Is Bilirubin a Liver Enzyme?
No, bilirubin is not a liver enzyme—it is a metabolic breakdown product of hemoglobin that is processed by the liver. 1
What Bilirubin Actually Is
Bilirubin is predominantly the by-product of the breakdown of the haem component of hemoglobin by the reticuloendothelial system, not an enzyme produced by hepatocytes. 1
It is a tetrapyrrolic compound originating from heme catabolism that exists in two forms: unconjugated (insoluble) and conjugated (water-soluble). 1, 2
Bilirubin is transported to the liver in its unconjugated form, where it is converted into conjugated bilirubin by the enzyme glucuronyltransferase in order to be excreted. 1
Why This Distinction Matters Clinically
While bilirubin is included in "liver function tests" or "liver blood test panels," it is fundamentally different from true liver enzymes like ALT, AST, alkaline phosphatase, and GGT, which are actual enzymatic proteins. 3
Bilirubin serves as a marker of liver function and biliary system patency, but it reflects the liver's ability to process and excrete a metabolic waste product rather than hepatocellular enzyme activity. 4
Elevated bilirubin can result from problems at multiple stages: excess production (hemolysis), impaired liver uptake, impaired conjugation (Gilbert's syndrome), or impaired excretion (cholestasis/obstruction). 1, 4
Common Clinical Pitfall
Do not confuse bilirubin with hepatocellular enzymes when interpreting liver tests. Isolated elevation of bilirubin without enzyme elevation (especially unconjugated hyperbilirubinemia) most commonly indicates Gilbert's syndrome, not liver disease. 1
Conversely, conjugated hyperbilirubinemia typically indicates parenchymal liver disease or biliary obstruction and requires prompt diagnostic evaluation with ultrasound as first-line imaging. 3