concentration, lowers their adhesion to endothelial cells, inhibits their chemotactic response to fMLP and induces distinctive morphological changes. Here we show that thymosin beta4 sulfoxide is generated by monocytes in the presence of glucocorticoids and acts as a signal to inhibit an inflammatory response. In vitro, thymosin beta4 sulfoxide inhibited neutrophil chemotaxis, and in vivo, the oxidized peptide, but not the native form, was a potent inhibitor of carrageenin-induced edema in the mouse paw. Thymosin beta4 is unique, because oxidation attenuates its intracellular G-actin sequestering activity, but greatly enhances its extracellular signaling properties. This description of methionine oxidation conferring extracellular function on a cytosolic protein may have far-reaching implications for future strategies of anti-inflammatory therapy.

Ozone inhibits guard cell K+ channels implicated in stomatal opening

Ozone (O3) deleteriously affects organisms ranging from humans to crop plants, yet little is understood regarding the underlying mechanisms. In plants, O3 decreases CO2 assimilation, but whether this could result from direct O3 action on guard cells remained unknown. Potassium flux causes osmotically driven changes in guard cell volume that regulate apertures of associated microscopic pores through which CO2 is supplied to the photosynthetic mesophyll tissue. We show in Vicia faba that O3 inhibits (i) guard cell K+ channels that mediate K+ uptake that drives stomatal opening; (ii) stomatal opening in isolated epidermes; and (iii) stomatal opening in leaves, such that CO2 assimilation is reduced without direct effects of O3 on photosynthetic capacity. Direct O3 effects on guard cells may have ecological and agronomic implications for plant productivity and for response to other environmental stressors including drought.

Decrease in peptide methionine sulfoxide reductase in Alzheimer's disease brain

Previous studies have shown that the pathophysiology of Alzheimer's disease (AD) is linked to oxidative stress. Oxidative damage to different biomolecular components of the brain is a characteristic feature of AD. Recent evidence suggests that methionine may act as an antioxidant defense molecule in proteins by its ability to scavenge oxidants and, in the process, undergo oxidation to form methionine sulfoxide. The enzyme peptide, methionine sulfoxide reductase (MsrA), reverses methionine sulfoxide back to methionine, which once again is able to scavenge oxidants. The purpose of this study was to measure the activity of MsrA in the brain of AD patients compared with control subjects. Our results showed that there was a decline in MsrA activity in all brain regions studied in AD and this decline reached statistical significance in the superior and middle temporal gyri (p < 0.001), inferior parietal lobule (p < 0.05), and the hippocampus (p < 0.05) in AD. An elevation of protein carbonyl content was found in all brain regions except the cerebellum in AD and reached statistical significance in the superior and middle temporal gyri and hippocampus. Messenger RNA analysis suggests that the loss in enzyme activity may be the result of a posttranslational modification of MsrA or a defect of translation resulting in inferior processing of the MsrA mRNA. Our results suggest that a decline in MsrA activity could reduce the antioxidant defenses and increase the oxidation of critical proteins in neurons in the brain in AD.

Molecular cloning and functional expression of a human peptide methionine sulfoxide reductase (hMsrA)

Oxidation of methionine residues in proteins to methionine sulfoxide can be reversed by the enzyme peptide methionine sulfoxide reductase (MsrA, EC 1.8.4.6). We cloned the gene encoding a human homologue (hMsrA) of the enzyme, which has an 88% amino acid sequence identity to the bovine version (bMsrA). With dot blot analyses based on RNA from human tissues, expression of hMsrA was found in all tissues tested, with highest mRNA levels in adult kidney and cerebellum, followed by liver, heart ventricles, bone marrow and hippocampus. In fetal tissue, expression was highest in the liver. No expression of hmsrA was detected in leukemia and lymphoma cell lines. To test if hMsrA is functional in cells, we assayed its effect on the inactivation time course of the A-type potassium channel ShC/B since this channel property strongly depends on the oxidative state of a methionine residue in the N-terminal part of the polypeptide. Co-expression of ShC/B and hMsrA in Xenopus oocytes significantly accelerated inactivation, showing that the cloned enzyme is functional in an in vivo assay system. Furthermore, the activity of a purified glutathione-S-transferase-hMsrA fusion protein was demonstrated in vitro by measuring the reduction of [3H]N-acetyl methionine sulfoxide.

