![]() Like these other pigments, some of the double-bonds in bilirubin isomerize when exposed to light. All of these contain an open chain of four pyrrolic rings. īilirubin is structurally similar to the pigment phycobilin used by certain algae to capture light energy, and to the pigment phytochrome used by plants to sense light. After conjugation with glucuronic acid, bilirubin is excreted. ![]() It is formed by oxidative cleavage of a porphyrin in heme, which affords biliverdin. Structure īilirubin consists of an open-chain tetrapyrrole. Īlthough bilirubin is usually found in animals rather than plants, at least one plant species, Strelitzia nicolai, is known to contain the pigment. A different breakdown product, urobilin, is the main component of the straw-yellow color in urine. Its breakdown products, such as stercobilin, cause the brown color of feces. It is responsible for the yellow color of healing bruises and the yellow discoloration in jaundice. Ultimately, bilirubin is broken down within the body, and its metabolites excreted through bile and urine elevated levels may indicate certain diseases. The production of biliverdin from heme is the first major step in the catabolic pathway, after which the enzyme biliverdin reductase performs the second step, producing bilirubin from biliverdin. ![]() For example, the molecules excreted in the urine differ from those in the feces. Heme then passes through various processes of porphyrin catabolism, which varies according to the region of the body in which the breakdown occurs. In the first step of bilirubin synthesis, the heme molecule is stripped from the hemoglobin molecule. This catabolism is a necessary process in the body's clearance of waste products that arise from the destruction of aged or abnormal red blood cells. Bilirubin ( BR) (from the Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates.
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