Lecture 1 (Alcohol metabolism & Oxidative stress)

Question 1

Does alcohol have a higher energy content than carbohydrates?

Answer 1

Yes, higher than carbohydrates and proteins but lower than fats

Question 2

Where is alcohol metabolised?

Answer 2

Liver.

The remainder will be excreted passively in urine and on breath

Question 3

How is alcohol (ethanol) metabolised?

Answer 3

Alcohol is oxidised (loss of hydrogen) to actealdehyde by alcohol dehydrogenase.
Then acetaldehyde is oxidised (loss of hydrogen) to acetate by aldehyde dehydrogenase.
Acetate is converted to acetyl-coA which enters the TCA(Krebbs)/fatty acid synthesis.

Small amounts of alcohol are oxidised by the cytochrome P450 enzyme (CYP2E1) or by catalase in the brain

Question 4

What is the recommended limit of alcohol?

Answer 4

14 units, spread across the week (same for men and women)

Question 5

How many grams of ethanol does 1 unit of alcohol contain and how quickly is it eliminated?

Answer 5

8 grams of ethanol per unit

Eliminated via 0 order kinetics at roughly 7 grams per hour

Question 6

What order does the metabolism of alcohol present?

Answer 6

Zero order kinetics
Rate of elimination is constant (you don’t get faster elimination if you have a higher concentration of ethanol in your blood)

Question 7

What are the roles of alcohol dehydrogenase and aldehyde dehydrogenase?

Answer 7

Alcohol dehydrogenase oxidises alcohol (ethanol) to acetaldehyde.
Aldehyde dehydrogenase oxidises acetaldehyde to acetate
Therefore they both reduce NAD+ to NADH.

Question 8

What is the effect of the accumulation of acetaldehyde in the liver?

Answer 8

Toxic, so can cause liver cirrhosis.
Contributes to the feeling of hangovers coupled with the dehydration from ethanol (as it inhibits secretion of ADH from the posterior pituitary)

Question 9

What are the consequences of obtaining energy from ethanol?

Answer 9

Uses up NAD+

• lactate can’t be converted to pyruvate via lactate dehydrogenase (enzyme requires NAD+ > NADH), therefore lactate accumulates in blood causing LACTIC ACIDOSIS
• lactate and uric acid use the same transporter in the kidney, with excess lactate, the ability to excrete uric acid is reduced therefore urate crystals accumulate in tissues = GOUT
• increased supply of acetyl-coA causing increase in synthesis of fatty acids/ketone bodies (come from fatty acids): FATTY LIVER
• limits the metabolism of glycerol, limiting gluconeogenesis: HYPOGLYCAEMIA
• limits fatty acid oxidation (metabolism of fatty acids to acetyl-coA units-catabolic), causing an increase in triacylglycerol syntheis causing accumulation

Question 10

How does the liver export fat?

Answer 10

Fat is packaged into lipoprotein particles.

(protein synthesis is required to make these, so if this is limite then fat will accumulate in the liver = FATTY LIVER

Question 11

What is gout?

Answer 11

Form of arthritis caused by excess uric acid in the bloodstream causing formation of uric acid crystals in joints (most commonly affects big toe)

Question 12

What damage can consumption of alcohol lead to?

Answer 12

• fatty liver
• alcoholic hepatitis (inflammation of liver)
• alcoholic cirrhosis

Question 13

How is acetaldehyde toxicity handled?

Answer 13

Aldehyde dehydrogenase has a low Km for acetaldehyde, so it acts quickly keeping levels of acetaldehyde low.
(low Km = high substrate affinity)

Question 14

What drug is used to treat alcohol dependence?

Answer 14

Disulfiram. It is an inhibitor of aldehyde dehydrogenase. Causing an accumulation of acetaldehyde making you feel sick. The patient then associates drinking with feeling sick, making them stop.

Question 15

Why is oxidative stress important?

Answer 15

It contributes to many disease states (MS, COPD, cancer, cardiovascular disease)

Question 16

What is oxidative stress?

Answer 16

An imbalance between our defence mechanisms and stress placed on cells from free radicals

Question 17

What is a free radical and what does it cause?

Answer 17

A free radical is an atom/molecule that contains one or more upaired electrons and it leads to oxidative stress. It pinches an electron off another molecule causing damage to the cell. (denoted by subscript dot)

Question 18

What is an ROS?

Answer 18

Reactive oxygen species. (also used to describe reactive nitrogen species)

Question 19

Give some examples of ROS’s:

Answer 19

-Oxygen is a biradical (has 2 unpaired electron) which are in different orbitals, so this molecule is quite stable
-if oxygen gains another electron from another species it forms SUPEROXIDE
-superoxide can then take 2 hydrogens and 1 electron to form HYDROGEN PEROXIDE (not a free radical but is toxic to cells)
-hydrogen peroxide then gains another electron to form WATER and a HYDROXYL RADICAL (most damaging as can pinch and electron off anything)
(hydroxyl radical can form water with the gain of an electron and proton)

Question 20

Why is hydrogen peroxide dangerous?

Answer 20

Hydrogen peroxide can react with Fe2+ to produce free radicals via The Fenton Reaction (converts hydrogen peroxide to hydroxyl radical)

Question 21

Give an example of a Reactive Nitrogen Species:

Answer 21

Nitric oxide (NO) can react with superoxide to form PEROXYNITRILE, which is a powerful oxidant.

Question 22

How can ROS damage DNA?

Answer 22

Removal of an electron from a base/sugar of DNA.
-modification of a base can lead to MISPAIRING and mutation
-modification of a sugar can cause a STRAND BREAK and mutation on repair
Failure to repair leads to accumulation of mutations which can lead to cancer

Question 23

What is 8-oxo-dG?

