Bilirubin Metabolism and Excretion

Trace bilirubin metabolism from senescent erythrocyte destruction through fecal and urinary elimination

Senescent RBCs → heme → biliverdin → unconjugated bilirubin → albumin-bound transport → hepatic uptake → glucuronic acid conjugation → conjugated bilirubin → bile → intestine → urobilinogen

urobilinogen then has multiple fates

• enterohepatic circulation

• kidney → urobilin → urine

• intestinal material → stercobilin → feces

What is the immediate precursor of bilirubin during heme degradation

biliverdin

which enzyme catalyzes heme → biliverdin

heme oxygenase

what products are released during the heme oxygenase reaction

biliverdin

Fe²+

CO

with O2 and NADPH involved in the reaction

Which enzyme converts biliverdin → bilurubin

biliverdin reductase

What reducing equivalent is used during conversion of biliverdin to bilirubin

NADPH + H+

Immediately after macrophage production, is bilirubin conjugated or unconjugated

unconjugated bilirubin (UCB)

Why does unconjugated bilirubin travel through blood bound to albumin

UCB is poorly water soluble, so it circulates as bilirubin-albumin complex until delivered to the liver

What happens to the bilirubin-albumin complex when it reaches the liver

bilirubin is taken up by the hepatocyte, where is can undergo conjugation with glucuronic acid

By what transport mechanism does hepatic uptake of bilirubin represent

facilitated diffusion

Why is unconjugated bilirubin also called “indirect bilirubin”

unconjugated bilirubin = indirect bilirubin

conjugated bilirubin = direct bilirubin

this distinction becomes crucial when interpreting jaundice labs

What enzyme conjugates bilirubin in hepatocytes

bilirubin UDP - glucuronosyltransferase (UDPGT)

What molecule is added to bilirubin during hepatic conjugation

glucuronic acid

what activated glucuronic acid donor is in the bilirubin conjugation pathway

UDP-glucuronic acid

How many glucuronic acid equivalents are ultimately attached to form the major conjugated products

2, producing bilirubin diglucuronide

What is the major functional consequence of conjugating bilirubin with glucuronic acid

it makes bilirubin sufficiently water soluble/polar for excretion into bile

a mutation eliminates UDGPT activity, which bilirubin fraction should accumulate, direct or indirect

indirect/ unconjugated bilirubin

Without UDGPT: UCB cannot be conjugated → UCB accumulates → unconjugated hyperbilirubinemia

this is the central defect in Crigler-Najjar syndrome type I

A patient conjugates bilirubin normally, but cannot efficiently secrete conjugated bilirubin into bile, which bilirubin fraction should increase

conjugated/direct bilirubin

the problem occurs after conjugation, so conjugated bilirubin accumulates and can regurgitate into blood

what happens to conjugated bilirubin after hepatocytes process it normally

conjugated bilirubin → actively secreted into bile → enters intestine

What do intestinal bacteria do to conjugated bilirubin

they remove glucuronic acid, and the resulting bilirubin is converted into urobilinogen

which bilirubin-derived compound is the major branching point between enterohepatic recycling, urinary excretion and fecal elimination

urobilinogen

What happens to intestinal urobilinogen that is reabsorbed

it enters portal blood and participates in the enterohepatic urobilinogen cycle

What happens to the portion of urobilinogen transported to the kidney

it is converted to yellow urobilin and excreted in urine

which bilirubin metabolite contributes to the characteristic yellow color of urine

urobilin

which bilirubin metabolite contributes to the characteristic brow color of feces

sterocobilin

what produces stercobilin from urobilinogen

intestinal bacteria oxidize urobilinogen → stercobilin

a patient has complete obstruction preventing conjugated bilirubin from reaching the intestine, why might the stool become pale

No CB reaches intestine

→ decreased intestinal bilirubin metabolism

→ decreased urobilinogen

→ decreased stercobilin

→ loss of normal brown fecal pigmentation

→ pale/clay colored stool

what is the reference value for unconjugated/indirect bilirubin

<1.0 mg/dL

what is the reference value for conjugated/direct bilirubin

<0.2 mg/dL

What is the reference value for total bilirubin

<1.2 mg/dL

At what total bilirubin level would hyperbilirubinemia be described

>1.2 mg/dL

Why does serum bilirubin concentration represent a balance rather than simply bilirubin production

the measured concentration reflects the relationship between:

bilirubin production ← → bilirubin processing/excretion

thus, hyperbilirubinemia can result from excess production OR impaired hepatic handling/exretion

define cholestatsis

impaired bile flow, resulting in increased concentrations of bilirubin, bile acids, cholesterol in blood

Which bilirubin fraction is responsible for kernicterus

unconjugated bilirubin

Why is severe nonconjugated hyperlipidemia neurologically dangerous in a newborn

at sufficiently high concentrations, UCB can enter the CNS and cause bilirubin - induced neurologic toxicity (kernicterus)

