Alcohol Metabolism & Oxidative Stress Flashcards
Where is alcohol metabolised
• Alcohol mainly metabolised on liver (90%)
○ Remainder passive excreted in urine and on breathe
• Smaller amounts of alcohol can also be oxidised by cytochrome P450 (CYP2E1) or by catalase in brain
What is the recommended limit of alcohol consumption
• Recommended limits - 14 units/week spread over at least 3 days
• One unit of alcohol = 8g (half pint of normal strength beer, small glass of wine)
○ Eliminated at rate of 7g per hour
Outline the steps of alcohol metabolism
• Alcohol oxidised by alcohol dehydrogenase to acetaldehyde and then to acetate by aldehyde dehydrogenase
○ Each step reduces NAD to NADH
• Acetaldehyde is a toxic metabolite - accumulation causes hangover
• Acetate converted to acetyl CoA and used in TCA Cycle or for fatty acid synthesis
Explain how alcohol can cause liver damage
Acetaldehyde toxicity normally kept to a minimum by aldehyde dehydrogenase
○ High acetaldehyde accumulation leads to liver damage
○ Excess NADH and acetyl CoA leads to changes in liver metabolism
§ Increased acetyl coA = increased synthesis of fatty acids and ketone bodies = increased synthesis of triacylglycerol = fatty liver
○ Alcoholic liver damage includes ‘fatty liver’, alcoholic hepatitis, alcoholic cirrhosis
○ Decrease in NAD/NADH ratio means inadequate NAD for conversion of lactate to pyruvate, leading to lactic acidosis as lactate accumulates in blood
§ Also leads to hypoglycaemia due to deficit in gluoconeogenesis
Explain the action of Disulfiram
• Disulfiram - treat chronic alcoholism
○ Inhibits aldehyde dehydrogenase
○ If patient drinks alcohol, acetaldehyde will accumulate causing symptoms of ‘hangover’
Describe the production of superoxide radicals and other reactive oxygen (ROS) and reactive nitrogen (RNS) species
• Molecular oxygen has 2 unpaired electrons in different orbitals
• When it adds another electron, it forms superoxide radical (O2•)
• Superoxide radical can readily generate other reactive oxygen species, including hydrogen peroxide and hydroxyl radical
○ Hydrogen peroxide not a free radical itself, but can react with metals to produce free radicals
○ Hydroxyl radical most reactive and very damaging, can react with anything
• Superoxide radical also reacts with nitric oxide to produce peroxynitrite (reactive nitrogen species)
Discuss damage caused by reactive oxygen and reactive nitrogen species
• Damage to DNA - reactive oxidation species reacts with base - modified base can lead to mispairing and mutation
○ Reactive oxidation species with sugar can cause strand break and mutation on repair
• Damage to protein - reacting with backbone leads to fragmentation and degradation
○ Reacting with sidechain leads to modified amino acid can result in subtle change in structure which would alter its function
○ Can react in inappropriate disulphide bond formation if ROS takes electrons from cysteines, causing misfolding, crosslinking and disruption of function
• Damage to lipids - lipid membranes can be damaged, resulting in a chain reaction forming and disrupting hydrophobic environment and membrane integrity (lipid peroxidation)
What are sources of reactive oxygen and nitrogen species
• Endogenous sources
○ Electron transport chain where oxygen can becomes superoxide
§ Electrons can escape chain and react with dissolved oxygen to form superoxide radical
○ Nitric oxide synthase (NOS)
§ iNOS - inducible nitric oxide synthase - produces high NO concentrations in phagocytes for direct toxic effect
§ eNOS - endothelial nitric oxide synthase
§ nNOS - neuronal nitric oxide synthase
§ Nitric oxide signalling molecule in vasodilation, neurotransmission - toxic at high levels
§ Respiratory burst - rapid release of superoxide and hydrogen peroxide by phagocytes in killing bacteria
○ NADPH oxidises
Exogenous sources - radiation, pollutants, drugs, toxins
Outline cellular defenses against reactive oxygen species
• Superoxide dismutase converts superoxide to hydrogen peroxide and oxygen
○ Superoxide targeted as strong initiator of chain reactions
• Catalase enzyme converts hydrogen peroxide to water and oxygen
○ Important in immune cells in protection against oxidative burst
○ Declining levels in hair follicles with age may explain grey hair
• Glutathione - tripeptide synthesised by body to protect against oxidative damage
○ Thiol group of cysteine donates electron to ROS
○ GSH then reacts with another GSH to form disulphide bond (GSSG)
§ Catalysed by glutathione peroxidase, which also converts hydrogen peroxide to water
§ Requires selenium
○ GSSG reduced back to GSH by glutathione reductase using electrons from NADPH to disulphide bond
○ NADPH from pentose-6-phosphate pathway needed
• Free radical scavengers - reduce free radical damage by donating hydrogen atom and its electron to free radicals
○ Vitamin E lipid soluble antioxidant and important for protection against lipid peroxidation
○ Vitamin C water soluble antioxidant and regenerates reduced form of vitamin E
Describe the role of reactive oxygen species in galactosaemia
Inability to metabolise galactose
○ Deficiency in galactokinase, uridyl transferase or UDP-galactose epimerase
○ Galactokinase deficiency only leads to cataract
○ Other 2 enzyme deficiency leads to build up of galactose-1-P, which causes tissue damage
○ Galactose forms galactitol instead using NADPH through aldose reductase
○ Using NADPH can compromise defences against ROS damage (cant reduce GSSG)
○ Symptoms include cataracts, hypoglycaemia, liver damage, jaundice, seizures and brain damage
Discuss the effects of G6PDH deficiency
○ NADPH required for reduction of oxidised glutathione (GSSG) back to reduce glutathione (GSH)
○ Lower GSH means less production against damage from oxidative stress
○ Oxidative stress due to infection, drugs can cause lipid peroxidation and protein damage (Heinz bodies)
§ Heinz bodies - clumps of precipitated haemoglobin visible on erythrocytes due to haemoglobin cross links
□ Binds to cell membrane and alters rigidity and increases mechanical stress when cells squeeze through small capillaries
□ Spleen removes bound Heinz bodies resulting in ‘blister cells’
Explain the dangers of high dosage of paracetamol
○ Normally, metabolised in liver to glucuronide and sulphate
○ In overdose, produces NAPQI (toxic metabolite)
○ NAPQI can have oxidative damage to liver cells - lipid peroxidation, damage to proteins, damage to DNA
○ Liver cells try to combat damage by using glutathione, however causes glutathione depletion
○ NAPQI damage is irreversible
Giving acetylcysteine replenishes glutathione levels and acts as a precursor for more to be made
