Proteins, lipids or DNA in the path of destruction Detection and meaning
Markers of oxidative stress
XIIth ISFRR International Conference Free Radical School, May 6, 2004
Albert van der Vliet, Ph.D. University of Vermont
Introduction
What is the role of oxidants in biology? What oxidants do we mean?
- Reactive oxygen species comprise various metabolites with highly
variable reactive properties (O2•–, H2O2, OH•, 1O2)
- In addition, analogous families of reactive nitrogen species (NO •, N2O3, NO2•, ONOO–, etc.) and reactive halogen species (HOCl, Cl2, HOBr,
NH2Cl, chloramines, bromamines, etc.).
Biological oxidants are important in physiology and pathology
- Involved in normal cell function (e.g. growth regulation, host defense),
but also in (age-related) disease?
- Is this due to altered production of oxidants? Oxidative modification of
specific cell targets? - How do we measure this?
Biological targets for oxidation
• Unsaturated fatty acids, phospholipids, lipoproteins • Nucleic acids, DNA/RNA • Proteins, peptides, amino acids
• Sugars, carbohydrates • Other small biomolecules (antioxidants, cofactors, etc.)
Lipid oxidation
Defined as “the oxidative deterioration of polyunsaturated fatty acids” (A.L. Tappel) Although general peroxidation mechanisms well characterized, highly variable range of products, due to:
- range of different biological lipid classes (phospholipids, cholesterol esters, triglycerides) - variable unsaturated fatty acids (16:1, 18:2, 20:4, 22:5); generally, the more unsaturated, the more oxidizable - non-enzymatic (e.g. hydroxyl radical, metal ion catalyzed) and enzymatic oxidation mechanisms (e.g. lipoxygenases, cyclooxygenases) - variable endproducts depending on secondary reactions, effects of antioxidants and repair/turnover
Radical-mediated lipid peroxidation: initiation and propagation
Linoleic acid (18:2)
OH• Bisallylic carbon radical
R
Hydrogen abstraction (Initiation) R
•
•
O O•
O2
Diene conjugation
R
Conjugated diene
Oxygen addition
R
Peroxyl radical HOO Lipid hydroperoxide LH
Hydrogen abstraction (Propagation) R +
Fe(II)
•O
H H H Ethane H LH H Alkoxyl radical
Fenton reaction
R
H H
O
Fragmenation
R
+
H Aldehyde
•
Reactive lipid oxidation products: a,b-unsaturated aldehydes
H H 2 a 1 H H b H H 2 a 1 O O
H
H 2
b
O HO 3
O
3
1
Malondialdehyde (b-hydroxy-acrolein)
H
3
Acrolein (2,3-propenal)
H
H b H3C
3
2 a 1
O
Crotonaldehyde (2,3-butenal)
b 9
H
8
7
6
5
4
OH
3
2 a 1
O
4-Hydroxy-2,3-nonenal (HNE)
Strong electrophiles because of unsaturated carbonyl group, react rapidly with nucleophilic targets (e.g. GSH, protein thiols) (Esterbauer et al., Free Rad. Biol. Med. 1991: 11, 81-128; Uchida, Free Rad. Biol. Med. 2000: 28, 1685-1696)
Reactive lipid oxidation products: cyclopentenone prostaglandins
Cyclopentenone prostaglandins of A and J series are generated by dehydration of prostaglandins E2 and D2. Highly reactive because of unsaturated carbonyl group which conjugates rapidly with thiols, which may be involved in its bioactive properties. (Milne et al., Chem. Res. Toxicol. 2004: 17, 17-25)
Arachidonic acid oxidation products: isoprostanes
Isoprostanes comprise four regioisomers with eight stereoisomers. Relatively stable endproducts that are currently viewed as most reliable marker of lipid oxidation in vivo (Lawson et al., J. Biol. Chem. 1999: 274, 2444-24444)
Lipid oxidation
Significance
- formation of bioactive substances, secondary effects on DNA/RNA or
proteins (receptor-mediated or alkylation) - alteration of membrane function, fluidity
Detection of stable endproducts
- Colorometric/fluorometric methods (TBARS, conjugated dienes)
- HPLC, GC/LC-MS (lipid hydroperoxides, alkanes, isoprostanes, chlorohydrins, nitrated lipids)
- Immunochemical methods, ELISA (isoprostanes, MDA-dG, HNE-His, etc.)
