Classification Term: 673

Bilirubin

C33H36N4O6 (584.2634716)

Bilirubin is a yellow bile pigment that is a degradation product of heme. It occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the bodys clearance of waste products that arise from the destruction of aged or abnormal red blood cells. Bilirubin has been found in all vertebrates and in certain plants including Strelitzia nicolai (PMID: 28573242). Bilirubin levels in humans are elevated in certain diseases such as jaundice and liver disease and it is responsible for the yellow color of bruises and the yellow discoloration in jaundice. Bilirubin breakdown products, such as stercobilin, cause the brown color of feces. A different breakdown product, urobilin, is the main component of the straw-yellow color in urine. Bilirubin consists of an open chain of four pyrroles (tetrapyrrole). It is formed by oxidative cleavage of a porphyrin in heme, which leads to biliverdin, a green tetrapyrrolic bile pigment that is also a product of heme catabolism. Biliverdin is then reduced to bilirubin via biliverdin reductase. After conjugation with glucuronic acid, bilirubin can be excreted in the urine. Bilirubin 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. Elevated bilirubin levels in humans are associated with Crigler-Najjar syndrome type I, which is an inborn error of metabolism. Crigler-Najjar syndrome is a rare genetic disorder characterized by an inability to properly convert and clear bilirubin from the body. Affected individuals cannot convert unconjugated bilirubin to the conjugated form because they lack a specific liver enzyme required to break down (metabolize) bilirubin. Since they cannot convert bilirubin, they develop abnormally high levels of unconjugated bilirubin in the blood (hyperbilirubinemia). Crigler-Najjar syndrome is caused by mutations in the UGT1A1 gene. The hallmark finding of Crigler-Najjar syndrome is a persistent yellowing of the skin, mucous membranes and whites of the eyes (jaundice). Elevation of both alanine aminotransferase and bilirubin levels in serum or plasma can be indicative of serious liver injury. High levels of bilirubin are indicative of jaundice, which is easily recognizable due to a yellowing of the skin and eyes. Bilirubin is also an antioxidant. Bilirubins antioxidant activity may be particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging superoxide during N-methyl-D-aspartic acid neurotransmission (PMID: 31353321). Bilirubin is a bile pigment that is a degradation product of heme. In particular, bilirubin is a yellow breakdown product of normal heme catabolism. Its levels are elevated in certain diseases and it is responsible for the yellow color of bruises. Bilirubin is an excretion product, and the body does not control levels. Bilirubin levels reflect the balance between production and excretion. Thus, there is no "normal" level of bilirubin. Bilirubin consists of an open chain of four pyrroles (tetrapyrrole); by contrast, the heme molecule is a closed ring of four pyrroles, called porphyrin. -- Wikipedia [HMDB]. Bilirubin is found in many foods, some of which are barley, mustard spinach, other bread, and sesbania flower. 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. 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.[3] In the first step of bilirubin synthesis, the heme molecule is stripped from the hemoglobin molecule. Heme then passes through various processes of porphyrin catabolism, which varies according to the region of the body in which the breakdown occurs. For example, the molecules excreted in the urine differ from those in the feces.[4] 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.[5][6] Ultimately, bilirubin is broken down within the body, and its metabolites excreted through bile and urine; elevated levels may indicate certain diseases.[7] It is responsible for the yellow color of healing bruises and the yellow discoloration in jaundice. The bacterial enzyme bilirubin reductase is responsible for the breakdown of bilirubin in the gut.[8] One breakdown product, urobilin, is the main component of the straw-yellow color in urine.[9] Another breakdown product, stercobilin, causes the brown color of feces. Although bilirubin is usually found in animals rather than plants, at least one plant species, Strelitzia nicolai, is known to contain the pigment.[10] Bilirubin is created by the activity of biliverdin reductase on biliverdin, a green tetrapyrrolic bile pigment that is also a product of heme catabolism. Bilirubin, when oxidized, reverts to become biliverdin once again. This cycle, in addition to the demonstration of the potent antioxidant activity of bilirubin,[14] has led to the hypothesis that bilirubin's main physiologic role is as a cellular antioxidant.[15][16] Consistent with this, animal studies suggest that eliminating bilirubin results in endogenous oxidative stress.[17] Bilirubin's antioxidant activity may be particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging superoxide during N-methyl-D-aspartic acid neurotransmission.[18] Bilirubin in plasma is mostly produced by the destruction of erythrocytes. Heme is metabolized into biliverdin (via heme oxygenase) and then into bilirubin (via biliverdin reductase) inside the macrophages. [11] Bilirubin is then released into the plasma and transported to the liver bound by albumin, since it is insoluble in water in this state. In this state, bilirubin is called unconjugated (despite being bound by albumin). [11] In the liver, unconjugated bilirubin is up-taken by the hepatocytes and subsequently conjugated with glucuronic acid (via the enzyme uridine diphosphate–glucuronyl transferase). In this state, bilirubin is soluble in water and it is called conjugated bilirubin. [11] Conjugated bilirubin is excreted into the bile ducts and enters the duodenum. During its transport to the colon, it is converted into urobilinogen by the bacterial enzyme bilirubin reductase.[8] Most of the urobilinogen is further reduced into stercobilinogen and is excreted through feces (air oxidizes stercobilinogen to stercobilin, which gives feces their characteristic brown color). [11] A lesser amount of urobilinogen is re-absorbed into portal circulation and transferred to the liver. For the most part, this urobilinogen is recycled to conjugated bilirubin and this process closes the enterohepatic circle. There is also an amount of urobilinogen which is not recycled, but rather enters the systemic circulation and subsequently the kidneys, where it is excreted. Air oxidizes urobilinogen into urobilin, which gives urine its characteristic color.[11][19] In parallel, a small amount of conjugated billirubin can also enter the systemic circulation and get excreted through urine. This is exaggerated in various pathological situations.[19]

