Haemoglobin is made up of heme, an iron containing porphyrin and globin, a protein. Normally the erythrocyte lives about 120 days. Ageing or damaged erythrocytes are destroyed by the macrophages of the reticuloendothelial system of the spleen. Other sites notably the, liver and the bone marrow, are also capable of destroying erythrocytes. As the life span of erythrocytes is not increased in splenectomized patients, these sites can completely take over the function in the absence of the spleen. The spleen, unlike other reticuloendothelial sites, is sensitive to subtle damage to the erythrocytes.
Heme splits into globin and hemin and globin. The amino acids released from the catabolism of globin chain are reused for protein synthesis. Hemin is acted upon by heme oxygenase to give biliverdin and iron. The iron is reused for haemoglobin synthesis. Biliverdin, released from the catabolism of protoporphyrin, is finally excreted as conjugated bilirubin in the bile (Figure 1).
Biliverdin is converted to bilirubin by biliverdin reductase. Bilirubin is water insoluble and needs to be conjugated with glycuronic acid in the liver to make it water soluble and make excretion in bile possible. Unconjugated bilirubin binds to albumin and is carried to the liver. The hepatocyte takes up the unconjugated bilirubin by both simple and facilitated diffusion and converts it to bilirubin diglucuronide in two steps. Some bilirubin monoglucuronide is also formed. The enzyme uridine diphophate glucuronyl transferase (UDPGT) facilitated bilirubin conjugation.
Bilirubin is secreted into bile against a concentration gradient. MRP-2 (multidrug resistance like protein 2) is one of the proteins involved in bilirubin secretion.
In the intestine bilirubin is converted to urobilinogen, a colourless compound, by the intestinal flora. Urobilinogen is converted to a pigment responsible for the colour of faeces, urobilin. Some urobilinogen is absorbed and excreted in urine.
Bilirubin metabolism is affected in conjugation defects, secretion defects and in states of increased haemoglobin catabolism.
- Bilirubin conjugation defects: Mutations in the UDGPT result in three syndromes. These are, in decreasing severity, Crigler-Najjar Syndrome I (CN-I), Crigler-Najjar Syndrome II (CN-II) and Gilbert’s syndromes. While CN-I is fatal except in those who undergo liver transplantation, Gilbert’s syndrome causes no symptoms otherthan jaundice. Patients with all three diseases have unconjugated hyperbilirubinemia which may range from usually >20mg/dL in CN-I, usually <20mg/dL in CN-II and <4mg/dL in Gilbert’s syndrome. Gilbert’s syndrome can mimic haemolysis. The absence of other evidence of red cell destruction, viz. increase in the LDH and decrease in the haptoglobin, and the absence reticulocytosis differentiates it from haemolysis.
- Bilirubin secretion defects: Defects in secretion of conjugated bilirubin include Dubin-Johnson syndrome, Rotor syndrome, benign recurrent intrahepatic cholestasis (types 1 and 2) and progressive familial intrahepatic cholestasis (types 1, 2 and 3). (links take you to the OMIM page for the disease)
Increased haemoglobin catabolism: Increased haemoglobin catabolism, seen in patients with haemolytic anaemia and states associated with ineffective erythropoiesis, increases bilirubin production and overwhelms the hepatic uptake/conjugation capacity. This increases the unconjugated bilirubin. Increased bilirubin production in patients with haemolytic anaemia is a result of increased erythrocyte destruction by the spleen. Typically, hyperbilirubinemia is associated with extravascular haemolysis. Splenomegaly may accompany unconjugated hyperbilirubinemia due to haemolysis because of the increased workload. Patients of haemolytic anaemia show elevated LDH levels and reticulocytosis. Patients with ineffective erythropoiesis, e.g. megaloblastic anaemia due to folate/B12 deficiency, have increased haemoglobin catabolism due to destruction of haemoglobinized precursors in the bone marrow. There is reticulocytopenia, the increase in LDH is more pronounced than haemolytic anaemia, there is no splenomegaly. Haemolysis and ineffective erythropoiesis may co-exist in megaloblastic crises of haemolytic anaemia. Unconjugated hyperbilirubinemia due to increased bilirubin production is associated with increased urinary urobilinogen, a features not seen in inherited syndromes of bilirubin conjugation (CN-I, CN-II and Gilbert’s syndrome). It is unusual for the bilirubin to increase beyond 4mg/dL only from increased haemoglobin breakdown. When higher values are encountered other reasons for an increased bilirubin must be sought. All patients with increased haemoglobin breakdown do not show hyperbilirubinemia.