Heme Metabolism and Bilirubin Fractions
Heme breakdown occurs in macrophages. First, heme oxygenase converts heme into verdoglobin and then into the green pigment biliverdin (releasing iron and carbon monoxide). Next, the enzyme biliverdin reductase reduces it to unconjugated (free) bilirubin, which has a yellow-red color.
It is important to understand the biochemical differences between the fractions:
- Unconjugated bilirubin: Highly hydrophobic, therefore transported in the blood only bound to albumin. Because of its attachment to a large protein, it is not filtered by the kidneys. This form is extremely toxic because at high concentrations it crosses the blood-brain barrier into the CNS and acts as an uncoupler of oxidative phosphorylation.
- Conjugated bilirubin: Formed in the endoplasmic reticulum of hepatocytes. Via the enzyme UDP-glucuronosyltransferase, UDP-glucuronic acid is attached to the molecule (forming mono- and diglucuronides). As a result, the pigment becomes hydrophilic, loses its toxicity, and is secreted into bile, or easily excreted in urine during pathologies.
Differential Diagnosis of Jaundice
Based on the pathophysiological mechanism, jaundice is divided into three main groups. To make an accurate diagnosis, pigment levels in the blood, urine, and stool are evaluated.
- Hemolytic (pre-hepatic). Caused by accelerated erythrocyte destruction (oxidant poisoning, incompatible blood transfusion, hereditary microspherocytosis, or enzyme defects such as pyruvate kinase deficiency). The liver cannot keep up with the massive pigment load. Serum unconjugated bilirubin spikes (2-3 fold). Stool darkens due to excess stercobilin, and urine urobilin levels rise.
- Hepatocellular (parenchymal). Occurs in hepatitis of various etiologies, accompanied by fever and fatigue. Damaged hepatocytes exhibit impaired pigment uptake and lose the ability to excrete it into the bile canaliculi. Both conjugated and unconjugated bilirubin levels rise in the blood. Urine darkens because conjugated bilirubin is filtered by the kidneys, while stool becomes pale (hypocholic).
- Obstructive (post-hepatic). Caused by impaired bile flow due to gallstones or postoperative strictures. Conjugated bilirubin fails to reach the intestine and regurgitates into the bloodstream.
| Parameter | Hemolytic | Hepatocellular | Obstructive |
|---|---|---|---|
| Blood (Bilirubin) | Markedly ↑ unconjugated | ↑ conjugated and unconjugated | ↑ conjugated and unconjugated |
| Urine Color | Normal (no bilirubin) | Dark (conjugated bilirubin present) | "Dark beer" (conjugated bilirubin present) |
| Stool Color | Intense (high stercobilin) | Pale (low stercobilin) | Clay-colored / acholic (no stercobilin) |
Hereditary Jaundice Syndromes
This group of conditions is caused by genetic defects in proteins involved in intrahepatic bilirubin metabolism.
- Gilbert syndrome: Impaired bilirubin uptake from the blood. The defect lies in hepatocyte membrane transport proteins (ligandins) or reduced activity of UDP-glucuronosyltransferase.
- Crigler-Najjar syndrome: A severe defect in the primary structure of UDP-glucuronosyltransferase. The enzyme responsible for bilirubin conjugation may be partially or completely absent.
- Rotor and Dubin-Johnson syndromes: Impaired excretory function. Due to mutations in ATP-dependent canalicular membrane transport systems, conjugated bilirubin cannot be properly secreted into bile.