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Comparative Sensitivity of Amphibians and Mammals to Acute Toxicity of Chlorpyrifos oxon

Comparative Sensitivity of Amphibians and Mammals to Acute Toxicity of Chlorpyrifos oxon

Timothy Anderson


Organophosphosphorus compounds (OPs) are used extensively as pesticides worldwide. The acute toxicity for OPs is initiated by inhibition of the enzyme acetylcholinesterase (AChE), leading to accumulation of acetylcholine and disruption of cholinergic signaling throughout the nervous system. A number of studies have reported that amphibians are less sensitive to acute OP toxicity compared to mammals, but the basis for these differences is unclear. Understanding differences in sensitivity to OPs could potentially provide insight into phylogenetic differences in AChE and cholinergic signaling as well as aid in risk assessment. This study compares the in vitro sensitivity of amphibian (Anaxyrus cognatus) and mammalian (Mus musculus) brain AChE to inhibition by paraoxon and chlorpyrifos oxon, the active metabolites of the pesticides parathion and chlorpyrifos. Enzyme sensitivity was evaluated by pre-incubating brain homogenate with one of a range of paraoxon or chlorpyrifos-oxon concentrations (0.001 - 100 µM, 20 min, 37°C), prior to adding substrate (acetythiocholine, 1 mM final) and measuring residual activity. In all cases, amphibian AChE was markedly less sensitive to inhibition in vitro. The IC50 (concentration that inhibits 50% of enzyme activity) for paraoxon was 275-fold higher (5.3 vs. 0.019 µM) in amphibian brain than in mouse brain. Similar but less extensive differences were noted with chlorpyrifos oxon, i.e., the IC50 for amphibian brain AChE were 31- (136.5 vs. 6.4 nM) higher than in the respective mammalian tissue. Substrate kinetics of AChE from the brain in both species was also evaluated. Vmax was similar between species. The Km for mammalian brain AChE, however, was 4-fold lower than that in amphibian brain (0.052 vs. 0.23 µM). We also estimated maximum tolerated dose (MTD) of chlorpyrifos oxon in both species in vivo. We found that the MTD for chlorpyrifos oxon in toads was 10-fold higher than in mice (77 vs. 7 mg/kg). These data suggest that phylogenetic differences in AChE and relative sensitivity to OP inhibitors may contribute to observed species differences in acute OP toxicity.