Describe the metabolism (detoxification) of xenobiotics.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open in two lines. Xenobiotics are foreign compounds such as drugs, pesticides, food additives and pollutants. The body, mainly the liver, changes them so they can be excreted.
2 · Draw the two phases. [Flow chart: fat-soluble xenobiotic → Phase I (cytochrome P450 adds –OH) → Phase II (conjugation) → water-soluble conjugate → urine or bile]
3 · Phase I.
Oxidation (hydroxylation) by cytochrome P450, using O₂ and NADPH.
Reduction and hydrolysis.
In liver smooth ER (microsomes); can sometimes activate a compound into a toxin.
4 · Phase II.
Glucuronidation (most common), e.g., bilirubin.
Sulphation, glutathione conjugation, acetylation.
Products are water-soluble and usually inactive.
5 · Effects of xenobiotics. Mostly detoxification, but also cell injury (paracetamol overdose), cancer (benzpyrene), allergy, and enzyme induction that changes other drug levels.
6 · Significance. Protects the body from foreign chemicals and explains drug interactions, side effects and why the liver is open to toxic damage.
7 · Nutrition link. Protein (for glutathione and enzymes) and iron support detox; grapefruit inhibits CYP3A4.
8 · Close in one line. Sum up: Phase I adds a handle, Phase II adds a tag, and the body flushes the product out.
Scoring tip: Name the reactions under each phase with one example each; that is what examiners look for.
Write-up practice
The check looks for key words only. It does not grade your answer and never writes it for you.