1. Dr. Shumaila Asim Lecture # 8
Bilirubin Metabolism
Dr. Shumaila Asim
Lecture # 8

2. Definition of bilirubin
Bilirubin is the water insoluble breakdown product of normal heme catabolism
It’s a yellow pigment present in bile ( a fluid made by the liver) , urine and feces .
Heme is found in hemoglobin, a principal component of RBCs [Heme: iron + organic compound “porphyrin”].
Heme source in body:
80% from hemoglobin
20% other hemo-protein: cytochrome, myoglobin)

3. Heme and bilirubin
Heme four pyrrols rings connected together to form (porphyrin).
Bilirubin consists of open chain of four pyrrols-like rings

4. Hemoglobin degrading and bilirubin formation
Spleen
Plasma
Protein and a.a pool
Iron pool
TO LIVER
Liver
globin
iron
Heme
Hemoglobin
Bilirubin
Binds with albumin
Conjugation
process

5. Transport of Bilirubin in Plasma
Bilirubin on release from macrophages circulates as unconjugated bilirubin in plasma tightly bound to albumin.
Albumin + free Bilirubn Bilirubin ~ Albumin Complex
unconjugated bilirubin
Why bound to albumin?
Significance:
★Increase the solubility of whole molecule
★ Prevent unconjugated bilirubin freely come into other tissue, cause damage.

6. Transport of Bilirubin in Plasma
Molar
Ratio
Bilirubin
H affinity binding sites
2:1
Bilirubin
Plasma protein
Albumin
L affinity binding sites
>2:1
can be replaced by
Other organic anions
PH
UB
Albumin has two binding sites for bilirubin---a high affinity site and a low affinity site.

7. Bilirubin formed in peripheral tissues is transported to liver by albumin
IN LIVER:
1)Uptake of bilirubin by liver paranchmal cells
2)conjugation of bilirubin with glucuronate in endoplasmic reticulum
3)secretion of conjugated bilirubin into bile

9. Uptake of bilirubin by liver
Bilirubin is only sparingly soluble in water
İts solubility in plasma is increased by noncovalent binding to albumin
Albumin has one high affinity site and one low affinity site for bilirubin
In 100 ml plasma = 25 mg bilirubin can be tightly bound to albumin at high affinity site

10. Antibiotics and other drugs compete with bilirubin to bind to high_affinity site of albumin.
In liver, bilirubin is removed from albumin and taken up at the sinusoidal surface of hepatocytes by a facilated transport system.
In the hepatocytes bilirubin binds to cytosolic proteins :
ligandin (a family of glutathione s-transferases) and protein γ_to be kept solubilized prior to conjugation.

11. Conjugation of bilirubin with glucuronic acid
Bilirubin is non-polar.
Hepatocytes convert bilirubin to a polar form by adding glucuronic acid to it (conjugation)
Enzyme: glucuronosyl transferase
Location:endoplasmic reticulum
Glucuronyl donor:UDP-GLUCURONIC ACID

13. Bilirubin monoglucuronide is an intermediate, subsequently converted to diglucuronide.
Phenobarbital induces UDP-glucoronoyl-transferase activity

15. Major differences between unconjugated and conjugated bilirubin
FEATURE
Unconjugated bilirubin
CONJUGATED BILIRUBIN
Normal serum level
More
Less (less than 0.25mg/dl)
Water solubility
Absent
Present
Affinity to lipids (alcohol solubilty)
Serum albumin binding
High
Low
Van den Bergh reaction
Indirect (Total minus direct)
Direct
Reanal excretion
Affinity to brain tissue
Present (kernicterus)

16. Excretion of bilirubin into bile
Bilirubin diglucuronide is actively transported against a concentration gradient into bile duct.
This energy-dependent, rate –limiting step is susceptible to impairment in liver disease. Uncojugated bilirubin is normally not excreated.

18. Formation of urobilins in the intestine
Bilirubin diglucuronide is hydrolysed and reduced by bacteria in the gut to yield urobilinogen, a colorless compound.
Most of the urobilinogens of the feces are oxidized by intestinal bacteria to stercobilin, which gives stools their characteristic brown color.
Some urobilinogen is reabsorbed from the gut into the portal blood and transported to the kidney, where it is converted to the yellow urobilin and excreted, giving urine its characteristic color. (bilinogen enterohepatic circulation)
Bilirubin diglucuronide
urobilinogen
urobilin
stercobilin
bilin

19. Catabolism of hemoglobin
BLOOD
CELLS
Stercobilin
excreted in feces
Urobilinogen
formed by bacteria
Urobilin
excreted in urine
Heme
Globin
Hemoglobin
KIDNEY CO
Biliverdin IX
Heme oxygenase O2
reabsorbed
into blood
INTESTINE
via bile duct to intestines
Bilirubin
(water-insoluble)
NADP+
NADPH
Biliverdin
reductase
Bilirubin diglucuronide
(water-soluble)
2 UDP-glucuronic acid
Bilirubin
(water-insoluble)
via blood to the liver
LIVER
unconjugated
Catabolism of hemoglobin

20. Summary of bilirubin metabolism
red cells are major source of hemeproteins
Breakdown of heme to bilirubin occur in macrophage of reticuloendithelial system ( tissue macrophages, spleen and liver).
Unconjugated bilirubin is transported through blood ( complex to albumin) to liver.
Bilirubin is taken into liver and conjugate with glucuronic acid.
Bile is secreted into intestine where glucuronic acid is removed and the resulting bilirubin is converted to urobilinogen.
A portion of urobilinogen is reabsorbed into blood, where it is converted to the yellow urobilin and excreted by kidneys.
Urobilinogen is oxidized by intestinal bacteria to the brown stercobilin.