Interactions of Hydroxylated Tetrabromodiphenyl Ethers with Phase II Enzymes

Abstract

Polybrominated diphenyl ethers (BDEs) 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 BDEs in the environment, their biotransformation has not been extensively studied, however it is known that they can be debrominated or monooxygenated 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). The present studies examined the human hepatic phase II metabolism of three natural product OH‐BDEs (6‐OH‐BDE47, 2‐OH‐BDE68, and 2‐OH‐6′methoxy‐BDE68) and a synthetic OH‐BDE (4‐OH‐BDE68), which was synthesized to study structure‐metabolism activity. Human liver microsomes and cytosol were prepared from de‐identified tissue bank samples from two female and five male donors aged 31 to 75 under an exempt protocol approved by the University of Florida IRB. Recombinant human SULT1A1, 1B1 and 1E1 enzymes were expressed and purified from bacterial expression systems. Sulfonation and glucuronidation of each OH‐BDE were studied using radiolabeled co‐substrates, 3′phosphoadenosine‐5′phospho‐35S‐sulfate or uridine diphospho‐β‐D‐14C‐glucuronic acid. The sulfated or glucuronidated products were extracted as ion‐pairs into ethyl acetate and quantitated or were separated by TLC and quantified by imaging the radioactivity. The OH‐BDEs studied were somewhat more efficiently glucuronidated than sulfonated. Of the compounds studied, 2‐OH‐BDE68 was most readily conjugated, and exhibited an efficiency (Vmax/Km) of glucuronidation of 0.256 ± 0.095 ml/min/mg protein, mean ± S.D., n=3, while that for sulfonation was 0.179 ± 0.030 ml/min/mg protein. For both pathways, Km values were in the low μM range. Studies with human SULT enzymes showed that sulfonation of all four substrates was readily catalyzed by SULT1B1 and SULT1E1, but not SULT1A1. OH‐BDEs inhibited SULT1A1 activity at concentration ranges between 0.2 μM – 20 μM. These OH‐BDEs also inhibited SULT1E1 sulfonation activity with Ki constants ranging between 0.725 nM–219 nM. However, assuming that the glucuronide and sulfate conjugates are non‐toxic and readily excreted, as is the case for most such conjugates, these studies suggest that OH‐BDEs should not accumulate in people to the same extent as the parent BDEs.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.