BILIRUBIN-metabolism and medical significance. Mikko H. & Piia L.

Transcription

1 BILIRUBIN-metabolism and medical significance Mikko H. & Piia L.

2 Bilirubin Bilirubin is a yellowish pigment you can find in bile Is degraded from heme of hemoglobin Most abundant porphyrin compound in vertebrates Since mammalian erythrocytes lack nuclei they are incapable of renewal and thus self-destruct in set intervals. In humans this is about 120 days before damaged or aged erythrocytes are destroyed, mainly in the retriculoendothelial cell of the spleen, bone marrow and liver.

3 Catabolism (1/5) The catabolism of all heme proteins is carried out in the microsomal fraction of cell by heme oxygenase, EC Heme oxygenase synthesis is substrate-inducible where heme serves both as a substrate and cofactor for the reaction. Amino acids from hemoglobin s globin portion are released from the molecule and catabolized or reused for protein synthesis. The heme portion undergoes degradation that starts with mixedfunction oxidase reaction which opens the ring and converts one of the methane bridge carbons to CO.

4 Catabolism (2/5) Next, iron is released from the resulting linear tetrapyrrole and transported to storage pools in bone marrow for reuse in erythrocyte production. The iron from the heme that reaches the heme oxygenase is usually oxidized to its ferric form (hemin).

5 Conversion of one mole of heme-fe 3+ to biliverdin, CO and Fe 3+ consumes: Fe 3+ -Heme + 3 O 2 + 7e - biliverdin + CO + Fe 3+ Biliverdin Heme B

6 Catabolism (3/5) The formed tetrapyrrole is reduced to insoluble bilirubin via biliverdin by biliverdin reductase, EC , which reduces the central methylene bridge of biliverdin to a methyl group, producing yellow pigment

7 Biliverdin + NADPH + H + (partial reduction by biliverdin reductase) Bilirubin (Insoluble) + NADP + Biliverdin Insoluble Bilirubin Bilirubin is sparingly water-soluble and it must be bound to serum albumin for its transportation to liver. In blood bilirubin forms a complex with serum albumin for transporation. Albumin has low-affinity site and high-affinity site for bilirubin binding. HA can bind 25mg of bilirubin/100ml of plasma.

8 Catabolism (4/5) In the uptake to liver, bilirubin is removed from albumin and taken up at the sinusoidal surface of hepatocytes by a large capacity, saturable facilitated transport system. The net uptake of bilirubin depends on its removal by subsequent metabolism. Once bilirubin enters a cell, it is bound to cytosolic proteins such as glutathione S-transferase (ligandin) to prevent it from re-entering bloodstream. Once in liver, bilirubin is solubilized by conjugating it with two molecules of glucuronic acid. Bilirubin is nonpolar and would perisist in cells, ie. bound to lipids, if not solubilized. Conjugation with glucuronic acid converts bilirubin in to a more polar form. Bilirubin spesific UDP-glucosyltransferase, EC , of the ER catalyses the stepwise transfer of two glucosyl moieties from UDPglucoronate to bilirubin

9 Reactions Bilirubin + UDP-glucuronate (UDP-GT) UDP + Bilirubin monoglucuronate Bilirubin monoglucuronate + UDP-glucuronate (UDP-GT) UDP + Bilirubin dinoglucuronate These reactions are comparable to other glycosyltrasferase reactions with the substrate being UDP-glucoronate

10 Bilirubin diglgucuronide

11 Catabolism (5/5) Formed solubilized compound, bilirubin diglucuronide is secreted in to the bile and finally excreted via the insestine. Secretion occurs via active transport mechanism which is propably rate-limiting to for the entire hepatic bilirubin metabolism. The protein involved is a multi-specific organic anion transporter (MOAT) located on the PM of the bile canaliculi. MOAT is a member of ATPbinding cassette transporters. The hepatic transport is inducible by the same drugs that can induce conjugation of bilirubin.

