Oxidative Stress and Antioxidants Flashcards
What are the types of reactive oxygen species (ROS)?
Superoxide radical - not very reactive, second messenger
Hydrogen peroxide - reactive, second messenger
Hydroxyl Radical - most reactive and destroying ROS
Singlet oxygen - exists only at high temperature or in response to radiation
What are reactive oxygen species?
Partially reduced, reactive forms of oxygen
What is the path of oxygen to other ROS?
Oxygen
Superoxide (O2-)
Hydrogen peroxide (H2O2)
Hydroxyl anion (OH-) and Hydroxyl radical (OH-)
H2O
SEPARATE PATH
Oxygen to Singlet Oxygen via energy transfer
What is the electron transport chain a source of?
Superoxide radical
Main source of leaking electrons is the semiquinone (UQH)
What is the function of cytochrome P450 oxidase?
Hydroxylation of variety of substrates
Active site is heme - iron center
NADPH is required
2 atoms of O2 are used
One O atom hydroxylates S and second O atom forms water
If second transfer fails, superoxide radical is formed
What is the location of cytochrome P450 oxidase?
Membranes of ER
What is MEOS and what is it a source of?
Microsomal Ethanol Oxidation System
Requires CYP450E1 enzyme and is alternate pathway of ethanol metabolism in liver
Active during chronic alcohol consumption
Source of hydrogen peroxide and hydroxyethyl radical (mutagenic)
What is the MEOS equation using CYP450E1 enzyme?
Ethanol + 2O2 + NADPH
to
Acetaldehyde + 2 H2O2 + NADP + Hydroxyethyl radical
What does hydroxyethyl radical lead to?
Liver cancer
What does hydrogen peroxide from MEOS lead to?
Lipid peroxidation of liver cell membranes
What is the NADPH-Dependent Oxidase System?
Found on membranes of phagocytic cells
Activated in response to bacterial infection and inflammation
Robust response to bacterial infection can lead to Respiratory burst which leads to formation of superoxide radical, hydrogen peroxide, and hydroxyl radicals
What is xanthine oxidase and what ROS does it produce?
Catalyzes oxidation of hypoxanthine and xanthine to uric acid
Hypoxanthine is from inosine which is from the purine degradation pathway
Source of hydrogen peroxide
What is the Fenton Reaction?
Free iron (Fe2+) and copper (Cu+) donate an electron to hydrogen peroxide
Source of Hydroxyl Radicals
What is superoxide dismutase?
Coverts superoxide radical into hydrogen peroxide
2O2- + 2H+ -> H2O2 + O2
What are antioxidant enzymes?
Catalase (peroxisomes)
Glutathione peroxidase
What does catalase in peroxisomes do?
Antioxidant enzyme
Detoxifies hydrogen peroxide
2H2O2 -> 2H2O + O2
What does glutathione peroxidase do?
Antioxidant enzyme
Detoxifies hydrogen peroxide and lipid peroxides
H2O2 + 2GSH -> 2H2O + GSSG
What are antioxidant vitamins?
Vitamin C (ascorbic acid)
Vitamin E (alpha-tocopherol)
Vitamin A retinoid (animals) and Beta-carotene (plants)
How does vitamin C act as a antioxidant and what is it?
Water soluble vitamin
Reduces superoxide radicals, organic radicals, and lipid peroxides
Becomes radical itself (GSH regenerates Vit C) and recycles vitamin E
How does vitamin E acts as a antioxidant and what is it?
Lipid soluble vitamin
Detoxifies free radicals and terminates lipid peroxidation in membranes
Plentiful vegetable oils
How does vitamin A retinoid and Beta-carotene acts as an antioxidant and what is it?
Lipid soluble vitamin
Conjugated double bonds in vitamin A are capable quenching the vibration energy of singlet oxygen
What is glutathione (GSH)?
Water soluble antioxidant
Tripeptide (glutamate, cysteine, and glycine)
Cysteine thiol group donates electrons to ROS
Oxidized form is GSSG
How does glutathione function as an antioxidant?
GSH serves as cofactor of glutathione peroxidase
Leads to GSH oxidation to GSSG
GSSG reduced back to GSH by glutathione reductase
Hydrogen peroxide is converted to water
Pentose phosphate pathway provides NADPH to recover GSH
What are polyphenols (flavenoids)?
Water soluble antioxidants from plant extracts
What is uric acid?
Major serum antioxidant
What is bilirubin as an antioxidant?
Lipid soluble, unconjugated bilirubin donates an electron to radicals
Oxidation coverts bilirubin back to biliverdin
How are antioxidants regenerated?
Radical abstracts electron from PUFA (polyunsaturated fatty acid)
PUFA radical is quenched by vitamin E
Vitamin E is regenerated by vitamin C
Vitamin C is regenerate by GSH
GSH is recovered by NADPH
NADPH comes from pentose phosphate pathway
What is oxidative stress?
Irreversible damage caused by attack of ROS upon protein, lipids, and DNA of cells
Results from imbalance between ROS (pro-oxidants) and antioxidants
What makes red blood cells vulnerable to oxidative stress?
High oxygen saturation
What are the major defense mechanism of red blood cells?
Glutathione peroxidase
Glutathione (GSH)
Pentose Phosphate Pathway - helps to destroy hydrogen and organic peroxides
Methemoglobin reductase - recovers hemoglobin and replaces electrons to the iron in the heme of methemoglobin
What is lipid peroxidation?
Hydroxyl radicals (OH) cause peroxidation of PUFAs in the membranes
What are the steps in lipid peroxidation?
OH radical (hydroxyl radicals) abstracts an electron from a PUFA producing a lipid radical
Lipid radicals react with oxygen to form lipid peroxyl radicals
Lipid peroxyl radicals are unstable and either abstract electron from other PUFA double bonds (propagation or chain reaction) or form lipid peroxides that break down into malondialdehyde (mutagenic)
What are major antioxidants for lipid peroxidation?
Vitamin E
Glutathione peroxidase (GPx)
What ROS damages DNA?
Hydroxyl radical damages DNA
Produces DNA double strand breaks - cuts chromosome in two
Can bind guanine bases forming 8-oxoguanine that interferes with replication and transcription
How do ROS cause protein damage?
ROS abstract electrons from thiol groups
This leads to formation of incorrect disulfide bonds
Proteins become inactive and are degraded
How do is oxidative stress prevented?
ROS and antioxidants must be in equilibrium
Not all ROS are bad - redox signaling and thyroid hormone synthesis requires H2O2
