Definition scleral icterus2/29/2024 ![]() Bilirubin is conjugated to glucuronic acid, taurine (and to a lesser extent glucose) primarily via glucuronosyltransferase to form bilirubin monoglyceride and diglucuronide. Within the hepatocytes bilirubin is bound by two proteins called y and z binding proteins which allow the process of conjugation to take place.Ĭonjugation renders bilirubin more water soluble, thereby facilitating the secretion of bilirubin into the bile. The process may exhibit saturation as the carrier is shared by conjugated bilirubin and a variety of organic anions including bile acids and bacterial endotoxin. Hepatocyte uptake of unconjugated bilirubin is a carrier mediated process. Transport of bilirubin by albumin ends with its disassociation at the hepatic sinusoidal membrane. Binding of bilirubin to albumin is disrupted by a low serum albumin concentration and competition from a variety of drugs such as sulfonamides, thyroxine and salicylates. This type of albumin-bound bilirubin is referred to as unconjugated, indirect reading, or lipid soluble bilirubin. Bilirubin is insoluble in water and the binding to albumin allows it to be transported and inhibits its diffusion into the tissues. Bilirubin is the only major breakdown product that requires excretion since the globin and heme molecules are reused.īilirubin is released from the mononuclear phagocyte system into the circulation where it circulates bound to albumin. Globin is enzymatically degraded to amino acids, iron is bound to transferrin and transported to the bone marrow, and the heme is converted to biliverdin by heme oxygenase and subsequently to bilirubin by biliverdin reductase. Reduction of heme occurs primarily in the liver and spleen where hemoglobin released from senescent red cells is phagocytosed by cells of the mononuclear phagocytic system. The remainder comes from catabolism of other pigments (e.g., cytochromes and catalases, particularly cytochrome p450). The bulk of bilirubin (approximately 80%) is produced as a result of the breakdown of senescent red cells. The intensity and distribution of tissue bile pigment staining depends on 4 factors: 1) the total serum bilirubin concentration 2) the form of bilirubin (unconjugated or conjugated) 3) capillary perfusion and the ability to detect a yellow hue through normal pink mucous membranes and 4) tissue composition, i.e., tissue high in fat is predisposed to deposition of lipid soluble unconjugated bilirubin while tissue high in elastic fibers, such as sclera and skin, is predisposed to deposition of water soluble conjugated bilirubin. Icterus results when the rate of bilirubin production exceeds its rate of elimination. Icterus (or jaundice) is defined as the clinical state in which hyperbilirubinemia is detectable as a yellowish discoloration of the plasma or soft tissues.
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