UDP-Glucuronyltransferase Kinetics for 3-Trifluoromethyl-4-nitrophenol (TFM) in Fish

Abstract

Studies were conducted to address glucuronidation of 3-trifluoromethyl-4-nitrophenol (TFM) in sea lampreys Petromyzon marinus, channel catfish Ictalurus punctatus, rainbow trout Oncorhynchus mykiss, and bluegills Lepomis macrochirus. The ability of these species to biotransform TFM was investigated by determining the kinetics of UDP-glucuronyltransferase (UDPGT; also known as glucuronosyltransferase) in vitro from hepatic microsomal preparations. Maximal velocity (Vmax, nmol/min·mg) for UDPGT activity toward TFM was significantly greater (P < 0.05) in bluegills (1.52), rainbow trout (1.82), and channel catfish (1.46) than in sea lampreys (0.68). Binding affinities (Km) of UDPGT for TFM varied significantly among species in the following order: Bluegill (58 μM) > rainbow trout (97 μM) > channel catfish (172 μM) > sea lamprey (261 μM). Analysis of Vmax/Km ratios, a measure of enzyme efficiency (nmol/min-mguM TFM), indicated that the efficiency of UDPGT activities in all species examined was influenced more by binding affinity (Km) than by the Vmax of the reaction. These calculated ratios were progressively lower for species that were previously reported to be more sensitive to aqueous TFM (i.e., to have lower LC50s, TFM concentrations lethal to half the test fish). Sea lampreys appear to have relatively low UDPGT activity and binding affinity for phenolic substrates. This, in part, may account for the sensitivity of the sea lamprey to aqueous TFM.