Metabolism of bilirubin & Jaundice

1. Jaundice
& Bilirubin
Lecture17 by Dr Mohammad Manzoor Mashwani
BKMC Mardan
Metabolism
Jaune (French word)- means Yellow
ICTERUS
Hematoidin

2. Definition of Jaundice
Bilirubin pigment has high affinity for elastic tissue and hence jaundice isparticularly noticeable in tissues rich in elastin content.
A rise of serum bilirubin between the normal and 2 mg/dl is generally not
accompanied by visible jaundice and is called latent jaundice.

3. Category
Pre-hepatic/ hemolytic
Hepatic/ hepatocellular
Post-Hepatic/ cholestatic
Unconjugated
Conjugated
(1) Production of bilirubin;
(2) Uptake;
(3) Conjugation;
(4) Excretion
(5) Bile flow.
Jaundice Mechanisms:
Jaundice occurs when the equilibrium between bilirubin production and clearance is disturbed.

4. Hepatic bile serves two major functions:
• I. Bile constitutes the primary pathway for the
elimination of bilirubin, excess cholesterol, and
xenobiotics (foreign substance) that are insufficiently water
soluble to be excreted in the urine.
• II. Secreted bile salts and phospholipid molecules
promote emulsification of dietary fat in the lumen
of the gut.
Jaundice results from the retention of bile.
Emulsification is the breakdown of large fat globules into smaller, uniformly
distributed particles. It is accomplished mainly by bile acids in the small intestine.
Emulsification is the first preparation of fat for chemical digestion by specific
enzymes.

5. The metabolism of bilirubin by the liver consists of four events:
1. Uptake (by the Liver) from the circulation;
2. Intracellular storage (Cytosolic protein binding and delivery to the endoplasmic reticulum.);
3.Conjugation with glucoronic acid;
4.Biliary excretion.
Hepatocellular processing of bilirubin involves the following sequence:
1. Carrier-mediated uptake at the sinusoidal membrane (Hepatocellular uptake).
2. Cytosolic protein binding and delivery to the endoplasmic reticulum.
3. Conjugation with one or two molecules of glucuronic acid by bilirubin uridine
diphosphate–glucuronosyl transferase.
4. Excretion of the water-soluble, nontoxic bilirubin glucuronides into bile.
Normal metabolism of bilirubin can be conveniently described under 4 main
headings—source, transport, hepatic phase and intestinal phase.

6. Bilirubin metabolism and elimination.
Bilirubin is the end product of heme degradation.
• The majority of daily production (0.2 to 0.3 gm, 85%)
is derived from breakdown of senescent red cells by
the mononuclear phagocytic system, especially in the spleen, liver, and bone marrow.
Most of the remainder (15%) of bilirubin is derived from the
turnover of hepatic heme or hemoproteins (e.g., the P-450 cytochromes, muscle myoglobin) and
from premature destruction of red cell precursors in the bone marrow (Ineffective
erythropoises)
Excessive destruction of erythroid progenitors in the bone marrow due to intramedullary
apoptosis (ineffective erythropoiesis) is an important cause of jaundice in hematologic
disorders.

7. Bilirubin metabolism and elimination.
• Intracellular heme oxygenase oxidizes heme to biliverdin (step1),
which is immediately reduced to bilirubin by biliverdin reductase.
Bilirubin thus formed outside the liver is released and bound to serum albumin (step 2).
Albumin binding is necessary to transport bilirubin because bilirubin is virtually insoluble in aqueous solutions at physiologic pH.
Hepatic processing of bilirubin involves carrier-media uptake at the sinusoidal membrane (step
3),
Conjugation with one or two molecules of glucuronic acid by bilirubin uridine
diphosphate (UDP)–glucuronyltransferase (UGT1A1, step 4) in the endoplasmic reticulum, and
excretion of the water-soluble, nontoxic bilirubin glucuronides into bile.
Most bilirubin glucuronides are deconjugated in the gut lumen by
bacterial β-glucuronidases and degraded to colorless urobilinogens (step 5).
5 STEPS

8. Bilirubin metabolism and elimination.
• The urobilinogens and the residue of intact pigment are largely excreted in
feces.
Approximately 20% of the urobilinogens formed are reabsorbed in the ileum and
colon, returned to the liver, and re-excreted into bile.
A small amount of reabsorbed urobilinogen is excreted in the urine.
Conjugated and unconjugated bile acids also are reabsorbed in the ileum and
returned to the liver by the enterohepatic circulation.
Stercobilin= stool +Yellow color
Urobilin= Urine + Yellow color

9. C34H32O4N4Fe
Heme B
Spleen Macrophage
5 steps
Prehepatic
Hepatic
Posthepatic
Sinusoid- Tiny endothelium-lined passages for blood in the tissue of an organ.
The perisinusoidal space (or space of Disse) is a location in the liver between
a hepatocyte and a sinusoid.

10. Spleen
Bilirubin (Hematoidin) is the potentially toxic
catabolic product of heme metabolism.
MPS/ RES
Mononuclear Phagocyte system/Reticuloendothelial system
Erythrocytes
generated in the bone
marrow are disposed
of in the spleen when
they get old or
damaged. This
releases hemoglobin,
which is broken
down to heme as the
globin parts are
turned into amino
acids. The heme is
then turned into
unconjugated
bilirubin in the
reticuloendothelial
cells of the spleen.

11. Bilirubin metabolism and elimination.
(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 and readily
excreted into bile.
(5) Gut bacteria deconjugate the bilirubin and degrade
it to colorless urobilinogens. The urobilinogens and
the residue of intact pigments are excreted in the
feces, with some reabsorption and excretion into
urine.