Biliverdin

C33H34N4O6 (582.2478224)

Biliverdin is a green pigment formed as a byproduct of hemoglobin breakdown. It consists of four linearly connected pyrrole rings (a tetrapyrrole). Biliverdin is formed when the heme group in hemoglobin is cleaved at its alpha-methene bridge. The resulting biliverdin is then reduced to bilirubin, a yellow pigment, by the enzyme biliverdin reductase. The changing color of a bruise from deep purple to yellow over time is a graphical indicator of this reaction. Biosynthesized from hemoglobin as a precursor of bilirubin. Occurs in the bile of amphibia and of birds, but not in normal human bile or serum. [HMDB] Biliverdin is a green pigment formed as a byproduct of hemoglobin breakdown. It consists of four linearly connected pyrrole rings (a tetrapyrrole). Biliverdin is formed when the heme group in hemoglobin is cleaved at its alpha-methene bridge. The resulting biliverdin is then reduced to bilirubin, a yellow pigment, by the enzyme biliverdin reductase. The changing color of a bruise from deep purple to yellow over time is a graphical indicator of this reaction. Biliverdin occurs in the bile of amphibia and of birds, but not in normal human bile or serum. Biliverdin. CAS Common Chemistry. CAS, a division of the American Chemical Society, n.d. https://commonchemistry.cas.org/detail?cas_rn=114-25-0 (retrieved 2024-07-01) (CAS RN: 114-25-0). Licensed under the Attribution-Noncommercial 4.0 International License (CC BY-NC 4.0).

