Bilirubin is a yellowish breakdown product of the heme. It is a part of the hemoglobin molecule that is in the red blood cells. It is thrown out of our body by means of bile or urine. Hence an increase in the level of bilirubin indicates the person could be suffering from certain diseases like jaundice. It is lipid soluble as it is a four ring structure known as tetrapyrrole. Bilirubin is split into two unconjugated versus conjugated bilirubin. Bilirubin when high is brown however when the level of bilirubin is slightly higher than normal it is yellowish. In some cases depending on the level of bilirubin that is elevated it may show even on our skin and sclera.
There are differences between unconjugated versus conjugated bilirubin where unconjugated bilirubin is not soluble with water and conjugated bilirubin is soluble with water. In order to explain this tetrapyrrole is digested by reticuloendothelial cells that then result in unconjugated bilirubin. The Unconjugated bilirubin combines with albumin and is carried to the liver. From the liver it then joined or "conjugated" to glucuronide by an enzyme called UDP-glucuronyl transferase.
Conjugated bilirubin reacts quickly as compared to unconjugated bilirubin. In order to produce Azobilirubin, which is a red-violet compound; dyes are added to blood sample. Conjugated bilirubin is known as direct bilirubin whereas unconjugated bilirubin is known as indirect bilirubin. Unconjugated bilirubin still produces Azobilirubin whereas conjugated bilirubin only produces it if dye is added.
Unconjugated bilirubin is fat soluble however conjugated bilirubin is water-soluble and hence can be excreted through kidneys. An increase in the level of conjugated bilirubin means an indication towards hepatobiliary disease. Unconjugated bilirubin generally reacts slowly in the absence of an accelerator namely ethanol. In order to calculate indirect bilirubin the formula of Total bilirubin - Direct bilirubin is used.
Direct bilirubin is measured without an accelerator. Unlike unconjugated bilirubin which binds with neural tissue and also leads to kernicterus (a serious disorder damages the basal ganglia and other parts of the central nervous system) if left untreated or other forms of toxicity, conjugated bilirubin does not bind significantly to neural tissue neither does it lead to any form of toxicity or kernicterus. A very popular or well-known disease that is caused by elevated level of bilirubin in the body is Gilbert's syndrome. This is mainly hereditary and is caused by elevated level of unconjugated bilirubin but is not serious and may lead to mild jaundice if exerted.