Title: Difference HBCD Stereoisomers are Metabolized Differently Author
Submitted to: International Toxicology Congress
Publication Type: Abstract Only
Publication Acceptance Date: April 15, 2010
Publication Date: July 17, 2010
Citation: Hakk, H. 2010. Different HBCD stereoisomers are metabolized differently. In: Abstracts of the XII International Congress of Toxicology, July 19-23, 2010, Barcelona, Spain. Vol. 196, Supplement 1, S33-S34. Technical Abstract: The requirement of flame retardancy in most consumer products has resulted in a vast array of chemical compounds that unfortunately have toxic effects and are environmental pollutants. Hexabromocyclododecane (HBCD) is an additive brominated flame retardant applied to extruded and high-impact polystyrene foams (<2.5% by weight) used as thermal insulation in buildings, and HBCD is the only suitable flame retardant for these applications. HBCD the second highest-volume BFR used in Europe. HBCD was first detected in fish and sediment samples in Sweden in 1998, and has been detected in a variety of environmental samples. Chemically HBCD is a complex mixture of stereoisomers, and the commercial mixture contains a majority of the gamma-HBCD (~81%) with minor amounts of alpha-HBCD (~10%) and beta-HBCD (~9%); however, alpha-HBCD is the most abundant isomer reported in biota. Two possible interpretations of these results would be that alpha-HBCD is the most bioavailable stereoisomer, or that metabolic biotransformations favor beta- and gamma-HBCD over alpha-HBCD. This report will present recent laboratory results that lend support to the later hypothesis. Stereoisomer-specific biotransformations have been observed in mice in which gamma-HBCD is readily converted to both beta- and alpha-HBCD, but where alpha-HBCD is resistant to such isomerization. The high molecular weight and aliphatic nature of HBCD lends it susceptible to intriguing metabolic pathways, and these will be discussed in context of the known metabolic pathways for PCDD/Fs, PBDEs, or PCBs. Possible conjugation of HBCD, urinary volatiles, and involvement of carrier protein systems will be discussed, as well as observed debromination, and oxidative and reductive metabolic events.