Significance of lipid oxidation: repair of oxidized lipids
(I)
Lipid-soluble antioxidants (vitamin E) prevent oxidative degradation of lipids and terminate lipid oxidation to form lipid hydroperoxides.
(II) Phospholipid hydroperoxide
can be reduced by phospholipid hydroperoxide GSH peroxidase (PHGPX), or excised by phospholipase A2 (PLA2; which preferentially cleaved oxidized fatty acids)
(III)
Cleaved fatty acid hydroperoxide can be reduced by GSH peroxidase (GPX), and reduced phospholipid hydroperoxide can be cleaved by PLA2.
(IV) Lyso-phospholipid (lacking one fatty acid
chain) can be repaired by acyltransferase
Ghirotti (1998) J. Lipid Res. 39: 1529-1542
Oxidative damage to DNA and its detection
•
•
Single and double strand breaks
Comet assay (Fairbairn et al. Mutat. Res. 1995: 339, 37-59)
Oxidative modification of DNA-bases (e.g. 8- OH-guanine, 5-OH-uracil, etc.)
GS/MS based methods (Dizdaroglu, Mutat. Res. 2003: 531, 109-126) Immunochemical methods (e.g. 8-oxodG)
•
Nitrosative deamination of DNA bases
Guanine - xanthine Cytosine - uracil (Burney et al. Mutat. Res. 1999: 424, 37-49)
•
Formation of DNA adducts with oxidized lipids or proteins (e.g. dG-MDA, dTTyr).
DNA base oxidation: oxidation of guanine by OH•
O
Guan(os)ine
O HN N• OH H H2N
HN
N
N
N
O HN
OH•
G8OH•
R
N•
OH H
H2N
N
N
H2N
N
N R
Oxidation Ring opening
O HN
R
Reduction
O
H N
O
HN
N
OH N
N R
H2N
H2N
N
N
FAPy-guanine
8-OH-guanine
R
Lipid oxidation products and addition to dG
O
deoxy-Guanosine
H2N
HN
N
N
N dR
Acrolein Crotonaldehyde Hydroxynonenal (HNE)
O
N N
OH N N H
O
N
HO
N H
N
N
dR
R’
N
N
dR
Acr-dG 1&2
Acr-dG3 Cro-dG HNE-dG
-R’ -H -CH3 -CH(OH)-CH2-CH2-CH2-CH2-CH3
DNA repair mechanisms
• Direct reversal of DNA modification
- photolyase (only in light-exposed cells) - removal of methyl groups (O6-alkylguanine-DNA-alkyltransferase)
• Base excision repair (small modifications not causing major helix disruption)
- removal of modified base (DNA-glycosylase) - AP site is “opened” by endonuclease - abasic suger replaced by correct nucleotide (DNA polymerase) - resealing (DNA ligase)
• Nucleotide-excision repair (bulky lesions or cross-links)
- Excision of damaged nucleotide with neighboring nucleotides (ATP-dependent nuclease) - Insertion of correct nucleotides (DNA polymerase)
• Recombinational repair (DNA replication) - sister chromatid exchange
- double-strand-break repair (nonhomologous DNA end joining; NHEJ)
Consequences of DNA oxidation
• Repair can be ineffective
This causes mismatch repair and mutations, e.g.: - adenine deamination: AT-GC transitions
- 8OH-dG: GC-TA transversions
• If DNA repair is excessive:
- Massive ADP-ribosylation by activation PARP - Causes depletion of NAD and consequently ATP (metabolic cell stress)
Poly(ADP-ribose) polymerase (PARP) and DNA repair
NH2 N N O OH OH N N P + N OH
CONH2
P
O OH
NAD+
CONH2 + N
Nicotinamide
NH2
N
N O N
N
P P
O
OH
OH
OH
OH
n
ADPr
DNA damage causes the activation of PARP, which causes poly(ADP-ribosyl)ation of itself as well as other DNA-bound proteins (transcription factors, histones), causing their dissociation to facilitate DNA repair. Excessive PARP activation causes depletion of NAD and ATP.
D’Amours et al. (1999) Biochem. J. 342: 249-268
Protein oxidation
Why is protein oxidation important?
- constitute most of the functional components in the cell (enzymes, ion
channels, structural proteins); oxidative modification may have direct consequences
- relatively abundant in cells/tissues compared to e.g. lipids or DNA
(e.g. per kg wet weight: 146 g protein, 2.6 g DNA, 49 g total lipid) - certain amino acids or protein functional groups susceptible to oxidation:
importance in oxidant signaling.
