Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2012
Publication Date: 11/21/2012
Citation: Hakk, H., Szabo, D.T., Huwe, J.K., Diliberto, J.J., Birnbaum, L.S. 2012. Novel and distinct metabolites identified following a single oral dose of alpha- or gamma-hexabromocyclododecane in mice. Environmental Science and Technology. 46:13494-13503. Interpretive Summary: Hexabromocyclododecane (HBCD) is one of the most abundant brominated flame retardants on the market and is used extensively in the construction industry. HBCD is persistent and bioaccumulative and, therefore, of concern as an environmental pollutant. A great unknown regarding HBCD is why the isomer pattern for the commercial mixture shifts shows a dramatic shift from gamma-HBCD to alpha-HBCD in biological samples. Two metabolism studies were conducted in female mice using these two HBCD isomers to address this question. Metabolites detected in feces or tissues were distinct between the two isomers. In mice exposed to alpha-HBCD, the metabolic pathway was simple oxidation. In contrast, gamma-HBCD was first metabolized in mice to lose a bromine atom, which was then oxidized and dehydrated. Urinary metabolites were also distinct between the isomers, and were very water-soluble. Our data showed that although interconversion of gamma-HBCD to alpha-HBCD occurred and gamma-HBCD was more readily metabolized, these observations were inadequate to explain the isomer pattern differences between commercial product and environmental samples. Therefore, another mechanism for this isomer shift probably exists, perhaps a non-biological one. Although the potential toxicity of these metabolites is unknown, the presence of distinct metabolites between the two HBCD isomers will allow future researchers to select appropriate metabolite markers that may aid in characterizing the source of HBCD exposure.
Technical Abstract: The metabolism of alpha- and gamma-hexabromocyclododecane (HBCD) was investigated in adult C57BL/6 female mice. Alpha- or gamma-[14C]HBCD (3 mg/kg bw) was orally administered with subsequent urine and feces collection for 4 consecutive days; a separate group of mice were dosed and sacrificed 3 hours post-exposure to investigate tissue metabolite levels. Extractable and non-extractable HBCD metabolites were quantitated in liver, blood, fat, brain, bile, urine and feces and characterized by LC/MS (ESI-). Metabolites identified were distinct between the two stereoisomers. In mice exposed to alpha-HBCD, four hydroxylated metabolites were detected in fecal extracts, and one of these metabolite isomers was consistently characterized in liver, brain, and adipose tissue extracts. In contrast, mice exposed to gamma-HBCD contained multiple isomers of monohydroxy-pentabromocyclododecene, dihydroxy-pentabromocyclododecene, and dihydroxy-pentabromocyclododecadiene in the feces while only a single monohydroxy-pentabromocyclododecane metabolite was measured in liver and adipose tissue. Both stereoisomers were transformed to metabolites which formed covalent bonds to proteins and/or lipids in the gut as evidenced by high fecal non-extractables. Although the potential toxicity of these free and bound metabolites remains to be determined, the presence of distinct metabolic products from the two main HBCD stereoisomers should allow biomarkers to be selected that may aid in characterizing sources of HBCD exposure.