Combined Actions and Interactions of Chemicals in Mixtures

Science Institute (RSI) have convened a group to develop a comprehensive
approach for grouping chemicals by a common mechanism of toxicity using the
organophosphorus pesticides as a case study (Mileson et al., 1998).
Three scenarios were considered:
• two compounds that cause the same effect and induce toxicity by the same
molecular mechanism,
• two compounds that cause different toxic effects and induce toxicity via
the same molecular mechanism
• two substances that cause the same toxic effect and induce toxicity by
different molecular mechanisms
Overall, the expert panel concluded that the anticholinesterase organophosphorus
pesticides are a group of structurally related compounds that share certain
characteristic toxicological actions, specifically the inhibition of
acetylcholinesterase by phosphorylation and the subsequent accumulation of
acetylcholine in the nervous system of animals. However, although the
anticholinesterase organophosphorus pesticides clearly share the mentioned
characteristics, they also produce a variety of clinical signs of neurotoxicity that are
not identical for all organophosphorus compounds.
Ortiz et al. (1994) examined the acute neurotoxicities of formulated products of
methyl parathion and permethrin in male rats with that of a commercially
formulated mixture of the compounds. Methyl parathion was found to increase the
LD50 of permethrin because of an inhibition of the carboxylesterase involved in the
main metabolic pathway of permethrin. The decrease in carboxylesterase activity
probably caused an increase in the concentration of permethrin, which again caused
a greater toxicity. They also found that permethrin decreased the methyl parathioninduced
inhibition of brain cholinesterase activity. Methyl paraoxon is a metabolite
of methyl parathion and also inhibits the activity of brain cholinesterase.
An inhibition of the brain cholinesterase activity of 90 % was found in rats treated
with methyl parathion but the activity was only inhibited by 40 % in animals
treated with a mixture of methyl parathion and permethrin. Ortiz et al. (1994)
therefore assumed that in rats treated with the mixture there was only enough
methyl paraoxon available for a partial inhibition of the cholinesterase activity.
Although they used commercially formulated products in their experiment, they
argued that the decreased inhibition found for brain cholinesterase activity in rats
was due to the action of the active ingredient in the permethrin formulation. This
argument was based on the experimental data on cholinesterase activity in brain of
rats treated with commercially formulated products of methyl parathion and
permethrin or with the formulation alone. They concluded that when a mixture of
commercially formulated products of permethrin and methyl parathion was
administered to rats at high doses the toxicity differed from that of either pesticide
given alone. More specifically, the two pesticides produced a synergistic effect at
doses corresponding to LD50 and LD90.
Marinovich et al. (1996) compared the effect of mixtures of dimethoate, diazinon
and azinophos-methyl in vitro on acetylcholinesterase in nervous cells with the
effect of the single pesticides. They examined the following three mixtures in
concentrations ranging from 0.4 to 100 µg/ml of the single compound:
A. dimethoate, diazinon and azinophos-methyl
B. benomyl and pirimiphos methyl
C. Dimethoate, azinophos-methyl, diazinon, pirimiphos methyl and benomyl
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