Diastereoselective reduction of protein-bound methionine sulfoxide by methionine sulfoxide reductase

Methionine sulfoxide (MetSO) in calmodulin (CaM) was previously shown to be a substrate for bovine liver peptide methionine sulfoxide reductase (pMSR, EC 1.8.4.6), which can partially recover protein structure and function of oxidized CaM in vitro. Here, we report for the first time that pMSR selectively reduces the D-sulfoxide diastereomer of CaM-bound L-MetSO (L-Met-D-SO). After exhaustive reduction by pMSR, the ratio of L-Met-D-SO to L-Met-L-SO decreased to about 1:25 for hydrogen peroxide-oxidized CaM, and to about 1:10 for free MetSO. The accumulation of MetSO upon oxidative stress and aging in vivo may be related to incomplete, diastereoselective, repair by pMSR.

Rundown of the hyperpolarization-activated KAT1 channel involves slowing of the opening transitions regulated by phosphorylation

Disappearance of the functional activity or rundown of ion channels upon patch excision in many cells involves a decrease in the number of channels available to open. A variety of cellular and biophysical mechanisms have been shown to be involved in the rundown of different ion channels. We examined the rundown process of the plant hyperpolarization-activated KAT1 K+ channel expressed in Xenopus oocytes. The decrease in the KAT1 channel activity on patch excision was accompanied by progressive slowing of the activation time course, and it was caused by a shift in the voltage dependence of the channel without any change in the single-channel amplitude. The single-channel analysis showed that patch excision alters only the transitions leading up to the burst states of the channel. Patch cramming or concurrent application of protein kinase A (PKA) and ATP restored the channel activity. In contrast, nonspecific alkaline phosphatase (ALP) accelerated the rundown time course. Low internal pH, which inhibits ALP activity, slowed the KAT1 rundown time course. The results show that the opening transitions of the KAT1 channel are enhanced not only by hyperpolarization but also by PKA-mediated phosphorylation.

Ca2+-dependent gating mechanisms for dSlo, a large-conductance Ca2+-activated K+ (BK) channel

The Ca2+-dependent gating mechanism of cloned BK channels from Drosophila (dSlo) was studied. Both a natural variant (A1/C2/E1/G3/IO) and a mutant (S942A) were expressed in Xenopus oocytes, and single-channel currents were recorded from excised patches of membrane. Stability plots were used to define stable segments of data. Unlike native BK channels from rat skeletal muscle in which increasing internal Ca2+ concentration (Cai2+) in the range of 5 to 30 microM increases mean open time, increasing Cai2+ in this range for dSlo had little effect on mean open time. However, further increases in Cai2+ to 300 or 3000 microM then typically increased dSlo mean open time. Kinetic schemes for the observed Ca2+-dependent gating kinetics of dSlo were evaluated by fitting two-dimensional dwell-time distributions using maximum likelihood techniques and by comparing observed dependency plots with those predicted by the models. Previously described kinetic schemes that largely account for the Ca2+-dependent kinetics of native BK channels from rat skeletal muscle did not adequately describe the Ca2+ dependence of dSlo. An expanded version of these schemes which, in addition to the Ca2+-activation steps, permitted a Ca2+-facilitated transition from each open state to a closed state, could approximate the Ca2+-dependent kinetics of dSlo, suggesting that Ca2+ may exert dual effects on gating.

Regulation of voltage-dependent K+ channels by methionine oxidation: effect of nitric oxide and vitamin C

Methionine oxidation is known to alter functional properties of a transient A-type potassium channel expressed in Xenopus oocytes. We show here that nitric oxide (NO) slows down the K+ channel inactivation time course by oxidizing a critical methionine residue in the inactivation ball domain of the channel protein. We also demonstrate that the channel protein is protected from methionine oxidation by the enzyme methionine sulfoxide reductase and the antioxidant vitamin C.

