Biirubin Determination
Bilirubin is a breakdown end product of heme containing proteins. About 80% of bilirubin is formed from the degradation of erythrocyte hemoglobin after 120 days of erythrocyte life in the blood. The remaining 20% of daily-produced bilirubin is formed from erythrocyte precursors in bone marrow and other heme proteins: myoglobin, cytochromes, and catalase.
Hemoglobin degradation begins with opening of the heme porphyrin ring, forming the biliverdin – iron – globin compound known as verdoglobin. Then the iron and globin split off, and bilirubin is formed by the reduction of biliverdin. In contrast to iron and globin, which can be metabolically reused, bilirubin is an end product and toxic product that should be efficiently eliminated from the body. As the bilirubin formed is water insoluble, it binds to albumin upon entering the blood as unconjugated or free bilirubin. Upon the bilirubin binding to albumins, it is transported to the liver, where it undergoes conjugation and excretion. Upon its separation from albumin on hepatocyte membrane, bilirubin binds to specific carriers and is intracellularly transferred to the site where it conjugates with glucuronic acid to form bilirubin diglucuronide and bilirubin monoglucuronide. During this process, the insoluble form of bilirubin is converted into its soluble form, known as conjugated or bound bilirubin. Upon conjugation, the bilirubin from hepatocytes is excreted via bile to the small intestine. Then it is converted, by the action of bacteria, into urobilinogen (stercobilinogen), which transforms by oxydation into urobilin (stercobilin). It is in part excreted in stool, while the rest returns to the liver via enterohepatic circulation and to the bile. A small part of urobilinogen is transported by circulation to the kidneys, wherefrom it is excreted in urine.
Plasma levels of bilirubin are maintained within very low limits (1.7 – 22 µmol/L) by concerted activities of the reticuloendothelial system and liver as
Hemoglobin degradation begins with opening of the heme porphyrin ring, forming the biliverdin – iron – globin compound known as verdoglobin. Then the iron and globin split off, and bilirubin is formed by the reduction of biliverdin. In contrast to iron and globin, which can be metabolically reused, bilirubin is an end product and toxic product that should be efficiently eliminated from the body. As the bilirubin formed is water insoluble, it binds to albumin upon entering the blood as unconjugated or free bilirubin. Upon the bilirubin binding to albumins, it is transported to the liver, where it undergoes conjugation and excretion. Upon its separation from albumin on hepatocyte membrane, bilirubin binds to specific carriers and is intracellularly transferred to the site where it conjugates with glucuronic acid to form bilirubin diglucuronide and bilirubin monoglucuronide. During this process, the insoluble form of bilirubin is converted into its soluble form, known as conjugated or bound bilirubin. Upon conjugation, the bilirubin from hepatocytes is excreted via bile to the small intestine. Then it is converted, by the action of bacteria, into urobilinogen (stercobilinogen), which transforms by oxydation into urobilin (stercobilin). It is in part excreted in stool, while the rest returns to the liver via enterohepatic circulation and to the bile. A small part of urobilinogen is transported by circulation to the kidneys, wherefrom it is excreted in urine.
Plasma levels of bilirubin are maintained within very low limits (1.7 – 22 µmol/L) by concerted activities of the reticuloendothelial system and liver as