Urobilinogen from bilirubin

Urobilin is an oxidation product of urobilinogen that is formed from bilirubin, a bile pigment, by human intestine. Eventually, all these compounds are natural degradation products of haemoglobin, the blood pigment. Tracing urobilin it was able to monitor municipal waste water effluents in a Japanese river system (Takada and Eganhouse 1998). 

Urobilinogen is derived from bilirubin that has undergone reduction in the intestine, and its excretion is an index of the degree of intestinal stasis and putrefaction. It disappears from the urine in... 

In jaundice secondary to hemolysis, the increased production of bilirubin leads to increased production of urobilinogen, which appears in the urine in large amounts. Bilirubin is not usually found in the urine in hemolytic jaundice (because unconjugated bihmbin does not pass into the urine), so that the combination of increased urobilinogen and absence of bihmbin is suggestive of hemolytic jaundice. Increased blood destruction from any cause brings about an increase in urine urobilinogen. 

Bilirubin digiucuronide is excreted from the liver via the bile into the intestine. Within the bowel, it is hydrolyzed, and the bilirubin is reduced to urobilinogen and stercobilinogen. These are excreted in urine as urobilin after reabsorption from the bowel, and in feces as stercobilin these pigments give urine and feces their characteristic colors. 

Bilirubin is produced by the transformation of haem (mainly from the destruction of red blood cells) via biliverdin (see Figure 2.8). This takes place in the liver, spleen and bone marrow. Bilirubin is transported to the liver in the serum attached to albumin, and at this stage is unconjugated. It is insoluble in water and hence cannot be excreted in this form. Hepatocytes transform unconjugated bilirubin into a water-soluble conjugated form which is excreted via the bile into the intestine. Here, some is converted to urobilinogen and excreted by the kidneys, the majority being converted to stercobilin and excreted in the faeces. 

Haem catabolism occurs in both the liver and gut. Microsomal haem oxygenase of the reticulo-endothelial cells and hepatic parenchymal cells convert haem to biliverdin, and thence rapidly to bilirubin via a reductase. In the gut reduction of methene and vinyl groups of bilirubin results in the formation of colourless urobilinogens which on oxidation produce orange stercobilins that appear in the faeces. All these pigments contain the 4 nitrogen atoms which were initially derived from glycine via haem. 

In conclusion, bile pigment metabolism can be divided into at least seven separate steps (1) hemoglobin breakdown in the reticuloendothelial cell (2) transfer of bilirubin from the reticuloendothelial to the hepatic cell by way of the blood (3) conversion of bilirubin to bilirubin glucuronide inside the hepatic cell (4) excretion of conjugated bile in the bile duct (5) urobilinogen formation (6) urobilinogen reabsorption and (7) re-excretion of urobilinogen in feces and urine (see Fig. 6-13). 

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