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Biota tissues often show enriched levels of organic environmental xenobiotics. Therefore, many analytical
working groups have been focusing on the distribution of these compounds in different tissues of various aquatic and terrestrial species. Our philosophy is spanning further, also including the aspects fate of
organic environmental xenobiotics and of the respective transformation products as well as implications for toxicological effects. A breakthrough for the former aspect was achieved by the
successful development and application of enantioselective chromatography to environmental samples, be it water biota, soil or air samples. A comprehensive appreciation of these experimental approaches is given in the
recent monograph by Kallenborn and Hühnerfuss.
The latter aspect has been and is being taken into account in several Ph.D. dissertations of our group by a close co-operation with the group of Prof. Westendorf, Universitätsklinikum Eppendorf, Institut für Toxikologie/Fachbereich Medizin.The basic concept
of this experimental approach is based upon the assumption that enzymatic transformation of chiral xenobiotics may occur highly enantioselectively thus giving rise to enantiomeric excesses, while abiotic processes are
expected to lead to racemic transformation products, if the parent compounds are entering the environment as racemates. In conclusion, enantioselective chromatography of environmental sample extracts should reveal
biotic and abiotic transformation processes, respectively. In 1991, the working group of Prof. Hühnerfuss (Kallenborn et al., 1991, see full list of references) was the first to report on the successful application of
enantioselective gas chromatography (GC)
to biota extracts with the aim of investigating the enantioselective metabolism of alpha-HCH in organisms of different trophic levels. The authors obtained common Eider ducks (Somateria mollissima (L.)) from the Oehe/Schleimünde wildlife refuge on the Baltic coast of
Germany. Organ samples were taken only from healthy animals that, upon diving, had become trapped in the fishing nets of local fisherman and were thereby drowned. The Eider duck was chosen, because it largely favours
blue mussels (Mytilus edulis L.) in its diet. Blue mussels,
in turn, are capable of strongly enriching pollutants and thus serve as “indicator organisms” to provide insight into the state of an aquatic environment. Thus, one of only few examples is encountered, where a simple ”food chain” can be assumed (water mussel Eider duck). Normally, one should rather consider a ”food web”.
The detailed analyses of the extracts of six common Eider duck tissues revealed that (+)-alpha-HCH was clearly enriched; almost enantiomerically pure (+)-alpha-HCH was present in the liver extracts. The
enantiomeric purity of (+)-alpha-HCH isolated from liver extracts was so high that after purification by HPLC, it can be used directly in model experiments. By contrast, the enantiomeric ratio
(+)-alpha-HCH/(-)-alpha-HCH was about 7 in muscle extracts and about 1.6 in kidney extracts, whereby the values for these organs were slightly larger or smaller for different common Eider ducks.. An exact
explanation for the appearance of different enantiomeric ratios of (+)-alpha-HCH in the organs of common Eider ducks cannot be given at the present time. It may be assumed, however, that the reason lies in the different
physiological functions of the organs. For muscle and kidney, whose main functions are ”locomotion” and ”excretion”, respectively, the content of extractable lipids is about 2 %; these organs can, therefore, store
lipophilic pollutants. Liver, which contains about 2.5 % of extractable matrix, 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 (+)-alpha-HCH found in the liver of common Eider ducks is almost enantiomerically pure, (-)-alpha-HCH is presumably more readily transformed enzymatically than
the (+)-enantiomer in the liver. Although such a nearly enantioselective transformation had already been observed previously for biogenic organic compounds, the study by Kallenborn et al. represents the first
demonstration for synthetic environmental pollutants. Additional and, at first glance, surprising results were presented by Möller et al. (see full list of references) 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 ecotoxicologists has to be taken into account when assessing the potential risk of environmental pollutants: the enantioselective
permeation through the blood-brain-barrier. The chiral xenobiotic that has been investigated most intensively in all environmental compartments is, without any doubt, alpha-HCH. However, meanwhile
several chiral xenobiotics with one or more stereogenic centres have been included in order to study the transformation of these compounds in different environmental compartments, however, enantioselective
chromatographic approaches, in particular GC and HPLC, have also been applied to the investigation of additional processes such as the enantioselective permeation through the blood-brain-barrier, photochemical
conversion processes, and air/water gas exchange and atmospheric long-range transport. Details can be found in the recent monograph by
Kallenborn and Hühnerfuss and in the full list of references. |
