Tissue Phase and Blood Transport
Heme degradation begins in cells of the reticuloendothelial system, predominantly in the spleen and bone marrow. In the endoplasmic reticulum of these cells, the heme oxygenase system operates. Utilizing oxygen ($O_2$) and NADPH+$H^+$ as a cofactor, it cleaves hemoglobin into an intermediate compound, verdoglobin, followed by the formation of biliverdin, a green pigment. During this reaction, ferric iron ($Fe^{3+}$), the protein moiety (globin), carbon monoxide ($CO$), and the oxidized form of $NADP^+$ are released.
Next, the enzyme biliverdin reductase uses NADPH+$H^+$ to reduce green biliverdin to red-yellow bilirubin. The resulting pigment is toxic and extremely poorly water-soluble. To safely reach the liver, it binds in the bloodstream to a transport protein—albumin. This complex is called indirect (unconjugated) bilirubin. Because of its tight binding to the protein, it does not give a direct reaction with the laboratory diazo reagent.
Hepatic Phase: Uptake and Conjugation
Upon reaching the liver via the bloodstream, indirect bilirubin is taken up by hepatocytes. This process occurs via facilitated diffusion, which strictly requires specialized intracellular carrier proteins—ligandin and protein Z.
Inside the hepatocyte, specifically within the endoplasmic reticulum membranes, the pigment is detoxified. Bilirubin serves as the substrate, and two molecules of UDP-glucuronic acid act as functional group donors. The reaction is catalyzed by UDP-glucuronosyltransferase enzymes.
- UDP-glucuronosyltransferase I attaches the first molecule to form bilirubin monoglucuronide.
- UDP-glucuronosyltransferase II completes the process, forming bilirubin diglucuronide.
The final product is direct (conjugated) bilirubin, which loses its toxicity, becomes water-soluble, and is actively secreted into bile. The activity of conjugation enzymes can be induced (increased) by ethanol or drugs such as phenobarbital.
Intestinal Phase and Excretion Pathways
With the flow of bile, direct bilirubin enters the duodenum, where the final stage of its metabolism begins. Here, influenced by intestinal microflora enzymes, glucuronic acid is cleaved off. The freed bilirubin is reduced by bacteria to colorless tetrapyrroles—urobilinogens (specifically forming stercobilinogen).
Metabolite excretion follows two main pathways:
- Via stool (95%). Most of the urobilinogen moves through the large intestine. Upon contact with atmospheric oxygen, it is oxidized into a brown pigment—stercobilin (urobilin), which gives stool its characteristic color. A substantial amount of pigment—200 to 300 mg—is excreted daily via this route.
- Via urine (5%). A small fraction of urobilinogen is reabsorbed from the intestine into the portal venous system. After passing through the liver and entering the systemic circulation, the pigment is filtered by the kidneys. Upon contact with oxygen in the urine, it is oxidized to straw-yellow urobilin, determining normal urine color. Only trace amounts—1 to 6 mg of pigment—are excreted daily via this pathway.