Describe bilirubin metabolism?

1 Answer
Feb 23, 2018

Bilirubin is the terminal product of heme metabolism.

Explanation:

enter image source here
Image source: https:/en.m.wikipedia.org/wiki/file:Heme__Breakdownpng

Bilirubin is a yellow compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates.

In normal circumstances, plasma bilirubin is mostly unconjugated and is tightly bound to circulating albumin. It is taken up by hepatocytes by facilitated diffusion, stored in hepatocytes bound to gluconorides by microsomes UGT1A1.

In the liver bilirubin is conjugated with glucoronic acid by the enzyme glucoronyltransferase, making it soluble in water. Much of it goes into the bile and thus out into the small intestines.

In the colon the colonic bacteria deconjugate and metabolise the bilirubin into colourless urobilinogen, which can be oxisidised to form urobilinogen and stercobilin. Urobilinogen is excreted by the kidneys to give the yellow colour to urine and stercobilin is excreted in the faeces giving stool its characteristic brown colour.

Bilirubin is produced by a two stage reaction that occurs in cells of the reticulo endothelial system, including phagocytes, the Kupffer cells of the liver, and the cells of the spleen and bone marrow. Heme is taken up into these cells and acted on by the enzyme hemeoxygenase, liberating the chelated iron from the heme structure and releasing an equimolar amount of carbon monoxide which is excreted by the lungs.

This reaction yields a green pigment known as biliverdin. Biliverdin is then acted upon by the enzyme biliverdin reductive, again releasing a molecule of carbon monoxide and producing the yellow bilirubin.

Increased production, reduced uptake, and low glucoronidation capacity can increase plasma unconjugated bilirubin levels. In case of inherited or acquired deficiencies of bilirubin storage or excretion, both conjugated and unconjugated bilirubin accumulate in the plasma.