Bilirubin is the catabolic product of heme metabolism. Understanding these mechanisms is necessary for interpretation of the clinical significance of high serum bilirubin concentrations.
- Bilirubin is formed by breakdown of heme present in hemoglobin, myoglobin, cytochromes, catalase, peroxidase and tryptophan pyrrolase.
- Once bilirubin is released into the plasma, it is taken up by albumin which serves as its transporter throughout the body. The binding affinity for albumin to bilirubin is extremely high. When the albumin-bilirubin complex reaches the liver, the highly permeable hepatic circulation allows the complex to reach the sinusoidal surface of the hepatocyte.
- Hepatic transport mechanisms: Bilirubin is taken up into the hepatocytes from the liver sinusoids by two different mechanisms: passive diffusion and receptor-mediated endocytosis.
- Conjugation is mandatory to render bilirubin aqueous soluble and facilitate its secretion across the canalicular membrane and excretion into bile.
- The process of conjugation alters the physiochemical properties of bilirubin giving it many special properties.
- Degradation in the digestive tract is the last step.