Biochemistry chapters
Haem breakdown and jaundice
How haem is made (and what goes wrong in the porphyrias and lead poisoning), how it is broken down to bilirubin, how the liver conjugates and excretes bilirubin, and how to tell pre-hepatic, hepatic and post-hepatic jaundice apart, including neonatal jaundice and the inherited hyperbilirubinaemias.
- Haem is made from glycine + succinyl-CoA in mitochondria and cytosol (mainly marrow and liver); ALA synthase (needs vitamin B6) is rate-limiting and inhibited by haem; lead inhibits ALA dehydratase and ferrochelatase (microcytic anaemia, basophilic stippling); acute intermittent porphyria (porphobilinogen deaminase) causes abdominal pain, neuropsychiatric symptoms and port-wine urine triggered by drugs such as barbiturates, while porphyria cutanea tarda (uroporphyrinogen decarboxylase) causes blistering photosensitivity.
- Haem → (haem oxygenase) biliverdin → unconjugated bilirubin (fat-soluble, carried on albumin, not in urine) → liver conjugation by UDP-glucuronyl transferase → conjugated bilirubin (water-soluble) into bile → gut bacteria → urobilinogen (some reabsorbed, in urine) and stercobilin (brown stool).
- Pre-hepatic (haemolytic): high unconjugated bilirubin, no bilirubin in urine, more urobilinogen; post-hepatic (obstructive): high conjugated bilirubin, dark urine with bilirubin, pale stools, absent urobilinogen, high ALP; hepatic: mixed, with high ALT; Gilbert (mild, fasting-related, unconjugated) is the commonest inherited cause.
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Haem synthesis
Haem (iron + protoporphyrin IX) is made mainly in the bone marrow (for haemoglobin) and liver (for cytochromes, including P450).
| Step | Enzyme | Location | Notes |
|---|---|---|---|
| Glycine + succinyl-CoA → δ-aminolevulinic acid (ALA) | ALA synthase | Mitochondria | Rate-limiting; needs pyridoxal phosphate (B6); inhibited by haem (feedback); induced by drugs (barbiturates) |
| 2 ALA → porphobilinogen (PBG) | ALA dehydratase | Cytosol | Inhibited by lead |
| 4 PBG → hydroxymethylbilane | PBG deaminase | Cytosol | Deficient in acute intermittent porphyria |
| → uroporphyrinogen III → coproporphyrinogen III | Uroporphyrinogen decarboxylase | Cytosol | Deficient in porphyria cutanea tarda |
| → protoporphyrin IX | (oxidases) | Mitochondria | (none specific) |
| Protoporphyrin IX + Fe²⁺ → haem | Ferrochelatase | Mitochondria | Inhibited by lead |
B6 deficiency (isoniazid) → sideroblastic anaemia.
Lead poisoning
Inhibits ALA dehydratase and ferrochelatase → microcytic anaemia with basophilic stippling of red cells, raised ALA in urine and zinc protoporphyrin; abdominal colic, constipation, peripheral neuropathy (wrist and foot drop), encephalopathy in children, lead lines at gums and growing bones. Sources: old paint, batteries, some cosmetics (surma/kohl), traditional medicines. Treat with chelation (succimer, EDTA, dimercaprol).
