Bilirubin Metabolism and Excretion: 8/23

Question 1

Enteroheptatic circulation of Bile

Answer 1

• Liver metabolizes and excretes into bile compounds and toxins.
• Important Components of Bile: Bilirubin, Bile Acids/Salts, Cholesterol
• majority of bile salts are recycled between the liver and small intestine through this circulation

Question 2

Bile Metabolism Overview

Answer 2

1- Phagocytic RBCs convert heme –>biliverdin –> Bilirubin (water-insoluble/unconjugated)

2. bilirubin is secreted into blood and is bound by albumin
3. It is picked up by liver hepatocytes, and uncojugated bilirubin is stripped of albumin and conjugated by UDP-GT (2 UDP glucuronic acid)
4. conjugated bilirubin is then secreted into the bile canaliculus as part of bile and is delivered to small intestine
5. bacteria in small intestine metabolize bilirubin, and it is excreted in feces (stercobillin) or is reabsorbed and excreted in urine (urobillin).

* most is excreted in feces.

Question 3

Bilirubin

Answer 3

• unconjugated/water insoluble

toxic breakdown product of Hb and generated in large quantities

• heme–>biliverdin–>bilirubin (in phagocyte)–> bilirubin+albumin –> hepatocyte where bilirubin is conjugated and excreted in bile.
• Biliberdin is converted to bilirubin by biliverdin reductase
• orange/yellow color

Question 4

Bilverdin

Answer 4

Hemoglobin –>Heme –> bilverdin–>bilirubin (in red blood cells)

• Heme is converted to biliverdin by Heme Oxygenase
• green color

Question 5

Conversion of Heme–> Bilirubin

Answer 5

• two step reaction: catalyzed by heme oxygenase and biliverdin reductase
• takes place in hepatocytes

Question 6

UDP- glucoronate

Answer 6

• necessary to convert bilirubin (water insoluble/unconjugated) in the liver to bilirubin diglucoronide (water soluble/conjugated).
• UDP-Glucoronyl Transferase (UDP-GT) is the enzyme that accomplishes this.
• Bilirubin diglucoronide can then be released into the bile and transported to the small intestines and excreted or reabsorbed in the blood to be excreted in the kidneys

Question 7

UDP- glucuonyl transferase

Answer 7

• Catalyzes the conjugation of Bilirubin –>Bilirubin Diglucoronide
• requires 2 UDP-Glucoronate molecules
• if this enzyme is defiicient –> high levels of unconjugated bilirubin in blood –> Jaundice

Question 8

bilirubin diglucuronate

Answer 8

Conjugated/Water soluble bilirubin that can be transported via bile to the small intestine to be excreted as (stercobillin) or can be reabsorbed into blood to be excreted in kidney as urine (urobilin)

Question 9

conjugated/direct vs unconjugated/indirect

Answer 9

Unconjugated/indirect = no sugar on it –> not water soluble

Conjugated/Direct = bilirubin + 2 sugars (glucocoronate) –> water soluble

• conjugation of bilirubin to bilirubin diglucoronide is necessary to safely excrete bilirubin in the fesces/urine.

Question 10

urobilinogen

Answer 10

Bilirubine diglucuronate in bile –> bilirubin diglucoronate in gut –> urobilinogen

• urobilinogen (colorless) leaks into blood and is excreted into urine as urobilin (yellow color)

Question 11

urobilin

Answer 11

• When Urobilinogen leaks into blood, it is excreted into urine as urobilin

Question 12

stercobilinogen

Answer 12

• The bulk of urobilinogen remains in the colon. The gut flora adds electrons to urobilinogen and makes it stercobilinogen (colorless)
• stercobilinogen + O2 –> Stercobilin (dark brown)
• If bile duct is obstructed: means high levels of bilirubin: results in clay-colored stools

Question 13

stercobilin

Answer 13

Stercobilinogen + O2 (clear) = Stercobilin (dark brown)

-excreted in feces

Question 14

Overview of Bilirubin Metabolism and Elimination

Answer 14

1. Normal bilirubin production from heme (0.2–0.3 gm/day) is derived primarily from the breakdown of senescent circulating erythrocytes
2. Extrahepatic bilirubin is bound to serum albumin and
   delivered to the liver.
3. Hepatocellular uptake and
4. Glucuronidation in the endoplasmic reticulum generate
   bilirubin monoglucuronides and diglucuronides, which are water soluble (conjugated/direct) and readily excreted into bile. Catalyzed by enzyme uridyldiphosphate glucuronyl
   transferase (UDPGT).
5. Gut bacteria deconjugate the bilirubin and degrade it to colorless stercobiliogen. The oxidized stercobilinogen called stercobili are excreted in the feces (dark brown color)
6. A small portion is reabsorbed and recycled back into bile
7. Another small portion urobilinogen is oxidzed to urubilins in blood and is filtered and excreted by kidney in urine (light yellow color)

Question 15

Diseases related to high amounts of indirect bilirubin….