12. Pathophysiology of Jaundice
• Both unconjugated (Indirect, free) bilirubin and
conjugated bilirubin (Direct, bilirubin glucuronides, combine with glucoronic acid)
may accumulate systemically.
• There are two important pathophysiologic
differences between the two forms of bilirubin:
• 1. Solubility in water &
• 2. Binding with Albumin

13. Unconjugated/Indirect/Free bilirubin
• Unconjugated bilirubin is virtually (almost)
insoluble in water (due to intramolecular hydrogen bonding) at physiologic
pH and exists in tight complexes with serum
albumin. This form cannot be excreted in the urine
even when blood levels are high.
Serum bilirubin estimation is based on van den Bergh diazo reaction by
spectrophotometric method. Water-soluble conjugated bilirubin gives direct van
den Bergh reaction with diazo reagent within one minute, whereas alcohol-soluble
unconjugated bilirubin is determined by indirect van den Bergh reaction. The
unconjugated bilirubin level is then estimated by subtracting direct bilirubin value
from this total value. However, unconjugated bilirubin also reacts slowly with
diazosulfanilic acid, so that the measured indirect bilirubin is an underestimate of
the true unconjugated concentration.
Pathophysiology of Jaundice
Alcohol soluble & water insoluble

14. Pathophysiology of Jaundice
Unconjugated bilirubin & Kernicterus
• Normally, a very small amount of unconjugated
bilirubin is present as an albumin-free anion in
plasma. This fraction of unbound bilirubin may
diffuse into tissues, particularly the brainin
infants, and produce toxic injury.

15. Pathophysiology of Jaundice
Unconjugated bilirubin
• The unbound plasma fraction may increase in
severe hemolytic disease or when protein-binding
drugs (sulfonamides, salicylates). displace bilirubin from
albumin. Hence, hemolytic disease of the newborn
(erythroblastosis fetalis) may lead to accumulation
of unconjugated bilirubin in the brain, which can
cause severe neurologic damage, referred to as
kernicterus.

16. Pathophysiology of Jaundice
Conjugated Bilirubin (Direct Bilirubin)
• Conjugated bilirubin is water-soluble, nontoxic,
and only loosely bound to albumin. Because of its
solubility and weak association with albumin,
excess conjugated bilirubin in plasma can be
excreted in urine.

17. Pathophysiology of Jaundice
Conjugated Bilirubin
With prolonged conjugated hyperbilirubinemia, a
portion of circulating pigment may become
covalently bound to albumin; this is termed the
bilirubin delta (biliprotein) fraction.

18. • Although the terms direct and indirect bilirubin are used
equivalently with conjugated and unconjugated bilirubin,
this is not quantitatively correct, because
• the direct fraction includes both conjugated
bilirubin and δ bilirubin.
• Furthermore, direct bilirubin tends to
overestimate conjugated bilirubin levels due to
unconjugated bilirubin that has reacted with diazosulfanilic acid,
leading to increased azobilirubin levels (and increased direct bilirubin).

20. Pathophysiology of Jaundice
• Serum bilirubin levels in the normal adult vary
between 0.3 and 1.2 mg/dL, and
• the rate of systemic bilirubin production is equal
to the rates of
• HEPATIC UPTAKE,
• CONJUGATION, and
• BILIARY EXCRETION.
Pathogenesis

21. Pathophysiology of Jaundice
• Jaundice becomes evident when the serum
bilirubin levels rise above 2.0 to 2.5 mg/dL; levels
as high as 30 to 40 mg/dL can occur with severe
disease.

22. Jaundice occurs when the equilibrium between
bilirubin production and clearance is disturbed by
one or more of the following mechanisms :
(1) Excessive extrahepatic production of bilirubin;
(2) Reduced hepatocyte uptake;
(3) Impaired conjugation;
(4) Decreased hepatocellular excretion; and
(5) Impaired bile flow.
The first three mechanisms produce unconjugated hyperbilirubinemia, and the
latter two produce predominantly conjugated hyperbilirubinemia.
3 types: pre-hepatic (haemolytic), hepatic, and post-hepatic
cholestatic.

23. Neonatal Jaundice
Because the hepatic machinery for conjugating and
excreting bilirubin does not fully mature until
about 2 weeks of age, almost every newborn
develops transient and mild unconjugated
hyperbilirubinemia, termed neonatal jaundice or
physiologic jaundice of the newborn.
This may be exacerbated by breastfeeding, as a result of the presence of bilirubin-
deconjugating enzymes in breast milk. Nevertheless, sustained jaundice in the
newborn is abnormal.

24. Cholestasis
• Cholestasis is defined as systemic
retention of not only bilirubin but also
other solutes eliminated in bile
(particularly bile salts and
cholesterol).
Cholestasis denotes a pathologic condition of impaired bile formation and bile
flow, leading to accumulation of bile pigment in the hepatic parenchyma.
It can be caused by extrahepatic or intrahepatic obstruction of bile channels, or
by defects in hepatocyte bile secretion.
Extrahepatic biliary obstruction frequently is amenable to surgical correction. By contrast, cholestasis caused by diseases of the intrahepatic biliary
tree or hepatocellular secretory failure (collectively termed intrahepatic cholestasis) cannot be treated surgically, and the patient’s condition may be
worsened by an operative procedure. Thus, there is some urgency in identifying the cause of jaundice and cholestasis.

26. Clinical features
• Jaundice, pruritus, skin xanthomas (focal accumulation of
cholesterol), or symptoms related to intestinal
malabsorption, including nutritional deficiencies of the fat-soluble
vitamins A, D, or K.A characteristic laboratory finding is elevated serum alkaline phosphatase and γ-glutamyl transpeptidase (GGT),
enzymes present on the apical membranes of hepatocytes and bile duct epithelial cells.

27. HCC