Difficulties with proteins
- High variability; 20 amino acids, thousands of different proteins.
- Highly variable abundance and turnover of proteins - Many types of oxidative modifications: oxidative fragmentation/crosslinking,
reversible and irreversible oxidations of amino acid side chains
Protein oxidation: redox signaling or oxidant injury
Reversible modifications (causing dysfunction of targets)
• Electron transfer or coordination reaction with transition metal ions (e.g. heme centers, iron-sulfur clusters): Aconitase, hemoglobin, cytochrome c, antioxidant enzymes (SOD, catalase) • Oxidation/nitros(yl)ation of cysteine residues: Cell signaling proteins ( G-proteins, Ras, phosphatases, etc.), transport proteins (e.g. Ca2+-ATPases), transcription factors (e.g. NF-kB), proteases (e.g. caspases) • Methionine oxidation: May represent important defense mechanisms against oxidative stress • Tyrosine nitration (reversed by denitrases?)
Irreversible modifications (destruction of targets)
• Oxidative protein fragmentation/crosslinking: usually due to radical-mediated hydrogen abstraction in protein backbone • Formation of protein carbonyls: Oxidation of amino acid residues, addition reactions with aldehydes or sugars • Aromatic amino acid modifications: Hydroxylation, nitration, halogenation
Amino acid residues most susceptible to oxidation
Amino acid
Cysteine Methionine Tryptophan
Oxidation products
disulfides, cysteic acid methionine sulfoxide, methionine sulfone 2-, 4-, 5-, 6-, 7-hydroxytryptophan, nitrotryptophan, kynurenine, 3-hydroxykynurenine, formylkynurenine 2,3-dihydroxyphenylalanine, 2-, 3-, and 4-hydroxyphenyalanine 3,4-dihydroxyphenylalanine, tyrosine-tyrosine cross-linkages Tyr-O-Tyr, cross-linked nitrotyrosine 2-oxohistidine, asparagine, aspartic acid glutamic semialdehyde a-aminoadipic semialdehyde 2-amino-3-ketobutyric acid Oxalic acid, pyruvic acid
Phenylalanine Tyrosine
Histidine Arginine Lysine Proline Glutamyl
Berlett and Stadtman, 1997
Sulfhydryl/thiol oxidation: reversible and irreversible
RSH
thiol
2e–
RSOH
sulfenic acid
2e–
RSO2H
sulfinic acid
2e–
RSO3H
sulfonic acid
1e–
RSH
O2 O2
RSSR
disulfide
RS•
thiyl radical
RSOO•
thiol peroxyl radical
RSO2OO•
sulfonyl peroxyl radical
RSO3H
RSH
RSH
O2
RSSR•–
disulfide anion radical
RSSR +
disulfide
O2•–
Reversal of protein cysteine oxidation: thioredoxin and glutaredoxin
THIOREDOXIN PATHWAY
Prot-SS Prot1-SS-Prot2 Prot-SOH Prot-Met=O Prot-[SH]2 Prot1-SH +Prot2-SH Prot-SH + H2O Prot-Met + H2O
GLUTAREDOXIN PATHWAY
Prot-SSG Prot-SH + GSH
GRO-[SH]2
GRO-SS Glutaredoxin
TR-[SH]2
TR-SS
Thioredoxin GSSG
2 GSH
TRR-SS
TRR-[SH]2 Thioredoxin reductase GR-[SH]2 GSH reductase GR-SS
NADPH + H+
NADP+
NADP+
NADPH + H+
Thiol oxidation and cell signaling: tyrosine phosphorylation
Ligand/growth factor
membrane
RTK
PO4-Y –
Receptor tyrosine kinase
– Y-PO4
Oxidase
protein –Y
protein –Y-PO4
PTP-S–
Protein tyrosine phosphatase
H2O2 Trx
PTP-SOH
Reversible sulfenic acid formation in oxidant signaling
Peroxiredoxins (Prx) are important in detoxification of H 2O2. Inactivation by formation of sulfinic acid (-SO2H) may allow H2O2 signaling (“floodgate” model). An enzymatic activity exists that reduces sulfinic acid (-SO2H) to the corresponding thiol (SH) (Georgiou and Masip, Science 2003: 300, 592-594).