Mesobilirubinogen

C33H44N4O6 (592.3260683999999)

Mesobilirubinogen (also known as I-urobilinogen) is a tetrapyrrole chemical compound that is closely related to two other compounds: urobilinogen (also known as D-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 is the parent compound of both stercobilin (the pigment that is responsible for the brown colour of feces) and urobilin (the pigment that is responsible for the yellow colour 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 hemoglobin 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 colour of the biliverdin molecule from blue-green (vert or verd for green) to yellow-red, which is the colour 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 colourless 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 colour in urine. Urobilinogen is an uribiniloid, the product of bilirubin reduction in multiple sequential reactions. Urobilinogens are colorless chromogens that may in turn be oxidized to respective yellow oxidation products, urobilins. Under normal conditions only small amounts of bilirubin can be found in stools of adults while urobilinoids are predominant bile pigments (50-250 mg/day). Only negligible amounts of fecal urobilinoids are present in the intestinal lumen of infants during the first months of their life, due to undeveloped intestinal microflora capable of reducing bilirubin. This presumably contributes importantly to the pathogenesis of neonatal jaundice. In adults, the urobilinoid production is highly efficient. At times, it is re-excreted in the urine, where it may be later oxidized to urobilin. (PMID: 16504607) [HMDB]

Bilirubin diglucuronide

C45H52N4O18 (936.3276452)

Bilirubin diglucuronide is a glucuronidated version of bilirubin, a tetrapyrrole compound produced via heme degradation. Heme is 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.

Stercobilinogen

C33H48N4O6 (596.3573667999999)

Stercobilinogen is a tetrapyrrole chemical compound that is the parent compound of stercobilin, the pigment that is responsible for the brown color of feces. Stercobilinogen is formed through the reduction of its parent compound uroblinogen. 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. Stercobilinogen (aso known as L-urobilinogen) is closely related to two other compounds: mesobilirubinogen (also known as I-urobilinogen) and urobilinogen (also known as D-urobilinogen). Specifically, urobilinogen can be reduced to form mesobilirubinogen, and mesobilirubinogen can be further reduced to form stercobilinogen. Confusingly, however, all three of these compounds are frequently collectively referred to as "urobilinogens".

Urobilinogen

C33H42N4O6 (590.3104192000001)

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 Ehrlichs reagent, which contains para-dimethylaminobenzaldehyde. Ehrlichs 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. Urobilinogen is a colourless product of bilirubin reduction. It is formed in the intestines by bacterial action. Some urobilinogen is reabsorbed, taken up by the hepatocytes into the circulation and excreted by the kidney. This constitutes the normal "intrahepatic urobilinogen cycle".

Urobilin

C33H42N4O6 (590.3104192000001)

Urobilin, also known as urochrome, is the tetrapyrrole chemical compound that is primarily responsible for the yellow color of urine. Urobilin is formed through the oxidation of its parent compound uroblinogen. Urobilin 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. Some of the urobilinogen produced by the gut bacteria is reabsorbed and re-enters the enterohepatic circulation. These urobilinogens are 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. Many urine tests monitor the amount of urobilin in urine, as this provides some useful insight into urinary tract function. Normally, urine would appear as either light yellow or colorless. A lack of water intake, for example following sleep or dehydration, reduces the water content of urine, thereby concentrating urobilin and producing a darker color of urine. Obstructive jaundice reduces biliary bilirubin excretion, which is then excreted directly from the blood stream into the urine, giving a dark-colored urine. This dark colored urine has a paradoxically low urobilin concentration.

(4E,15Z)-Bilirubin

C33H36N4O6 (584.2634716)

