[bil-uh-roo-bin, bil-uh-roo-bin] Bilirubin (formerly referred to
as hematoidin) is the yellow breakdown product of normal heme
catabolism. Heme is formed from hemoglobin, a principal
component of red blood cells. Bilirubin is excreted in bile, and
its levels are elevated in certain diseases. It is responsible
for the yellow colour of bruises and the yellow discolouration
in jaundice. Bilirubin reduction in the gut leads to a product
called urobilinogen which is then oxidized to urobilin which is
excreted in the urine.        Bilirubin is created by the
activity of biliverdin reductase on biliverdin. Bilirubin, when
oxidized, reverts to become biliverdin once again. This cycle,
in addition to the demonstration of the potent antioxidant
activity of bilirubin, has led to the hypothesis that
bilirubin's main physiologic role is as a cellular antioxidant.
An indication of the bile pigments rapid passage to the large
intestines giving them no time to be transformed into
stercobilin (Stercobilin is a substance that