Heme degradation:

Bile pigments exist in both the animal and plant kingdoms, and are building through breakdown of the cyclic tetrapyrrole structure of heme. In the animals this pathway is an excretory  system  through that  the heme  from  the hemoglobin  of aging  red blood  cells,  and  other  hemoproteins,  is erased  from  the  body.  In the  plant kingdom,  moreover,  heme  is broken  down  to form  bile  pigments  that  have main roles to play in coordinating  light responses  in higher plants (the phycobiliprotein phytochrome),  and in light harvesting in algae (the phycobiliproteins phycoerythrin and phycocyanin).

In all organisms, the degradation of heme starts with a reaction carried out through a single general enzyme. This enzyme, heme oxygenase, is present mainly in the spleen and liver of vertebrates, and carries out the oxidative ring opening of heme to produce the green bile pigment biliverdin and a linear tetrapyrrole. The Heme oxygenase is a member of the cytochrome P450 family of enzymes and needs O₂ and NADPH.  In  birds,  reptiles  and  amphibians  this  water- soluble  pigment  is  the  last  product  of  heme  degradation   and  is  excreted directly. In mammals, thus, an additional conversion to the red-orange bilirubin takes place; a reaction catalyzed through biliverdin reductase. The changing color of a bruise is a visible indicator of these degradative reactions. The bilirubin, like other lipophilic molecules like as free fatty acids, is then send in the blood bound to serum albumin. In the liver its water solubility is

Figure:  Degradation of heme to the bile pigments biliverdin and bilirubin. M  methyl (CH₃), V    vinyl (CH    CH₂), P     propionyl (CH₂CH₂CH₂OH).

increased through conjugation  to two molecules  of glucuronic  acid, a sugar residue which differs from glucose in having a COO^-   group at C-6 rather than a CH₂OH group. The conclusion bilirubin diglucuronide is secreted into the bile and then into  the  intestine, where it is further metabolized through bacterial enzymes and at last excreted in the feces.

When the blood holds excessive amounts of the insoluble bilirubin and it is deposited in the skin and the whites  of the eyes and resulting in a yellow discoloration.  This situation, known as jaundice it is indicative either of impaired liver function and obstruction of the bile duct or excessive breakdown of erythrocytes.