Bilirubin Bilirubin metabolism metabolism and and jaundice jaundice Jayanta Roy Chowdhury Jayanta Roy Chowdhury Professor of Medicine and Molecular Professor of Medicine and Molecular Genetics Genetics Albert Einstein College of Medicine Albert Einstein College of Medicine
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Bilirubin metabolism and jaundice jaundice Jayanta Roy Chowdhury Professor of Medicine and Molecular Genetics Albert Einstein College of Medicine.
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BilirubinBilirubin metabolism metabolism andand
jaundicejaundice
BilirubinBilirubin metabolism metabolism andand
jaundicejaundice
Jayanta Roy ChowdhuryJayanta Roy Chowdhury
Professor of Medicine and Molecular GeneticsProfessor of Medicine and Molecular GeneticsAlbert Einstein College of MedicineAlbert Einstein College of Medicine
Pathophysiological importance of bilirubin metabolism
It is the end product of heme degradation.
Serum bilirubin level is an important clinical marker of hepatobiliary excretory function.
Bilirubin is an endogenous model for plasma carriage and hepatic throughput of organic anions.
Hepatic uptake, storage, conjugation and excretion of bilirubin are finely balanced. Therefore, enhancement of bilirubin throughput requires coordinated induction of multiple genes, which may be mediated by nuclear receptors.
Sources of bilirubinSources of bilirubinSources of bilirubinSources of bilirubin
Early bilirubin(15-20%)0- 3 days
Late bilirubin(65%)
40- 80 days
Non-Hbsources (liver)
Increased erythropoiesis
Erythrocytesources
Early and late labeled peaks of radioisotope incorporation into bilirubinAfter injection of labeled porphyrin precursor (14C-glycine)
Opening of the heme ring and Enzyme-catalyzed formation of bilirubin
C
NH
MV
ONH
M CH2
CH2
O OH
CH2
NH
NH
CH2
CH2
C
OOH
M MV
O
The linear structure of bilirubin:Two dipyrroles joined by a central methene bridge
C
NH
MV
ONH
M CH2
CH2
O OH
CH2
NH
NH
CH2
CH2
C
OOH
M MV
O
Bilirubin contains several polar groups (shown in red):Yet, it is insoluble in water.
C
NH
MV
ONH
M CH2
CH2
O OH
CH2
NH
NH
CH2
CH2
C
OOH
M MV
O
Water insolubility of bilirubin is explained by internal hydrogen bonding.
C
NH
MV
ONH
M CH2
CH2
O OH
CH2
NH
NH
CH2
CH2
C
OOH
M MV
O
This is explained by internal hydrogen bonding.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
This is explained by internal hydrogen bonding.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
This is explained by internal hydrogen bonding.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
As a consequence of hydrogen bonding, all polar groups are engaged.
The central methene bridge becomes buried.
C
C
Ridge-tile structure of bilirubin
Conjugation with glucuronic acid makes bilirubin water soluble
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
The internal hydrogen bonds of bilirubin are The internal hydrogen bonds of bilirubin are disrupted by conjugation of the propionic aciddisrupted by conjugation of the propionic acidcarboxyl group with glucuronic acidcarboxyl group with glucuronic acid
NH
MV
ONH
M CH2
CH2
NH
NH
CH2
CH2
C
O
M MV
O
CH2 CO-GlucA
GlucA-
The internal hydrogen bonds of bilirubin are The internal hydrogen bonds of bilirubin are disrupted by conjugation of the propionic aciddisrupted by conjugation of the propionic acidcarboxyl group with glucuronic acidcarboxyl group with glucuronic acid
Phototherapy changes the configuration of bilirubin makingit transiently water soluble
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
Internal hydrogen bonds are disrupted transiently upon exposure of bilirubin to light.
C
C
The dipyrrole carbon bridges switch direction.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
Thus a carbon atom comes in the way of the hydrogen bonds.
The dipyrrole carbon bridges switch direction.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
Thus a carbon atom comes in the way of the hydrogen bonds.
The dipyrrole carbon bridges switch direction.
C
C
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
C
C
The bulky carbon atom disrupts the hydrogen bonds by steric hindrence.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
C
C
The bulky carbon atom disrupts the hydrogen bonds by steric hindrence.
