Rational Approach to the Yellow Cat

Jaundice or icterus is the yellow staining of serum or tissues due to the excessive accumulation of bile pigment or bilirubin. Icterus is clinically detectable as yellow stained tissue when the serum bilirubin concentration exceeds 2 mg/dl (34 mol/L), whereas serum is discolored when the serum bilirubin concentration exceeds 0.6 mg/dl (10 mol/L). Hemoglobin constitutes more than 90% of all heme in the body, and is the most important source of bilirubin. Heme is enzymatically cleaved by heme oxygenase following phagocytosis by cells of the mononuclear-phagocytic system, predominantly of the bone marrow and spleen, forming biliverdin. Biliverdin reductase reduces biliverdin to fat-soluble bilirubin, the majority of which is released into the circulation, where it is bound to albumin for transport to hepatic sinusoidal membranes. When delivered to the liver, bilirubin undergoes active uptake, protein binding, transcellular transport, and eventual conjugation to glucuronic acid. The conjugated bilirubin is now water-soluble and is stored in the gall bladder until it is released into the duodenum to facilitate micelle formation and fat absorption. Following expulsion of the bile into the gastrointestinal tract, conjugated bilirubin undergoes deconjugation and hydrogenation by colonic bacteria. This produces a colorless pigment, mesobilirubinogen, which is oxidized to urobilinogens. These may be oxidized to urobilin (yellow-orange) or stercobilin (brown fecal pigment). Eighty to 90% of the urobilinogen is excreted in the feces as stercobilin, while only a small amount (10 to 20%) of the urobilinogen is reabsorbed to undergo enterohepatic cycling.  […]