1. Disturbances in Bilirubin Metabolism
Ravneet Bal, Navdeep Sahota, Ananya Garg
November 6th, 2018
PHM 142 Presentation
Instructor: Dr. J. Henderson

2. What is Bilirubin?
A yellow bile compound excreted by the liver due to the breakdown of red blood cells
The heme from the hemoglobin of senescent red blood cells is broken down into biliverdin which is then broken down into bilirubin
The tetrapyrrole molecule is conjugated in the liver using glucuronyl transferase to increase its water solubility
Conversion from the unconjugated form to the conjugated form of bilirubin occurs in the liver, thus showing why it is a central player in diseases linked to the liver such as jaundice

3. Mechanism - Heme Metabolism
Each Hb has four heme molecules that carry O2 in the body
When RBCs die, the cellular components are re-used
One of the major proteins that is reused is heme and converted to bilirubin

4. Mechanism - Bilirubin Metabolism
Unconjugated bilirubin is transported into the hepatocyte by OATP1B1, 1B3, 2B1 (Organic Anion Transporting Polypeptide)
Unconjugated bilirubin forms a complex with UDPGA (Uridine diphosphoglucuronic acid)
This complex is converted to the conjugated form of bilirubin by the enzyme UGT1A1 (UDP-glucuronosyltransferase 1A1)
Conjugated bilirubin is then transported out of liver into the bile canaliculus by MRP2 (Multidrug Resistance-associated Protein) and then into the small intestine along with other bile components

5. Mechanism - Bilirubin Excretion
In the intestinal lumen, conjugated bilirubin is then deconjugated by intestinal or bacterial beta-glucuronidase
Free bilirubin is broken down into urobilinogen by bacterial dehydrogenases
90% of urobilinogen is further oxidized into urobilin and stercobilin which are excreted in the feces
The remainder either undergoes enterohepatic circulation (re-absorbed and re-conjugated in the liver) or is excreted into the urine

6. Disruption in Mechanism Leads to Hyperbilirubinemia
Occurs when there are higher than normal levels of bilirubin present in the blood
Caused by interruptions in pre-hepatic, intra-hepatic and post-hepatic mechanism
Types:
Unconjugated Hyperbilirubinemia
Hemolysis
Gilbert’s syndrome
Crigler-Najjar syndrome (Type 1 and 2)
Conjugated Hyperbilirubinemia
Dubin-Johnson syndrome
Rotor syndrome

7. Jaundice
Condition characterized by increased levels of bilirubin in the blood (hyperbilirubinemia)
Common symptoms include yellowing of the skin and the whites of the eye, as well as the presence of dark urine
Can be fatal in infants
Commonly due to the ineffectiveness of the liver to remove bilirubin from the plasma or its inability to excrete it to the intestines from the bile duct
Main Causes:
Hemolysis
Impaired hepatic uptake (typically drug-induced)
Problem with glucuronyl transferase action
Defective bilirubin excretion
Extrahepatic obstruction

8. Pre-Hepatic Hyperbilirubinemia: Jaundice
Impaired Hepatic Uptake
Primarily drug-induced
Through protein binding or blocking of the uptake receptor, bilirubin is not able to enter the liver to be conjugated
For example, Rifampicin, an antibiotic used to treat bacterial infections
Hemolysis
Hemolysis describes the process of red blood cells rupturing and releasing its contents into the plasma
Bilirubin levels in the plasma are increased as a result, as is characteristic in patients with jaundice

9. Unconjugated Hyperbilirubinemia
Hemolysis
Increased breakdown of RBCs as a result of
Inherited RBC defects (sickle cell anemia, spherocytosis, thalassemia)
Erythroblastosis fetalis (Rh incompatibility between mother and fetus)
Acquired RBC defects (virus, certain medications)
Results in more hemoglobin converted to unconjugated bilirubin
Excess unconjugated bilirubin cannot be adequately processed by the liver causing increased concentrations in the blood
Treatment: Phototherapy

10. Intra-Hepatic Hyperbilirubinemia: Jaundice
Impaired Glucuronyl Transferase Action
Absence or deficiency in the enzyme, glucuronyl transferase, prevents bilirubin from being converted from its unconjugated form to its water-soluble conjugated form
Bilirubin is unable to be metabolized as a result in conditions such as Crigler-Najjar syndrome where activity of this enzyme is impaired or absent

11. Unconjugated Hyperbilirubinemia
Gilbert’s Syndrome
Common
Autosomal recessive
Bilirubin uptake reduced in hepatocyte
Partial deficiency in bilirubin glucuronidation
Approximately 30% reduction in UGT1A1 activity
Mild symptoms; therefore, does not require treatment
Crigler-Najjar Syndrome Type 1
Rare
Loss of UGT1A1 activity due to point mutations, deletions, or insertions in any of the five exons
Treatment: phototherapy, plasmapheresis, orthotopic liver transplantation
Crigler-Najjar Syndrome Type 2
Uncommon
Reduced UTG1A1 activity due to single amino acid substitution in the exon; therefore, milder
Treatment: phenobarbital, may require phototherapy

