What Is Bilirubin?
Bilirubin is an organic compound that is a metabolite of heme (heme iron).
It has the chemical formula C33H36N4O6 and a chain-like molecular structure consisting of four pyrrole rings connected in a linear chain. In humans and animals, it is excreted through bile and urine.
It has a molecular weight of 584.66 and a melting point of 192°C. It is a reddish yellow to yellowish red powder at room temperature. It is almost insoluble in water, but soluble in benzene, chloroform, chlorobenzene, carbon disulfide, acids, and alkalis. It is slightly soluble in ethanol and ether.
Uses of Bilirubin
Bilirubin levels are used as an indicator of liver function in blood tests. Bilirubin before being processed by the liver is called indirect bilirubin, and after being processed is called direct bilirubin: the sum of both is called total bilirubin.
The normal level of total bilirubin is 0.2 to 1.2 mg/dL, and when this level is exceeded, hepatitis, cirrhosis, and liver cancer disease are suspected. Bilirubin is a pigment formed when hemoglobin in red blood cells breaks down.
Bilirubin is processed (conjugated) in the liver and excreted as bile in the duodenum, but when liver function is impaired, bilirubin cannot be processed, and a large amount of bilirubin remains in the blood. This is called jaundice, which is a yellowing of the skin, sclera, and mucous membranes.
Even though bilirubin is a pigment molecule, it is not widely used as a pigment. It is found in animal bile and is sometimes used in various studies.
Characteristics of Bilirubin
Bilirubin has an open chain-like structure with four pyrrole rings and is classified as a type of tetrapyrrole. This structure contrasts with the four pyrroles, forming a large porphyrin ring in heme.
It functions as a dye molecule and has an absorption maximum wavelength of 451-455 nm. Bilirubin is very similar in structure to phycobilin, a pigment used by seaweeds to absorb light.
It is also known that when exposed to light, the double bond in bilirubin is isomerized, increasing its water solubility, and this has been applied in phototherapy to treat neonatal jaundice.
Types of Bilirubin
Bilirubin in vivo is divided into two types: before it is transported to the liver (indirect bilirubin) and after it is processed by the liver (direct bilirubin).
Although bilirubin is a biological substance, it is also sold as an isolated compound. It is generally sold as a reagent product for research and development, which requires refrigeration (-20°C) for storage and transport. It is available in quantities of 100 mg and 1 g.
Other Information on Bilirubin
Metabolism of Bilirubin
Senescent or damaged red blood cells are broken down in the spleen. Bilirubin is a breakdown product of heme, a major component of red blood cells which is metabolized during this process.
Heme is broken down into biliverdin by heme oxygenase (HMOX) and biliverdin is reduced by the action of biliverdin reductase, leading to bilirubin.
Bilirubin is then transported to the liver by binding to a plasma protein, albumin, where it undergoes glucuronidation by glucuronyltransferase in the liver.
The conjugation makes bilirubin more soluble in water. This reaction is mediated by uridine diphosphate-glucuronyltransferase (UDPGUTF). Bilirubin is secreted by the liver in this form as bile.