Porphyrias
| Feature | Acute intermittent porphyria (AIP) | Porphyria cutanea tarda (PCT) |
|---|---|---|
| Enzyme | Porphobilinogen deaminase | Uroporphyrinogen decarboxylase |
| Inheritance | Autosomal dominant | Mostly acquired (with inherited susceptibility) |
| Build-up | ALA and PBG (not photosensitising) | Uroporphyrin (photosensitising) |
| Features | Attacks of severe abdominal pain, vomiting, neuropathy (motor weakness), psychiatric symptoms, tachycardia, hyponatraemia; urine darkens (port-wine) on standing; no skin lesions | Blistering of sun-exposed skin (backs of hands), fragility, hyperpigmentation, hypertrichosis; tea-coloured urine; the commonest porphyria |
| Triggers | Drugs (barbiturates, sulphonamides, griseofulvin, oral contraceptives, some antiepileptics), alcohol, fasting, infection, menstruation | Alcohol, hepatitis C, iron overload, oestrogens |
| Treatment | IV haem (haem arginate), glucose (both suppress ALA synthase), avoid triggers, analgesia (opioids are safe) | Avoid sunlight and alcohol, venesection, low-dose hydroxychloroquine |
Haem breakdown
- Old red cells are destroyed by macrophages (spleen, liver, marrow) after about 120 days.
- Haemoglobin → globin (amino acids reused) + haem.
- Haem oxygenase opens the ring → biliverdin (green) + iron (reused) + carbon monoxide.
- Biliverdin reductase → unconjugated bilirubin (yellow).
About 250–350 mg bilirubin is formed daily; most from red cells, the rest from other haem proteins and ineffective erythropoiesis.
Bilirubin handling
| Stage | Details |
|---|---|
| Transport | Unconjugated bilirubin is fat-soluble, carried on albumin; not filtered by the kidney (so not in urine). Drugs such as sulphonamides and aspirin can displace it (kernicterus risk in neonates) |
| Uptake | Into hepatocytes |
| Conjugation | UDP-glucuronyl transferase adds glucuronic acid → bilirubin diglucuronide (conjugated, water-soluble) |
| Excretion | Actively secreted into bile (MRP2 transporter): the rate-limiting step of bilirubin handling |
| In the gut | Bacteria convert it to urobilinogen; most is oxidised to stercobilin (brown stool); some urobilinogen is reabsorbed (enterohepatic circulation) and a little is excreted in urine (as urobilin, yellow) |
The van den Bergh reaction distinguishes direct (conjugated) from indirect (unconjugated) bilirubin.
Types of jaundice
Jaundice becomes visible when serum bilirubin is above about 2–3 mg/dL (about 35–50 µmol/L); normal is below about 1 mg/dL.
| Feature | Pre-hepatic (haemolytic) | Hepatic (hepatocellular) | Post-hepatic (obstructive, cholestatic) |
|---|---|---|---|
| Causes | Haemolysis (thalassaemia, sickle cell, G6PD, malaria), ineffective erythropoiesis, Gilbert | Hepatitis (viral, alcohol, drugs), cirrhosis | Gallstones in the CBD, carcinoma of the head of the pancreas, cholangiocarcinoma, strictures, primary biliary cholangitis |
| Serum bilirubin | Unconjugated ↑ | Both ↑ | Conjugated ↑ |
| Urine bilirubin | Absent ("acholuric") | Present | Present (dark urine) |
| Urine urobilinogen | ↑ | Variable (often ↑ early) | Absent (complete obstruction) |
| Stool colour | Normal or dark | Normal or pale | Pale (clay-coloured) |
| ALT/AST | Normal | Markedly ↑ | Normal or mildly ↑ |
| ALP, GGT | Normal | Mildly ↑ | Markedly ↑ |
| Other | Anaemia, reticulocytes ↑, LDH ↑, haptoglobin ↓ | Prolonged PT (poor synthesis) | Itching (bile salts), steatorrhoea, vitamin K deficiency (PT corrects with vitamin K) |
Haemolytic jaundice: unconjugated, no bilirubin in urine, more urobilinogen. Obstructive jaundice: conjugated, dark urine, pale stool, no urobilinogen, high ALP.
Courvoisier's law: a palpable, non-tender gallbladder with painless jaundice suggests cancer of the head of the pancreas rather than gallstones.
Neonatal jaundice
Physiological jaundice (common):
- Appears after 24 hours, peaks around days 3–5 (term), fades by about 2 weeks.
- Causes: high red cell mass and shorter red cell life, immature UDP-glucuronyl transferase, increased enterohepatic circulation.