Exposure to diazo reagents resultin “direct” and “indirect” van den Burgh reaction, roughlycorresponding to conjugated andunconjugated fractions of bilirubin.
NH
MV
ONH
M CH2OH
CH2
NH
NH
CH2
CH2
C
OOH M MV
O
CH2 C
O
C
C
In unconjugatedbilirubin, the central carbonbridge is buried byhydrogen bonds. Therefore, the van den Burgh reaction
is “indirect”.
NH
MV
ONH
M CH2
CH2
NH
NH
CH2
CH2
CO
M MV
O
CH2 CO-GlucA
GlucA-
In conjugatedbilirubin, the central carbonbridge is accessible. Therefore, the van den Burgh reaction
is “direct”.
Bilirubin throughput: schema of a hepatocyteBilirubin throughput: schema of a hepatocyte
Sinusoidalsurface
Canalicularsurface
Tight junction
Liversinusoid
Fenestratedendothelium
Bilirubin circulates bound to serum albumin.
BBalb
Albumin- binding: Keeps bilirubin soluble
Prevents tissue deposi- tion.
Prevents renal excretion
Drugs that displace bilirubin from albumin may precipitate kernicterus: Sulfonamides Coumadin, etc.
Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin.
BBalb
Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin.
BBalb
Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin.
BB
alb
Bilirubin circulates bound to serum albumin. At the sinusoidal surface of hepatocytes, it dissociates from albumin.
BB
alb
Bilirubin enters through the sinusoidal surface, probably by facilitated diffusion. Uptake is energy independent and bidirectional.
BB
Bilirubin uptakeis reduced: In neonates
In cirrhosis
From drug effect: novobiocin
In some cases of Gilbert syndrome
What is the mechanism of What is the mechanism of facilitated diffusion of bilirubin?facilitated diffusion of bilirubin?
• Zucker has proposed that no transporter protein Zucker has proposed that no transporter protein is needed. is needed.
• In a recent report, organic anion transport In a recent report, organic anion transport protein 2 (oatp2) has been implicated in bilirubin protein 2 (oatp2) has been implicated in bilirubin uptake. uptake.
• However, our recent studies show that althoughHowever, our recent studies show that although oatp2 transports organic anions, such as BSP, oatp2 transports organic anions, such as BSP, it is not sufficient for bilirubin transport. it is not sufficient for bilirubin transport.
B
Inside the hepatocyte, bilirubin binds to cytosolic proteins termed ligandins, which are the same as glutathione-S- transferases (GSTs).
GSTs
B
GST bindinginhibits theefflux of bilirubin,thereby increasingits net uptake
B
GSTs
B
B
Conjugation of bilirubin with glucuronic acid is catalyzed by UGT1A1, which transfers glucuronic acid from UDP-glucuronic acid to bilirubin
GSTsUDPUDPGAGA UDPUDP
BBGAGAUGT1A1
B
Conjugation with glucuronic acid makes bilirubin water-soluble and non-toxic.
Glucuronidation is essential for biliary excretion of bilirubin.
• UGTs are ER proteins that convert many internal and UGTs are ER proteins that convert many internal and exogenous toxins to non-toxic metabolites.exogenous toxins to non-toxic metabolites.
• UGT’s are a family of enzymes concentrated in the liver.UGT’s are a family of enzymes concentrated in the liver.
• One UGT isoform, UGT1A1, conjugates bilirubin and is One UGT isoform, UGT1A1, conjugates bilirubin and is essential for its excretion. essential for its excretion.
Serum bilirubin 8-18 mg/dl:Serum bilirubin 8-18 mg/dl:Kernicterus is rare Kernicterus is rare
Serum bilirubin normal to Serum bilirubin normal to 5 mg mg/dl 5 mg mg/dl (increases during fasting, (increases during fasting, intercurrent illness, etc.intercurrent illness, etc.No cerebral toxicity.No cerebral toxicity.
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Unconjugated HyperbilirubinemiaUnconjugated HyperbilirubinemiaInherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
A disease of canalicular A disease of canalicular excretion of multiple organic excretion of multiple organic anions, but not bile salts.anions, but not bile salts.
Hepatic storage disorderHepatic storage disorder
• Inherited deficiency or abnormality of MRP2 causes Dubin-Johnson syndrome
• Biliary excretion of many organic anions, but not most bile acids, is deficient in Dubin-Johnson syndrome. Abnormality of biliary excretion causes the retention of a pigment in the liver.
