Bilirubin is the end product of the
catabolism of heme, derived principally from circulating hemoglobin.
METABOLISM OF BILIRUBIN
The first step in the degradation of
the heme group to bilirubin is the cleavage of its α – methane bridge to form
biliverdin, a linear tetrapyrrole. This reaction is catalysed by heme
oxygenase. Two aspects of this reaction are noteworthy. First, this enzyme is a
monooxygenase; O2 and NADPH are required for the cleavage reaction. Second, a
methane – bridge carbon is released as carbon monoxide (Stryer,2000). This
endogenous production of CO pose a special problem in the evolution of oxygen carriers (Katzung, 1998).
The central
methane bridge of biliverdin is then reduced by biliverdin reductase to form
bilirubin.
Bilirubin is transported in the
plasma bound tightly to plasma-albumin in the ratio of 1:1 above which
dissociation occurs. In pathological conditions that affect the liver,
concentration of free bilirubin known as unconjugated or indirect reacting
bilirubin increases. Once in the liver, unconjugated bilirubin which is largely
water –insoluble is converted to the water soluble (direct –reacting)
conjugated bilirubin, which is easily excreted into the bile from where it gets
into the gastro-intestinal tract (Katzung, 1998).With in the guts, some
bilirubin is excreted with feaces as stercobilirubin and gives colour to
faeces. A fraction is hydrolysed back to unconjugated bilirubin and reabsorbed
into plasma through enterohepatic circulation.