Bile pigments are formed through the degradation of heme, a component of hemoglobin found in red blood cells.
Here's a breakdown of the process:
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Heme Release: When red blood cells are broken down (a process called hemolysis), hemoglobin is released. From hemoglobin, heme is released.
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Heme Oxygenase Action: The enzyme heme oxygenase breaks down heme. This crucial step releases iron (Fe2+ or Fe3+), carbon monoxide (CO), and biliverdin.
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Biliverdin Reduction: Biliverdin reductase then reduces biliverdin to bilirubin. Bilirubin is the primary bile pigment.
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Bilirubin Glucuronidation (in the Liver): Bilirubin is relatively insoluble in water. To make it more water-soluble for excretion, it's conjugated with glucuronic acid in the liver by the enzyme UDP-glucuronosyltransferase (UGT). This process is called bilirubin glucuronidation, resulting in bilirubin diglucuronide (conjugated bilirubin).
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Excretion: Conjugated bilirubin is then excreted into bile and passed into the small intestine.
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Urobilinogen Formation (in the Intestine): In the intestine, bacteria convert bilirubin into urobilinogen.
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Further Metabolism: Urobilinogen can undergo several fates:
- Some is reabsorbed into the bloodstream and excreted by the kidneys as urobilin (giving urine its yellow color).
- Most is converted by bacteria in the intestine to stercobilin, which is excreted in feces (giving feces its brown color).
In summary, heme degradation leads to the formation of biliverdin, which is then converted to bilirubin. The liver then conjugates bilirubin for excretion, and intestinal bacteria further modify it into urobilinogen and stercobilin. The entire process is catalyzed by various enzymes, primarily heme oxygenase and biliverdin reductase. Bile pigments, especially bilirubin, possess antioxidant properties, counteracting the pro-oxidant effects of heme and iron released during the initial degradation steps.