Thiol oxidation by RNS: formation of S-nitrosothiols
O2
NO•
Fe(III)
O2•–, H+
N2O3
RS–
Fe(II)-NO+
RS–
ONOOH
RS–
RS– RS–
RSNO
RS–
RS•
RS–
RSNO2
RS–
RSSR
Regulation of protein function by S-nitros(yl)ation
Protein target
Channels/Transporters
NMDA receptor Ryanodine receptor Na+/K+-ATPase Metallothionin
Functional effect
Inhibition Activation or inhibition Unknown
Reference
Lipton, 1993 Xu, 1998; Eu, 2000 Jaffrey, 2001
Transport/storage Serum albumin
Bioactivation Zinc release?
Inhibition Activation Inhibition Inhibition Inactivation Inhibition Inhibition Activation Unknown
Stamler, 1992; Nedospasov, 2000 Kroncke, 1994; Pearce, 2000
So, 1998; Park, 2000 Akhand, 1999 Arstall, 1998; Konorev, 2000 Xian, 2000; Caselli, 1995 Mohr, 1996; Molina y Vedia, 1992 Ruiz, 1998; Perez-Mato, 1999 Dimmeler, 2002 Lander, 1997 Jaffrey, 2001
Protein kinases/phosphatases
JNK Src Creatine kinase Protein phosphatases
Metabolic enzymes
GAPDH Methionine adenosyl transferase Thioredoxin p21 ras Hsp 72
Signaling proteins Proteases Tissue plasminogen activator
Caspases 1-8
Activation Inhibition
Inhibition Inhibition
Stamler, 1992 Dimmeler, 1997, Mannick, 1999
Nikitovic, 1998; Melino, 2000 Matthews, 1996; Marshall, 2001
Transcription factors
AP-1 p50 (F NF-kB)
Detection of thiol oxidation and nitros(yl)ation
• Sulfenic acids (RS-OH) Indirect methods based on unique reactivity toward nucleophiles (dimedone, TNB, benzylamine). In general, difficult to detect because very unstable (Poole et al., 2004). • Glutathionylation, mixed disulfides (RS-SG, R1S-SR2)
35S-GSH
incorporation, labeling with biotinylated glutathione ethyl ester.
Immunochemical detection with anti-GSH antibody. Can be confirmed by reversibility with reducing agents (DTT). • Sulfinic or sulfonic acids (RS-O2H, RS-O3H) HPLC/MALDI (Hamann et al., 2002), immunochemical detection of “hyperoxidized” cysteines (Woo et al., J. Biol. Chem. 2003: 278, 47361-47364).
• S-nitrosothiols (RS-NO)
Chemiluminescence detection of NO after selective chemical reduction (KI/I 2; e.g. Feelisch et al., FASEB J. 2002: 16, 1590-1596)
Immunochemical detection with anti-S-nitrosocysteine antibodies (e.g. Gow et al., J. Biol. Chem. 2002: 277, 9637-9640)
Indirect detection of thiols after selective reduction (e.g. biotin switch method; Jaffrey, 2001).
Methionine oxidation: reversible and irreversible
O
H N
O H N H N
O
oxidation
S
oxidation
reduction
H3C
irreversible
S O
S
H3C
O
Methionine
H3C
O
Methionine sulfoxide
Methionine sulfone
Methione oxidation is reversed by methionine sulfoxide reductase (Msr). Two forms exist, MsrA and MsrB that specifically reduce S- and R -stereoisomers of Met(O), respectively. Msr deletion increases susceptibility to oxidant toxicity and shortens lifespan. Methionine oxidation is involved in regulating proteases inhibitors, signaling proteins, and ion channels (Hoshi and Heinemann, 2001).
Irreversible protein oxidation: protein carbonyls
Mechanisms of carbonyl formation
• Oxidative protein cleavage by a-amidation or by oxidation of glutamyl side chains
• Direct oxidation of lysine, arginine, proline or threonine
• Reaction with lipid oxidation products (aldehydes)
• Reaction of reducing sugars or their oxidation products with e.g. lysine (advanced glycation end products; AGE’s)
Detection of carbonyl formation
• Reaction with 2,4-dinitrophenylhydrazine (DNPH)
• Detection of chromophore by UV/Vis spectroscopy • Immunological detection using a-DNP antibodies
Berlett and Stadtman (1997) J. Biol. Chem. 272: 20313-20316.
