Characterization of Bucolome N-Glucuronide Formation: Tissue Specificity and Identification of Rat UDP-Glucuronosyltransferase Isoform (s)

Bucolome N-glucuronide (BCP-NG, main metabolite of bucolome (BCP) is the first N-glucuronide of barbituric acid derivatives isolated from rat bile. The objective of this study was to identify the main tissue producing BCP-NG and the molecular species of BCP-NG-producing UGT. Four target tissues were investigated: the liver, small and large intestines, and kidney. To identify the UGT molecular species responsible for BCP-NG formation, yeast microsomes expressing each rat UGT isoform were prepared. BCP-NG formation was detected in all microsomal fractions of the 4 tissues. The liver microsomal BCP-NG-producing activity was the highest, followed by that in the small intestinal microsomes, showing about 41% of the liver microsomal activity level. BCP-NG-producing activity (min-1) was determined in yeast microsomal fractions expressing rat UGT isoforms, and the activity was detected in UGT1A1 (0.059), UGT1A2 (0.318), UGT1A3 (0.001), UGT1A7 (0.003), UGT2B1 (0.004), UGT2B3 (0.091), and UGT2B6 (0.031), showing particularly high levels for UGT1A1 and UGT1A2 among the UGT1A isoforms. It was clarified that UGT1A1, widely distributed in rat tissues, is the molecular species responsible for BCP-NG formation.