Critique of the Assay and Significance of Bilirubin Conjugation

Hepatic Accumulation and Intracellular Binding of Conjugated Bilirubin

“…They confirmed earlier data (5,38) indicating that the monoglucuronide was the only bilirubin conjugate produced by the microsomal conjugating mechanism. In addition, they presented evidence that the conversion of bilirubin monoglucuronide to the diglucuronide occurs by a transesterifieation step not requiring UDPglucuronicacid as a cofactor, and catalyzed by an enzyme preparation localized to the liver cell plasma membrane, and present in highest concentration in the canalicular-enriched membrane fractions.…”

“…[3H]Bilirubin (4)(5)uCi) was injected intravenously and all bile was collected into preweighed glass scintillation vials. For the first 20 min, vials were changed at 2-min intervals, and capped tightly.…”

“…These studies indicate that (a) both bilirubin and its monoglucuronide accumulate within the liver cell as ligands with the glutathione S-transferase; and (b) INTRODUCTION Hepatic excretion of organic anions is believed to proceed by several discrete but interrelated processes: uptake by the hepatocyte, "storage", conjugation, and excretion into bile (1,2). In particular, bilirubin is selectively transported into the hepatocyte (3), bound there to specific cytoplasmic proteins (4), conjugated to more polar compounds (5), and actively transferred to the bile canaliculus (2,6). The presence of intrahepatocellular macromolecules with high affinity for bilirubin has been localized to two protein fractions designated as Y (ligandin) and Z (4,7,8) of which the former appears to be quantitatively more important with respect to bilirubin binding (4).…”

Unconjugated Bilirubin and an Increased Proportion of Bilirubin Monoconjugates in the Bile of Patients with Gilbert's Syndrome and Crigler-Najjar Disease

“…1). Provided certain conditions are met, the azopigment fraction a0-EA (pH 2.7) multiplied by 2 equals the amount of monoconjugates expressed as a percentage of the total conjugated bilirubin-IXa (38,39). This ao-EA (pH 2.7) fraction can assume the extreme values of 0 (no monoconjugate) and 50% (only monoconjugates), but theoretically should never exceed 50%.…”

Mechanism of Bilirubin Diglucuronide Formation in Intact Rats

“…It is well established that formation of bilirubin monoglucuronide (BMG) is catalyzed by microsomal uridine diphosphate (UDP)-glucuronosyltransferase [UDP-glucuronate /3-glucuronosyltransferase (acceptor unspecific), EC 2.4.1.17], with UDP-glucuronic acid (UDPGlcUA) serving as sugar donor (5). It remained unexplained, however, whv on incubation of liver homogenate or microsomiies frolm humans, rats, dogs, and cats with UDPGlcUA, the reaction product was largely BMG (6), while in these species bile contains predominantly bilirubin diglucuronide (BDG) (2,3,(7)(8)(9). Because of this apparent inconsistenicy between the findings in vivo and in vitro, it has been postulated that conversion of B.MG to BDG is catalyzed by a separate enzyme that differs fromii bilirubin UDP-glucuronosyltransferase.…”