Session 1 ILOs - Alcohol metabolism, Oxidative stress, Protein and amino acid metabolism

Question 1

Describe how alcohol is metabolised

Answer 1

• Metabolised in the liver (some excreted passively - urine and breath)
• Alcohol is oxidised to acetaldehyde by alcohol dehydrogenase (NAD+ -> NADH)
• Acetaldehyde is then oxidised to acetate by aldehyde dehydrogenase (NAD+ -> NADH)
• Acetate is then conjugated to acetyl CoA and can be metabolised in the TCA cycle

Question 2

Explain the mechanism of Disulfiram

Answer 2

Disulfiram works by blocking aldehyde dehydrogenase, leading to a build up of the metabolite acetaldehyde which causes the symptoms fo a hangover.

This creates an immediate negative association between drinking and being hungover - classical conditioning.

Question 3

Describe the production of superoxide radicals

Answer 3

During oxidative phosphorylation about 0.1 - 2% of electrons do not reach the end of the electron transport chain and they prematurely reduce oxygen to from superoxide radicals (O2.-)

Important: is a source of the other reactive oxygen species (hydrogen peroxide, hydroxyl radical, nitric oxide and peroxynitrite)

Question 4

Discuss other reactive oxygen (ROS) and reactive nitrogen (RNS) species

Answer 4

Hydrogen peroxide:

• Formed from Superoxide
• Not actually a free radial but can react with Fe2+ to produce free radicals (hydroxyl radial)

Hydroxyl radical:

• Formed from Hydrogen peroxide and reaction with Fe2+ (or other molecules)
• Most reactive and damaging
• Reacts with anything

Peroxynitrite:

• Formed from a reaction between nitric oxide and superoxide
• Not free radial but it’s a powerful oxidant and can damage cells

Question 5

Outline defences against reactive oxygen species

Answer 5

Superoxide dismutase (SOD):
- Converts superoxide to hydrogen peroxide and oxygen

Catalase:
- Converts the hydrogen peroxide to water and oxygen (superoxide dismutase and catalyse work together!!)

Glutathione:

• A tripeptide synthesised in the body
• The cysteine group donates an electron to the reactive oxygen species
• The glutathione (GSH) then binds to another GSH to form oxidised GSSG - requires selenium
• GSSG reduced back to GSH by glutathione reductase (requires NADPH)

Vitamin C and E

• Vit E Donates an electron
• Vit C regenerates the reduced form of Vit E

Question 6

Explain the role of oxidative stress in disease state examples

Answer 6

Galactosaemia:

• Occurs when the NADPH gets used up (galactose is redirected down different pathway)
• Reduced defence against ROS
• Crystallin protein is denatured in the lens of the eye

G6PDH deficiency:
- Occurs when there is a deficiency in G6PDH in the pentose phosphate pathway which is responsible for regenerating NADPH
- Unable to regenerate GSH from GSSG (NADPH not available for glutathione reductase)
- Less H2O2 removed
= lipid peroxidation and protein damage (HEINZ BODIES)

Question 7

Explain how alcohol can because liver damage

Answer 7

Acetaldehyde is toxic, but is normally readily broken down by aldehyde dehydrogenase

However, on prolonged/excessive alcohol consumption, there can be Acetaldehyde accumulation causing liver damage. Excess NADH and acetyl CoA lead to changes in liver metabolism.

Increased Acetyl CoA leads to:
- Increased synthesis of fatty acids and ketone bodies
- increased synthesis of triacylglycerol
= resulting in fatty liver

Can also develop alcoholic hepatitis or alcoholic cirrhosis
(NADH mainly causes systemic problems)

The oxidation of alcohol to acetaldehyde by alcohol dehydrogenase and acetaldehyde to acetate by aldehyde dehydrogenase both produce NADH.

The decreased NAD+/NADH ratio favours the formation of triacylglycerols since there in inadequate NAD+ for fatty acid oxidation.

The increase in triacylglycerol (TAG) synthesis and a reduced capacity of the liver to export these due to lower lipoprotein synthesis; TAGs accumulate in the liver cells, leading to a ‘fatty liver’

Question 8

Explain lipid peroxidation and give an example of a disease where it is prevalent

Answer 8

Initiation
- Free radicals can extract hydrogen from lipids in the lipid membrane and this can form a lipid radical

Propagation
- The lipid radial reacts with oxygen to form a lipid peroxyl radial = results in a chain reaction

Result: membrane integrity fails (damage to lipid membrane)

Atherosclerosis - occurs in LDL particles which are engulfed by macrophages which settle in vessel walls

Question 9

Explain how metabolism of Paracetamol can be toxic (overdose)

What is the antidote to Paracetamol overdose?

