1. Oxidative Stress Flashcards
How is alcohol oxidised?
By alcohol dehydrogenase to acetaldehyde and then to acetate by aldehyde dehydrogenase
Small amounts can be oxidised by cytochrome P450 2E1 enzyme, or by catalase in brain
What is the rate of metabolism of alcohol?
One unit of alcohol = 8g
Eliminated at rate of around 7g per hour
What causes hangover?
Accumulation of toxic metabolite acetaldehyde
What can excess NADH and Acetyl-CoA lead to?
Changes in liver metabolism
What is disulfiram used for and how does it work?
Used as an adjunct in the treatment of chronic alcohol dependence
It is an inhibitor of aldehyde dehydrogenase
If patient drink alcohol, acetaldehyde will accumulate causing symptoms of a hangover
What are ROS?
Reactive oxygen species
What are RNS?
Reactive nitrogen species
What do ROS and RNS do to cells?
Damage nucleic acids, proteins and lipids
What are free radicals?
An atom or molecule that contains one or more unpaired electrons and is capable of independent existence
How can ROS cause damage to DNA?
Reacts with base:
Modified base can lead to mispairing and mutation
Reacts with sugar:
Can cause strand break and mutation on repair
Can lead to cancer
How does ROS damage protein side chains?
Can add/modify carbonyls, hydroxylated adducts, ring opened species, dimers, disulphide bond (Cys)
Causes change in protein structure
Leading to protein degradation, loss of function or gain of function
What is the role of disulphide bonds in proteins?
Important role in folding and stability of some proteins
Formed between thiol groups of cysteine residues
When does inappropriate disulphide bond formation occur?
If ROS takes electrons from cysteines causing misfolding, crosslinking and disruption of function
How do ROS damages lipids?
Free radical extracts hydrogen atom from a polyunsaturated fatty acid in membrane lipid - damaged membrane
Lipid radical formed which can react with oxygen to form a lipid pyroxylin radical
Chain reaction formed as lipid pyroxylin radical extracts hydrogen form nearby fatty acid
What are endogenous sources of biological oxidants?
Electron transport chain
Nitric oxide synthases
NADPH oxidases
What are exogenous sources of biological oxidants?
Radiation
Pollutants
Dugs
Toxins
What is the electron transport chain a source of?
ROS
What happens to electrons that escape the electron transport chain?
React with dissolved O2 to form superoxide
What is iNOS?
Inducible nitric oxide synthase
Produces high NO concentrations in phagocytes for direct toxic effect
What is eNOS?
Endothelial nitric oxide synthase
Signalling
What is nNOS?
Neuronal nitric oxide synthase
Signalling
What reaction does nitric oxide synthase catalyse?
Arginine to citrulline + NO radical
What is nitric oxide radical used for?
Signalling molecule:
Vasodilation
Neurotransmission
S-Nitrosylation
What is respiratory burst?
Rapid release of superoxide and H2O2 from phagocytic cells (neutrophils and monocytes)
ROS and peroxynitrite destroy invading bacteria
Part of antimicrobial defence system
What is chronic granulomatous disease?
Genetic defect in NADPH oxidase complex causes enhanced susceptibility to bacterial infections
E.g. atypical infections, pneumonia, abscesses, impetigo, cellulitis
What is the role of superoxide dismutase (SOD)?
Converts superoxide to H2O2 and oxygen
Primary defence because superoxide is strong initiator of chain reactions
What is the role of catalase?
Converts H2O2 to water and oxygen
Widespread enzyme, important in immune cells to protect against oxidative burst
What is the reduced form of glutathione?
GSH
Glycine-cysteine-glutamate
I
SH
What is the oxidised form of glutathione?
GSSG
Thiol group of Cys donates e- to ROS
GSH then reacts with another GSH to form disulphide
What enzyme is used to catalyse GSH to GSSG?
Glutathione peroxidase which requires Selenium
What enzyme is used to catalyse GSSG to GSH?
Glutathione reductase, catalyse transfer of electrons from NADPH to disulphide bond
What is NADPH used for in cellular defences?
NADPH from pentose phosphate pathway is essential for protection against free radical damage
What is NADPH required for?
Reducing power for biosynthesis
Maintenance of GSH levels
Detoxification reactions
What is the rate limiting enzyme of the pentose phosphate pathway?
Glucose 6-phosphate dehydrogenase
What is the role of vitamin E in cellular defences?
Lipid soluble antioxidant
Donates electrons
Important for protection against lipid peroxidation
What is the role of vitamin C in cellular defences?
Water soluble antioxidant
Important role in regenerating reduced form of vitamin E
What are vitamin C and E in terms of cellular defences?
Free radical scavengers - reduce free radical damage by donating hydrogen atom to free radicals in a nonenzymatic reaction
What are Heinz bodies?
Dark staining within RBC resulting from precipitated haemoglobin
Bind to cell membrane altering rigidity
Spleen removes bound Heinz bodies resulting in blister cells
Clinical sign of G6PDH deficiency