Which brain region does the neonatal section specifically identify as vulnerable to excessive UCB

basal ganglia

what severe UCB concentration range is associated with kernicterus

severe levels are around 15-20 mg/dL or higher, with neonatal pathology noting that UCB exceeding albumin capacity at approximately 2-=25 mg/dL can enter the basal ganglia

What are the 4 broad categories of jaundice

1. prehepatic

2. hepatic

3. post hepatic

4. neonatal

What is the central pathophysiologic problem in pre-hepatic jaundice

excessive bilirubin production before hepatic processing, classically from hemolysis

what is the central pathophysiologic problem in hepatic jaundice

abnormal hepatocyte function, impairing one or more of the following

• bilirubin uptake

• conjugation

• secretion/excretion

What is the central pathophysiologic problem in post-hepatic jaundice

bilirubin has been conjugated normally, but the bile flow is obstructed after hepatic processing, preventing CD from reaching the intestine

Why does extensive hemolysis predominately increase unconjugated bilirubin

increased RBC destruction

→ increased heme breakdown

→ increased bilirubin production

→ bilirubin is generated faster than the liver can conjugate it

→ increased UCB

Is the liver intrinsically dysfunctional in most cases of hemolytic jaundice

NO

the problem is excessive bilirubin load

List disorders as potential causes of hemolytic/pre-hepatic jaundice

• sickle cell

• spherocytosis

• pyruvate kinase deficiency

• G6PD deficiency

• Malaria

• Thalassemia

Why does urobilinogen increase during hemolysis

increased hemolysis

→ increased bilirubin production

→ liver still conjugates more bilirubin

→ increased CB delivered to intestine

→ increased intestinal urobilinogen

→ increased enterohepatic/urinary urobilinogen

Why is stool generally still pigmented in hemolytic jaundice

bile flow into the intestine is not obstructed, bilirubin reaches the gut and can still generate stercobilin

Patient has jaundice, marked increased RBC destruction, elevated indirect bilirubin and increased urinary urobilinogen, where is the defect relative to the liver

pre-hepatic

What bilirubin fraction predominates in physiologic neonatal jaundice

unconjugated bilirubin

What enzymatic immaturity produces physiologic neonatal jaundice

hepatic UDGPT is not yet fully induced/developed

draw the mechanism of physiologic neonatal jaundice

immature neonatal liver

→ insufficient UDGPT activity

→ decreased bilirubin conjugation

→ increased unconjugated bilirubin

→ transient jaundice

why is physiologic neonatal jaundice generally described as transient

as hepatic conjugation capacity and UDGPT activity mature, bilirubin handling improves

What type of light is identifies as neonatal phototherapy

blue fluorescent light

does phototherapy work primarily by increasing UDGPT expression

no, it changes bilirubin itself through isomerization

what molecular change does phototherapy induce

trans-bilirubin → water-soluble cis-bilirubin isomer

why does photoisomerization help an infant whose hepatic conjugation system is immature

the resulting bilirubin isomer is more polar/water soluble, facilitating elimination despite deficient/immature normal conjugation

A neonate has elevated UCB because UDGPT is not fully developed. Why is phototherapy mechanistically useful rather than merely symptomatic?

It effectively bypasses the solubility problem by converting bilirubin into a more water-soluble isomer that can be eliminated more readily.

What pharmacologic agent is identified as increasing hepatic bilirubin metabolism

phenobarbital

Distinguish the mechanisms of phototherapy and phenobarbital

phototherapy

→ bilirubin photoisomerization

→ more water-soluble form

phenobarbital

→ increases liver metabolism

→ lowers bilirubin

What is the mechanism for breast-milk associated neonatal jaundice

a chemical in breast milk inhibits UDGPT activity/conjugation, increasing UCB

What mechanism causes neonatal jaundice from maternal-infant blood incompatibility

maternal antibodies

→ destruction of infant RBCs

→ increased hemolysis

→ increased bilirubin production

→ unconjugated hyperbilirubinemia

why is blood-group-incompatibility jaundice mechanistically different from physiologic neonatal jaundice

physiologic

→ hepatic UDGPT immaturity

→ conjugation problem

blood incompatibility

→ antibody-mediated RBC destruction

→ hemolytic/ore-hepatic bilirubin overproduction

If severe neonatal hyperbilirubinemia does not adequately respond to phototherapy, what treatment is indicated

blood exchange transfusion

What is the fundamental enzyme defect shared by Crigler-Najjjar types I and II

deficient UDGPT- mediated bilirubin conjugation

What distinguishes Crigler-Najjar type I enzymatically from type II

Type I: No UDGPT expression

Type II: reduced UDGPT expression

Which bilirubin fraction is elevated in Crigler-Najjar type I

unconjugated bilirubin

Approximately how high can UCB become in Crigler-Najjar type I

>30 mg/dL

Why is Crigler-Najjar type I potentially fatal in childhood

complete UDGPT absence

→ profound UCB accumulation

→ UCB enters CNS

→ KERNICTERUS

→ severe neurological toxicity / death

What is the treatment for Crigler-Najjar type I

• liver transplant

• exchange transfusions

Approximately what UCB level is associated with Crigler- Najjar type II

<20 mg/dL

Why is Crigler-Najjar type II less severe than type I

Type II retains some UDGPT expression, whereas type I has NONE

What is the treatment for Crigler-Najjar type II

phenobarbital

A child with severe unconjugated hyperbilirubinemia >30 mg/dL and develops kernicterus. Phenobarbital is not presented as the definitive treatment. What is the diagnosis