- Unconjugated.
Pathological jaundice (investigate): appears in the first 24 hours (haemolysis: Rh or ABO incompatibility, G6PD deficiency), rises fast or very high, lasts beyond 2 weeks (term) or 3 weeks (preterm) (breast milk jaundice, hypothyroidism, biliary atresia: conjugated, pale stools), or is conjugated.
Kernicterus: unconjugated bilirubin (free, not albumin-bound) crosses the immature blood-brain barrier and stains the basal ganglia → lethargy, poor feeding, high-pitched cry, opisthotonos, later choreoathetoid cerebral palsy, deafness, gaze palsy.
Treatment: phototherapy (blue light changes bilirubin into water-soluble isomers excreted without conjugation); exchange transfusion for very high levels; IV immunoglobulin in immune haemolysis.
Inherited hyperbilirubinaemias
| Syndrome | Defect | Bilirubin | Features |
|---|---|---|---|
| Gilbert syndrome | Mildly reduced UDP-glucuronyl transferase (UGT1A1 promoter) | Unconjugated | Commonest; mild jaundice with fasting, stress, illness or exercise; benign; normal liver tests otherwise |
| Crigler-Najjar type I | Absent UDP-glucuronyl transferase | Unconjugated (very high) | Kernicterus in neonates; phototherapy, liver transplant |
| Crigler-Najjar type II | Severely reduced enzyme | Unconjugated | Milder; responds to phenobarbitone (induces the enzyme) |
| Dubin-Johnson syndrome | Defective MRP2 (canalicular excretion) | Conjugated | Benign; black liver (pigment) |
| Rotor syndrome | Defective hepatic storage/uptake | Conjugated | Benign; liver not pigmented |
Unconjugated: Gilbert, Crigler-Najjar. Conjugated: Dubin-Johnson (black liver), Rotor.
Expect: substrates and rate-limiting enzyme of haem synthesis, B6, lead poisoning (enzymes, stippling), AIP vs PCT (enzymes, features, triggers, treatment), steps of haem breakdown, albumin binding and conjugation, urobilinogen and stercobilin, laboratory differences between haemolytic, hepatic and obstructive jaundice, physiological vs pathological neonatal jaundice, kernicterus and phototherapy, and Gilbert, Crigler-Najjar, Dubin-Johnson and Rotor syndromes.
Quick revision
- Haem: glycine + succinyl-CoA; ALA synthase (B6) rate-limiting.
- Lead: ALA dehydratase and ferrochelatase; basophilic stippling.
- AIP: PBG deaminase; pain, neuropathy, psychiatric; drugs trigger; haem arginate.
- PCT: uroporphyrinogen decarboxylase; blistering photosensitivity.
- Haem → biliverdin → unconjugated bilirubin (albumin) → UDP-GT → conjugated → bile → urobilinogen, stercobilin.
- Haemolytic: unconjugated, no urine bilirubin, ↑ urobilinogen. Obstructive: dark urine, pale stool, ↑ ALP.
- Gilbert: mild unconjugated, fasting. Dubin-Johnson: conjugated, black liver.
Written by The House Job from standard biochemistry references (Lippincott Illustrated Reviews: Biochemistry).
FAQ
What is the rate-limiting enzyme in haem synthesis?
ALA synthase, which needs vitamin B6 and is inhibited by haem.
How can urine tests separate haemolytic from obstructive jaundice?
In haemolytic jaundice the urine has no bilirubin but more urobilinogen; in obstructive jaundice the urine contains bilirubin (dark) and urobilinogen is absent.
Why does unconjugated bilirubin not appear in urine?
It is fat-soluble and tightly bound to albumin, so it is not filtered by the kidney.
What is Gilbert syndrome?
A common, harmless inherited reduction in bilirubin conjugation that causes mild unconjugated jaundice during fasting, illness or stress.