Regioselectivity of Human UDP-Glucuronosyl-transferase 1A1 in the Synthesis of Flavonoid Glucuronides Determined by Metal Complexation and Tandem Mass Spectrometry

A three-part tandem mass spectrometric strategy that entails MSn analysis and a post-column LC-MS cobalt complexation method is developed to identify flavonoid monoglucuronide metabolites synthesized using the 1A1 isozyme of human UDP-glucuronosyltransferase (UGT). Ten flavonoid aglycons were used as substrates, spanning the subclasses of flavones, flavonols, and flavanones. The products were characterized by LC-MS and LC-MSn, with post-column cobalt complexation employed to pinpoint the specific sites of conjugation. The dissociation of complexes of the form [Co(II) (flavonoid glucuronide − H) (4,7-diphenyl-1,10-phenanthroline)2]+ allowed identification of the products and differentiation of isomers. The correlation between glycosylation site and elution order is used to provide additional structural confirmation. Flavonoids lacking a 3′ hydroxyl group were glucuronidated only at position 7, while those containing this functionality also formed 3′-O-glucuronides and sometimes 4′-O-glucuronides, thus supporting the conclusion that the presence or absence of the 3′-OH group is the major determinant of the regioselectivity of glucuronidation. Moreover, the specific distribution of multiple glucuronide products (7-O, 3′-O, 4′-O) is governed by the subclass of flavonoid.