Detoxification By The Liver

Question 1

Where does detoxification take place?

Answer 1

Most (but not all) detoxification takes place in the liver

• inactivation and facilitated elimination of drugs and other xenobiotics
• formation of active metabolites with similar or occasionally enhanced activity
• activation of pro-drugs
• toxification of less toxic xenobiotics

Question 2

What are cytochrome P450 enzymes?

Answer 2

There are at least 10 main groups of cytochrome-P450 enzymes encoded by a superfamily of at least 55 different genes.

Question 3

What are some features in common of cytochrome P450 enzymes?

Answer 3

• They are present in the smooth Endoplasmic Reticulum (hence called “microsomal” enzymes)
• They oxidise the substrate and reduce oxygen
  hey have a cytochrome reductase subunit which uses NADPH
• They are inducible – enzyme activity may be increased by certain drugs, some dietary components, and some environmental toxins eg smoking
• They generate a reactive free radical compound

Question 4

What is meant by induction of cytochromes?

Answer 4

One drug can induce numerous cytochrome isoenzymes

Question 5

What is one of the most commonest mechanisms of drug interactions?

Answer 5

Via cytochrome P450
Cytochrome P450 enzymes are inducible, which may accelerate the breakdown of some medications

Question 6

What can cytochrome P450 be inhibited by?

Answer 6

Cytochrome P450 enzymes can be inhibited by various drugs and foodstuffs (usually takes effect quicker than induction)

It can result in increased blood concentrations of certain medications (less breakdown)

Question 7

What are some inducers affecting clozapine?

Answer 7

Smoking
(Increased metabolism of drugs)
Barbiturates
Carbamazepine
Primidone
Rifampicin
Cruciferous vegetables, eg cabbage, broccoli
Grilled meat

Question 8

What agents are known to inhibit clozapine?

Answer 8

Conversely agents including ciprofloxacin
Resulting in increased concentrations of medications such as clozapine

Question 9

What dietary components may inhibit cytochrome P450?

Answer 9

Grapefruit juice
- A lot of medications are metabolised in Phase I by CYP3A4.
- Most statins are metabolised by CYP3A4.
- By inhibiting metabolism of simvastatin and atorvastatin, grapefruit juice causes increased blood levels with increased risk of side-effects.

Question 10

What is the inactive metabolite of phenobarbital?

Answer 10

Glucuronides etc

Question 11

What is the active metabolite of codeine?

Answer 11

Morphite

Question 12

What is the active metabolite of diazepam?

Answer 12

Oxazepam

Question 13

What is the reactive intermediate of benzo[a]pyrene?

Answer 13

Reactive metabolite (carcinogenic)

Question 14

What is the reactive intermediate of paracetamol?

Answer 14

NAPQI (toxic)

Question 15

What happens during the inactivation of xenobiotic?

Answer 15

An illustration is Phenobarbital which is a barbiturate derivative with both sedative and anti-epileptic activity, metabolised in classic Phase I & Phase II reactions

Phenobarbital is relatively lipophilic; drug distributes into fat tissue

The amount that remains in the plasma is mostly bound to plasma proteins

Only a small fraction of the drug is found freely dissolved in the blood plasma

Elimination of the unmodified drug is thus very slow, and most of the drug is excreted after enzymatic conjugation.

Question 16

How is codeine metabolised to morphine?

Answer 16

Codeine is a morphine molecule with one hydroxyl group replaced by a methyl group which makes the compound less susceptible to first-pass metabolism (in gut mucosa and liver).

Codeine is active, and is de-methylated in the liver to morphine which is also active.

Question 17

How is diazepam converted to oxazepam?

Answer 17

Diazepam is demethylated in the liver (a phase-1 reaction) to nordiazepam (an active metabolite).

Nordiazepam is hydroxylated (also a phase-1 reaction) to oxazepam.

Oxazepam is also an active metabolite, and can be prescribed as a shorter-acting sedative.

Oxazepam is metabolised by conjugation (phase-2) and excreted without any phase-1 step.

Question 18

What are examples of Phase II reactions?

Answer 18

Glycoside conjugation - glucuronidation
Sulphate - sulphation
- Glutathione (GSH)
- Methylation
- Acylation
- Phosphate conjugation

Question 19

What is responsible for most Phase II reactions?

Answer 19

Transferase enzymes

Question 20

What are microsomal enzymes?

Answer 20

Location - smooth endoplasmic reticulum

Sites - liver then kidney, lungs, intestinal mucosa

Enzymes – mono-oxygenases, (CYPs,FMOs); UGTs etc

Reactions - majority of drug biotransformation reaction ;
oxidative, reductive & hydrolytic and glucuronidation

Note - they are inducible by drugs, diet etc…

Question 21

What are non microsomal enzymes?

Answer 21

Location - cytoplasm and mitochondria of hepatocytes, other tissues

Enzymes - protein oxidases, esterases, amidases, conjugases

Reactions - non specific enzymes that catalyze few oxidative,
a number of reductive & hydrolytic reactions and also conjugation reactions other than glucuronidation.

Note - Not inducible but having polymorphism (acetyltransferase & pseudocholinesterase)

Question 22

How is ethanol metabolised?

Answer 22

Metabolism of ethanol doesn’t fit the category of phase I and Phase II.

Ethanol doesn’t need to be conjugated for excretion.
Only between 2% and 10% is usually excreted in the urine, because it is used in the liver as a dietary fuel.

The major route is via alcohol dehydrogenase (ADH)

Question 23

What is another route of the metabolism of ethanol?

Answer 23

The other route is via the so-called Microsomal Ethanol Oxidising

System MEOS (predominantly CYP2E1), which also produces acetaldehyde as an intermediate.

The cytochrome P450 isoenzyme CYP2E1 has high affinity for alcohol.

MEOS refers to the combined ethanol-oxidising activity of all CYP450 enzymes.