Liver physiology, Detoxification by the Liver

Question 1

What are Xenobiotics?

Answer 1

• Foreign substances with no nutritional value and need to be excreted
• Can be toxic

Question 2

Where do Xenobiotics come from?

Answer 2

Most of ingested (e.g. drugs) but also air pollution etc.

Question 3

What problems can Xenobiotics cause?

Answer 3

• They damage proteins, lipids, and can bind to DNA (carcinogens)
• React with O2 and release free radicals

Question 4

What are the Phase I biotransformation reactions?

Answer 4

• They are non-synthetic
• They are the addition or exposure of a functional group

Question 5

What do Phase I biotransformation reactions produce?

Answer 5

• Most commonly produces a hydroxyl group (-OH) but also sulfhydryl, amine, and carboxyl groups
• This only produces a small increase in hydrophilicity

Question 6

What are the Phase II biotransformation reactions?

Answer 6

• They are conjugation reactions with endogenous molecules; glucuronic acid, sulphate, glutathione which forms covalent bonds
• They are biosynthetic reactions

Question 7

What do Phase II biotransformation reactions produce?

Answer 7

• These form covalent bonds with molecules entering the liver
• Produce a large increase in hydrophilicity

Question 8

Where does detoxification occur?

Answer 8

• Most, but not all, occurs in the liver
• Within the Smooth ER of the cells

Question 9

How many groups of Cytochrome P450 enzymes?

Answer 9

• At least 10 main groups
• Encoded by a superfamily of 60 different genes

Question 10

What are some features of the Cytochrome P450 enzymes? (6)

Answer 10

1. Present in smooth ER (hence microsomal enzymes)
2. Oxidise substrates and reduce oxygen
3. Have a cytochrome reductase subunit – uses NADPH
4. They are inducible – get increased production of enzymes upon exposure to specific molecules
5. Generate a reactive free radical
6. Have affinity for carbon monoxide and oxygen

Question 11

Why is CYP3A4 important?

Answer 11

• CYP3A4 is present in the highest concentration and responsible for 2/3rds all known drugs
• It is involved in the metabolism of about 50% of all clinically prescribed drugs

Question 12

How are the cytochromes induced?

Answer 12

• Via enzymes
• The molecule bind to an intracellular receptor within the cytoplasm
• This molecule-receptor complex migrates to the nucleus
• Increases transcription of mRNA for cytochrome-P450s
• This increases the effect of the CYP
• One substance can induce a number of enzymes

Question 13

What drugs induce the CYP3A4 to metabolise the drug?

Answer 13

Paracetamol, codeine

Question 14

What drugs induce the CYP2D6 to metabolise the drug?

Answer 14

Amitriptyline (also codeine)

Question 15

How do cytochrome P450 breakdown drugs?

Answer 15

• CYPs contain a haem component which is capable of oxidising molecules (-OH addition) by becoming reduced themselves
• The reductases use NADPH to become active
• This reductases reduce CYPs allowing the oxidation of the foreign molecule

Question 16

What does the reaction of the cytochrome with the drug produced?

Answer 16

• The reaction forms water and has an intermediate of a haem free radical
• Overall, the addition of the –OH group increases the solubility of the molecule

Question 17

What are some examples of Phase I reactions?

Answer 17

• Oxidation
• Reduction
• Hydrolysis

Question 18

What are some examples of Phase II reactions?

Answer 18

Glycoside conjugation – glucuronidation (most common)
Sulphate - sulphation
Glutathione (GSH)

Question 19

What 5 problems can occur when metabolism occurs?

Answer 19

1. Complete inactivation and elimination
2. Formation of another active compound – can have similar or new activity (e.g. codeine breaking down into morphine)
3. Activation of pro-drugs
4. Toxification of less toxic xenobiotics
5. Active drug to reactive intermediates

Question 20

What is the Phase III reactions?

Answer 20

Phase III is the removal of drugs/metabolites by transporter-mediated elimination via the liver, gut, kidney and lung

Question 21

How does smoking have an effect on CYP?

Answer 21

• CYP1A2 can be induced by smoking
• Consequently this increased activity of CYP1A2 has effects on the metabolism of other molecules
  E.g. Clozapine – dose has to be tightly controlled depending on how much the patient is smoking

Question 22

How does grapefruit have an effect on CYP?

Answer 22

Grapefruit juice has effects on medications e.g. statins
Contains products that inhibit CYPs
This means that statins become more potent
This means that grapefruit shouldn’t be eaten

Question 23

What CYP can cause paracetamol to be harmful?

Answer 23

The harmful intermediate NAPQI is created from CYP2E1 (remaining 10%)
If the normal pathways are overwhelmed CYP2E1 becomes more significant and more NAPQI is created
Normally NAPQI is metabolised by glutathione-S-transferase but this can be overwhelmed by high NAPQI levels
Hepatocytes then become damaged

Question 24

How can ethanol and alcohol be harmful with CYPs?

Answer 24

Alcohol is polar but also slightly lipid soluble.
Can be excreted but is more commonly used as a fuel
Metabolised into acetaldehyde (toxic),
Further metabolism to acetate by ALDH which can be used in the Krebs Cycle
This second metabolism becomes overwhelmed and acetaldehyde builds up

Question 25

How does the chronic use of alcohol have an effect on liver and CYPs?

Answer 25

Microsomal Ethanol Oxidising System (MEOS) also produces acetaldehyde
Chronic alcohol use increases CYP2E1 levels 5-10 fold therefore alcohol is metabolised quicker
Acetaldehyde is produced quicker and in larger quantities – more toxic
Results in liver damage from the production of free radicals

Question 26

Where are microsomal enzymes found?

Answer 26

• In the smooth ER
• Liver, kidney, lugs, intestinal mucosa

Question 27

What is an example of a microsomal enzyme?

Answer 27

mono-oxygenases

Question 28

What reactions are microsomal enzymes used in?

Answer 28

• Majority of drug biotransformation reaction, oxidative, reductive and hydrolytic
• They are inducible by drugs, diet

Question 29

Where are non-microsomal enzymes found?

Answer 29

Cytoplasm and mitochondria of hepatocytes and other tissues

Question 30

What is an example of a non-microsomal enzyme?

Answer 30

protein oxidases, esterases, amidases

Question 31

What reactions are involved with non-microsomal enzymes?

Answer 31

• Non-specific enzymes catalyse oxidative, reductive and hydrolytic reactions and conjugation reactions