S1 - Alcohol metabolism and oxidative stress

Question 1

Describe the breakdown of alcohol and the enzymes that are involved.

Answer 1

Alcohol -> acetaldehyde -> acetate
1st enzyme: alcohol dehydrogenase
2nd enzyme: aldehyde dehydrogenase

Question 2

What is the drug disulfiram used for and how does it work?

Answer 2

Used for alcoholism.
Inhibits the enzyme aldehyde dehydrogenase - causes a build of acetaldehyde, which is a toxic substance to the body and causes hangover symptoms. This makes the patient feel very ill whenever they drink alcohol and will hopefully deter them.

Question 3

What happens to the acetate that is produced from alcohol metabolism?

Answer 3

It is conjugated to coenzyme A to form acetyl CoA.

Acetyl coA is used to synthesise fatty acids and ketone bodies.

Question 4

Where does alcohol metabolism occur?

Answer 4

In the liver, which is why you get liver change/damage with excess alcohol.

Question 5

What causes the changes in liver metabolism e.g. Fatty liver, alcoholic hepatitis, cirrhosis, gout, lactic acidosis?

Answer 5

Decreased NAD+:NADH (because in alcohol metabolism NAD+ is converted to NADH at each stage).
Increased Acetyl CoA.

Question 6

What is the definition of oxidative stress?

Answer 6

An imbalance between the manifestation of reactive oxygen species and the body’s ability to detoxify reactive molecules or repair the damage.

Question 7

Which free radical is the most damaging to the body?

Answer 7

OH

Question 8

Name some endogenous sources of ROS

Answer 8

Electron transport chain (e- can react with O2 to form O2- free radical), peroxidases.

Question 9

Name some exogenous sources of ROS.

Answer 9

Radiation, pollutants, drugs.

Question 10

What affect do ROS have on lipids?

Answer 10

Lipid peroxidation:
ROS takes an H+ from a polyunsaturated lipid, forming a lipid free radical. Lipid free radical reacts with O2 to form lipid peroxyl radical, which reacts with another H+ on a fatty acid. Chain reaction that disrupts the bilayer.

Question 11

What marker can be used to measure oxidative stress?

Answer 11

8-oxo-dG, it is formed when an amino acid is modified by oxidative stress.

Question 12

What is the respiratory burst?

Answer 12

An antimicrobial defence mechanism where O2-(free radical) and H2O2 are rapidly released from monocytes and neutrophils. These ROS alongside peroxynitrite (ONOO-) destroy bacteria.

Question 13

What enzyme is essential for the respiratory burst?

Answer 13

NADPH oxidase (converts NADPH -> NADP+ in the respiratory burst).

Question 14

What is chronic granulomatous disease?

Answer 14

A genetic defect in NADPH oxidase - leads to an increased susceptibility to bacterial infections because the respiratory burst isn’t as effective.

Question 15

Describe the 3 cellular defence mechanisms against oxidative species…

Answer 15

1. Superoxide disputable and catalase - both enzymes alter the pathway in which ROS is produced - so that OH f.r isn’t made.
2. GSH and GSSG - 2 x GSH produce GSSG, allowing the thiol group of cysteine on GSH to donate e- to ROS. This bit requires glutathione peroxidase, which needs selenium to work. GSSG then forms GSH again, requiring glutathione reductase and NADPH. NADPH is obtained from the pentose phosphate pathway.
3. Free radical scavengers - vitamins A, C and E donate H+ to free radicals to make them less damaging.

Question 16

What is the pentose phosphate pathway?

Answer 16

A pathway that produces 5-carbon sugars needed for nucleotide production and NADPH, needed for oxidative stress production (GSH production). The rate limiting enzyme is Glucose-6-phosphate dehydrogenase.
NADPH is produced from NAD+ in most steps.

Question 17

What is galactossaemia? Name the 3 enzymes that could be deficient.

Answer 17

A deficiency in either galactokinase, uridyl transferase or UDP-galactose epimerase. This means that galactose can’t be broken down in the normal way. Instead, it is converted to galacticol by aldose reductase. However, this uses up NADPH, therefore compromises ROS defences.

Question 18

Why is cataracts a common consequence of galactossaemia?

Answer 18

Galacticol build up leads to cataracts due to protein denaturation in the eye lens.

Question 19

What is G6PDH deficiency?

Answer 19

A deficiency in the enzyme Glucose-6-phosphate dehydrogenase, which is the rate limiting enzyme in the pentose phosphate pathway. This means that less NADPH can be produced in the pentose phosphate pathway, and therefore less GSH is produced = less protection against oxidative damage.

Question 20

What is the clinical sign of G6PDH deficiency? Explain the clinical sign?

Answer 20

Heinz bodies.
They are dark stained RBC due to precipitation of Hb, caused by protein damage due to ROS. The altered Hb makes the RBC membrane more rigid, making it harder for cells to squeeze through capillaries. Heinz bodies will be removed by the spleen as part of the RES - results in blister cells.

Question 21

Describe the metabolism of therapeutic levels of paracetamol.

Answer 21

At therapeutic levels, paracetamol is metabolised by the liver by conjugation with gluconaride or sulphate. The products are relatively non-toxic.

Question 22

How does the metabolism of paracetamol change with an overdose?

Answer 22

The normal metabolic pathways become quickly saturated when an overdose is taken. In these conditions, paracetamol produces NAPQI, which is very toxic to hepatocytes.
NAPQI is a strong oxidising agent, and undergoes conjugation with glutathione.
Decreased glutathione leads to destruction of liver cells and liver failure (since glutathione is an important anti-oxidant in hepatocytes). If left untreated, this will cause death.

Question 23

What treatment could be given for a paracetomol overdose?

Answer 23

Acetylcysteine -> if given < 8 hours after overdose, prognosis is good as it replenishes the glutathione levels, so they liver can safely metabolise the overdose. If treatment isn’t given, paracetomol overdose can lead to death.