Answer 15

• high indirect bilirubin = hemolytic disease/anemia (massive hemolysis, liver cannot handle it). Could be the result of G6PD deficiency, PK deficiency, or Vit K toxicity
  1. crigler-Najjar syndrome
  2. gilbert syndrome:
  3. kernicterus

Question 16

Diseases Related to high amounts of DIRECT bilirubin…

Answer 16

• increased direct bilirubin is due to intra/extrahepatic bile duct obstruction, gall stone, or tumor, or direct canalicular transport
• direct bilirubin is water soluble and spills into blood and kidney and in urine–> urine being a dark brown color
• urinary bilirubin (pathological) is not the same as urobilinogen (urobilin)

Question 17

Crigler-Najar Syndrome

Answer 17

• due to high amounts of indirect bilirubin
• gene for enzyme glucuronyl transferase (UDPGT) is defective, cannot conjugate bilirubin
• rare genetic condition in children –> severe congenital jaundice
• bilirubin is greater than 20-50mg/dl
• results in death (6mos - 1yr)

Question 18

Gilbert Syndrome

Answer 18

• increased indirect bilirubin. slightly 2-3mg/dl
• does not cause jaundice, benign syndrome
• gene for glucuronyl Transferase (UDPGT) has a point mutation, works fine, but not optimally
• common, 6-8% of population

Question 19

Dubin-Johnson Syndrome

Answer 19

• due to increased direct bilirubin
• chronic benign, defective canalicular transport –> black coloration of liver
• moderate disease with brown urine
• has jaundice, but lives a normal life

Question 20

Rotor Syndrome

Answer 20

• due to high amounts of direct bilirubin
• RARE form of asymptomatic direct hyper-bilirubinemia
• moderate, multiple defects in hepatocellular uptake and excretion of bilirubin pigment
• problem with storage of bilirubin in liver cells resulting in leakage of bilirubin in blood
• LIVER is NORMAL , has moderate jaundice, lives a normal life

Question 21

Kernicterus

Answer 21

• condition in newborn due to massive destruction of RBC’s. When child is born, most of the UDPGT enzyme has not been produced yet.
• high concentraion of unconjugated/indirect bilirubin (exceeding 15-20mg/dl) in newborn blood.
• Bilirubin enters CNS–>mental retardation, motor dysfunction, brain damage, cerebral palsy
• bilirubin crosses blood brain barrier through use of albumin

Question 22

Jaundice/Icterus

Answer 22

• results of excess bilirubin in blood–> yellow skin and eyes
• Jaundice=hyperbilirubinemia with increased bilirubin >2.0mg/dl
• Icterus: bilirubin >2.5 mg/dl –> yellow coloration of scler/skin

*bilirubin is an excretion product, there are not normal levels of it

Question 23

Hyperbilirubinemia

Answer 23

Icterus/Jaundice: the result of excessive quantities of uncojugated or conjugated bilirubin accumulated in exctracellular fluid.

–> Kernicterus= excessive [] of conj. bilirubin

• yellow discoloration of skin and yellow in sclera and mucous membranes
• Normal Blood Bilirubin Values: Unconjugated <1 mg/dl, conjugated <.2 mg/dl = 1.2 mg/dl total

Hyperbilirubinemia = above normal level 1.2 mg/dl

• there is a greater amount of unconjugated due to the fact that it takes a long time for liver to conjugate bilirubin, and then it is almost immediately excreted

Question 24

Diseases due to increased DIRECT and INDIRECT bilirubin…..

Answer 24

• due to liver malfunction/damage
• cirrhosis, alcoholic cirrhosis, hepatitis virus

Question 25

Pre-Hepatic Jaundice

Answer 25

• excessive bilirubin presented to liver for metabolism and capable of excreting.
  -overcomes ability of liver to clear.
  -in most cases, liver function is normal.
  Causes: hemolytic process
  Lab findings:
  -↑ in serum unconjugated bilirubin.(Total bili usually does not exceed 5 mg/dL)
  -negative urine bilirubin
  -negative urinary urobilinogen

Question 26

Hepatic Jaundice

Answer 26

-abnormal hepatocyte function, cannot deal with normal load of bilirubin

Cause: enzyme mutation/impaired hepatocellular uptake (Gilbert’s syndrome)
-findings: serum total bilirubin < 3.0 mg/dL, primarily composed of unconjugated
bilirubin, ↑ urinary bilirubin

Cause: enzyme mutation/defective conjugation (Crigler-Najjar type I syndrome)
-findings: serum unconjugated bilirubin often > 5.0 mg/dL; ↑ urinary bilirubin

Cause: defective secretion by hepatocyte (Dubin-Johnson syndrome)
-findings: ↑ serum conjugated bilirubin

Cause: hepatitis with lowered conjugation or excretion
-findings: ↑ serum direct and indirect bilirubin with total levels of 5-10 mg/dL.

Question 27

Posthepatic Jaundice

Answer 27

Posthepatic- impaired excretion of bilirubin
Cause: mechanical obstruction of the flow of bile into the intestines due to gallstones or tumors
-findings:
↑ serum AND urine conjugated bilirubin
↓ level of urobilin/stercobilin in stool (clay-colored stools)
Negative urinary urobilinogen

Question 28

Neonatal Hyperbilirubinaemia… Why do newborns have jaundice?

Answer 28

Why do newborns have jaundice? At birth massive destruction of RBC’s is due to wrong type of Hb. HbA and HbF (Hbf doesn’t respond to 2,3 BPG) at birth all the HbF have to be destroyed and replaced with HbA. At birth massive RBC’s destruciton –> bilirubin goes to liver to be conjugated.

Very high levels of bilirubin at birth may be caused by: neonatal hyperbilirubinaemia, at birth the enzyme UDP-GT is induced only at birth so the enzyme expression is not enough to break down or conjugate all the bilirbuin–> this gives rise to unconjugated bilirubinimia and jaundice

Photo therapy works – bilirubin with all those unconjugated double bonds absorb light and will fragment into water soluble fragments and be excreted into the urine.