Overexpression of peptide-methionine sulfoxide reductase in Saccharomyces cerevisiae and human T cells provides them with high resistance to oxidative stress

The yeast peptide-methionine sulfoxide reductase (MsrA) was overexpressed in a Saccharomyces cerevisiae null mutant of msrA by using a high-copy plasmid harboring the msrA gene and its promoter. The resulting strain had about 25-fold higher MsrA activity than its parent strain. When exposed to either hydrogen peroxide, paraquat, or 2,2'-azobis-(2-amidinopropane) dihydrochloride treatment, the MsrA overexpressed strain grew better, had lower free and protein-bound methionine sulfoxide and had a better survival rate under these conditions than did the msrA mutant and its parent strain. Substitution of methionine with methionine sulfoxide in a medium lacking hydrogen peroxide had little effect on the growth pattern, which suggests that the oxidation of free methionine in the growth medium was not the main cause of growth inhibition of the msrA mutant. Ultraviolet A radiation did not result in obvious differences in survival rates among the three strains. An enhanced resistance to hydrogen peroxide treatment was shown in human T lymphocyte cells (Molt-4) that were stably transfected with the bovine msrA and exposed to hydrogen peroxide. The survival rate of the transfected strain was much better than its parent strain when grown in the presence of hydrogen peroxide. These results support the proposition that the msrA gene is involved in the resistance of yeast and mammalian cells to oxidative stress.

Modulation by chloramine-T of 4-aminopyridine-sensitive transient outward current in rabbit atrial cells

The effects of an oxidizing agent, chloramine-T, on the 4-aminopyridine-sensitive transient outward current (ITO) were investigated in rabbit atrial myocytes by using patch-clamp techniques. Extracellular application of chloramine-T at 20 microM irreversibly slowed the time course of inactivation of the whole-cell ITO, and increased the peak by 19.3% (n = 19) at +40 mV. At 100 microM, chloramine-T decreased the peak by 22.5% (n = 9) of the control, and subsequently induced a glibenclamide-sensitive time-independent outward K+ current. Under superfusion with dithiothreitol (3 mM), chloramine-T (100 microM) produced no change in ITO. The chloramine-T-induced slowing of ITO inactivation was partially reversed by subsequent application of 3 mM dithiothreitol. In single-channel recordings with the cell-attached patch configuration, chloramine-T (20 microM) increased the open probability of the ITO channel from 0.15 to 0.46 at a potential 100 mV positive to the resting potential, and the mean open lifetime from 5.1 ms to 7.0 ms (n = 5). The unitary current amplitude was not affected. As a result, chloramine-T increased the ensemble current in amplitude and slowed its decay. These results indicated that: (1) inactivation of the native A-type channels of rabbit heart is susceptible to oxidation; and (2) oxidation of ITO channels may contribute to the genesis of arrhythmias.

Oxidation regulates cloned neuronal voltage-dependent Ca2+ channels expressed in Xenopus oocytes

Functional modifications of neuronal P/Q-type voltage-dependent Ca2+ channels expressed in Xenopus oocytes by oxidation were examined electrophysiologically. Oxidation by external H2O2 enhanced the whole-oocyte currents through the Ca2+ channels composed of the alpha1A, alpha2/delta, and beta3 subunits at negative voltages (<0 mV) without markedly affecting the currents at more positive voltages. Single-channel analysis showed that oxidation accelerates the overall channel opening process. The effect of H2O2 to enhance the Ca2+ channel activity did not require heterologous expression of the alpha2/delta subunit, and it was not mimicked by a cysteine-specific oxidizing agent. The results suggest that oxidative stress may regulate the activity of neuronal Ca2+ channels and that regulation by oxidation may be important in some clinical situations, such as in reperfusion injury after ischemic episodes.

Oxidation regulates cloned neuronal voltage-dependent Ca2+ channels expressed in Xenopus oocytes

Functional modifications of neuronal P/Q-type voltage-dependent Ca2+ channels expressed in Xenopus oocytes by oxidation were examined electrophysiologically. Oxidation by external H2O2 enhanced the whole-oocyte currents through the Ca2+ channels composed of the alpha1A, alpha2/delta, and beta3 subunits at negative voltages (<0 mV) without markedly affecting the currents at more positive voltages. Single-channel analysis showed that oxidation accelerates the overall channel opening process. The effect of H2O2 to enhance the Ca2+ channel activity did not require heterologous expression of the alpha2/delta subunit, and it was not mimicked by a cysteine-specific oxidizing agent. The results suggest that oxidative stress may regulate the activity of neuronal Ca2+ channels and that regulation by oxidation may be important in some clinical situations, such as in reperfusion injury after ischemic episodes.