Reactive Oxygen-intermediates in biology and medicine- Lecture 72 Flashcards
What is a free radical?
a species which contains an unpaired electron in its outer orbital
very reactive
What diseases have ROS been implicated in?
neurodegenerative diseases (CNS- MAO, high levels of Fe)
cancer (DNA damage, mutations)
free radical theory of aging
foam cells and cardiovascular disease
smoking and lung injury
reperfusion and transplant injury
Others (sepsis, inflammation, oxygen toxicity, radiation injury, etc.)
What is oxidative stress?
imbalance between the prodoxidant and antioxidant levels in the cells and tissues
_____ are ROS which can cause tissue damage and whose levels may be increased by _____.
Prooxidants
certain drugs, infection, external exposures, tissue injury
What is the function of antioxidants?
inhibit formation of FOS or remove/ scavenge the generated ROS
What can cause oxidative stress?
increase in prooxidant formation
and/or
decrease/deficiency in antioxidant levels
What is the triplet state of oxygen?
molecular oxygen in its ground state
2 unpaired electrons in its outer orbital (diradical) that are spinning in the same direction
What is the singlet ground state of oxygen?
absorption of energy by triplet oxygen inverts the spin of one of the electrons to produce this
very reactive
What can the two electrons of the reactive singlet ground state of oxygen insert into?
the double bonds of:
aromatic amino acids of proteins (Phe, Tyr, Try)
unsaturated fatty acids
the purine and pyrimidine bases of DNA and RNA
*this disrupts the normal structures and interactions
____ is the most important reactive oxygen species in the biological system.
Superoxide anion radical
Describe the dismutation reaction of superoxide.
superoxide + superoxide + 2H+ –> H2O2 + O2
via superoxide dismutase (SODs) OR can occur non-enzymatically
What kinds of reactions involve free radicals?
Hydrogen abstraction addition to double bond termination dismutaion/disproportionation Haber-Weis Rxn Fenton Rxn
The _____ radical is the most powerful ROS produced in biological and chemical systems and can react with any biochemical or macromolecule.
OH
Why is OH radical central to the toxicity produced by ROS.
it reacts and inactivates or disrupts proteins, lipids, DNA, and RNA
Describe the Haber-Weis reaction.
H2O2 + superoxide –> O2 + OH- + OH(radical)
slow thermodynamically if not catalyzed by iron or copper
Describe the Fenton reaction.
H2O2 + Fe2+ –> OH(radical) + OH- + Fe3+
Fe3+ + superoxide –> Fe2+ + O2
Name the systems that produce ROS.
mitochondrial respiratory chain (complexes I and III) NADPH oxidases cytochrome P450s-Cyp2E1 reprofusion injury (xanthine dehydrogenase --> xanthine oxidase) oxidative enzymes autooxidizable biochemicals autooxidizable hemes external agents metal overload
What kinds of oxidative enzymes produce ROS?
monoamide oxidase, urate, glycolate, D-aa, oxidases, peroxisomal fatty acyl CoA, prostaglandin synthesis
Why are ROS toxic?
interactions with proteins
interaction with lipids
interaction with DNA
Why does ROS interactions with proteins result in toxicity ?
oxidized SH groups
oxidized methionine to methionine sulfoxide
fragment aromatic rings (Phe and Tyr) or indole ring of Try to carbonyls
nitration of tyrosine residues
disrupt peptide bonds
disrupt covalent bonds and non-covalent interactions
Why does ROS interaction with lipids result in toxicity?
form lipid radicals and lipid hydroperoxides
destroy cellular membranes
form reactive aldehydes
form protein-aldehyde adducts
Why do ROS interaction with RNA/DNA result in toxicity?
cleave phosphodiester bonds
cause base excision
oxidize bases
all result in base mispairing
Describe the lipid peroxidation reaction.
Initiation: LH --> L(radical) Propagation: L(radical) + O2 --> LOO(radical) LOO(radical) + LH --> LOOH + L(radical) Degradation: LOOH --> malondialdehyde (MDA) + 4-hydroxynonenal (4-HNE)
How can you halt the propagation step of the lipid peroxidation reaction?
Vitamin E (alpha-tocopherol) is a major antioxidant that reacts with lipid and lipid alkoxyl radical s to restore the lipid and form a vitamin E radical can be regenerated by ascorbic acid (Vitamin C)
What are the beneficial effects of ROS and free radicals?
activation of certain txn factors (eg. activation of Nrf2, NF-kB, AP1 via H2O2)
macrophages and neutrophils release ROS as a weapon against immunologic targets
enzymatic reactions
How do phagocytes use ROS?
NADPH oxidase (produces superoxide) reactions myeloperoxide reactions (produce bleach)
Describe chronic granulomatous disease.
failure to promote a respiratory burst largely due to deficiencies in some of the subunits which make up the NADH oxidase
What mechanisms are in place to protect the body from damage from ROS.
superoxide dismutase catalase peroxidases GSH + Gluthathione peroxidase (SE) + Gluthathione reductase (G6PDH generates NADPH for reduction of GSSG) Glutathione transferase metallothionein, ferritin ceruloplasmin alpha-tocopherol (vitamin E), ascorbate (vitamin C), uric acid, caretenoids, GSH, SAM heme oxygenase
Describe the process of reperfusiontransplant injury.
when ATP is no longer being synthesized (due to ischemia of the tissue), it is converted to hypoxanthine which can be converted to xanthine via xanthine oxidase (which is converted from xanthine dehydrogenase when large amounts of hypoxanthine accumulates)
when O2 is re-added during reprofusion, you get injury of the tissue because it can further be converted to ROS
How can reperfusion/transplant injuries be prevented?
addition of buffers containing protectants (antioxidants) + chelators + a xanthine oxidase inhibitor (normally allopurinol) with the reperfusion of O2
Why does our mitochondria have the most ROS?
auto-oxidation of quinones CoQ + E- + H+ --> semiquinone radical semiquinone radical + e- + H+ --> ubiquinol (CoQH2) OR semiqinone + O2 --> CoQ + O2(radical)
