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Bilirubin Metabolism – Formation and Significance

Bilirubin metabolism describes the breakdown of hemoglobin into bilirubin and its processing in the liver, bile, and intestine. Disruptions can lead to jaundice.

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Things worth knowing about "Bilirubin Metabolism"

Bilirubin metabolism describes the breakdown of hemoglobin into bilirubin and its processing in the liver, bile, and intestine. Disruptions can lead to jaundice.

What Is Bilirubin Metabolism?

Bilirubin metabolism refers to the biochemical processes by which bilirubin is produced, transported, transformed, and excreted in the human body. Bilirubin is a yellowish-orange bile pigment formed during the breakdown of the red blood pigment hemoglobin. When bilirubin metabolism is disrupted, it can cause a visible yellowing of the skin and eyes known as jaundice (icterus).

Formation of Bilirubin

Red blood cells (erythrocytes) have a limited lifespan of approximately 120 days. After their breakdown – primarily in the spleen, but also in the liver and bone marrow – the released hemoglobin is degraded in several steps:

Hemoglobin is first split into heme and globin.
Heme is converted by the enzyme heme oxygenase into biliverdin, releasing carbon monoxide and iron in the process.
Biliverdin is then reduced by biliverdin reductase to form unconjugated bilirubin (indirect bilirubin).

Transport in the Blood

Unconjugated (indirect) bilirubin is water-insoluble and is therefore transported in the blood bound to the carrier protein albumin. Because it is not water-soluble, it cannot be excreted by the kidneys in this form.

Processing in the Liver

In the liver, unconjugated bilirubin is taken up by liver cells (hepatocytes) and conjugated with glucuronic acid by the enzyme UDP-glucuronosyltransferase. This process, known as conjugation, makes bilirubin water-soluble. The resulting conjugated bilirubin (direct bilirubin) is then secreted into the bile.

Excretion via Bile and the Intestine

Conjugated bilirubin travels with bile through the bile ducts into the small intestine, where it is converted by intestinal bacteria into urobilinogen and other metabolites:

A portion of urobilinogen is reabsorbed in the intestine and re-excreted by the liver (enterohepatic circulation).
A small fraction enters the bloodstream and is excreted by the kidneys in the urine as urobilin – this gives urine its characteristic yellow color.
The majority of urobilinogen remains in the intestine and is converted to stercobilin, which gives stool its brown color.

Clinical Significance and Disorders

An elevated bilirubin level in the blood (hyperbilirubinemia) leads to jaundice. Depending on the cause, three types are distinguished:

Pre-hepatic jaundice: Excessive breakdown of red blood cells (e.g., in hemolytic anemia) produces more unconjugated bilirubin than the liver can process.
Hepatic jaundice: Liver disease (e.g., hepatitis, liver cirrhosis) impairs the conjugation or excretion of bilirubin.
Post-hepatic jaundice: An obstruction in the bile ducts (e.g., gallstones, pancreatic cancer) prevents conjugated bilirubin from reaching the intestine.

Bilirubin metabolism is also clinically relevant in newborns: Neonatal jaundice (icterus neonatorum) often occurs in the first days of life because the newborn liver is not yet fully mature and cannot conjugate bilirubin sufficiently.

Diagnostics

To assess bilirubin metabolism, the following values are measured in the blood:

Total bilirubin: Normal value below 1.2 mg/dL in adults.
Direct (conjugated) bilirubin: Normal value below 0.3 mg/dL.
Indirect (unconjugated) bilirubin: Calculated as total bilirubin minus direct bilirubin.

Additional investigations such as liver function tests (AST, ALT, GGT, ALP), a full blood count, and imaging procedures (ultrasound, MRI) can be used to determine the underlying cause of elevated bilirubin levels.

References

Devlin, T.M. – Textbook of Biochemistry with Clinical Correlations. Wiley-Liss, 7th edition, 2010.
Fauci, A.S. et al. – Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition, 2022.
World Health Organization (WHO) – Neonatal jaundice. Available at: https://www.who.int (accessed 2024).

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