The physiology of bilirubin is similar in the premature and term infant.
Red blood cells are continually being broken down in the reticuloendothelial system and replaced. This process releases heme, which is catabolized by the enzyme heme oxygenase 1 to biliverdin and thence to bilirubin. This form of bilirubin is called unconjugated or indirect bilirubin. Bound to serum albumin the bilirubin is transported to the liver.
Bilirubin conjugation and elimination
After uptake into the hepatocyte, unconjugated bilirubin is glucuronated to form water soluble mono- and diglucuronides, known as conjugated or direct bilirubin. The enzyme controlling the conjugation process is UDP-glucuronosyltransferase 1A1 (UGT). Direct bilirubin is excreted via bile to the bowel from which it is excreted from the body.
Genetic control of bilirubin conjugation
The enzyme UGT1A1 is encoded by the gene UGT1A1. This gene contains a noncoding promoter region and a coding region. Polymorphisms of the promoter region, such as the (TA)n polymorphism, result in diminished expression of a normally formed enzyme and are associated with Gilbert syndrome. Coding area mutations result in an abnormally structured enzyme with no or little conjugating ability, such as is seen in Crigler-Najjar syndrome.
In the newborn bowel, conjugated bilirubin may be unconjugated by the enzyme ?-glucuronidase and reabsorbed into the bloodstream, thereby adding to the bilirubin pool and straining the already deficient bilirubin ...