Answer 23

When deoxyguanosine has been damaged by ROS it produces 8-oxo-dG.
A measurement of the amount of this can give indication of the amount of oxidative stress the patient is experiencing as it accumulates in DNA

Question 24

How can ROS damage proteins?

Answer 24

Reaction with backbone:
-fragmentation leading to protein degredation
Reaction with side chains forming:
-dimers
-hydroxylated adducts
-carbonyls
-disulphide bonds between cysteine residues which are unnatural causing a change in protein structure

This can lead to loss/gain of function of proteins, the loss of function proteins will be degraded.

Question 25

What is the structure of insulin?

Answer 25

2 disulphide bonds b/w an alpha and beta chain.
(If oxidative stress occurs a cysteine not normally involved in a disulphide bond, could form a bond if an electron is pinched from it, disruption the functio of the protein)

Question 26

How can ROS damage lipids?

Answer 26

Lipid peroxidation (significant in the aetiology of atherosclerosis)

• free radical pinches an electron from a phospholipid in the cell membrane
• this sets up a chain reaction causing thenext phospholipid to pinch an electron off its neighbour
• hydrophobic nature of bilayer is disrupted and this can damage the cell

Question 27

What are endogenous sources of biological oxidants?

Answer 27

ETC, nitric oxide synthases, NADPH oxidases

Question 28

Role of NADPH oxidase:

Answer 28

Catalyses production of superoxide radical

Question 29

What are exogenous sources of biological oxidants?

Answer 29

Radiation (cosmic rays/UV/X-Ray)
Pollutants
Drugs (primaquine- treats malaria)
Toxins (herbicides)

Question 30

How does the ETC act as an oxidant?

Answer 30

An electron going along the ETC could escape, and react with molecular oxygen to form superoxide and then eventually produce a hydroxyl radical.

Question 31

How does nitric oxide synthase as as an oxidant?

Answer 31

Converts arginine to citrulline & nitric oxide.

Nitric oxide can be a signalling molecule: vasodilation/NT

Question 32

What are the different types of nitric oxide synthases?

Answer 32

INOS: (inducible) at high levels produces NO in phagocytes for direct toxic effect to bacteria
ENOS: (endothelial)
nNOS: (neuronal) produces at low levels in controlled manner for signalling role

Question 33

What is the respiratory burst?

Answer 33

WBC’s contain INOS, producing NO which can react with superoxide (produced by NADPH oxidase in WBC) forming PEROXYNITRITE.

Or the superoxide can form hydrogen peroxide. Hydrogen peroxide can react with chloride to produce hypochlorite which can kill bacteria

Question 34

What does superoxide dismutase and catalase do?

Answer 34

SOD converts superoxide to hydrogen peroxide and oxygen.
CATALASE converts hydrogen peroxide to water and oxygen.
(these are abundant around mitochondria due to leakage of electrons from ETC)

Question 35

What is glutathione?

Answer 35

Tripeptide consisting of glycine/cysteine/glutamate. The cysteine has a sulfhydryl (-S-H i.e. thiol) which can donate an electron to a free radical. This allows the cysteine to form a disulphide bond with another glutathione molecule.
Free radical donation is catalysed by GLUTATHIONE PEROXIDASE
(oxidised form isn’t protective just in that state)

Question 36

How is glutathione regenerated?

Answer 36

Need NADPH to reduce the oxidised form of glutathione

pentose phosphate pathway produces NADPH

Question 37

What are some free radical scavengers?

Answer 37

Vitamin E is lipid soluable so important against lipid peroxidation. Vitamin C is a water soluable antioxidant that regenerates the vitamin E back to its reduced form after vitamin E give an electron to a free radical.

Question 38

What is galactosemia?

Answer 38

Deficiency in enzyme which deals with the conversion of galactose to enter glycolysis, so galactose is converted to galactitol by ALDOSTERONE REDUCTASE which uses NADPH

Question 39

Why is galactosemia dangerous?

Answer 39

Increased level of aldosterone reductase consumes NADPH compromising defences against ROS (glutathione regeneration). This can cause crystallin protein in the lens of the eye to become denatured= CATARACT

Question 40

Why is glucose-6-P hydrogenase deficiency important?

Answer 40

It is the first enzyme in the pentose phophate pathway, therefore a limitation of this enzyme limits the amount of NADPH produced. If people with this deficiency have an added drug (primaquine) which is an oxidant drug, it increases oxidative stress further. Lack of NADPH means we lack the ability to regenerate glutathione, leaving cells susceptible to oxidative stress.

Question 41

Why are RBC’s affected greatly by oxidative stress?

Answer 41

They carry oxygen, and the pentose phosphate pathway is their only source of NADPH as they lack mitochondria. Patients with G6PDH have HEINZ BODIES in their RBC’s.

Question 42

What are Heinz bodies?

Answer 42

Precipitated RBC’s which have been damaged by oxidative stress. They bind to the cell membrane. Spleen removes heinz bodies from cell resulting in BLISTER CELLS, or just removes the whole cell.

Question 43

How is paracetamol metabolised?

Answer 43

Normally converted to glucuronide and sulphate, but if you take an overdose, those pathways are saturated so metabolism is diverted down an alternative pathway producing NAPQI which is an oxidant and has direct toxic effects causing lipid peroxidation and damage to proteins/DNA.
Cell is using up glutathione to protect itself causing glutathione depletion leaving the cell more susceptible to oxidative stress.

Question 44

What is the antidote to overdose of paracetamol?

Answer 44

Acetylcysteine. Boosts levels of glutathione.