BILIRUBIN METABOLISM

PREPARED

BY

ASOGWA INNOCENT KINGSLEY

ML-508

Introduction

• Bilirubin is the orange-yellow pigment derived from senescent red blood cells.

• It is a toxic waste product in the body.

• It is extracted and biotransformed mainly in the liver, and excreted in bile and urine.

• It is a bile pigment

• Elevations in serum and urine bilirubin levels are normally associated with Jaundice.  

Erythrocytes become “old” as they lose their flexibility and become pikilocytes (spherical), increasingly rigid and fragile. Once the cell become fragile, they easily destruct during passage through tight circulation spots, especially in spleen, where the intra-capillary space is about 3 micron as compared to 8 micron of cell size

RBCs useful life span is 100 to 120 days,After which they become trapped and fragment in smaller circulatory channels, particularly in those of the spleen. For this reason, the spleen is sometimes called the “red blood cell graveyard.”

Dying erythrocytes are engulfed and destroyed by macrophages.

Formation of Bilirubin

• Primary site of synthesis:-

SPLEEN: The Graveyard

of Red Blood Cells

• Secondary site of synthesis:-

LIVER & BONE MARROW

An average person produces about 4 mg/kg of bilirubin per day.

The daily bilirubin production from all sources in man averages from 250 to 300 mg.

TOTAL BILIRUBIN

HEMOGLOBIN FROM SENESCENT

RBC’S DESTROYED IN RETICULOENDOTHELIAL

CELLS OF LIVER, SPLEEN & BONE MARROW

RBC PRECURSORS DESTROYED IN THE

BONE MARROW

85% 15%

CATABOLISM OF HEME-CONTAINING

PROTEINS (MYOGLOBIN, CYTOCHROMES &

PEROXIDASES)

Extravascular Pathway for RBC Destruction

(Liver, Bone marrow, & Spleen)

Hemoglobin

Globin

Amino acids

Amino acid pool

Heme Bilirubin

Fe2+

Excreted

Phagocytosis & Lysis

Recycled

Within cells of the RE system, heme degraded to bilirubin in a two-step process .

• a. The porphyrin ring is opened and the iron atom is removed by the action of

• heme oxygenase to produce the green-colored intermediate biliverdin.

• b. Subsequent reduction converts biliverdin to bilirubin, which has a redorange color.

Pathophysiology

RBCs Breakdown

Hemoglobin Produces& Breakdown

Heme

Biliverdin

Bilirubin

Heme Oxygenase

BiliverdinReductase

• The globin is recycled or converted into amino acids, which in turn are recycled or catabolized as required.

• Heme is oxidized, with the heme porphyrin ring being opened by the endoplasmic reticulum enzyme, heme oxygenase.

• The oxidation occurs on a specific carbon producing equimolar amounts of the biliverdin, iron , and carbon monoxide (CO). This is the only reaction in the body that is known to produce CO.

• Most of the CO is excreted through the lungs, with the result that the CO content of expired air is a direct measure of the activity of heme oxygenase in an individual.

III IVI II

Oxidation Heme Oxygenase

In the first reaction, a bridging methylene group is cleaved by heme oxygenase to form Linear Biliverdin from Cyclic Heme molecule.

Fe 2+ is released from the ring in this process.

• In the next reaction, a second bridging methylene (between rings III and IV) is reduced by biliverdin reductase, producing bilirubin.

I

I III

III IV

IV

II

II

Reduction Biliverdin Reductase

• biliverdin causing a change in the color of the molecule from blue-green (biliverdin) to yellow-red (bilirubin).

• The latter catabolic changes in the structure of tetrapyrroles are responsible for the progressive changes in color of a hematoma, or bruise, in which the damaged tissue changes its color from an initial dark blue to a red-yellow and finally to a yellow color before all the pigment is transported out of the affected tissue.

• Peripherally arising bilirubin is transported to the liver in association with albumin, where the remaining catabolic reactions take place.

Bilirubin is not very water-soluble, so most of it is carried to the liver bound to albumin.

In cells of the liver, bilirubin undergoes modification to increase its water solubility so that it can be excreted more easily.

a.Bilirubin is conjugated to two molecules of glucuronic acid, creating bilirubin diglucuronide.

b. Bilirubin diglucuronide is transported out of the hepatocytes into the bile canaliculi and is thus excreted in bile.

In Blood

• The bilirubin synthesized in spleen, liver & bone marrow is unconjugated bilirubin.

• It is hydrophobic in nature so it is transported to the liver as a complex with the plasma protein, albumin.

Unconjugated bilirubin

– Lipid soluble – : limits excretion– 1 gm albumin binds 8.5

mg bilirubin– Fatty acids & drugs can

displace bilirubin– Indirect positive reaction

in van den Bergh test

Role of Blood Proteins in the Metabolism of Bilirubin

1. Albumin

Dissolved in Blood

Blood

Liver

Ligandin

(-) charge

Ligandin

(-) charge

Ligandin Prevents bilirubin from going back to plasma

In Endoplasmic Reticulum

In the microsomes of the endoplasmic reticulum, unconjugated bilirubin is converted to water soluble mono- or di- conjugates by sequential covalent coupling with glucuronic acid.

Bilirubin is conjugated in a two step process to form bilirubin mono- & di- glucuronide

Conjugation with Glucoronates

BILIRUBIN DIGLUCORONIDE

Excretion of Bilirubin

In the Intestine

• In the small intestine, conjugated bilirubins are poorly reabsorbed, but are partly hydrolyzed back to unconjugated bilirubin by catalytic action of bacterial ß-glucuronidases.

• In the distal ileum and colon, anaerobic flora mediate further catabolism of bile pigments:

a) hydrolysis of conjugated bilirubin to unconjugated bilirubin by bacterial β-glucuronidases;

b) multistep hydrogenation (reduction) of unconjugated bilirubin to form colorless urobilinogens; and

c) oxidation of unconjugated bilirubin to brown colored mesobilifuscins.

• Urobilinogens is a collective term for a group of 3 tetrapyrroles;

– Stercobilinogen (6H)– Mesobilinogen (8H)&,– Urobilinogen (12H)

• Upto 20 % of urobilinogen produced daily is reabsorbed from the intestine & enters the entero-hepatic circulation.

Urobilinogen Structure

• Most of the reabsorbed urobilinogen is taken up by the liver & is re-excreted in the bile.

• A small fraction (2 % - 5 %) enters the general circulation & appears in the urine.

• In the lower intestinal tract, the 3 urobilinogens spontaneously oxidize to produce the corresponding bile pigments;– Stercobilin– Mesobilin &– Urobilin;

which are orange-brown in color and are the major pigments of stool.