The high lipid solublity of bilirubin determines its behavior and its further metabolism.
Its lipid solubility dictates
that it must be transported in the blood by a carrier; the physiological carrier is serum albumin.
that it is soluble in the lipid bilayers of cell membranes.
Bilirubin must be conjugated to a water-soluble substance
This increased its water solubility, decreases its lipid solubility and eases its excretion. Conjugation is accomplished by attaching two molecules of glucuronic acid to it in a two step process.
The substrates are
bilirubin (or bilirubin monoglucuronide)
UDP-glucuronic acid.
The reaction is a transfer of two glucuronic acid groups sequentially to the propionic acid groups of the bilirubin. The major product is bilirubin diglucuronide (pronounce).
Bilirubin diglucuronide is excreted in the bile. It is subject to subsequent transformations to other species by the intestinal flora.
The clinical determination of plasma bilirubin distinguishes between conjugated (direct) and unconjugated (indirect) bilirubin.
The reaction, called the van den Bergh reaction, is a coupling of bilirubin with a diazonium salt to form a colored complex.
Only conjugated bilirubin is water soluble and reacts directly. This is called the DIRECT bilirubin.
To measure the unconjugated bilirubin bound to albumin, alcohol is added to release it into solution, where it can now react. This is called the INDIRECT bilirubin.
