The molecular weights of UDP-glucuronyltransferase determined with radiation-inactivation analysis. A molecular model of bilirubin UDP-glucuronyltransferase.

The molecular masses of the UDP-glucuronyltransferases were determined by means of radiation-inactivation analysis of sodium dodecyl sulfate-treated lyophilized rat liver microsomal preparations using a calibrated 60Co source. Bilirubin glucuronidation with formation of bilirubin monoglucuronide is catalyzed by a 41,500-Da enzyme; conversion of bilirubin mono- to diglucuronide is catalyzed by a 175,000-Da enzyme. The glucuronidation of estrone and testosterone is mediated by enzymes with molecular masses of 73,500 and 142,000 Da, respectively, and the glucuronidation of p-nitrophenol and phenolphthalein is mediated by enzymes with molecular masses of 109,000 and 159,000 Da, respectively. Our results show that UDP-glucuronyltransferase consists of a heterogenous group of enzymes with strikingly different molecular masses. Our data furthermore suggest that these enzymes may be oligomers composed of one to four subunits with similar molecular masses. Based on these findings, a molecular model of bilirubin UDP-glucuronyltransferase is proposed, consisting of four subunits. For bilirubin diglucuronide formation, the complete tetrameric enzyme is required, whereas formation of monoglucuronide can be mediated by a single subunit. The monomeric monoglucuronide-forming enzyme is resistant to sodium dodecyl sulfate, treatment whereas the tetrameric diglucuronide-forming enzyme is labile, but once inactivated, the diglucuronide-forming enzyme can be reconstituted by decreasing the sodium dodecyl sulfate concentration by means of dialysis.