The document summarizes the catabolism and degradation of heme. Heme is cleaved from hemoglobin in erythrocytes, which are degraded by macrophages in the liver and spleen. Heme is broken down by heme oxygenase into biliverdin, iron, and carbon monoxide. Biliverdin is further reduced to bilirubin by biliverdin reductase. Bilirubin is transported to the liver bound to albumin and conjugated with glucuronic acid, making it water soluble. Conjugated bilirubin is excreted in bile and broken down by gut bacteria, resulting in urobilinogen and its derivatives that color urine and feces.

1. Made by Khemal Ramoliya
➢ CATABOLISM OF HEME
➢ DEGRADATION OF HEME TO
BILE PIGMENTS
➢ HEME DEGRADATION
• Erythrocytes have a life span of 120 days.
• At The end of this period, they are removed from The
circulation.
• Erythrocytes are taken up & Degraded by the macrophages of
the Reticuloendothelial (RE) system in the spleen and Liver.
• The hemoglobin is cleaved to the protein Part globin and
non-protein heme.
• About 6 g of Hemoglobin per day is broken down, and
Resynthesized in an adult man (70 kg)

2. Fate of globin
• The globin may be utilized as such for the Formation of
hemoglobin or degraded to the Individual amino acids.
• The amino acids undergo their own Metabolism, including
participation in fresh Globin synthesis.
Sources of heme
• lt is estimated that about 80% of the heme that is subjected for
degradation comes from the erythrocytes & the rest (20%) comes
from immature RBC, myoglobin & cytochromes

3. Heme oxygenase
• A complex microsomal enzyme, heme oxygenase utilizes NADPH
& O2 and cleaves the methenyl bridges between the two pyrrole
rings (A and B) to form biliverdin.
• Simultaneously, ferrous iron (Fe) is oxidized to ferric form (Fe) &
released.
• The products of heme oxygenase reaction are biliverdin (a green
pigment), Fe & carbon monoxide (CO).
• Heme promotes the activity of this enzyme.
• Biliverdin is excreted in birds and amphibia while in mammals it is
further degraded.

5. Biliverdin Reductase
• Biliverdin Is methenyl bridges (between the pyrrole rings C and D)
are reduced to methylene group to form bilirubin yellow pigment.
• Catalyzed by an NADPH dependent soluble enzyme, biliverdin
reductase.
• One gram of hemoglobin on degradation finally yields about 35
mg bilirubin.
• Approximately 250-350 mg of bilirubin is daily produced in
human adults.
• The term bile pigments is used to collectively represent bilirubin
and its derivatives.

6. Transport of bilirubin to liver
• Bilirubin is lipophilic & therefore insoluble in aqueous solution.
• Bilirubin is transported in the plasma in a bound (non-covalently)
form to albumin.
• Albumin has two binding sites for bilirubin-a high affinity site & a
low affinity site.
• Approximately 25 mg of bilirubin can bind tightly to albumin (at
high affinity sites) per 100 ml of plasma.
• The rest of the bilirubin binds loosely (at the low affinity sites) to
albumin & enter the tissues.
• Certain drugs & antibiotics (e.g sulfonamides, salicylates) can
displace bilirubin from albumin, bilirubin enter the CNS &
damages neurons.

7. • Albumin-bilirubin complex
• Albumin-bilirubin complex enters the liver, bilirubin dissociates &
is taken up by sinusoidal surface of the hepatocytes by a carrier
mediated active transport.
• Inside the hepatocytes, bilirubin binds to a specific intracellular
protein namely ligandin.
Conjugation of Bilirubin
• In the liver, bilirubin is conjugated with 2 molecules of
glucuronate supplied by UDP glucuronate.
• Catalysed by bilirubin glucuronyltransferase.
• It results in the formation of water soluble bilirubin diglucuronide.
• When bilirubin is in excess, bilirubin monoglucuronide also
accumulated in body.

8. Excretion of Bilirubin into Bile
• Conjugated bilirubin is excreted into the bile canaliculi against a
concentration gradient which then enters the bile.
• The transport of bilirubin diglucuronide is an active, energy-
dependent & rate limiting process.
• This step is easily susceptible to any impairment in liver function
Fate of bilirubin
• Bilirubin glucuronide are hydrolysed in the intestine by specific
bacterial enzymes namely Bita-glucuronidase to liberate bilirubin.
• And then converted to urobilinogen (colourless compound). A
small part of it reabsorbed into circulation. Urobilinogen is
converted into urobilin (yellow) in kidney & excreted.
• The color of urine is due to urobilinogen. AA major part of
urobilinogen is converted bacteria to stercobilin which is excreted
along with feces. The characteristic brown color of feces is due to
stercobilin.