Combined Actions and Interactions of Chemicals in Mixtures

chemicals in the mixture can activate different enzymes, and thereby modify the
genotoxic response of other compounds by activation/deactivation. Some examples
are given below.
Activation of the hepatocarcinogen 2,6 dinitrotoluene (DNT) is a process involving
both hepatic and intestinal enzymes (Chadwick 1995). Pre-treatment of Fischer 344
rats with Aroclor 1254 or creosote (a complex mixture of PAH, phenols and
heterocyclic compounds), potentiate the genotoxic effect of DNT, measured as
hepatic DNA adduct formation. Also a higher level of DNT related mutagenic
metabolites were excreted in urine after pre-treatment with either of the two
complex mixtures. Using the 32 P-postlabeling technique three of fourDNT derived
DNA adducts were significantly increased in rats treated with creosote compared to
animals treated with DNT alone. Data from the study indicated that the creosote-
DNT interaction resulted from altered enzyme activity in the intestinal tract as well
as in the liver. Optimum bioactivation of DNT requires:
• reduced nitroreductase (NR) activity in the small intestine (NR deactivate
DNT in the small intestine before it is absorbed, therefore reduced activity
permits more unaltered DNT to reach the liver for biactivation of hepatic
enzymes)
• elevated caecal ß-glucuronidase activity (2,6-dinitrobenzoyl alcohol
glucuronide is a key intermediate in the bioactivation of DNT, while
elevated ß-glucuronidase activity will release more DNT metabolites for
further activation).
• elevated hepatic cyt-P450 isozymes (Aroclor 1254 is a well known inducer
of different cyt-P450 isozymes and creosote is reported to induce a 50 fold
increase in cyt-P450 activity (Schoor et al. 1991).
Combinations of arylamines and organophosphate isomers have, for instance, been
studied in the BHK cell transformation system. A supraadditive response was
generally obtained, in spite that the compounds would be expected to act in a
simple additive way. It was concluded that the potentiation most probably was
caused by competitive inhibition of cellular detoxification processes (Ashby &
Styles 1980). Pre-treatment of Chinese hamster V79 cells with N-acetylcystein
increases the intracellular level of glutathione, and this implies increased levels of
mutagenicity of MNNG (N-methyl-N-nitro-N-nitrosoguanidine), which is activated
by intracellular thiol or amino groups (Romert & Jenssen 1987). More often,
genotoxic effects are decreased by increases in the intracellular levels of
glutathione. The mutagenic effects of benzo[a]pyrene is, for instance, decreased by
glutathione-S-transferase mediated conjugation with glutathione. Rat hepatoma
cells (H4IIE) are able to detoxify benzo[a]pyrene, but MCF-7 cells (human
mammary carcinoma cells) have been shown to fail to form the conjugates and
they do not detoxify benzo[a]pyrene (Jenssen 1986). Depletion of glutathione does
most probably preferentially serve as a basis for synergistic effects concerning
genotoxic compounds.
Peroxisome proliferators constitute structurally disparate groups of rodent
hepatocarcinogens. It is generally accepted that these compounds are epigenetic
carcinogens. They are known to induce various isoforms of cytochrome P-450
(CYP2B1/2 and CYP4A1) and at the same time depress GST activities.
Cyclophosphamide is an indirect acting mutagen, which requires activation of
CYP2B1/2. The reactive electrophilic metabolite is subsequently detoxified by
glutathione S-transferase. The peroxisome proliferator nafenopin potentiated the
cytotoxicity and genotoxicity of cyclophosphamide both in the liver and bone
marrow (Voskoboinik 1997). Although the mechanisms of these synergistic effects
are not fully understood, there is evidence that the modulation of
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