Levels of Hexabromocyclododecane in Harbor Porpoises and Common Dolphins from Western European Seas, with Evidence for Stereoisomer-Specific Biotransformation by Cytochrome P450

Commercial hexabromocyclododecane (HBCD) is a high-production-volume flame-retardant applied in polystyrene
foams. It contains three stereoisomers, of which γ-HBCD
always dominates. Here we report on the levels of
HBCD in blubber of harbor porpoise and common dolphin
from different European seas. The highest total (Σ)HBCD levels were measured in harbor porpoises stranded
on the Irish and Scottish coasts of the Irish Sea (median
concentration 2.9 μg (g of lipid)-1) and the northwest coast
of Scotland (median concentration 5.1 μg (g of lipid)-1).
The median levels in other areas were, for the harbor porpoise
south coast of Ireland, 1.2 μg (g of lipid)-1, for the
coasts of The Netherlands, Belgium, and France north of
Calais (southern North Sea), 1.1 μg (g of lipid)-1, for the east
coast of Scotland (northern North Sea), 0.77 μg (g of
lipid)-1, and, for Galicia (Spain), 0.1 μg (g of lipid)-1. The
median levels for the common dolphin were, for west coast
of Ireland, 0.9 μg (g of lipid)-1, for the French coast of
the English Channel between Normandy and Brest, 0.4 μg
(g of lipid)-1, and, for Galicia, 0.2 μg (g of lipid)-1. A
subset of 10 harbor porpoise and 9 common dolphin blubber
samples representing all areas were analyzed by LC/MS
to determine the diastereomeric composition of their HBCD
residues. All samples showed exclusively the peak of
α-HBCD. To test if biotransformation by the cytochrome
P450 system could explain the observed compositional
difference with technical HBCD mixtures, a number of in
vitro assays with microsomal preparations of liver were carried
out. We had to revert to material stored at −80 °C from
laboratory rats and a fresh harbor seal found dead in the
Dutch Wadden Sea, since such liver samples of cetaceans
were not in our possession. The in vitro assays showed that
β- and γ-HBCDs were indeed significantly metabolized
when incubated in the presence of NADPH as electron
donor, compared to a set of reference samples which were
identical except for the addition of NADPH. In contrast,
the peak of α-HBCD did not decrease significantly in the
presence of NADPH. In separate microsomal assays with
β- and γ-HBCDs, new peaks of brominated compounds
(signal at m/z = 79 or 81) with masses of [M + O] were
formed only when NADPH was added. This confirms the
process of cytochrome P450 mediated biotransformation.
Although rat and harbor seal belong to different families of
the mammalia than the cetaceans, we propose that
biotransformation by the cytochrome P450 system is also
the most likely process to explain the exclusive accumulation
of α-HBCD in harbor porpoise and common dolphin.