Hepatic disposition and biliary excretion of bilirubin and bilirubin glucuronides in intact rats. Differential processing of pigments derived from intra- and extrahepatic sources.

Abstract

Mechanisms for transport of bilirubin and its conjugates in hepatocytes have not been defined. We investigated the hepatic processing of bilirubin glucuronides and their precursors, and characterized the disposition of bile pigments arising from intraversus extrahepatic sources. Tracer doses of purified radiolabeled biliverdin, bilirubin, bilirubin monoglucuronide (BMG) or diglucuronide (BDG) were administered intravenously to intact normal or jaundiced homozygous Gunn rats. Rapid sequential analysis of radiolabeled BMG and BDG in bile revealed comparable excretion patterns following biliverdin and bilirubin injection, with BDG as the major pigment. Biliary excretion of radiolabeled conjugates from injected BMG was more rapid, with BMG predominating. Excretion of injected BDG in normal rats and BMG or BDG in Gunn rats was virtually identical to that of unaltered BMG in normal rats. Model independent analysis by deconvolution provided objective comparison of the disposition of radiolabeled pigments from the different sources. These findings indicate that bilirubin glucuronides formed in the liver from endogenous (hepatic) and exogenous (extrahepatic) sources of bilirubin follow a similar excretory pathway. BMG formed endogenously is converted preferentially to BDG, whereas circulating BMG is excreted predominantly unchanged. Exogenous conjugated bilirubins are excreted more rapidly than those generated intrahepatically, by a transcellular pathway that is largely independent of the conjugation system.

Hepatic Uptake of Bilirubin and Its Conjugates by the Human Organic Anion Transporter SLC21A6

“…3 H]Bilirubin was obtained biosynthetically in rats in a procedure similar to the one described by Crawford et al (20). Two Harlan Sprague-Dawley rats were given an intravenous or intraportal injection of 500 Ci of [3, (6,7) and purified using radio-HPLC.…”

Estimating the Differences of UGT1A1 Activity in Recombinant UGT1A1 Enzyme, Human Liver Microsomes and Rat Liver Microsome Incubation Systems <i>in Vitro</i>

“…It involves the conjugation of a glucuronyl moiety to either of the two propionic acid side chains of bilirubin, which are located on the C-8 and C-12 carbons of the two central pyrrole rings. This creates two bilirubin monoglucuronide (BMG) isomers (BMG1 and BMG2), which can be further glucuronidated to form an 8,12-diglucuronide (BDG 2,3) ; Fig. 1).…”

Biliary excretion of a stretched bilirubin in UGT1A1‐deficient (Gunn) and Mrp2‐deficient (TR−) rats

“…The synthesis and properties of 2,2P- [1,4- (3) have been described previously [17]. Sodium 2,2P-[(2,17-diethenyl-1,10,19,22,23,24-hexahydro-3,7,13,18-tetramethyl-1,19-dioxo-21H-biline-8,12-diyl)bis[(1-oxo-3,1-propanediyl)imino]]bis-ethanesulfonate (bilirubin ditaurine amide, 4, disodium salt; sold as`bilirubin conjugate'), obtained from Porphyrin Products (Logan, UT, USA), showed one main peak on high performance liquid chromatography (HPLC) and was used without further puri¢cation.…”

“…The natural dicarboxylic acids bilirubin ( 1 ) and biliverdin ( 2 ) are paradigmatic examples of small endogenous molecules (molecular weight ∼600) whose metabolism seems to be highly dependent on these factors. With almost identical constitutional structures, molecular weights, and p K a s [1,2], both compounds are anions at physiologic pH and both are cleared efficiently from blood by the liver [3]. Yet their hepatic metabolism is very different.…”

“…In contrast, bilirubin taken up by the liver is not secreted in bile, despite the presence of organic anion transporters in the canalicular membrane. Rather, it is converted by the glucuronosyl enzyme uridinediphosphoglucuronosyl transferase (UGT)1A1 to monoglucuronides, which are subsequently secreted rapidly from the liver into bile [3]. The presence of the ATPase Mrp2 (multidrug resistance‐associated protein 2) in the canalicular membrane of the hepatocyte is essential for their secretion since mutant rats that lack it (TR − or Eisai rats) do not excrete bilirubin mono‐ or diglucuronides efficiently in bile [6–8].…”