NEONATAL JAUNDICE

Jaundice is a common cause of parental concern in new­borns, though most cases are benign and transient, due to developmental immaturity in bilirubin metabolism (physiological jaundice) rather than a pathological disease.

Normal bilirubin metabolism may be divided into following steps:

a. Bilirubin production: About 3#8725;4^th of the bilirubin is derived from the breakdown of hemoglobin in reticuloendothelial system, while rest comes from myoglobin or heme-containing enzymes. One gram of hemoglobin produces 34 mg of bilirubin.

b. Transportation to liver: After production, bilirubin is transported to liver, mostly bound with albumin and partly as free bilirubin. The albumin-bind bilirubin cannot cross the blood-brain barrier (BBB) and hence, not neurotoxic. However, free unconjugated bilirubin is fat-soluble and readily crosses the BBB with potential risk of kernicterus.

c. Hepatic conjugation: In liver, bilirubin dissociates from albumin to enter hepatocytes and combines with Ligandin (Y-protein) to reach the site of conjugation, i.e. endoplasmic reticulum. Here, it conjugates with glucuronic acid in presence of an enzyme Uridine diphosphate glucoronyl transferase (UDPGT), to form mono-/di-glucuronides, i.e. conjugated bilirubin. Unlike unconjugated bilirubin, conjugated bilirubin is water soluble and cannot cross BBB (non-neurotoxic).

d. Excretion of conjugated bilirubin: Conjugated bilirubin is excreted in bile to reach the intestines, where part of it is further metabolized for excretion as stercobilinogen in stools and urobilinogen in urine. Normal gut flora is essential for conversion of conjugated bilirubin into stercobilinogen/ urobilinogen.

e. Enterohepatic circulation: Part of the conjugated bilirubin excreted in gut, reverts back to unconjugated form by enzyme ^-glucuronidase and re-absorbed again for liver metabolism.

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