Two main functions of rat liver mitochondria namely the respiration rate and generation of membrane potential, were analysed simultaneously to evaluate the toxicity of the dinitroaniline herbicide pendimethalin. The chromatography grade pendimethalin enhanced the mitochondrial respiration in a concentration-dependent manner and markedly decreased the membrane potential
starting from 8.2×10–5 M up to 5.47×10–4 M (23–154 mg/ml). A higher toxicity of technical grade pendimethalin (Stomp – a mixture of pure ingredient and adjuvant(s)) was observed at lower concentrations of the pure ingredient (pendimethalin) than those of the single chromatography grade chemical. These data indicate that: 1) pure and technical grade pendimethalin preparations act as uncouplers of oxidative phosphorylation in mitochondria (enhance respiration and diminish membrane potential), 2) these effects are weaker but comparable (of the same concentration order) to those of 2,4-dinitrophenol, and 3) technical grade pendimethalin is more toxic to oxidative phosphorylation in mitochondria than pure chemical pendimethalin (active ingredient) itself. These data clearly disclaim the statement presented in the manual “Recognition and Management of Pesticides Poisonings” (US EPA 1999) that pendimethalin does not act as an uncoupler of oxidative phosphorylation.

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