Alcohol metabolism + Oxidative stress Flashcards
where is alcohol metabolised
- 90% in liver
- 10% excreted passively in urine and breath
how is alcohol metabolised
- alcohol oxidised to acetaldehyde by alcohol dehydrogenase using NAD+
- acetaldehyde oxidised to acetate by aldehyde dehydrogenase using NAD+
- acetate converted to acetyl Co-A which is used in TCA cycle or fatty acid synthesis
- smaller amounts of alcohol can be oxidised by CYP2E1 in liver or catalase in brain
recommended limit of alcohol
14 units/week spread over at least 3 days
rate of alcohol metabolism
eliminated at rate of ~7g/hour (1 unit = 8g)
how does alcohol cause liver damage
- acetaldehyde toxicity normally kept minimal as aldehyde dehydrogenase has very low Km for acetaldehyde and removes it as soon as it is formed
- prolonged and excessive alcohol consumption causes sufficient acetaldehyde accumulation to cause liver damage
- decrease in NAD+/NADH ratio and increased acetyl Co-A leads to changes in liver metabolism (fatty liver, alcoholic hepatitis, alcoholic cirrhosis)
effects of decrease in NAD+/NADH ration on liver cell metabolism
inadequate NAD+ for fatty acid oxidation
- contributes to fatty liver
inadequate NAD+ for conversion of lactate to pyruvate
- accumulation of lactate in blood causing lactic acidosis
- increased lactate reduces kidney’s ability to excrete uric acid as they share same transporter, causing urate crystals to accumulate in tissues producing gout
inadequate NAD+ for glycerol metabolism
- combined with inability to use lactate leads to deficit in gluconeogenesis causing fasting hypoglycaemia
effects of increased availability of acetyl Co-A on liver cell metabolism
increased synthesis of fatty acids and ketone bodies
- increased synthesis of triacylglycerols which cannot be transported from liver cells due to lack of lipoprotein synthesis so cause fatty liver
- production of ketone bodies can be sufficient to cause ketoacidosis
treatment of alcohol dependence
disulfiram
- inhibitor of aldehyde dehydrogenase
- if patient drinks alcohol then acetaldehyde accumulates in blood causing symptoms of a hangover such as nausea
effects of damage to liver cells caused by toxic effects of acetaldehyde
- leaky plasma membranes causing loss of enzymes (transaminases and gamma glutamyl transpeptidase)
- reduction in capacity of liver cells to take up and conjugate bilirubin leads to hyperbilirubinaemia causing jaundice
- reduction in capacity to produce urea leads to hyperammonaemia and increased glutamine levels
- reduced protein synthesis leads to decreased synthesis of albumin (oedema), clotting factors (increased clotting time) and lipoproteins (lipids accumulate in liver)
indirect effects of excessive alcohol consumption
poor dietary habit
- vitamin and mineral deficiencies
- inadequate protein and carbohydrate intake
direct effect of alcohol on GI tract
- high concentrations of alcohol damage cells lining GI tract and compound effects of poor diet
- loss of appetite, diarrhoea, impaired absorption of nutrients
- neurological symptoms of thiamine and pyridoxine deficiencies + haematological problems of folic acid deficiency
- thiamine deficiency leads to Wernicke-Korsakoff syndrome with mental confusion and unsteady gait
chronic pancreatitis
- constant pain in upper abdomen that radiates to back
- weight loss caused by malabsorption of food due to insufficient production of pancreatic enzymes
- diabetes if insulin producing pancreatic β cells are damaged causing hyperglycaemia and glucosuria
why is oxidative stress an important topic to understand
significant component in a wide range of diseases:
- cardiovascular disease
- Alzheimer’s
- rheumatoid arthritis
- multiple sclerosis
- Parkinson’s
- cancer
- pancreatitis
- ischaemia/reperfusion injury
- COPD
- Crohn’s disease
what is a free radical
atom, molecule or ion that contains one or more unpaired electrons and is capable of independent existence
what is used to denote a free radical
superscript dot
how do free radicals cause damage
- very reactive so tend to acquire electrons from other atoms, molecules or ions
- reaction generates second radical propagating the damage
what are the reactive oxygen species (ROS)
- superoxide (O2’)
- hydrogen peroxide (H2O2)
- hydroxyl radicals (‘OH)
what are the reactive nitrogen species
- nitric oxide (NO’)
- peroxynitrite (ONOO-)
ROS damage to DNA
ROS reacts with base
modified base can lead to mispairing and mutation
ROS reacts with sugar
can cause strand break and mutation on repair
failure to repair mutation could lead to cancer
why is mitochondrial DNA sensitive to ROS damage
- situated near inner mitochondrial membrane where ROS are formed
- not protected by histones
what can be used as a measurement of oxidative damage
amount of 8-oxo-dG in cells
ROS damage to proteins
ROS reacts with side chains
- modified amino acid
- change in protein structure
- loss of function, gain of function, protein degradation
ROS reacts with protein backbone
- fragmentation
- protein degradation
what is the most significant change to protein structure as a result of ROS
inappropriate disulphide bond formation can occur if ROS takes electrons from cysteine residues causing misfolding, crosslinking and disruption of function
where do disulphide bonds form
between thiol groups of cysteine residues
ROS damage to lipids
lipid peroxidation - significant in development of atherosclerosis
- ROS reacts with polyunsaturated fatty acid in membrane lipid
- lipid radical formed which reacts with oxygen to form lipid peroxyl radical