(4E,15Z)-Bilirubin IXa belongs to the class of organic compounds known as bilirubins. These are organic compounds containing a dicarboxylic acyclic tetrapyrrole derivative. Bilirubin (BR) is a yellow compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. (4E,15Z)-Bilirubin IXa is a linear tetrapyrrole, and a product of heme degradation. It is a member of the class of compounds known as biladienes. Biladienes consist of two linear tetrapyrroles in which the carbon bridges contain two more double bonds than bilane. (4E,15Z)-Bilirubin IXa is an isomer of bilirubin, which appears to have antioxidant effects. Bilirubins antioxidant activity may be particularly important in the brain, where it prevents excitotoxicity and neuronal death by scavenging superoxide during N-methyl-D-aspartic acid neurotransmission. (4E,15Z)-Bilirubin IXa is formed by oxidative cleavage of a porphyrin in heme, which first produces biliverdin. Biliverdin is then reduced to bilirubin by biliverdin reductase. Some of the double-bonds in bilirubin isomerize when exposed to light. The E,Z-isomers of bilirubin, such s (4E,15Z)-Bilirubin IXa formed upon light exposure are more soluble than the unilluminated Z,Z-isomer. Altered levels of (4E,15Z)-Bilirubin IXa in human serum have been used as a biomarker of acrylamide exposure (PMID: 28163100). 4E,15Z-Bilirubin IXa is an isomer of bilirubin, which has antioxidative effects. When bilirubin reacts with reactive oxygen species, oxidized metabolites of bilirubin are formed, such as biliverdin and propentdyopents. A decrease in serum bilirubin concentration and an increase in serum and urinary oxidized metabolites of bilirubin may indicate the protective action of bilirubin against reactive oxygen species. (PMID 10986860) [HMDB] COVID info from COVID-19 Disease Map Corona-virus Coronavirus SARS-CoV-2 COVID-19 SARS-CoV COVID19 SARS2 SARS

Bilirubin glucuronide

C39H44N4O12 (760.2955584)

Bilirubin glucuronide is a natural human metabolite of bilirubin generated in the liver by UDP glucuonyltransferase. Glucuronidation is used to assist in the excretion of toxic substances, drugs or other substances that cannot be used as an energy source. Glucuronic acid is attached via a glycosidic bond to the substance, and the resulting glucuronide, which has a much higher water solubility than the original substance, is eventually excreted by the kidneys. Bilirubin glucuronide is a natural human metabolite of bilirubin generated in the liver by UDP glucuonyltransferase.

Stercobilin

C33H46N4O6 (594.3417176)

Stercobilin is the tetrapyrrole chemical compound that is primarily responsible for the brown color of feces. It was originally isolated from feces in 1932. Stercobilin is formed through the reduction of its parent compound stercblinogen. 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. Stercobilin is responsible for the brown color of human feces. Stercobilin is then excreted in the feces. It is a microbial metabolite.

(4E,15E)-Bilirubin

C33H36N4O6 (584.2634716)

(4E,15E)-Bilirubin is an isomer of bilirubin and is less lipophilic and more polar than the naturally occurring Z-Z isomer (PMID: 426785). Bilirubin is a bile pigment that is a degradation product of heme. In particular, bilirubin is a yellow breakdown product of normal heme catabolism. Its levels are elevated in certain diseases and it is responsible for the yellow colour of bruises. Bilirubin is an excretion product and the body does not control its levels. Bilirubin levels reflect the balance between production and excretion. Thus, there is no "normal" level of bilirubin. Bilirubin consists of an open chain of four pyrroles (tetrapyrrole). In contrast, the heme molecule is a closed ring of four pyrroles, called porphyrin (Wikipedia).

(2R,3Z)-Phycocyanobilin

C33H38N4O6 (586.2791208)

21H-Biline-8,12-dipropanoicacid, 3,18-diethenyl-1,19,22,24-tetrahydro-2,7,13,17-tetramethyl-1,19-dioxo-

C33H34N4O6 (582.2478224)

(3Z)-phytochromobilin

C33H34N4O6 (582.2478224)

(3z)-phytochromobilin is practically insoluble (in water) and a weakly acidic compound (based on its pKa). (3z)-phytochromobilin can be found in a number of food items such as spelt, lotus, oat, and breakfast cereal, which makes (3z)-phytochromobilin a potential biomarker for the consumption of these food products.

biliverdin-IX-alpha

C33H32N4O6 (580.2321732)

Biliverdin-ix-alpha is practically insoluble (in water) and a weakly acidic compound (based on its pKa). Biliverdin-ix-alpha can be found in a number of food items such as pineappple sage, potato, pitanga, and kumquat, which makes biliverdin-ix-alpha a potential biomarker for the consumption of these food products.