Bilirubin is the result of the catabolism of haemoglobins. Structurally, it is an open-chain tetrapyrrole. The porphyrin rings in the haemoglobin are cleaved in the red blood cells by oxidation, and biliverdin is obtained as the product. The biliverdin is then reduced to bilirubin. It is predominantly found in the liver.
The chief role of bilirubin is as a cellular antioxidant. It prevents toxicity in the brain by scavenging free superoxide molecules.
The bilirubin exists in two forms in the liver: direct bilirubin and indirect bilirubin.
Direct bilirubin, also known as conjugated bilirubin, is the modified form of the product obtained after the catabolism of haemoglobin.
The bilirubin is conjugated with glucuronic acid by the enzyme glucuronyltransferase. This conjugation makes the direct bilirubin less toxic and easy to excrete. Direct bilirubin is soluble in water.
The total bilirubin of our body is the sum of direct and indirect bilirubin. Normal bilirubin values for adults are:
Total bilirubin: 0.3 to 1.9 mg/dL
Direct bilirubin: 0.0 to 0.3 mg/dL
Indirect bilirubin, also known as unconjugated bilirubin, is the direct product of the breakdown of haemoglobin. The unmodified bilirubin binds to albumin, facilitating easy transport from blood to the liver.
However, this form of indirect bilirubin is highly toxic; therefore, it is converted to direct bilirubin in the liver with the help of enzymes, which is less toxic.
Indirect bilirubin is highly soluble in lipids and is lipophilic. It is, however, insoluble in water.
Higher levels of bilirubin can be caused due to several reasons such as,
- Haemolytic anaemia, a condition where our body starts getting rid of too many RBCs.
- Bleeding in skin due to some injury.
- Bleeding in the lungs
- Genetic malfunctioning.
Direct vs Indirect Bilirubin
Direct bilirubin is the conjugated form of bilirubin that is obtained as a catabolism product of haemoglobin.
Indirect bilirubin is the direct product of the catabolism of haemoglobin.
Highly soluble in water.
Highly soluble in lipids.
It is less toxic to the human body.
It is very toxic to the human body.
It does not require any protein carrier for transportation.
It binds to albumin for transportation from blood to the liver.
Levels of Bilirubin
Higher levels of bilirubin pose a concern for the human body and need medical attention. Lower levels of bilirubin, however, are of no concern.
- Hyperbilirubinemia: Very high levels of bilirubin cause hyperbilirubinemia. In this condition, there is a buildup of bilirubin in our blood. Reasons for high bilirubin levels include haemolysis, Gilbert’s syndrome, Rotor syndrome, consumption of specific drugs, etc. High levels of bilirubin can damage brain tissues and lead to a seizure.
- Jaundice: It is another disease caused by high levels of bilirubin. It causes discolouration of eyes, skin and mucous membranes. It is caused by gallstones, tumours and hepatitis.
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Frequently Asked Questions on Difference between Direct and Indirect Bilirubin
What is the difference between direct bilirubin and total bilirubin?
Total bilirubin is the sum of direct and indirect bilirubin, whereas direct bilirubin is conjugated bilirubin.
What does it mean if both direct and indirect bilirubin is high?
High levels of both direct and indirect bilirubin indicate damage to the liver and its enzymes. It indicates malfunctioning of the liver.
Why is direct bilirubin higher than total bilirubin?
Due to the presence of monoclonal immunoglobulins, the measurement of direct bilirubin is sometimes falsely recorded as being higher than total bilirubin.