Human Biochemistry, Volume 1 - Murray R. 1993

Protein and Amino Acid Metabolism
Porphyrins and Bile Pigments
Intestinal Metabolism of Bilirubin

Once bilirubin reaches the ileum and colon, glucuronides are hydrolyzed by specific bacterial Enzymes (ß-glucuronidases). Subsequently, the intestinal microflora reduces the pigment to form a group of colorless tetrapyrrole compounds known as urobilinogens (Fig. 33.15). A small fraction of urobilinogen is reabsorbed in the ileum and colon and returns to the Liver, establishing the intrahepatic urobilinogen cycle. In pathological conditions, such as the accumulation of excessive amounts of Bile pigments or liver diseases that disrupt this intrahepatic cycle, urobilinogen may be excreted in the urine.

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Fig. 33.15. Structure of certain bile pigments.

Under normal conditions, the majority of colorless urobilinogens produced in the colon by intestinal microflora are oxidized into urobilins (colored compounds) and excreted in the feces (Fig. 33.15). The characteristic darkening of feces upon exposure to air results from The oxidation of remaining urobilinogens into urobilins.