EU-SICHERHEITSDATENBLATT Dieselkraftstoff ... - Schmierstoffe

dihydrodiol. These dihydroxylated intermediates may then be processed through either an
ortho cleavage type of pathway, in which ring fission occurs between the two
hydroxylated carbon atoms, or a meta- cleavage type of pathway, which involves
cleavage of the bond adjacent to the hydroxyl groups, leading to central intermediates
such as protocatechates and catechols. These compounds are further converted to
tricarboxylic acid cycle intermediates (van der Meer et al. 1992). For the lower molecular
weight PAHs, the most common route involves the fission into a C3 compound and a
hydroxyl aromatic acid compound. The aromatic ring can thereafter either undergo direct
fission or can be subjected to decarboxylation, leading to the formation of a
dihydroxylated compound. This compound can be dissimilated as described above. When
degraded via these pathways, the low molecular weight PAHs can be completely
mineralized to CO 2 and H 2 O (Volkering and Breure, 2003).
The following observations were made by Volkering and Breure (2003);
- Many bacterial and fungal species have the enzymatic capacity to oxidise PAHs
- The aerobic transformation of PAHs always involve incorporation of oxygen into the
molecule
- The initial step is usually performed via a dioxygenase and forms a dihydrodiol
Their conclusions were that the transformation products of PAHs were in general more
polar than the parent and that there were metabolites that were potentially harmful to
mammals and could also have important environmental consequences.
-bonding orbitals, which results in the
absorption of light in the UV/far blue region of the spectrum that is present in solar radiation
(Nikolaou et al. 1984; Newsted and Giesy 1987; Larson and Berenbaum 1988; Krylov et al.
1997). Absorption of light energy can alter the toxicity of these compounds through two
different mechanisms: photosensitization and photomodification (Krylov et al. 1997).
Photosensitization generally leads to the production of singlet oxygen, and other reactive
oxygen species (ROS), which are capable of damaging biological molecules (Foote 1991).
Photomodification of PAHs is defined here as photooxidation and photolysis, and results in
the formation of new compounds with increased polarity, and in many cases, increased
toxicity (Zhu et al. 1995; Duxbury et al. 1997; Mallakin et al. 2000; Brack et al. 2003;
Shimada et al. 2004; Lampi et al. 2006).
Photosensitization has been widely documented in environmental toxicology, particularly
with respect to homo- and heterocyclic PAHs (Mekenyan et al. 1994; Wiegman et al.
2002, Diamond et al. 2006). These compounds are ideal photosensitizers, and are able to
absorb environmentally relevant wavelengths of radiation (Krylov et al. 1997). The
photosensitized production of highly reactive, and biologically damaging singlet oxygen
is an important, and well-studied aspect of PAH phototoxicity (Larson and Berenbaum
1988).
Singlet oxygen-induced biological damage is not the only mechanism of PAH
phototoxicity. Many PAHs can undergo subsequent reactions with oxygen (Mallakin et
al. 2000), forming new compounds that may be more toxic and/or mutagenic than the
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