STUDIES ON THE IN-VITRO REACTIVITY OF CLOFIBRYL AND FENOFIBRYL GLUCURONIDES - EVIDENCE FOR PROTEIN-BINDING VIA A SCHIFF-BASE MECHANISM

Clofibryl and fenofibryl acyl (ester) glucuronides (CAG and FAG) are m ajor metabolites in humans of the hypolipidaemic drugs clofibrate and fenofibrate, respectively. We have investigated three inter-related as pects of the reactivity of CAG and FAG in human serum albumin (HSA) so lution, human plasma and in buffer at pH 7.0: namely (a) rearrangement via acyl migration to glucuronic acid esters of clofibric acid (CA) a nd fenofibric acid (FA), (b) hydrolysis of the parent glucuronide and rearrangement products to yield CA and FA and (c) the formation of cov alent adducts with albumin and plasma protein. CAG was more reactive t han FAG in all media, especially the protein solutions. The reactivity of both glucuronides was accelerated in protein solution compared wit h buffer and this was more marked in plasma than in HSA solution. The predominant reaction during the initial stages of the incubation was f ormation of isomeric rearrangement products. In the protein solutions, CA and FA were the major reaction products after 24 hr, compared to t he rearranged isomers in buffer. Protein binding of C-14 to HSA was ma rkedly higher after incubation of CAG and FAG labelled on the glucuron yl moiety compared with the label on the aglycone. This is consistent with the covalent binding of CAG and FAG to protein proceeding via the formation of a Schiff s base rather than by transacylation.