Answer 9

Normally, Paracetamol is metabolised with conjugation with Glucuronide or Sulphate

However, in an overdose, these enzymes become saturated. Paracetamol is diveerted and results in toxic metabolite NAPQI (has direct toxic effects). It conjugates with Glutathione which leaves cells susceptible to ROS (lipid peroxidation, damage to proteins, damage to DNA)

Antidote: Acetylcysteine = helps to replenish glutathione levels. But needs to be given Acetylcysteine within 8 hours for good prognosis.

Question 10

Explain nitrogen balance

Answer 10

Nitrogen balance is when nitrogen intake is the same as nitrogen output

Positive nitrogen balance = intake > output
- (pregnancy or growth state, or recovering from malnutrition)

Negative nitrogen balance = intake < output
- never normal (trauma, malnutrition or infection)

Question 11

Explain defects in amino acid metabolism

Answer 11

Excessive breakdown of proteins can occur in Cushing’s Syndrome (due to excess cortisol) - weakens skin structure leading to striae/stetch marks

Question 12

Define the terms glucogenic and ketogenic amino acids and give examples

Answer 12

Glucogenic amino acids - can be used to make glucose in gluconeogensis
e.g. Alanine

Ketogenic amino acids - can be used to make ketone bodies
e.g. Leucine

Both glucogenic and ketogenic:
e.g. Phenylalanine

Question 13

Describe how ammonia is metabolised

Answer 13

Ammonia is combined with CO2 to form carbamoyl phosphate, which combines with citrulline in the mitochondria - which can then enter the urea cycle

Aspartate enters the cycle and can be excreted as urea (glutamate can either enter through conversion to aspartate or by forming the ammonia which then enters the cycle as above)

Question 14

Explain why creatinine is measured

Answer 14

Creatinine is formed from the breakdown of creatine and creatine phosphate in muscle.

It’s measured because it is usually produced at a constant rate, in proportion to muscle mass.

Uses:

• Provide an estimate of muscle mass
• As an indicator of renal function

Question 15

Explain protein turnover

Answer 15

Dietary protein is broken down into free amino acids which can then be used to make cellular proteins by synthesis (only as last resort are cellular proteins broken back down into free amino acids by proteolysis)

Amino acids can also be broken down further in the liver - using the components:

• Carbon skeleton -> glucogenic amino acids -> gluconeogensis (energy)
• Carbon skeleton -> ketogenic amino acids -> ketone bodies (energy)
• Amino group -> urea -> urine

Question 16

Describe how amino acids are catabolised

Answer 16

Need to remove nitrogen from amino acids which can then be excreted as urea or incorporated into other compounds - either by transamination or deamination pathways.

Transamination:
- Involves the transfer of amino group from one compound to another
Most aminotransferases use alpha-ketoglutarate to move amino group to glutamate (AST uses oxaloacetate to move amino group to glutamate) - requires Vit B6 derivative
- Key: Alanine aminotransferase (ALT) converts alanine to glutamate using alpha-ketoglutarate
- Key: Aspartate aminotransferase (AST) converts glutamate to aspartate using oxaloacetate

Deamination:

• Liberates the amino acid group as ammonia (in liver and kidneys)
• Involves D (and L) amino acid oxidases which are enzymes that convert amino acids to keto acids and NH3
• Ammonia is converted to urea in the urea cycle or excreted directly in urine

Question 17

Briefly explain the relationship between NADPH and glutathione

Answer 17

There is a recycling system between NADPH and glutathione.

• NADPH reduces oxidised glutathione to its reduced form, via the enzyme GSH reductase
• The reduced glutathione is then available to be oxidised by reactive oxygen species, thus removing ROS

Question 18

Briefly explain how respiratory burst is produced by some leucocytes

Answer 18

Some cells of the immune system, such as neutrophils and monocytes, when stimulated can rapidly produce a release of ROS which is known as a respiratory burst (sometimes called oxidative burst)

The respiratory burst is produced by a membrane-bound enzyme complex termed NADPH oxidase

This enzyme is present in the cell membrane and it transfers electrons from NADPH across the membrane to couple these to molecular oxygen to generate superoxide radicals