12. Intra-Hepatic Hyperbilirubinemia: Jaundice
Defective Bilirubin Excretion
Conjugated bilirubin is unable to efficiently be excreted from the liver
This can be due to mutations in the MRP2 receptor, as is seen in Dubin Johnson syndrome
Since the MRP2 receptor is responsible for transport of the conjugated bilirubin from the liver to the bile canaliculus, a mutation in this receptor will prevent the excretion of bilirubin into the bile

13. Conjugated Hyperbilirubinemia
Dubin-Johnson Syndrome
Rare
Autosomal recessive
Characterized by a deficiency in the ATP dependent transporter MRP2 leading to reduced expression in canalicular membrane
Increased conjugated bilirubin concentration in hepatocyte
Upregulation of MRP3 which functions to increase the transport of conjugated bilirubin back to the plasma
Mild symptoms; therefore, no treatment required
Rotor syndrome
Caused by a mutation in SLCO1B1 and SLCO1B3 genes
Leads to either defective production of transport proteins or impaired functioning of these transport proteins
Mild symptoms; therefore no treatment required

14. Post-Hepatic Hyperbilirubinemia: Jaundice
Extrahepatic obstruction
The major extrahepatic ducts can be blocked, which leads to back diffusion of the water- soluble conjugated bilirubin from leaky tight junctions, bringing it from the bile back into the blood
The blood therefore sees an increased in conjugated bilirubin levels as a result
Adapted from Gilmore and Garvey, 2013.

15. Summary
Bilirubin is a yellow bile compound excreted by the liver due to the breakdown of red blood cells
The heme from the hemoglobin of senescent red blood cells is broken down into biliverdin which is then broken down into bilirubin
Unconjugated bilirubin is transported into the hepatocyte by OATP
Unconjugated bilirubin forms a complex with UDPGA and is converted to the conjugated form by UGT1A1
Conjugated bilirubin transported out of liver into bile canaliculus by MRP2 and then into small intestine
In the intestine, conjugated bilirubin is then deconjugated and broken down to be excreted in the feces or to undergo enterohepatic circulation
Hyperbilirubinemia occurs when there are higher than normal levels of bilirubin present in the blood
Two types of hyperbilirubinemia: conjugated and unconjugated
Jaundice is a disorder characterized by increased levels of bilirubin in the blood
Common symptoms of jaundice include yellowing of the skin and the whites of the eye, as well as the presence of dark urine

16. References
Beckingham, I. J., & Ryder, S. D. (2001). ABC of diseases of liver, pancreas, and biliary system. Investigation of liver and biliary disease. BMJ (Clinical research ed.), 322(7277), 33-6.
Bosma, P. J. (2003). Inherited disorders of bilirubin metabolism. Journal of Hepatology, 38(1), 107–
Gilmore, I., & Garvey, C. J. (2013). Jaundice. Medicine, 41(2), doi: /j.mpmed
Kalakonda A, John S. (2017). Physiology, Bilirubin. StatPearls. Retrieved November 1, 2018, from
MacGill, M. (2018, July 24). High bilirubin levels: Meaning, symptoms, and tests. Retrieved from
Memon, N., Weinberger, B. I., Hegyi, T., & Aleksunes, L. M. (2015). Inherited disorders of bilirubin clearance. Pediatric research, 79(3),
Novo, C., & Welsh, F. (2017). Jaundice. Surgery (Oxford), 35(12), doi: /j.mpsur
Orfei, E. (n.d.). Bilirubin Metabolism. Retrieved November 02, 2018, from
Phototherapy for Jaundice in Newborns. (2017, May 4). Retrieved November 1, 2018, from
Roy-Chowdhury, J., Roy-Chowdhury, N., & Jansen, P. L. M. (2006). Bilirubin Metabolism and its Disorders. In Zakim and Boyer's Hepatology (pp ). Elsevier Inc.. DOI: /B
ROCHE, S. P., & Kobos, R. (2004). Jaundice in the Adult Patient. American Family Physician, 2(69), Retrieved November 1, 2018, from
“Rotor Syndrome.” Genetic and Rare Diseases Information Center, U.S. Department of Health and Human Services, rarediseases.info.nih.gov/diseases/218/rotor-syndrome.

17. References
Images
Bilirubine [Digital image]. (n.d.). Retrieved November 1, 2018, from
Defective Bilirubin Excretion [Image]. (n.d.). Retrieved November 5, 2018, from
[Enterohepatic Circulation]. (2013, August 11). Retrieved November 02, 2018, from
[Extravascular hemolysis]. (2013). Retrieved November 01, 2018, from anemia/extravascular-hemolysis-new/
Image [Jaundice depicted on child's face]. (n.d.). Retrieved November 1, 2018, from types.html
Glucoronyl transferase [Enzyme activity image]. (n.d.). Retrieved November 5, 2018, from
Cui, Y., König, J., Leier, I., Buchholz, U., & Keppler, D. (2000). Hepatic Uptake of Bilirubin and Its Conjugates by the Human Organic Anion Transporter SLC21A6. Journal of Biological Chemistry,276(13), doi: /jbc.m