Glucuronidation Of Hydroxylated Bromodiphenyl Ethers In Liver Microsomes Of A Marine And A Freshwater Fish

Abstract

There are two sources of hydroxylated bromodiphenyl ethers in aquatic environments, biotransformation products of bromodiphenyl ether (BDE) flame retardants, and natural products biosynthesized by marine bacteria. Regardless of source, BDEs and their hydroxylated metabolites (OH-BDEs) are lipophilic molecules that can bioaccumulate in animals. OH-BDEs are potential substrates for glucuronidation and sulfonation, as demonstrated with human liver samples (Cisneros et. al. Chemosphere, 226, 132-139, 2019). The objective of the present study was to determine how well fish can eliminate selected OH-BDEs by glucuronidation. The four OH-BDEs studied here were natural products, 4-OH-BDE47, 2’-OH-BDE68, 6-OH-2’MeO-BDE68 or closely related to natural products, 4’-OH-BDE68. Liver microsomes were prepared from laboratory-maintained adult Channel Catfish, Ictalurus punctatus, two females and one male, held in freshwater tanks, and from wild-caught adult Red Snapper, Lutjanus campechanus, from the Gulf of Mexico, two females and two males. To determine rates of glucuronidation, uridine-diphosphate-14C-glucuronic acid (UDPGA), 1 mM, was incubated with liver microsomes, 0.05 mg, in the presence of 0.1M Tris-Cl buffer, pH 7.6, magnesium chloride, 5 mM, and varying concentrations of OH-BDE, 2.5 to 50 µM for 5 min. The amount of protein and incubation time were determined from preliminary studies to show linear formation of the glucuronide products. After 5 min, reactions were stopped by addition of tetrabutylammonium dihydrogen phosphate and acetic acid, and the ion-pair of the glucuronide was extracted into ethyl acetate for determination of 14C, following a previously published procedure. In both fish species, the rates of glucuronidation with increasing substrate concentration followed Michaelis-Menten kinetics. The Km values for the OH-BDEs ranged from 3 to 35 µM and were generally somewhat lower in the catfish than the red snapper, though this only reached statistical significance, p<0.05, for 2’-OH-BDE68. Vmax values ranged from 0.9 to 5.4 nmol glucuronide per min per mg protein and for each substrate were variable between individual fish. Mean glucuronidation enzyme efficiency was highest for 2’-OH-BDE68 in the channel catfish, 715 ± 119 µL.min-1.mg-1 (mean ± S.D., n = 3), significantly higher than for red snapper, 239 ± 74 µL.min-1.mg-1 (n=4). The lowest glucuronidation efficiencies were found for 6-OH-BDE47, 145 ± 57 µL.min-1.mg-1, n=3 in Channel Catfish and 68 ± 37 µL.min-1.mg-1, n=4, in Red Snapper. Because of the variability, the efficiencies of 6-OH-BDE47 glucuronidation were similar between the two species. The glucuronidation efficiencies reported here for two fish species are of similar orders of magnitude to those observed with human liver microsomes. Both fish species were able to efficiently catalyze glucuronidation of the studied OH-BDEs, suggesting these compounds should be readily excreted from exposed Channel Catfish or Red Snapper.