Stercobilin (SB)

Stercobilin is a tetrapyrrolic bile pigment and is one end-product of heme catabolism. Stercobilin results from breakdown of the heme moiety of hemoglobin found in erythrocytes. Macrophages break down senescent erythrocytes and break the heme down into biliverdin, which rapidly reduces to free bilirubin. Bilirubin binds tightly to plasma proteins (especially albumin) in the blood stream and is transported to the liver, where it is conjugated with one or two glucuronic acid residues into bilirubin diglucuronide, and secreted into the small intestine as bile. In the small intestine, some bilirubin glucuronide is converted back to bilirubin via bacterial enzymes in the terminal ileum. This bilirubin is further converted to colorless urobilinogen. Any urobilinogen that remains in the colon is converted to stercobilinogen and finally oxidized to stercobilin, which is responsible for the brown color of human feces. Stercobilin is then excreted in the feces.