Biotransformation of Hexabromocyclododecanes
(HBCDs)
with LinBAn HCH-Converting Bacterial Enzyme
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Abstract
Hexabromocyclododecanes (HBCDs) and hexachlorocyclohexanes
(HCHs)
are polyhalogenated hydrocarbons with similar stereochemistry. Both
classes of compounds are considered biologically persistent and bioaccumulating
pollutants. In 2009, the major HCH stereoisomers came under regulation
of the Stockholm convention. Despite their persistence, HCHs are susceptible
to bacterial biotransformations. Here we show that LinB, an HCH-converting
haloalkane dehalogenase from <i>Sphingobium indicum</i> B90A,
is also able to transform HBCDs. Racemic mixtures of α-, β-,
and γ-HBCDs were exposed to LinB under various conditions. All
stereoisomers were converted, but (−)α-, (+)β-,
and (+)γ-HBCDs were transformed faster by LinB than their enantiomers.
The enantiomeric excess increased to 8 ± 4%, 27 ± 1%, and
20 ± 2% in 32 h comparable to values of 7.1%, 27.0%, and 22.9%
as obtained from respective kinetic models. Initially formed pentabromocyclododecanols
(PBCDOHs) were further transformed to tetrabromocyclododecadiols (TBCDDOHs).
At least, seven mono- and five dihydroxylated products were distinguished
by LC-MS so far. The widespread occurrence of HCHs has led to the
evolution of bacterial degradation pathways for such compounds. It
remains to be shown if LinB-catalyzed HBCD transformations in vitro
can also be observed in vivo, for example, in contaminated soils or
in other words if such HBCD biotransformations are important environmental
processes