A rat liver microsomal carboxyesterase and a bilirubin UDP-glucuronyl transferase are responsible for the formation of bilirubin glucuronides from bilirubin dimethyl ester

Abstract

The hepatic clearance of non-conjugated bilirubin from the circulation is an essential homeostatic requirement of mammalian species. Failure of this hepatic function can lead to irreversible brain damage, kernicterus and even death in both the neonatal infant and some adults [ 11. The definition of one of the steps in the clearance mechanism, the conjugation of bilirubin to more polar metabolites, is critical for the understanding of certain hereditary hyperbilirubinemias. The lifelong unconjugated hyperbilirubinemia of the Crigler-Najjar syndrome of man and its animal analogue, the Gunn rat, are examples of essentially a complete inability to form the ester glucuronide of bilirubin in vivo and also when hepatic microsomes are assayed in vitro [2,3]. In [4], infusion of the dimethyl ester of bilirubin (DMB) in Gunn rats resulted in the biliary excretion of both monoand diglucuronides of bilirubin. Bilirubin glucuronides were formed in vitro by liver microsomes from both the Gunn rat and a patient with the Crigler-Najjar syndrome if DMB was used as substrate [4]. The in vitro yields of bilirubin glucuronides from DMB are small and not always reproducible using Gunn rat liver microsomes (G. B. O., B. B., unpublished), therefore we decided to use normal Wistar rat liver to determine what enzymatic pathways are responsible for the biotransformation of DMB to bilirubin glucuronides. Here, we report that by following the procedure for isolation and purification of Wistar rat liver microsomal bilirubin UDP-glucuronyltransferase in [5] and by further measurement of carboxyesterase activities in these purified fractions, we are able to demonstrate that a microsomal esterase and a bilirubin UDP-glucuronyltransferase catalyse the biotransformation of DMB to bilirubin glucuronides. The relevance of this work to the study of the genetic deficiency of bilirubin UDP-glucuronyltransferase in Gunn rat liver is discussed.