Crigler-Najjar syndrome type I

A patient has inherited unconjugated hyperbilirubinemia but retains reduced UDGPT expression and responds to phenobarbital. Which syndrome?

Crigler-Najjar syndrome type II

What molecular defect is associated with Gilber syndrome

mutations involving the UDGPT gene, resulting in decreased UDGPT activity

which bilirubin fraction predominates in Gilbert syndrome

unconjugated

what total serum bilirubin concentration is typical of Gilbert syndrome

<3.0 mg/dL

Why is Gilbert syndrome substantially milder then Crigler-Najjar type I

Gilbert syndrome involved DECREASED UDGPT activity, not complete absence of UDGPT expression

What treatment is recommended for Gilbert syndrome

No treatment needed

Gilbert syndrome is described as mild and transient

Rank the three UDGPT-related disorders from greatest enzymatic impairment to mildest based on the lecture.

Crigler-Najjar I — no expression

↓

Crigler-Najjar II — reduced expression

↓

Gilbert syndrome — decreased activity/mild defect

Is the fundamental defect in Dublin-Johnson syndrome failure to conjugate bilirubin

No, bilirubin is conjugated, but there is a defective SECRETION OF CONJUGATED BILIRUBIN into bile by hepatocytes

Which bilirubin fraction is elevated in Dublin-Johnson syndrome

conjugated/direct bilirubin

What gross hepatic finding is specifically associated with Dublin-Johnson syndrome

black liver due to pigment deposition

Patient has inherited direct hyperbilirubinemia and a black appearing liver, what diagnosis should immediately come to mind

Dublin-Johnson syndrome

What is the molecular mechanism for Rotor syndrome

mutations affecting two bilirubin transport proteins, impairing bilirubin transport/handling by the liver

What bilirubin pattern is associated with Rotor syndrome

increased conjugated bilirubin

increased unconjugated bilirubin

What liver histology is associated with Rotor syndrome

normal liver histology

which finding most directly separates Dublin-Johnson from Rotor: UDGPT absence, black liver, kernicterus or hemolysis

black liver → dublin- johnson

rotor is described as having normal liver histology

Why can viral hepatitis increase both direct and indirect bilirubin

hepatocyte disfunction can impair both

• conjugation → increased UCB

• Excretion/secretion → increased CB

What total bilirubin range is associated with viral hepatitis

roughly 5-10 mg/dL

Why can intrahepatic cholestasis cause conjugated hyperbilirubinemia despite intact conjugation

bilirubin is successfully conjugated

→ secretion into bile is impaired

→ CB regurgitates/leaks back into blood

→ increased serum direct bilirubin

What urine and stool findings can accompany intrahepatic cholestasis

• dark urine

• pale/clay-colored stool

In post-hepatic jaundice, has bilirubin conjugation occured normally before the obstruction

yes, the lesion is downstream of conjugation

What are some causes of extrahepatic obstruction

• gallstones

• tumor

• bile duct cancer

• biliary atresia

which bilirubin fraction predominates in complete post-hepatic obstruction

Conjugated/direct bilirubin

Why does conjugated bilirubin rise in blood during common bile duct obstruction

CB cannot enter intestine

→ backs up/regurgitates into bloodstream

→ conjugated hyperbilirubinemia

Why does urine become dark in post-hepatic obstruction

conjugated bilirubin regurgitates into blood and is subsequentially excreted in urine

Why does urinary urobilinogen disappear in complete biliary onstriction

No CB reaches intestine

→ little/no intestinal bilirubin available

→ little/no urobilinogen formation

→ urinary urobilinogen absent

Combine the classic laboratory and physical findings of the post-hepatic obstruction

• increased conjugated bilirubin

• urinary bilirubin present/increased

• urinary urobilinogen absent

• dark urine

• pale/clay colored stool

Why is the combination of dark urine and pale stool particularly useful for recognizing obstructive jaundice

dark urine = CB is diverted backward into blood → kidney excretion

pale stool = CB fails to reach intestine → decreased stercobilin

Patient has jaundice, abdominal pain, nausea, elevated direct bilirubin, urinary bilirubin present, and urinary urobilinogen is absent, what is the most likely mechanism

decreases secretion of bile into the intestine due to obstruction