Biology of Disease - CH 0576

Hyperbilirubinaemia & Jaundice

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Introduction• Jaundice is not a disease state in itself• It is a non-specific symptom which is

a feature of a range of disease states• The clinical approach to jaundice

must be based on a clear understanding of the metabolism of bilirubin and an appreciation of the potential blocks which can occur in the pathways.

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Bilirubin Metabolism

• Several factors lead to the removal of aged red cells from the circulation:– They have lost their ability to deform– They have lost enzyme activities– They have compromised their ATP levels– They are unable to staunch the flow of

Ca2+ ions into the cell, across the plasma membrane

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Bilirubin Metabolism

• The aged red cells are removed from the circulation by the RES cells of the spleen and the bone marrow.

• The aged cells are unable to negotiate the torturous route through the red pulp of the spleen.

• Following phagocytosis, the bulk of the red cell constituents are recycled

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Bilirubin Metabolism

• Recycling includes;– Fe transported on transferrin– Amino acids enter the plasma pool– Lipids from membranes enter the

plasma pool

• The only part of the haemoglobin molecule which is not reutilised is the tetra-pyrolle ring from HAEM.

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Bilirubin Metabolism

• Haem is the portion of the Hb molecule which is catabolised to form bilirubin.

• Bilirubin is then handled by the liver prior to its excretion in urine and in faeces.

• Haem also forms the prosthetic group in molecules other than haemoglobin.

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Bilirubin Metabolism

• Molecules, other than Hb, containing haem as a prosthetic group include:– Cytochromes– Myoglobin– Peroxidases

• These all contribute to bilirubin production, via haem breakdown.

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Bilirubin Metabolism

• Daily bilirubin production in Man averages between 250 - 350 mg.

• 85% of this amount is the result of the breakdown of Hb from aged red cells.

• The remaining 15% from other sources includes the destruction of red cell precursors in the marrow.

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Bilirubin Metabolism

• Initial stage in the degredation of haem is under the control of the microsomal enzyme, haem oxygenase, acting along with NADPH-cytochrome c reductase.

• This reaction involves the cleavage of the tetra-pyrolle ring at the methene bridge.

• During this step, iron is liberated.

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Bilirubin Metabolism• The iron passes out of the RES cell

and is bound onto the transport protein, transferrin, for passage back to the red marrow.

• The carbon atom from haem is excreted at the lungs in the form of carbon monoxide.

• Assessment of CO indicates level of red cell breakdown.

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Bilirubin Metabolism

• The product of this initial stage in bilirubin production is a colourless, non-toxic molecule : biliverdin.

• This is the main excretory product in the lower animal phyla.

• Mammals then undertake an apparent retrograde step, by converting this into a toxic molecule : bilirubin.

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Bilirubin Metabolism• This conversion step must have

some evolutionary advantage:– bilirubin crosses the placenta, whereas

biliverdin doesn’t.

• The conversion of bilverdin involves the action of a cytoplasmic enzyme in the RES cell : biliverdin reductase.

• Bilirubin is transported from the RES cell to the liver for processing.

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Bilirubin Metabolism• Bilirubin is transported, in the plasma,

bound onto the protein albumin.• Albumin has two binding sites for

bilirubin, of greatly varying affinities.• Bilirubin is displaced from the lower

affinity binding site by certain drugs including:– barbiturates, sulphonamides, salicylates.

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Bilirubin Metabolism

• Bilirubin displaced from albumin has a very high affinity for lipid-rich tissues, and it tends to be taken up by the brain.

• Bilirubin is very toxic to the CNS and causes enlargement and oedema of the brain.– KERNICTERUS.

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Bilirubin Metabolism

• This is a potential very serious problem in neonates, which often have a mild ‘physiological jaundice’.

• It is a potentially very damaging side effect in HDN.

• Adults are protected from this problem by a fully functioning blood-brain barrier.

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Unconjugated Bilirubin• Bilirubin, bound onto albumin, prior to

processing in the liver is often referred to as ‘unconjugated’.

• Unconjugated bilirubin, bound onto albumin, is non-toxic and remains in solution for its transportation, in the blood, to the liver for processing.

• A small amount, 20g/L is free in plasma.

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Liver Handling of Bilirubin• On arrival at the liver the bilirubin is

dissociated from the albumin and is actively transported across the sinusoidal membranes into the hepatocyte.

• This is a carrier mediated process.• In the liver cell the unconjugated

bilirubin is bound onto two soluble cytoplasmic binding proteins.

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Liver Handling of Bilirubin• The two proteins are:

– Z-protein– Ligandin.

• This binding greatly increases the solubility of the bilirubin, allowing it to be transported across the aqueous cytoplasm, to the ER of the liver cell.

• The next stage is CONJUGATION.

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Liver Handling of Bilirubin

• Within the ER of the hepatocytes the bilirubin is conjugated with UDP-glucuronic acid to form glucuronides.

• This process is under the control of two ER enzymes termed the glucuronyl transferases.

• Di- and monoglucuronides are produced under various circumstances.

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Liver Handling of Bilirubin

• Conjugated bilirubin (mono- or di-) is actively transported across the canalicular membrane, forming part of the secretion, bile.

• Conjugated bilirubin is non-toxic.• Bile passes down the bile ductules

and into the common hepatic duct, and, if required, into the intestine.

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Bilirubin Metabolism• Once within the small intestine the

glucuronides are acted upon by the bacterial flora of the gut.

• Bacterial -glucuronidases hydrolyse the glucuronides into free bilirubin and glucuronic acid.

• Further bacterial action leads to the conversion of bilirubin into a range of products : urobilinogens.

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Bilirubin Metabolism

• The majority , about 80% of these are excreted in faeces, following conversion to stercobilinogens.

• The remaining 20% of the urobilinogens are reabsorbed into the entero-hepatic circulation.

• The majority of these are re-exreted by the liver in bile.

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Bilirubin Metabolism

• Between 2% and 5% of the reabsorbed urobilinogens enter the general circulation and are excreted via the kidneys, in urine.

• Around 1 - 2 mg of bile pigments are excreted in the urine daily, with about 250 - 350 mg excreted in faeces daily.