12 Testing Blood test Measures level of bilirubin in blood No eating and drinking 4 h before test Patient gets possibly adviced to stop medication many drugs may change bilirubin level in blood Urine test urine collection bag normally no bilirubin in urine

13 Reason Increased urine bilirubin levels may be caused by bilary tract disease chirrosis gallstones in the biliary tract hepatitis liver disease tumors of the liver or gallbladder

14 Result high bilirubin may cause jaundice yellow skin color, eyes or mucus membranes caused by too many red blood cells dying or breaking down and going to liver liver is overloaded or damaged bilirubin from liver is unable to move into digestive tract

15 Example of diseases Crigler Najjar syndrome Gilbert s Syndrome

16 Crigler Najjar syndrome, type 1 inherited disorder bilirubin cannot be broken down enzyme normally converting bilirubin to easily removable form is in malfunction mode without enzyme bilirubin builds up in body causes jaundice and damages muscles, nerves and brains is caused by mutations in UGT1A1 gene inherited as autosomal recessive treatment is regular phototherapy for lifetime phototherapy is effective the best under age 4- skin thickness blocks the light also calcium compounds and blood transfusions have been used for some people, liver transplantation might be considered

17 Crigler Najjar syndrome, type 2 based on an attempt on bilirubin level reduction aggressive phototherapy and phenobarbital as a common treatment for severe disease intravenous fluids, albumin and calcium gluconate is possibly recommended in worst case scenario, patients are treated with plasmapheresis or liver transplantation may be more relevant with type 1, more severe version commonly, type 2 disease is recommended to be treated with longterm serum bilirubin level reduction can be made with phenobarbital continued administration

18 Gilbert s Syndrome caused by genetic mutation only 30% of UGT1A1 enzyme is produced only enzyme detoxifying bilirubin enzymatic shortage causes excess of bilirubin (as well as other toxins) in serum most Gilbert s Syndrome patients have mutations also in UGT1A6 and UGT1A7 enzymes causes related toxins circulate in body for longer time

19 Jaundice (Icterus) Pre-hepatic Hepatocellular Post-hepatic Neonatal jaundice

20 Pre-hepatic jaundice Caused by hemolysis increasing Increased bilirubin production leads to boost on urine-urobilinogen production Bilirubin does not usually exists in urine for the reason unconjugated bilirubin is not hydrophilic

21 Hepatocellular jaundice may be caused by several diseases, for example alcoholic liver disease, hepatitis or drug-induced hepatitis. Necrosis of liver cells leads to suppressed ability on bilirubin metabolism and excretion unconjugated bilirubin keeps entering liver cells conjugates as usually and returned into blood bilirubin in plasma is becoming more and more conjugated type eventually, more conjugated than unconjugated type occurs no urobilinogen gets formed and urine turns dark

22 Post-hepatic jaundice Caused by interruption to the bile drainage caused by, for example, gallstones No urobilinogen is found in urine

23 Neonatal jaundice Usually harmless, found commonly in infants during second day after birth Lasts approximately until day 8 or 24 Higher bilirubin levels causes brain damage More common for reason less time is spent outdoors Sunbathing as a treatment UV-B (leads also to production of vitamin D)

24 References GARD (2011) Crigler Najjar syndrome, type 1 [online] National Center for Advancing Translational Sciences [cited: ] Available at: GARD (2011) Crigler Najjar syndrome, type 2 [online] National Center for Advancing Translational Sciences [cited: ] Available at: Gilbert s Syndrome.com, Gilbert s Syndrome [online] [cited: ] Available: MedlinePlus (2015) Bilirubin blood test [online]. U.S. National Library of Medicine. [cited: ] Available: MedlinePlus (2015) Bilirubin-urine [online]. U.S. National Library of Medicine. [cited: ]Available: MedlinePlus (2015) Crigler-Najjar syndrome [online]. U.S. National Library of Medicine. [cited: ]Available at: MedlinePlus (2015) Jaundice [online]. U.S. National Library of Medicine. [cited: ] Available: Wikipedia (2016) Jaundice [online]. Wikimedia Foundation, Inc. [cited: ] Available: Rodwell, Bender (2015) Harper s Illustrated Biochemistry, 30 th edition Matthews, Van Holde (2013) Biochemistry, 4 th edition