• Inherited deficiency or abnormality of MRP2 causes Dubin-Johnson syndrome
• Biliary excretion of many organic anions, but not most bile acids, is deficient in Dubin-Johnson syndrome. Abnormality of biliary excretion causes the retention of a pigment in the liver.
• However, serum bilirubin is only mildly elevated (3-5 mg/dl), suggesting the existence of alternative pathways for excretion of bilirubin glucuronides.
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
• Dubin Johnson syndrome:Dubin Johnson syndrome:
• Rotor syndromeRotor syndrome
Excretory defect for Excretory defect for multiple organic anionsmultiple organic anions
Hepatic storage disorderHepatic storage disorder
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
• Dubin Johnson syndrome:Dubin Johnson syndrome:
• Rotor syndromeRotor syndrome
Benign, rare autosomal Benign, rare autosomal recessive disorder.recessive disorder.1:1300 in Sephardic Jews1:1300 in Sephardic Jews
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
• Dubin Johnson syndrome:Dubin Johnson syndrome:
• Rotor syndromeRotor syndrome
Accumulation of pigmentsAccumulation of pigments
No pigmentationNo pigmentation
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
• Dubin Johnson syndrome:Dubin Johnson syndrome:
• Rotor syndromeRotor syndrome
Injected BSP is taken up, Injected BSP is taken up, conjugated and regurgitated conjugated and regurgitated back to plasma (“double hump” back to plasma (“double hump” curve)curve)
BSP clearance is slower, BSP clearance is slower, but the double hump curve but the double hump curve is not seenis not seen
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
Inherited disorders of bilirubin metabolism causing Inherited disorders of bilirubin metabolism causing
Mixed (unconjugated and conjugated) Mixed (unconjugated and conjugated) hyperbilirubinemiahyperbilirubinemia
Urinary porphyrin excretion Urinary porphyrin excretion pattern is similar to that in pattern is similar to that in many cholestatic diseaess.many cholestatic diseaess.
500-
400-
300-
200-
100-
0-
Normal
Normal
Normal
Normal
D-J
D-J hetero
RotorRotorRotorRotor
Rotor hete
ro
Rotor hete
ro
Rotor hete
ro
Rotor hete
ro
g o
f co
pro
por
ph
yrin
per
g c
reat
inin
e
Urinary coproporphyrin excretion pattern in Urinary coproporphyrin excretion pattern in Dubin-Johnson and Rotor syndromesDubin-Johnson and Rotor syndromes
Coproporphyrin I: Hatched barCoproporphyrin III: open bar
Clinical evaluationClinical evaluationClinical evaluationClinical evaluationNormal liver enzymesNormal liver enzymesNormal bile salt levelsNormal bile salt levelsNormal liver enzymesNormal liver enzymesNormal bile salt levelsNormal bile salt levels Abnormal liver enzymesAbnormal liver enzymes
Bilirubin: nearly allBilirubin: nearly all indirect-reactingindirect-reacting
Large direct-reactingLarge direct-reacting componentcomponent
•Hepatitis riskHepatitis risk•DrugsDrugs•AlcoholAlcohol•SGPT>alk. phosSGPT>alk. phos• Pro.-time: Pro.-time: not corrected not corrected with vitamin K with vitamin K• AlbuminAlbumin
• History suggestsHistory suggests obstruction obstruction• SGPT<alk. phosSGPT<alk. phos• Pro.-time: Pro.-time: corrected with corrected with vitamin K vitamin K• CholesterolCholesterol
• Inherited disorders ofInherited disorders of bilirubin conjugation: bilirubin conjugation: Gilbert syndrome Gilbert syndrome Crigler syndrome, Crigler syndrome, types I and II types I and II
Summary and implicationsSummary and implications• Bilirubin throughput by the hepatocyte involves four discernible steps:
Process Uptake Storage Conjugation Excretion
Involvedmolecule
Unidentified GSTs UGT1A1 MRP2
• The four steps are finely balanced. Therefore,
Reduction at any step may cause hyperbilirubinemia.
Enhancement of the throughput requires induction of multiple genes, probably coordinated by nuclear receptors, such as the constitutive androstene receptor (CAR).
Thank you for your attention!Thank you for your attention!