Reactive Oxygen Species Flashcards
Primary ROS
- Superoxide (*O2-)
- Free radical with moderate reactivity
- Formed from one electron reduction of oxygen
Secondary ROS (H2O2)
- Hydrogen peroxide (H2O2)
- Oxidant, not a free radical
- Low reactivity but can generate the hydroxyl radical
Secondary ROS (*OH-)
- Hydroxyl radical
- Unpaired electron is much more unstable than superoxide, highest reactivity
- Electron stealer to form water
Generation of superoxide
- Occurs in mitochondria as a byproduct of ATP synthesis
- Premature leakage of oxygen during ETC
- Complex III: one electron transfer to produce superoxide
- Complex I can also generate superoxide
What can lead to increased production of superoxide?
- High membrane potential: slower electron transfer, greater reduction level of electron carriers and electron leak
- High NADH/NAD ratio: overreduction of ETC
- Electron transport chain damage
- Xenobiotics: may block ETC thus increasing reduction
- Electron backflow in complex I: ischemia, reperfusion injury leads to overreduction
NADPH oxidases
- Superoxide production in cells such as phagocytes: kill invading pathogens
- NADPH + 2O2 –> NADP+ +2*O2- + H+
Xanthine oxidase
- In liver and endothelial cells
- Catalyzes purine metabolism
- Xanthine + O2 –> Uric acid + *O2-(+H2O2)
Monoamine oxidase
- Type b catalyzes dopamine catabolism in neuronal cells
- Dopamine –> DOPAC + NH3 + H2O2
Fenton reaction
- Conversion of H2O2 to hydroxyl radical
- Transfers an electron from free metal ions (ex: Fe2+)
- Presence of iron is catalytic: overaccumulation can lead to oxidative stress
Ionizing radiation
- Can produce hydroxyl radical
- Induces homolytic fission of O-O bond in H2O2.
- Kills tumor cells
Primary reactive nitrogen/oxygen species
- Nitric oxide
- Can react with superoxide to form peroxynitrite (ONOO-) which is very reactive and can also form hydroxyl radical
Generation of NO
-Nitric oxide synthase: metabolizes arginine to citrulline with formation of NO.
Five major reactive oxygen species
- Superoxide
- Hydrogen peroxide
- Hydroxyl radical
- Nitric oxide
- Peroxynitrite
Places where oxidative damage may occur
- DNA: favored for OH caused by fenton reaction, nucleic acids bind iron well
- Protein
- Lipid
Indicator for extent of DNA damage in the cell
- 8 hydroxyldeoxyguanosine resulting from guanosine oxidation
- Can mispair with deoxyadenosine and lead to G to T transversion
Lipid peroxidation
- OH is active in mediating lipid oxidation
- PUFA highly susceptible: oxygens close to double bonds are highly reactive
- Lipid peroxidation can initiate a free radical chain reaction in the membrane to produce lipid peroxide which causes membrane damage
- Highly reactive aldehydes also accumulate
Protein carbonylation
- Hydroxyl radicals can directly oxidize amino acid side chains and cause protein damage
- Can also add reactive carbonyl function groups on proteins: most common are reactive aldehydes generated from lipid peroxidation
- Can cause conformational change, loss of activity, and/or protein turnover
H2O2 and thiols
-H2O2 can oxidize some protein cysteinyl residues to form disulfide cross links with other cysteines
Superoxide dismutase
- Converts two molecules or superoxide into one molecule of O2 and one H2O2
- Sod1: cytosolic, contain Cu and Zn
- Sod2: mitochondrial, contains Mn
Mutation in Sod1
- Amyotrophic lateral sclerosis
- Sod1 is misfolded and becomes cytotoxic
Glutathione peroxidase
- Converts H2O2 to water while consuming two molecules of reduced glutathione
- Contains micronutrient selenium
Glutathione reductase
- Regenerates reduced glutathione
- NADPH dependent: depends on pentose phosphate pathway and G6PDH
Peroxiredoxin pathway
- Uses small sulfhydryl containing proetin peroxidoxin to detoxify H2O2
- Forms water and oxidized peroxiredoxin which is then reduced by thioredoxin
- Thioredoxin reduced by thioredoxin reductase
Catalase
- Heme containing enzyme
- Catalyzes decomposition of H2O2 to water and oxygen
- Usually located in peroxisome
Mechanism of antioxidants
- Scavenge free radicals
- Protecting proteins against sulfhydyl oxidation
Coenzyme Q10 (ubiquinone)
- In vivo synthesized antioxidant
- Scavenges RO2 free radicals and inhibits lipid peroxidation
Glutathione
- In vivo synthesized antioxidant
- Keeps sulfhydryls of proteins reduced
- Donates electron to free radicals
