Ligandin, bilirubin transport

Unconjugated bilirubin is taken into the hepatocytes by binding to membrane transport proteins and transported through the liver cells to the SER by proteins called ligandins. The SER is the location of a specific bilirubin-UDP-glucuronosyl transferase... 

Z Uptake of bilirubin by the liven Bilirubin is only slightly soluble in plasma and, therefore, is transported to the liver by binding non-covalently to albumin. [Note Certain anionic drugs, such as salicylates and sulfonamides,1 can displace bilirubin from abu-min, permitting bilirubin to enter the central nervous system (CNS). This causes the potential for neural damage in infants.] Bilirubin dissociates from the carrier albumin molecule and enters a hepatocyte, where it binds to intracellular proteins, particularly the protein ligandin. 

In about 90% of all neonates, jaundice occurs after the first 2-5 days of life and rarely exceeds 6 mg/dl serum bilirubin. In premature infants, bilirubin levels can rise to 10-12 mg/dl. The cause is related to a number of factors .) reinforced degradation of haemoglobin as a result of the short erythrocyte survival span of 70-90 days (120 days in adults), (2.) reduction in cellular transport proteins, above all ligandin, (i.) deficiency of uri-dyltransferase and glucuronosyltransferase, and (4.) increasing intestinal absorption of meconium bilirubin. 

After the reduction of biliverdin, the bilirubin that is formed then undergoes a series of transport and transformation steps which ultimately lead to its excretion in the intestinal tract. From the sites of its production, bilirubin is released into the plasma where it efficiently binds to albumin, which acts as a plasma-transport system. The bilirubin-albumin complex is carried in the plasma to liver cells (hepatocytes), where the bilirubin is released from its albumin carrier protein and transported across the cell bilayer membrane into the hepatocyte. Once inside, the bilirubin is bound in the cytoplasm to anion-binding proteins such as ligandin. The latter carries the bilirubin to membrane-bound enzymes (localised in the endoplasmic reticulum of the liver cell) which catalyse the esterification of bilirubin, the ester groups (mainly p-D-glucuronoside, but also smaller amounts of fi-D-xylo-pyranosides and p-D-glucopyranosides) being transferred from their uridine diphosphate nucleotides. 

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