A team led by Dutch researcher Jan Boon from the Royal Netherlands Institute for Sea Research (Royal NIOZ) has discovered that one isomer of the toxic substance HBCD accumulates in dolphins and porpoises. The animals metabolise the other two isomers. HBCD has partially replaced other flame retardants already banned in Europe. However, according to the researchers it is still questionable whether HBCD is also less environmentally harmful. The results have just been published in the scientific journal 'Environmental Science & Technology' (1 April 2005 issue).
HBCD, hexabromocyclododecane, is a much-used flame retardant in insulating foam and furniture upholstery. It disrupts the thyroid function and the functioning of the nervous system in mammals. The researchers only found the alpha isomer of HBCD in the blubber of porpoises and dolphins. Boon's team were Bart Zegers from the Institute for Biodiversity and Ecosystem Dynamics (Universiteit van Amsterdam), Timo Hamers from the Institute for Environmental Studies (Vrije Universiteit Amsterdam) and Graham Pierce from the University of Aberdeen.
The porpoises and dolphins investigated were stranded on the coasts of Scotland, Ireland, the Netherlands, Belgium, France and northwest Spain. By far the most HBCD was found in the blubber of animals from the Irish Sea and the West Coast of Scotland. Therefore the researchers suspect that this HBCD originates from a source discharging into the Irish Sea.
The alpha isomer of HBCD forms at most fifteen percent of the HBCD formulation used in commercial flame retardants. The three isomers differ in how the bromine atoms are bound to the carbon atoms of HBCD and a new analytical technique can distinguish between the different isomers. Using this technique, Boon and his colleagues discovered that only the alpha isomer accumulates in sea mammals such as dolphins. They used liver material from laboratory rats and a recently dead seal washed up on a beach on Texel, the island off the Dutch coast where Royal NIOZ is located, to investigate the reason for these differences in bioaccumulation between the different HBCD isomers.
The beta and gamma isomers of HBCD are metabolised by the animals. The enzyme complex cytochrome P450 in the liver plays an important role in this. This complex adds oxygen to the HBCD but it cannot react with the alpha isomer. This is due to subtle differences in the spatial structure of the alpha, beta and gamma isomer molecules. As it is not metabolised, the alpha isomer accumulates in the fat-rich tissues of sea mammals, such as the blubber.
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