MEH 1 - Alcohol Metabolism + Oxidative Stress

Question 1

Why does alcohol + fat have a higher energy content than proteins and carbohydrates?

Answer 1

Alcohol + fat are more reduced (contains less oxygen)

Question 2

Where + how is alcohol metabolised?

Answer 2

• 90% by the liver, remainder passively in urine and on breath.
• Ethanol –>Acetaldehyde (via alcohol dehydrogenase)
• Acetaldehyde –>Acetate (via aldehyde dehydrogenase)
• Acetate conjugated to CoA to Acetyl-CoA and metabolised in TCA cycle or utilised for FA synthesis

NB: Acetaldehyde is a toxic metabolite. Accumulation = “hangover”.

Question 3

How does excessive alcohol consumption cause liver damage?

Answer 3

Causes acetaldehyde accumulation causes fatty liver, alcoholic hepatitis + alcoholic cirrhosis.

Question 4

What are the 4 metabolic consequences of chronic alcohol consumption + how they happen?

Answer 4

1) Lactic Acidosis - Decrease in NAD/NADH ratio, inadequate NAD+ for conversion of lactate into pyruvate. Lactate accumulates in blood.
2) Urate crystal accumulation causing Gout - same pathway, lactate accumulates, kidneys ability to excrete uric acid reduced.
3) Hypoglycaemia - Inadequate NAD for glycerol metabolism, deficit in gluconeogenesis.
4) Fatty liver - Increased synthesis of fatty acids and ketone body due to increased AcetylCoA. Increased synthesis of TAG’s –> fatty liver.

(Slide 7)

Question 5

How is disulfiram used to treat alcohol dependance?

Answer 5

• Inhibits aldehyde dehydrogenase
• Acetaldehyde accumulates, causing symptoms of hangover
• Pavlovian association between drinking and negative side effects.

Question 6

What is “oxidative stress” + what can it cause?

Answer 6

• An imbalance between cellular damage caused by ROS /RNS and cellular defences (antioxidants).
• Significant component to a wide range of disease states

Question 7

What is a free radical?
How is it denoted?
Why are they dangerous?

Answer 7

• An atom or molecule containing one or more unpaired electrons.
• A superscript dot
• Very reactive, and acquire electrons from other atoms/ions. Reaction w/molecule generates a 2nd radical propagating damage.

Question 8

Name 4 ROS and describe how they’re formed.

Which one is the most damaging?

Answer 8

1) Molecular oxygen (2 unpaired electrons)
2) Superoxide (O2-) - addition of 1e to molecular oxygen
3) Hydrogen Peroxide (H2O2) - addition of 1e + 2H+ to O2-. Not technically a free radicle.
4) Hydroxyl (OH) - addition of 1e + 1H+ to H2O2.

• Hydroxyl radicle most damaging - reacts with anything.

Question 9

Name 2 RNS and how they’re formed.

Answer 9

1) Nitric Oxide (NO)

2) Peroxynitrite (ONOO-) - superoxide reacts w/NO. It is not a free radical but is a powerful oxidant.

Question 10

What are the 2 main ways in which ROS cause damage to DNA?

Answer 10

1) Reacts w/base - modified base leads to mispairing and mutation
2) Reacts w/sugar - causing strand break and mutation on repair

• Failure to repair DNA can lead to cancer.

Question 11

Describe the mechanism by which ROS damage proteins

How do formation of disulphide bonds affect proteins?

Answer 11

1) ROS reacts w/protein backbone - leads to fragmentation and protein degradation
2) ROS reacts w/side-chain - leads to modified AA, e.g.: disulphide bonds –> change in protein structure –> loss or gain of function/protein degradation.

• DS bonds play important role in folding and stability. Inappropriate DS bonds between thiol groups of cysteine residues causes misfolding, crosslinking and disruption of function.

Question 12

How do ROS cause damage to lipids?

Answer 12

• Via lipid peroxidation
• Free radical extracts H atom from polyunsaturated fatty acid in membrane lipid, forming lipid free radical. (initiation)
• This reacts with oxygen to form lipid peroxyl radical (propagation)
• Forms a chain reaction - lipid peroxyl radicals extracts hydrogen from nearby fatty acids
• Hydrophobic environment of bilayer disrupted and membrane integrity fails.

Question 13

Give examples of endogenous and exogenous biological oxidants.

Answer 13

Endogenous = Electron transport chain, nitric oxide synthases, NADPH oxidases

Exogenous = Radiation, pollutants, drugs + toxins.

Question 14

Explain how the ETC is a source of ROS.

Answer 14

• Electrons travel down PTC’s, but can sometimes escape chain and reaction with dissolved oxygen to form superoxide radicals

Question 15

Explain how Nitric oxide synthases (NOS) provide a source of ROS.

Answer 15

• NOS convert arginine to citruline and a NO radical
• This radical can have toxic effects at a high level, but is also a signalling molecule in vasodilation + neurotransmission.

Question 16

Explain how respiratory burst provides a source of ROS.

Answer 16

• There is rapid release of superoxide and H2O2 from phagocytic cells (e.g.: neutrophils).
• This can form peroxynitrite and hypochlorite radicals which destroy invading bacteria.

Question 17

Explain how superoxide dismutase and catalase provide cellular defence against oxidative stress.

Answer 17

SOD = converts superoxide to H2O2 and oxygen. Primary defence as O2- is strong initiator of chain reactions.

Catalase = converts H2O2 (from SOD) into water and oxygen. Important in immune cells to protect against oxidative burst.

Question 18

Explain how glutathione provides a cellular defence against oxidative stress
Why is NADPH essential for protection against free radicals?

Answer 18

• Thiol groups of Cys from GSH donates electrons to ROS. GSH then reacts w/another GSH to form disulphide (GSSH) via glutathione peroxidase.
• GSSH is reduced back to GSH by glutathione reductase which transfers electrons from NADPH to the disulphide bonds.
• Therefore, NADPH from pentose phosphate pathway essential for protection against FR damage (as it converts oxides GSH back into reduced GSH).

Question 19

Explain how vitamin E + C are important free radical scavengers, and what their individual roles are.

Answer 19

Vit E = lipid soluble antioxidant, protects against lipid peroxidation.
Vit C = water soluble antioxidant, regenerates reduced form of Vit E.

• Free radical scavengers reduce damage by donating H atom and an electron to free radicals in a non-enzymatic reaction.

Question 20

How does galactosemia lead to cataracts?

Answer 20

• Lack of enzymes available to break down galactose
• Galactose acted upon by increased aldose reductase activity to form galactitol, which uses up NADPH
• Compromised defences against ROS damage (unable to regenerate GSH)
• Inappropriate disulphide bond formation, loss of integrity of crystallin protein in lens of eye = cataracts.

Question 21

Explain how G6PDH deficiency leads to oxidative stress.

What kinds of oxidative stress does this cause?

Answer 21

• G6PDH in PP pathway unable to regenerate NADPH
• GSSG unable to be converted back into GSH, leading to oxidative stress.
• Lipid peroxidation + protein damage - particularly aggregated of cross-linked Hb (Heinz bodies), which leads to haemolysis.

Question 22

What is the toxic metabolite produced in the conjugation metabolism of paracetamol?
What usually happens to this metabolite?
What is used to treat paracetamol overdos?

Answer 22

• NAPQI
• It is acted upon by glutathione to complete metabolism
• In O.D - glutathione depleted, leading to NAPQI accumulation (and oxidative stress)
• Treated by acetylcysteine - which replenishes glutathione levels.