Combined Actions and Interactions of Chemicals in Mixtures

cyclophosphamide-metabolising enzymes by nafenopin was enough to potentiate
the toxicity of cyclophosphamide.
7.2.7.2 Toxicodynamic interactions
DNA interaction and binding
As mentioned earlier reactive electrophilic chemicals or compounds, which are
metabolically activated to electrophiles, can react with DNA and cause DNA
damage, which, if the damage is misrepaired, can lead to mutations. Because
different electrophiles will attack different positions in DNA additive or synergistic
effects are most likely when the reactive species behave very similarly. This can be
exemplified by a study with quite similar “cooked-food” mutagens, which showed
that the combination effect of these mutagens was additive at high doses but
synergistic at lower doses (Ito et al. 1991).
However, the DNA binding effect of genotoxic chemical compounds can be
enhanced when other compounds alter the DNA conformation. Sakai (1994) has
shown that the DNA binding of the genotoxic and carcinogenic 1-nitropyrene (1-
NP) was enhanced by zinc acetate. It was suggested that the enhancing effect was
due to alteration of the DNA conformation from the B-form to the Z-form making
the binding of 1-NP easier. The main components of hard metals cobalt (Co) and
tungsten carbide (WC) and Co-WC mixtures have been investigated for genotoxic
effect in several experiments. In one in vitro study (Van Goethem et al. 1997) with
human leukocytes Co-WC showed synergistic effect at the highest dose tested
measured as DNA strand breaks in the COMET-assay. This might suggest that the
carbide allowed some uncoiling of the chromatin loops or induced the formation of
slowly migrating DNA fragments (Anard et al. 1997). By uncoiling the chromatin,
WC might amplify the clastogenic effect of cobalt in the mixture.
Classes of genotoxins that interact with DNA by intercalation may have altered
genotoxic responses in cells that contain pre-existing adducts, which can severely
distort normal DNA conformation. Results presented by (Said et al. 1999) suggest
that consecutive exposure to genotoxins may not always give rise to additive
effects, especially if the mixture contains different classes of genotoxins. Aflatoxin
B1-8,9-epoxide (AFB1-8,9-epoxide) and N-acetoxy-acetylaminofluorene (N-AcO-
AAF), both direct acting mutagens, were tested in the Salmonella/mammalian
microsome assay in the strains TA98 sensitive to (N-AcO-AAF) and TA100,
sensitive to AFB1-8,9-epoxide. Pre-treatment of the frameshift strain with the
basepair-substitution mutagen AFB1-8,9-epoxide enhanced the mutagenicity
induced by subsequent exposure to the frameshift mutagen N-AcO-AAF ~2-3times
above theoretical values for aditivity. Pre-treatment of base substitution strain
TA100 with N-AcO-AAF inhibited the mutagenicity following exposure of AFB1-
8,9-epoxide by a factor 3 below the theoretical additive value. The mechanism
behind these interactions is still not clear. One possible explanation for the
inhibitory effect of N-AcO-AAF on the mutagenicity of AFB1-8,9-epoxide in
TA100 could be, that prior adduction of Salmonella genomic DNA by N-AcO-
AAF reduced the number of binding sites available for reaction with AFB1-8,9epoxide.
This could be due to steric interference and/or conformational alterations
in the DNA polymer, which precluded reaction. Such an interaction was described
by Ross et al. (1999). Conversely it was found that initial modification of DNA by
AFB1-8,9-epoxide did not alter subsequent binding of N-AcO-AAF.
DNA repair
In general, two fundamental reactions are involved in cellular responses to DNA
damage: repair of DNA damage and tolerance to DNA damage. Most DNA
alterations are quickly corrected by a variety of repair processes. DNA repair may
be defined as those cellular responses associated with the restoration of normal
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