Oxidative Stress Flashcards
Describe some diseases caused by oxidative stress
- multiple sclerosis
- cardiovascular disease
- alzheimer’s disease
- rheumatoid arthritis
- crown’s disease
- COPD
- ischaemia
- cancer
- pancreatitis
- parkinson’s disease
List some sources of biological oxidants
Endogenous:
- electron transport chain
- peroxidases
- nitric oxide synthases
- lipooxygenases
- NADPH oxidases
- xanthine oxidase
- monoamine oxidase
Exogenous:
radiation
pollutants
drugs
toxins
Describe some reactive nitrogen species
- Nitric Oxide
- Nitric Oxide + Superoxide = Peroxynitrite (ONOO-)
- Peroxynitrite is not a free radical, but a strong oxidising agent
Describe some reactive oxygen species
- Superoxide
- Hydrogen Peroxide (not a free radical, but reacts to form free radicals e.g by reacting with Fe2+. Readily diffusable)
- Hydroxyl radical (most damaging radical)
How do ROS interact with DNA?
- ROS reacts with base leading to mispairing and mutation (can lead to cancer)
- ROS reacts with sugar (ribose or deoxyribose) causing strand to break or mutation on repair
- Levels of 8-oxo-2-deguanosine is a marker of oxidative stress in cells
How do ROS interact with proteins
ROS can react with sidechain leading to a modified amino acid:
carbonyls hydroxylated adducts ring opened species dimers (e.g. di-tyrosine) disulphide bond (cys)
Ultimately protein structure is changed, leading to possible loss or gain of function
ROS can also react with backbone:
this leads to fragmentation and protein degradation
Describe the action of free radicals
- The electrons of atoms, molecules and ions usually associate in pairs, which move in orbitals.
- A free radical contains one or more unpaired electrons and is capable of independent existence
- Free radicals are usually highly reactive and tend to acquire electrons from other atoms, molecules or ions
- Reactions of radicals with radicals typically generates a second radical
How can ROS affect disulphide bonds?
- Disulphide bonds play important role in folding and stability of some proteins (usually secreted proteins or in extracellular domains of membrane proteins)
- Formed between thiol groups of cysteine residues
- Inappropriate disulphide bond formation can occur if ROS takes electrons from cysteine causing misfolding, cross linking and disruption of function
How can ROS affect lipid membranes?
- Free radical extracts hydrogen atom from a polyunsaturated fatty acid in membrane lipid
- Lipid radical formed which can react with oxygen to form a lipid peroxyl radical
- Chain reaction formed as lipid peroxyl radical extracts -hydrogen from nearby fatty acids
- Hydrophobic environment of bilayer disrupted and membrane integrity falls
How is the electron transport chain a source of ROS?
- NADH and FADH2 supply electrons (e-) from metabolic substrates
- e- pass through ETC and reduce oxygen to form H20 at Complex IV
- occasionally electrons can escape chain and react with dissolved oxygen to form superoxide
Describe the action of nitrogen oxide synthase
- Converts arginine to citrulline. NADPH oxidised in process.
- Nitric Oxide is toxic at high concentrations, however it is a signalling molecule. It is involved in vasodilation, neurotransmission and S-Nitrosylation
iNOS: phagocytosis (toxic effects)
eNOS: (signalling)
nNOS: (signalling)
Describe the process of respiratory burst
- Rapid release of superoxide and H202 from phagocytic cells
- ROS and peroxynitrite destroy invading bacteria
- Part of antimicrobial defence system
- NADPH provides reducing power to form superoxide
Outline the pathophysiology of Chronic Granulamatous Disease
Genetic defect in NADPH oxidase complex causes enhanced susceptibilty to bacterial infections:
- Atypical infections
- Pneumonia
- Abscesses
- Impetigo
- Cellulitis
Describe the action of Superoxide Dismutase
- Converts superoxide to H2O2 and oxygen
- Primary defence as superoxide is a strong initiator of chain reactions
- 3 isoenzymes: Cu+, Zn2+ (cytosolic or extracellular), Mn2+ in Mitochondria
Describe the action of catalase
- Converts hydrogen peroxide to water and oxygen
- widespread enzyme, important in immune cells to protect against oxidative burst.
- secondary defence
Describe the action of gluthathione
- Tripeptide that protects against oxidative damage
- Thiol group of Cysteine donates electron to ROS.
- GSH then reacts with another GSH to form Glutathione Disulphide (GSSG). Glutathione peroxidase catalyses this reaction (requires selenium)
- GSSG reduced back to GSH by glutathione reductase which catalyses the transfer of electrons from NADPH to disulphide bond. NADP+ formed.
- NADPH from pentose phosphate pathway is therefore essential for protection against free radical damage
Describe the action of free radical scavengers, with examples
Free radical scavengers reduce free radical damage by donating hydrogen atom (and its electron) to free radicals in a non enzymatic reaction
Vitamin E:
- lipid soluble antioxidant
- important for protection against lipid peroxidation
Vitamin C:
- water soluble antioxidant
- important role in regenerating reduced form of vitamin E
-Other free radical scavengers include caretenoids, uric acid, flavenoids, melatonin
