Alcohol Metabolism And Oxidative Stress Flashcards
Alcohol metabolism to acetate?
Other minor ways of oxidising alcohol?
Build up of intermediates?
Most (>90%) alcohol is metabolised by liver
Remainder excreted passively in urine and on breath
Alcohol oxidised by alcohol dehydrogenase to acetaldehyde and then to acetate by aldehyde dehydrogenase. Both require NAD+
Acetate converted to acetyl~CoA and used in TCA cycle or for fatty acid synthesis
Smaller amounts of alcohol can also be oxidized by the cytochrome P450 2E1 enzyme (CYP2E1), or by catalase in brain.
Build up of acetaldehyde can cause liver cirrhosis
Units of alcohol for men and women
Recommended units - 14 units/ week spread over 3 days for both men and women
Rate of alcohol metabolism - 1 unit of alcohol = 8g, half pint of normal strength beer, small glass of wine
Eliminated at rate of ~7g per hour
Liver damage due to excessive alcohol consumption
Different clinical symptoms for increased alcohol oxidation
Acetaldehyde toxicity normally kept to a minimum by aldehyde dehydrogenase
Prolonged and excessive alcohol consumption can cause sufficient acetaldehyde accumulation to cause liver damage - leading fatty liver, alcoholic hepatitis and alcoholic cirrhosis
Excess NADH and Acetyl-CoA lead to changes in liver metabolism
E.g. for increase in NADH - lactate acidosis (due to lactate accumulation due not enough to NAD+ to convert it to pyruvate), Hypoglycaemia (due to deficit of gluconeogenesis due to lack of pyruvate)
For Increase Acetyl-CoA - increased synthesis of FA and ketone bodies leading to fatty liver - AFLD
How to treat alcohol dependence
How the drug works
Disulfram can be used as an adjunct in the treatment of chronic alcohol dependence
It is an inhibitor of aldehyde dehydrogenase
If patient drinks alcohol acetaldehyde will accumulates causeing symptoms of a hangover
Therefore leading to a large build up of acetaldehyde causing nausia, therefore the person starts to associate alcohol with throwing up
Oxidative stress?
Balance between defence mech vs stress placed on cells
Reactive oxygen species (ROS) & Reactive
Nitrogen species (RNS)?
Damage: What do ROS & RNS do to cells? - Nucleic acids, Proteins and Lipids
Sources of ROS & RNS - Internal sources and External sources
Defences against oxidative damage (antioxidants) - Antioxidant enzymes and Small molecule antioxidants
Oxidative stress has a component in a wide range of diseases
It’s an imbalance between our defence mechanisms and the stress placed on the cells by free radical production - we experience free radicals all the time (from metabolism and the environment) and our defence mech can usually cope with this with no issue - problem occurs when we have a compromise of our defence mech which can tip the balance, or we get excessive production of free radicals which can also tip the balance
Free radicles?
Electrons of atoms, molecules & ions usually associate in pairs.
Each pair moves within a defined region of space (an orbital).
A free radical is an atom or molecule that contains one or more unpaired electrons and is capable of independent (“free”) existence
A superscript dot used to denote free radical
Free radicals (usually) very reactive and tend to acquire electrons from other atoms, molecules or ions
Reaction of a radical with a molecule typically generates a second radical thereby propagating damage.
ROS?
Umbrella term for reactive oxygen series and reactive nitrogen species
Oxygen pairs with an electron to form a superoxide
This can further combine with 2 H+ ions and another electron to form hydrogen peroxide (which is toxic to cells)
Finally this combined with another electron and H+ ion makes water and a hydroxyl radical (v damaging - can react with DNA and lipids to form water - but it leaves behind the damaged molecule)
ROS damaging:
DNA
Proteins
Lipids
Disulphide bonds?
DNA - 2 main types of damage
ROS reacts with base - modified base can lead to mispairing and mutation
ROS reacts with sugar (ribose or deoxyribose) - can cause strand break and mutation on repair
Proteins - If ROS interacts with a protein it can interact with both the protein backbone (leads to fragmentation) or its side chains (leads to modified AAs)
Overall this can lead to gain of function, or loss of function leading to protein degradation or change in protein structure
Disulphide bonds - Play key role in folding and stability of some proteins (usually secreted proteins or EC domains of membrane proteins)
Formed between thick groups of cysteine residues
Inappropriate disulphide bond formation can occur if ROS takes electrons from cysteine causing misfolding, crosslinking and disruption of function
Lipids - Free radical extracts hydrogen atom from a polyunsaturated fatty acid in membrane lipid
Lipid radical formed which can rest with oxygen to form a lipid peroxyl radical
Chain reactions formed as lipid peroxyl radical extracts hydrogen from nearby FA
Hydrophobic environment of bilayer disrupted and membrane integrity fails - commonly occurs in building and rupturing of the plaque under the artery endothelial cells in atherosclerosis
The ETC as a source of ROS:
NADH and FADH2 supply electrons (e−) from metabolic substrates
e − pass through ETC and reduce oxygen to form H2O at Complex IV
Occasionally electrons can accidently escape the chain and react with dissolved O2 to form superoxide
This could then go on to change DNA, lipids and proteins - but our protective mech in the M can prevent this from happening, therefore preventing serious damage
Nitrogen oxide synthase as a source of free radicals?
NOS ( a RNS)
We produce NO from arginine using NOS
NOS converts arginine to citrulline and NO whilst converting NADPH +O2 to NADP + H2O (NO is a useful signalling molecule)
3 types of NOS - iNOS (inducible NOS), eNOS (endothelial NOS) and nNOS (neuronal NOS)
E and nNOS produced at low levels for signalling, iNOS produced in large concentrations (high enough to be toxic) - used in the immune response to bacteria - trying to kill them with NO
Respiratory burst?
Rapid release of superoxide and H2O2 from phagocytise cells (e.g. neutrophils and monocytes0
These ROS and Peroxynitirites destroy invading bacteria - this burst is used as part of the anti microbial defence system - i.e. phagolysosome engulfs invading bacteria and releases the above
Cellular defences for free radicals?
Superoxide dismutase (SOD) and catalase -
SOD converts superoxide to H2O2 and oxygen - this acts a primary defence because superoxide is a strong initiator of chain reactions
Catalase converts H2O2 to water and oxygen - used as a widespread enzyme, especially in immune cells to protect against oxidative burst
Glutathione -
Tripeptide synthesised by body to protects against oxidative damage
Thiol group of Cys donates e− to ROS. GSH then reacts with another GSH to form disulphide (GSSG).
Glutathione peroxidase requires Selenium
GSSG reduced back to GSH by glutathione reductase which catalyses the transfer of electrons from NADPH to disulphide bond
NADPH from pentose phosphate pathway is therefore essential for protection against free radical damage
Pentose Phosphate pathway -
Starts from Glucose-6-P and acts as an important source of NADPH which is required for the reducing power in biosynthesis, maintenance of GSH levels, and detox reactions
This pathway also produces the C5 sugar ribose which is required for the synthesis of nucleotides and DNA/RNA
No ATP is synthesised and no CO2 is produced (rate limiting enzyme is glucose 6 -PDH
Galatosaemia and G6PDH deficiency
See pictures in notes
Heinz bodies?
Dark staining within RBC resulting from precipitated Hb
They bind to the cell membrane altering rigidity (increasing it), therefore increased mechanical stress when cells squeeze through the small capillaries
Spleen removes bound Heinz bodies resulting in blister cells
A good indicator of G6PDH deficiency
Paracetamol metabolism
See picture from notes
