Protein Metabolism

Glycolysis &
Citric Acid Cycle
Lipid Metabolism &
Review Metabolism
 Elmhurst College
Transamination Urea Cycle Energy Summary  Chemistry Department
Oxidative Deamination Bilirubin - Heme Catabolism  PKU  Virtual ChemBook



Hemoglobin Catabolism and Bilirubin

Introduction:

The catabolism of hemoglobin is outlined in the graphic on the left. Red blood cells are continuously undergoing a hemolysis (breaking apart) process. The average life-time of a red blood cell is 120 days. As the red blood cells disintegrate, the hemoglobin is degraded or broken into globin, the protein part, iron (conserved for latter use), and heme (see middle graphic). The heme initially breaks apart into biliverdin, a green pigment which is rapidly reduced to bilirubin, an orange-yellow pigment (see bottom graphic). These processes all occur in the reticuloendothelial cells of the liver, spleen, and bone marrow. The bilirubin is then transported to the liver where it reacts with a solubilizing sugar called glucuronic acid. This more soluble form of bilirubin (conjugated) is excreted into the bile.

The bile goes through the gall bladder into the intestines where the bilirubin is changed into a variety of pigments. The most important ones are stercobilin, which is excreted in the feces, and urobilinogen, which is reabsorbed back into the blood. The blood transports the urobilinogen back to the liver where it is either re-excreted into the bile or into the blood for transport to the kidneys. Urobilinogen is finally excreted as a normal component of the urine.

 

Types of Jaundice:

Various conditions of jaundice result from the accumulation of bilirubin in the blood. A jaundice condition is characterized by yellow colored skin due to the presence of bilirubin.

Hemolytic Jaundice:

Excessive hemolysis or breakdown of red blood cells causes the formation of higher than normal amounts of bilirubin. Bilirubin made in the liver goes into bile and then into the gall bladder and into the intestines where most is excreted. The liver works normally, but could eventually be damaged from overwork. Usually the liver can handle the excess and the bilirubin is excreted via intestines and does not usually spill over into the kidneys. Urobilinogen levels are likely to be elevated in the blood and urine.

Hepatic Jaundice:

Hepatic jaundice is caused by damage or disease in the liver. Heme enters the liver but it does not take out as much bilirubin as is normal. Bilirubin builds up in the blood and spills over into the kidneys which filter it out into the urine. The amount of urobilinogen in the urine will be either normal or low if not enough bilirubin is being removed by the liver into bile and the intestines.

Biliary Obstruction:

If bilirubin cannot reach the intestinal area because of a blockage in the bile duct, than bilirubin builds up in the blood because it cannot get out of the liver. Bilirubin is then removed by the kidneys into the urine. Little if any, urobilinogen will be found in the urine since little or no bilirubin is reaching the intestines.


 
 Quiz: Lab tests on urine may be made for both bilirubin and urobilinogen.
Complete the types of results expected for the various jaundice
conditions. Answer with positive, negative, lower, higher, etc.
 LAB TEST

 Normal

Biliary
Obstruction

Hepatic
Diseases

Hemolytic
Diseases
 Bilirubin
Urobilinogen