Radioassay of UDP-glucuronyltransferase-catalysed formation of bilirubin monoglucuronides and bilirubin diglucuronide in liver microsomes.

A radioassay for specific determination of the rates of UDP-glucuronic acid-dependent conversion of bilirubin into the two isomeric (C-8, C-12) bilirubin monoglucuronides and bilirubin diglucuronide is described and illustrated by its application to rat liver microsomes. The method is based on measurement of the relative amounts of radiolabel in unesterified bilirubin and its mono- and di-esterified reaction products after incubation with [14C]bilirubin as substrate. This analysis is performed by the alkaline-methanolysis procedure, combined with one of two t.l.c. systems developed in order to enhance the sensitivity, accuracy and precision of the radioassay. Results for rates of total bilirubin glucuronide formation obtained with the new assay and the standard enzyme assay based on the ethyl anthranilate diazo-method were identical. However, the sensitivity of the latter technique is approx. 10-fold lower than that of the radioassay.