respectively, the content of extractable lipids is about 2%; these organs can, therefore, store lipophilic pollutants. Liver, which contains about 2.5% of extractable
organic matrix (EOM) which mainly consists of lipids, peptides and larger molecules, serves as a “detoxification organ” and, therefore, is not only capable of storing
toxic compounds but can also metabolise them to substances that the body can
tolerate or excrete. Since the (+)-α-HCH found in the liver of common Eider ducks is
almost enantiomerically pure, (À)-α-HCH is presumably more readily transformed
enzymatically than the (+)-enantiomer in the common eider liver. Although such a
nearly enantioselective transformation had already been observed previously for
biogenic organic compounds, the study by Kallenborn et al. (1991) represents the
first evidence for the enantiomer-selective degradation of chiral synthetic pollutants
in vertebrates.
Additional and, at first glance, even more surprising, results were reported by
Möller et al. (1994) who analysed brain tissue of the same Eider duck animals that
had already been investigated by Kallenborn et al. with regard to liver, kidney and
muscle tissues. It turned out that an additional enantioselective process that thus far
escaped the attention of environmental toxicologists has to be considered when
assessing the potential risk of environmental pollutants in vertebrates and higher
organisms. The permeation through the blood–brain barrier seems to be of highly
enantiomer-selective character for biogenic substances and anthropogenic chiral
pollutants.
After these early investigation by Kallenborn et al. who demonstrated the
enantioselective metabolisation of α-HCH in common Eiders (Somateria mollissima
(L.)) (Kallenborn et al. 1991), increased attention has been paid to the chromatographic enantiomer separation of chiral xenobiotics and their metabolites in environmental biota samples.
Today, the enantiomer distribution profiles in higher organisms are considered an
indicator parameter for bioaccumulation and bioactivity of chiral pollutants in the
respective food web and the investigated organisms (Müller and Kohler 2004; Wong
2006; Smith 2009). In Table 8.7, a selection of relevant chiral environmental
pollutants is listed, as well as the references, in which a successful enantiomer
separation of the respective standard compounds is reported. Table 8.8 summarises
environmental pollutants together with the respective biota samples and the references, in which results about these compounds with regard to enantioselective
analyses and processes can be found.
As it is already clarified in the previously discussed reports, the chiral pollutant
that has been investigated earliest and most intensively during the past decades and
throughout all environmental compartments is still, without any doubt, α-HCH. Its
chemical structure and its main microbial metabolism were already described earlier.
In this chapter, the emphasis is placed on enzymatic transformation and enantiomerselective uptake of α-HCH in higher organisms.
Already at lower trophic levels (microbial transformation in water, mussel),
preferential depletion of one α-HCH enantiomer, be it the (+)- or the (À)-enantiomer, has been verified. These first results encouraged to perform a more systematic
investigation that addressed the problem as to whether or not different enzymatic
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8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants
organic matrix (EOM) which mainly consists of lipids, peptides and larger molecules, serves as a “detoxification organ” and, therefore, is not only capable of storing
toxic compounds but can also metabolise them to substances that the body can
tolerate or excrete. Since the (+)-α-HCH found in the liver of common Eider ducks is
almost enantiomerically pure, (À)-α-HCH is presumably more readily transformed
enzymatically than the (+)-enantiomer in the common eider liver. Although such a
nearly enantioselective transformation had already been observed previously for
biogenic organic compounds, the study by Kallenborn et al. (1991) represents the
first evidence for the enantiomer-selective degradation of chiral synthetic pollutants
in vertebrates.
Additional and, at first glance, even more surprising, results were reported by
Möller et al. (1994) who analysed brain tissue of the same Eider duck animals that
had already been investigated by Kallenborn et al. with regard to liver, kidney and
muscle tissues. It turned out that an additional enantioselective process that thus far
escaped the attention of environmental toxicologists has to be considered when
assessing the potential risk of environmental pollutants in vertebrates and higher
organisms. The permeation through the blood–brain barrier seems to be of highly
enantiomer-selective character for biogenic substances and anthropogenic chiral
pollutants.
After these early investigation by Kallenborn et al. who demonstrated the
enantioselective metabolisation of α-HCH in common Eiders (Somateria mollissima
(L.)) (Kallenborn et al. 1991), increased attention has been paid to the chromatographic enantiomer separation of chiral xenobiotics and their metabolites in environmental biota samples.
Today, the enantiomer distribution profiles in higher organisms are considered an
indicator parameter for bioaccumulation and bioactivity of chiral pollutants in the
respective food web and the investigated organisms (Müller and Kohler 2004; Wong
2006; Smith 2009). In Table 8.7, a selection of relevant chiral environmental
pollutants is listed, as well as the references, in which a successful enantiomer
separation of the respective standard compounds is reported. Table 8.8 summarises
environmental pollutants together with the respective biota samples and the references, in which results about these compounds with regard to enantioselective
analyses and processes can be found.
As it is already clarified in the previously discussed reports, the chiral pollutant
that has been investigated earliest and most intensively during the past decades and
throughout all environmental compartments is still, without any doubt, α-HCH. Its
chemical structure and its main microbial metabolism were already described earlier.
In this chapter, the emphasis is placed on enzymatic transformation and enantiomerselective uptake of α-HCH in higher organisms.
Already at lower trophic levels (microbial transformation in water, mussel),
preferential depletion of one α-HCH enantiomer, be it the (+)- or the (À)-enantiomer, has been verified. These first results encouraged to perform a more systematic
investigation that addressed the problem as to whether or not different enzymatic
132
8 Enantiomer-Specific Fate and Behaviour of Chiral Contaminants