Katherine Viviana Cisneros , University of Florida, Gainesville, FL
Vinayak Agarwal , Georgia Institute of Technology, Atlanta, GA
Margaret O James , Medicinal Chemistry, University of Florida, Gainesville, FL
Polybrominated diphenyl ethers (PBDEs) have been used as flame retardants since the 1970s, and some congeners are classified as persistent and bioaccumulative. There is evidence that hydroxy and hydroxy-methyl tetra-BDEs are also natural products, as their synthesis has been demonstrated in marine bacteria, sponges, and macroalgae. Despite the prevalence of PBDEs in the environment, their biotransformation has not been extensively studied, however it is known that they can be debrominated or follow an oxidative pathway at varying rates and to varying degrees depending on degree of bromination and species of animal, thus producing metabolites that have proven to be more reactive or toxic, such as hydroxylated-polybrominated diphenyl ethers (OH-BDEs). Structurally related molecules such as triclosan have been shown to potently inhibit estrogen sulfotransferase, and are implicated as endocrine disruptors, so it is important to understand the properties of OH-BDEs and their likely fate in people. In this study, the glucuronidation and sulfonation of four OH-tetraBDEs, named by the convention established with polychlorinated biphenyls, were studied in human liver microsomes and cytosol fractions from male and female individuals aged 45-75. Linear conditions of product formation were established with 6OH-BDE47 and used for each studied substrate. To follow product formation we used radiolabeled co-substrates at saturating concentrations of 1mM 14C-UDPGA or 20 µM 35S-PAPS. Several concentrations of each substrate were studied, to calculate apparent kinetic constants. Following incubations of 10 to 15 min, conjugates were extracted as their ion-pairs into ethyl acetate for quantitation. Each of the OH-tetraBDEs studied, 6OH-BDE47, 2OH-BDE68, 4OH-BDE68 and 2-OH-6’MeOBDE were more readily glucuronidated than sulfonated. For example, the efficiency of glucuronidation of 6OH-BDE47 (Vmax/Km) was 111± 27 µl/min/mg protein (mean ± S.D., n=3), while the efficiency of sulfonation was 6.2 ± 2.3. Some of the studied substrates showed substrate inhibition for glucuronidation at concentrations above 50 to 100 µM, but this was not observed for sulfonation at concentrations up to 75 µM. This work shows that the OH-BDEs are substrates for human liver UGTs and SULTs and should be eliminated through these conjugation pathways.

Supported in part by University of Florida Clinical and Translational Science Institute TL1 grant.