Irreversible protein modification by lipid oxidation products
H
H
R H
O
a,b-unsaturated carbonyls (e.g. acrolein, HNE, cyclopentenone prostaglandins)
O
H N
Histidine
N
O H N S
Cysteine
O
H N
Lysine
N R O
HN
R
O
R O
Strong electrophilic products react with various amino acids by Michael addition. Can be detected by specific antibodies,e.g. against HNE-modified proteins (HNE-His) (Uchida, 1999)
Irreversible oxidative amino acid modifications by ROS/RNS
Tyrosine
O
Tyrosine
O
Tryptophan
O
Phenylalanine
O H N
H N
CH2
H N
CH2
H N
O NHCHO
OH
NO2
HO
2-OH-phenylalanine
OH COO–
NH3+
OH
N-formyl-kynurenine
3-NO2-tyrosine
o,o’-dityrosine
Stable oxidation markers: Tyrosine oxidation, chlorination, and nitration
NH3+
NH3+ COO–
NH3+ COO– OH
COO–
Ox
Tyr•
OH
Tyrosine
OH
O
•
Tyrosyl radical
NO2•
NH3+
COO–
COO–
o,o’-dityrosine
Ox
HOCl, Cl2
NH3+ COO–
NH3+
NH3+ COO–
OH
OH
Cl
OH
3,4,-diOH-Phe (DOPA)
NO2
3-Cl-tyrosine (RHS)
OH
3-NO2-tyrosine (RNS)
Features of tyrosine nitration
Initially thought to represent a selective marker for ONOO – formation, now known as collective marker for RNS formation (specifically NO2•).
Tyrosine nitration occurs with some selectivity, depending on protein and factors such as surface exposure and electrostatic factors. Protein nitration within mitochondria may be dynamic (involving denitration and renitration), suggesting some functional role (Aulak et al., Am. J. Physiol. 2004: 286, H30-H38)
Tyrosine nitration affects the pKa of tyrosine and adds a bulky group, which collectively could affect functional properties of tyrosine in proteins.
ONOO– NO2•
H OH N O– O O
Tyrosine pKa ≈ 10
3-Nitrotyrosine
pKa ≈ 7.5
Proposed consequences of tyrosine nitration on protein function
Enzyme
Mn-SOD Fe-SOD Tyrosine hydroxylase GSH-S -Transferase COX-1 Cytochrome P450 Glutamine synthetase Cytochrome c Ribonucl. reductase a1-proteinase inhibitor Surfactant protein A
Nitrotyrosine analysis
HPLC/UV, amino acid sequencing, MS/MS analysis of peptides HPLC/ESI-MS Western blot, protein digestion, HPLC/UV analysis HPLC of tryptic fragments, MALDI/MS and HPLC/ECD Protein digestion, HPLC/UV and amino acid sequencing GC/MS and HPLC of intact protein, sequencing of tryptic fragments HPLC and UV analysis of tryptic peptide fragments Protein digestion, HPLC/UV and MS analysis of peptides HPLC/UV analysis, protein digestion and amino acid sequencing Amino acid sequencing of peptide fragments Protein digestion, HPLC and MS analysis
Tyrosine role
Catalytic activity? Catalytic activity?
Reference
MacMillan-Crow, 1998 Yakamura, 1998 Soulere, 2001
Unknown GSH binding and activation Tyr radical in active site Unknown, electron transfer? Enzyme (in)activation Catalytic activity? Catalytic activity Catalytic domain Receptor binding, lipid aggregation
Ara, 1998 Wong, 2001 Goodwin, 1998 Roberts, 1998 Berlett, 1998 Cassina, 2000 Guittet, 2000 Mierzwa, 1987 Greis, 1996
Does irreversible protein modification affect protein function in vivo?
Biomarker
Carboxymethyl-lysine DOPA o- and m-Tyr dityrosine N-formyl-kynurenine aliphatic hydroxyls nitrotyrosine chlorotyrosine protein carbonyls
Normal levels (range)
pmol/mg pmol/mg µmol/mol Phe µmol/mol Tyr pmol/mg pmol/mg µmol/mol Tyr µmol/mol Tyr nmol/mg
Fold increase in disease
2-5 5-6 2-6 100-500 10 1-10 10-100 10-100 2-3
Quantitatively, these modifications may be less significant than e.g. cysteine or methionine oxidation.
Removal of oxidized proteins: proteosomal degradation
Ubiquitin ligation and proteasomal degradation require ATP, and will be impaired in energy depleted cells (e.g. as a result of excessive repair of oxidative DNA damage).
