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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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