Glucuronate transport

Bilirubin formed in peripheral tissues is transported to the hver by plasma albumin. The further metabolism of bihtubin occuts primarily in the hver. It can be divided into thtee processes (1) uptake of bilirubin by hver parenchymal cells, (2) conjugation of bilirubin with glucuronate in the endoplasmic reticulum, and (3) secretion of conjugated bilirubin into the bile. Each of these processes will be considered separately. 

Jedlitschky, G., Leier, I., Buchholz, U., Barnouin, K., Kurz, G. and Keppler, D. (1996) Transport of glutathione, glucuronate, and sulfate conjugates by the MRP gene-encoded conjugate export pump. Cancer Research, 56, 988-994. 

Bilirubin is the waste product derived from haem catabolism. In order to be eliminated from the body, mainly via the gut, bilirubin must be processed through the liver (see Section 6.4). Bilirubin is, however, insoluble in water, so to reach the liver from the spleen where a substantial amount of red cell destruction occurs, bilirubin must first be bound to albumin. As blood perfuses the liver, bilirubin is transported into the hepatocyte where it is conjugated with glucuronic acid prior to excretion. 

The chemical is removed before it can properly reach the cytoplasm or important organelles. The substrates for this transporter are structurally diverse but tend to be organic, weakly basic (cationic), or uncharged hydrophobic or amphipathic substances. Thus, the chemical diffuses into the cell and is then pumped out. Substrates include anions conjugated with glutathione (GSH), glucuronic acid, and sulfate. 

Figure 22-1. Production of bilirubin (BR). The degradation of I e l 3-hcme by molecular oxygen and NAD PH, catalyzed by microsomal heme oxygenase, produces biUverdin, CO, and Fe+2. Subsequent reduction of biliverdin by NADPH, catalyzed by biliverdin reductase, produces bilirubin. Bilirubin that is produced in phagocytes from degradation of senescent erythrocytes is transported to liver for conjugation with glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. In some cells, the bilirubin is used as an antioxidant, where it recycles through the biliverdin reductase reaction.

Within hepatocytes, bilirubin forms a complex with an abundant cytosolic family of enzymes, the glutathione 5-transferases, that have bilirubinbinding sites and function to transport bilirubin to the surface of the endoplasmic reticulum, where one or two glucuronic acid moieties are added to the propionic acid side chains of bilirubin (Fig. 22-1).This reaction, catalyzed by bilirubin-UDP-glucuronyltransferase, involves transfer of glucuronic acid from UDP-glucuronide to bilirubin. The effect of attachment of one or... 

Figure 22-3. Transport and hepatic metabolism of bilirubin. Bilirubin that is produced in phagocytes is transported to liver as an albumin-bilirubin complex. Uptake into the hepatocytes takes place in liver sinusoids. Within the hepatocyte, bilirubin is transported to the endoplasmic reticulum (microsomes) bound to glutathione S-transferase (GST). Bilirubin is made water soluble by addition of one or two glucuronic acid moieties obtained from UPD-glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. The product, conjugated bilirubin, is transported across the bile canalicular membrane for secretion into the biliary system, with subsequent movement into the intestines.

Figure 12.2. Interrelationships among Phase I (hydroxylation). Phase II (glucuronic acid, sulfate, and glutathione conjugation), and Phase III (ABC transporter-mediated efflux) detoxification processes leading to the inactivation and elimination of xenobiotics.

HA is an unsulfated glycosaminoglycane composed of repeating disaccharide units of D-glucuronic acid and A-acetylglucosaminc linked a-( 1 —4) and p-( 1—3), respectively. HA has special importance because it is a component of the ECM [53] in the soft tissues of mammals, where it mainly ensures water retention [54], This enables the transport of nutrients to, and removal of waste from, cells that do not have a direct blood supply, such a cartilage cells. Moreover, HA is present in the synovial joint fluid, the vitreous humor of the eye, cartilage, blood vessels, and the unbilical cord. More detailed information about the biological functions and physicochemical properties of HA can be found elsewhere [55, 56],... 

Modulation of liver and kidney function. Nutrients and xenobiotics (such as secondary metabolites) are transported to the liver after resorption in the intestine. In the liver, the metabolism of carbohydrates, amino acids, and lipids takes place with the subsequent synthesis of proteins and glycogen. The liver is also the main site for detoxification of xenobiotics. Lipophilic compounds, which are easily resorbed from the diet, are often hydroxylated and then conjugated with a polar, hydrophilic molecule, such as glucuronic acid, sulfate, or amino acids (312). These conjugates, which are more water soluble, are exported via the blood to the kidney, where they are transported into the urine for elimination. 

A sophisticated approach using active site-directed pho-toafflnity substrate analogs has also been applied to study the inward transport in isolated microsomal vesicles. These experiments provided convincing and elegant evidence for the translocation of UDP-glucuronate (48) and of FAD (12) by photo incorporation of the probes into the luminally oriented enzymes in intact microsomes. The drawback of the method is that it is not suitable to determine of the rate or capacity of transport. 

Radominska A, Berg C, Treat S, Little JM, Lester R, Gollan JL, Drake RR. Characterization of UDP-glucuronic acid transport in rat liver microsomal vesicles with photoaffinity analogs. Biochim. Biophys. Acta 1994 1195 63-70... 

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