Urobilinogen is a tetrapyrrole chemical compound that is that is the parent compound of both stercobilin (the pigment that is responsible for the brown color of feces) and urobilin (the pigment that is responsible for the yellow color of urine). Urobilinogen is formed through the microbial degradation of its parent compound bilirubin. Urobilinogen is actually generated through the degradation of heme, the red pigment in haemoglobin and red blood cells (RBCs). RBCs have a life span of about 120 days. When the RBCs have reached the end of their useful lifespan, the cells are engulfed by macrophages and their constituents recycled or disposed of.  Heme is broken down when the heme ring is opened by the enzyme known as heme oxygenase, which is found in the endoplasmic reticulum of the macrophages. The oxidation process produces the linear tetrapyrrole known as biliverdin along with ferric iron (Fe3+), and carbon monoxide (CO). In the next reaction, a second methylene group (located between rings III and IV of the porphyrin ring) is reduced by the enzyme known as biliverdin reductase, producing bilirubin. Bilirubin is significantly less extensively conjugated than biliverdin. This reduction causes a change in the color of the biliverdin molecule from blue-green (vert or verd for green) to yellow-red, which is the color of bilirubin (ruby or rubi for red). In plasma virtually all the bilirubin is tightly bound to plasma proteins, largely albumin, because it is only sparingly soluble in aqueous solutions at physiological pH. In the sinusoids unconjugated bilirubin dissociates from albumin, enters the liver cells across the cell membrane through non-ionic diffusion to the smooth endoplasmatic reticulum. In hepatocytes, bilirubin-UDP-glucuronyltransferase (bilirubin-UGT) adds 2 additional glucuronic acid molecules to bilirubin to produce the more water-soluble version of the molecule known as bilirubin diglucuronide. The bilirubin diglucuronide is transferred rapidly across the canalicular membrane into the bile canaliculi where it is then excreted as bile into the large intestine.  The bilirubin is further degraded (reduced) by microbes present in the large intestine to form a colorless product known as urobilinogen. Urobilinogen that remains in the colon can either be reduced to stercobilinogen and finally oxidized to stercobilin, or it can be directly reduced to stercobilin. Some of the urobilinogen produced by the gut bacteria is reabsorbed and re-enters the enterohepatic circulation. This reabsorbed urobilinogen is oxidized and converted to urobilin. The urobilin is processed through the kidneys and then excreted in the urine, which causes the yellowish color in urine. Urobilinogen (also known as D-urobilinogen) is closely related to two other compounds: mesobilirubinogen (also known as I-urobilinogen) and stercobilinogen (also known as. L-urobilinogen). Specifically, urobilinogen can be reduced to form mesobilirubinogen, and mesobilirubinogen can be further reduced to form stercobilinogen. Confusingly, all three of these compounds are frequently collectively referred to as "urobilinogens".
Urobilinogen content can be determined by a reaction with Ehrlich's reagent, which contains para-dimethylaminobenzaldehyde. Ehrlich's reagent reacts with urobilinogen to give a pink-red color. Low urine urobilinogen may result from complete obstructive jaundice or treatment with broad-spectrum antibiotics, which destroy the intestinal bacterial flora. Low urine urobilinogen levels may also result from congenital enzymatic jaundice (hyperbilirubinemia syndromes) or from treatment with drugs that acidify urine, such as ammonium chloride or ascorbic acid. Elevated urine levels of urinobilinogen may indicate hemolytic anaemia, a large hematoma, restricted liver function, hepatic infection, poisoning or liver cirrhosis.