- chain reaction formed as lipid peroxyl radicals react with nearby fatty acids
- hydrophobic environment of bilayer disrupted and membrane integrity fails
endogenous sources of biological oxidants
- electron transport chain
- peroxidases
- nitric oxide synthases
- lipooxygenases
- NADPH oxidases
- xanthine oxidase
- monoamine oxidase
exogenous sources of biological oxidants
- radiation - cosmic rays, UV light, X-rays
- pollutants
- drugs - primaquine (anti-malarial)
- toxins - paraquat (herbicide)
how is the electron transport chain a source of ROS
- NADH and FADH2 supply electrons from metabolic substrates
- e- pass through ETC and reduce oxygen to form water at complex IV
- occasionally electrons can accidentally escape chain and react with dissolved oxygen to form superoxides
how is nitric oxide synthase a source of ROS
converts arginine to citrulline and nitric oxide
- nitric oxide has toxic effects at high levels as it is used in immune cells to destroy invading bacteria
- nitric oxide is a signalling molecule for vasodilation, neurotransmission, S-Nitrosylation
3 types of nitric oxide synthase
- iNOS - indicible nitric oxide synthase produces high NO concentrations in phagocytes for direct toxic effect
- eNOS - endothelial nitric oxide synthase (signalling)
- nNOS - neuronal nitric oxide synthase (signalling)
what is respiratory/oxidative burst
- rapid production of ROS (superoxide and H2O2) from phagocytic cells such as neutrophils and monocytes
- ROS and peroxynitrite destroy invading bacteria or fungal cells and cell also destroyed
- important part of body’s immune response to infection
- produced by membrane-bound enzyme complex NADPH oxidase
how does NADPH oxidase produce respiratory burst
- present in cell membranes of phagosomes
- transfers electrons from NADPH to molecular oxygen to generate superoxide radicals
- superoxide radicals form peroxynitrite and hydrogen peroxide
- myeloperoxidase converts hydrogen peroxide to bleach
- bacteria are engulfed and killed by respiratory burst
chronic granulomatous disease
genetic defect in NADPH oxidase complex so can’t produce superoxide radicals so ability of phagocytes to destroy invading bacteria compromised so enhanced susceptibility to bacterial infections
3 cellular defences against ROS
- superoxide dismutase and catalase
- glutathione
- free radical scavengers
superoxide dismutase and catalase as cellular defences
superoxide dismutase
catalyses conversion of superoxide to hydrogen peroxide and oxygen
- primary defence as superoxide strong intiator of chain reactions
- 3 isoenzymes: Cu+-Zn2+ (cytosolic), Cu+-Zn3+ (extracellular), Mn2+ (mitochondrial)
catalase
converts hydrogen peroxide to moleular oxygen and water
- widespread enzyme
- important in immune cells to protect against oxidative burst
what is glutathione (GSH)
tripeptide (Gly-Cys-Gly) synthesised by body as an antioxidant to protect against oxidative damage
how does glutathione protect against oxidative damage
- thiol group of Cys residue donates electron to ROS and reacts with another GSH forming a disulphide bond to form GSSG
- catalysed by enzyme glutathione peroxidase which requires selenium
- GSSG reduced back to GSH
how is GSH regenerated from GSSG
- GSSG reduced back to GSH by glutathione reductase which catalyses transfer of electrons from NADPH to disulphide bond of GSSG
- NADPH from pentose phosphate pathway essential to regenrate GSH to protect against oxidative damage
what are the free radical scavengers
- vitamin E
- vitamin C
- carotenoids
- flavonoids
- melatonin
- uric acid
how do free radical scavengers reduce ROS damage
donate hydrogen atom and its electron to free radicals in a nonenzymatic reaction
vitamin E and vitamin C as free radical scavengers
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
what is oxidative stress
imbalance between oxidants and defences
- production of oxidative stress is excessive
- levels of antioxidants are low
galactosaemia and oxidative stress
- increased activity of aldose reductase (galactose to galactitol) consumes excess NADPH
- insufficient levels of NADPH limit ability to recycle GSSG back to GSH so cell more susceptible to oxidative damage
- crystallin protein in lens of eye denatured leading to cataracts
G6PDH deficiency and oxidative stress
- production of NADPH from pentose phosphate pathway is limited
- insufficient levels of NADPH limit ability to recycle GSSG back to GSH so cell more susceptible to oxidative damage
what are Heinz bodies
- dark staining within RBC die to precipitated haemoglobin due to oxidative damage
- bind to cell membrane altering rigidity
- increased mechanical stress when squeezing through capillaries
- spleen removes Heinz bodies causing blister cells
- clinical sign of G6PDH deficiency
metabolism of paracetamol (acetaminophen)
- at prescribed dosage it is safely metabolised in the liver
- conjugation with glucoronide or sulphate yielding non-toxic metabolites
- pathway becomes saturated if toxic dose ~10g is ingested
metabolism of toxic dose of paracetamol
- paracetamol produces metabolite NAPQI (N-acetyl-p-benzo-quinone imine) which causes oxidative damage to liver cells
- NAPQI undergoes conjugation with glutathione so depletes levels of antioxidant in hepatocytes
- direct toxic effects of NAPQI cause covalent binding in hepatic proteins
- causes destruction of liver cells and liver failure occurs over several days, resulting in death
treatment for paracetamol overdose
acetylcysteine
- replenishes glutathione allwing liver to safely metabolise NAPQI
- good prognosis if treatment within 8 hours after overdose
