Bilirubin

Bilirubin is a yellow-orange pigment formed when heme is broken down. The liver modifies it and excretes it into bile, which carries it toward elimination.

Heme oxygenase: Heme oxygenase is an enzyme that opens the heme ring, producing biliverdin, carbon monoxide, and iron. It catalyzes the first step in the pathway that produces bilirubin.
Jaundice: Jaundice is yellow discoloration of the skin and eyes caused by elevated bilirubin in tissues. Visible jaundice is the most familiar sign of bilirubin accumulation.
Heme: Heme is an iron-containing ring molecule that forms the oxygen-binding core of hemoglobin and other proteins. Most bilirubin is generated when heme from aging red blood cells is dismantled.
Urobilinogen: Urobilinogen is a largely colorless intestinal product formed when gut bacteria metabolize bilirubin. It is a downstream bilirubin product that can be reabsorbed or converted into urinary pigments.
Biliverdin: Biliverdin is a green tetrapyrrole pigment formed during heme breakdown and reduced to bilirubin. This immediate precursor is converted into bilirubin by biliverdin reductase.
Neonatal jaundice: Neonatal jaundice is elevated bilirubin in newborns, often reflecting immature bilirubin processing. Newborns commonly develop bilirubin elevations because conjugation capacity is still developing.
Hemoglobin: Hemoglobin is the oxygen-carrying protein in red blood cells, built from globin chains and heme groups. Its heme groups supply a major share of the body’s bilirubin.
Stercobilin: Stercobilin is a brown pigment formed in the intestine from bilirubin-derived compounds. It gives stool much of its characteristic color after bilirubin reaches the gut.
Unconjugated bilirubin: Unconjugated bilirubin is the water-insoluble form carried in blood bound to albumin. Newly formed bilirubin circulates in this form before liver uptake.
Kernicterus: Kernicterus is permanent neurological injury caused by unconjugated bilirubin entering the brain. Severe unconjugated hyperbilirubinemia can damage vulnerable regions of the newborn brain.

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