From: Goldberg (2003) Nature 426: 895-899
Irreversible protein modification and degradation
Protein modification/oxidation
Structural change Increased exposure of hydrophobic regions
Proteasome
Protein aggregation Accumulation of unfolded or mutated proteins
Protein degradation Repair
Loss of cell function Cell death/Apoptosis
Summary and major conclusions
• Lipid oxidation may produce bioactive intermediates that can act as second messengers in oxidant signaling. Can form adducts with specific targets in DNA or proteins.
• Reversible protein oxidations (transition metal ions, cysteine or methionine residues) are important in cell signaling. Because of their reversible nature, quantitation of these modifications has been difficult.
• Irreversible protein modifications are more easily quantified and immunochemical and analytical methods have been developed to detect them. Useful as diagnostic markers of oxidant production, but biological significance still unclear, because of protein degradation and turnover. Nevertheless, various irreversible protein modifications accumulate with age.
• Biological consequence of excessive oxidative injury and repair in many cases due to metabolic disturbance in the cell (ATP depletion and/or Ca 2+ influx) leading to apoptotic or necrotic cell death.
Metabolic disorder induced by oxidative stress
ROS RNS
NAD(P)H
ATP-Syn NOS PARP (DNA injury) Mitochondrial electron transfer
XO
Ca2+
Ca2+ATPase DYm
ATP
Recommended reading
Adams J (2003) The proteasome: structure, function, and role in the cell. Cancer Treat. Rev. 29 (Suppl 1): 3-9. Berlett BS and Stadtman ER (1997) Protein oxidation in ageing, disease, and oxidative stress. J. Biol. Chem. 272: 20313-20316. Bjelland S, Seeberg E (2003) Mutagenicity, toxicity and repair of DNA base damage induced by oxidation. Mutat. Res. 531: 37-80. Poole LB, Karplus PA, and Claiborne A. (2004) Protein sulfenic acids in redox signaling. Ann. Rev. Pharmacol. Toxicol. 44: 325-347. Giulivi C, Traaseth NJ, and Davies KJ (2003) Tyrosine oxidation products: analysis and biological relevance. Amino Acids 25: 227-232. Goldberg AL (2003) Protein degradation and protection against misfolded or damaged proteins. Nature 426: 895-899. Grune T, Merker K, Sandig G, and Davies KJ (2003) Selective degradation of oxidatively modified protein substrates by the proteasome. Biochem. Biophys. Res. Commun. 305: 709-718. Halliwell B and Gutteridge JMC (1999) Free radicals in biology and medicine. Oxford Science Publishers, New York. Hamann M, Zhang T, Hendrich S, Thomas JA (2002) Quantitation of protein sulfinic and sulfonic acid, irreversibly oxidized protein cysteine sites in cellular proteins. Meth. Enzymol. 348: 146-156. Heinecke JW (1999) Mass spectrometric quantification of amino acid oxidation products in proteins: insights into pathways that promote LDL oxidation in the human artery wall. FASEB J. 13: 1113-1120.
Recommended reading
Hoshi T, Heinemann SH (2001) Regulation of cell function by methionine oxidation and reduction. J. Physiol. 531: 1-11. Ischiropoulos H (2003) Biological selectivity and functional aspects of protein tyrosine nitration. Biochem. Biophys. Res. Commun. 305: 776-783. Jaffrey SP, Snyder SH (2001) The biotin switch method for the detection of S-nitrosylated proteins. Sci. STKE 86: PL1 Lawson JA, Bokach J, and FitzGerald GA (1999) Isoprostanes: formation, analysis and use as indices of lipid peroxidation in vivo. J. Biol. Chem. 274: 24441-24444. Levine RL (2002) Carbonyl modified proteins in cellular regulation, ageing and disease. Free Rad. Biol. Med. 32: 790-796. Moskovitz J, Bar-Noy, Williams WM, Requena J, Berlett BS, and Stadtman ER (2001) Methionine sulfoxide reductase (MsrA) is a regulator of antioxidant defense and lifespan in mammals. Proc. Natl. Acad. Sci. USA 98: 12920-12925. Roberts LJ 2nd, Morrow JD (2002) Products of the isoprostane pathway: unique bioactive compounds and markers of lipid oxidation. Cell Mol. Life Sci. 59: 808-820. Stamler JS, Lamas S, Fang FC (2001) Nitrosylation, the prototypic redox-based signaling mechanism. Cell 106: 675-683. Uchida K (2000) Role of reactive aldehyde in cardiovascular diseases. Free Rad. Biol. Med. 28: 16851696. Wong PS, van der Vliet A (2002) Quantitation and localization of tyrosine nitration in proteins. Meth. Enzymol. 359: 399-410.