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|Section2= |Section3= }} Bilirubin (formerly referred to as haematoidin) is the yellow breakdown product of normal heme catabolism, caused by the body's clearance of aged red blood cells which contain hemoglobin. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases. It is responsible for the yellow color of bruises and the yellow discoloration in jaundice. It is also responsible for the brown color of feces, via its conversion to stercobilin, and the background straw-yellow color of urine via its breakdown product, urobilin. It has also been found in plants. == Chemistry == Bilirubin consists of an open chain of four pyrrole-like rings (tetrapyrrole). In heme, these four rings are connected into a larger ring, called a porphyrin ring. Bilirubin can be "conjugated" with a molecule of glucuronic acid which makes it soluble in water (see below). This is an example of glucuronidation. Bilirubin is very similar to the pigment phycobilin used by certain algae to capture light energy, and to the pigment phytochrome used by plants to sense light. All of these contain an open chain of four pyrrolic rings. Like these other pigments, some of the double-bonds in bilirubin isomerize when exposed to light. This is used in the phototherapy of jaundiced newborns: the E,Z-isomers of bilirubin formed upon light exposure are more soluble than the unilluminated Z,Z-isomer, as the possibility of intramolecular hydrogen bonding is removed. This allows the excretion of unconjugated bilirubin in bile. Some textbooks and research articles show the incorrect geometric isomer of bilirubin. The naturally occurring isomer is the Z,Z-isomer. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「bilirubin」の詳細全文を読む スポンサード リンク
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