ToxSci Advance Access originally published online on June 6, 2007
Toxicological Sciences 2007 98(2):604; doi:10.1093/toxsci/kfm116
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Published by Oxford University Press 2007.
Letter to the Editor
Cellular and Molecular Toxicology Branch, Neurotoxicology Division, National Health and Environmental Effects Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711
E-mail: padilla.stephanie{at}epa.gov
Received April 9, 2007; accepted April 17, 2007
I read with great interest Age-Related Brain Cholinesterase Inhibition Kinetics Following in vitro Incubation with Chlorpyrifos-Oxon and Diazinon-Oxon by Kousba, Poet, and Timchalk (Vol. 95, no. 1, January 2007). In the Abstract the authors state "Neonatal brain ChE inhibition kinetics exhibited a marked age-related sensitivity to CPO [chlorpyrifos oxon]...." This is in direct opposition to a conclusion my laboratory reached years ago when investigating, in vitro, the age-related sensitivity of rat brain to inhibition by chlorpyrifos-oxon (Mortensen et al., 1998
). We demonstrated that acetylcholinesterase in the adult and neonatal (postnatal day 5) rat brain had similar sensitivity to inhibition by chlorpyrifos-oxon. We also showed that age differences in the inhibitory potency seen in plasma and liver were not due to the inherent sensitivity of acetylcholinesterase but to extrinsic factors in the tissue homogenate that either hydrolyzed or bound the chlorpyrifos-oxon. Acetylcholinesterase in neonatal plasma was approximately 10x more sensitive to chlorpyrifos-oxon inhibition as adult plasma, but this age-related difference disappeared when acetylcholinesterase was separated by immunoprecipitation from extrinsic factors present in the plasma. The only reason neonatal plasma acetylcholinesterase appeared more sensitive to chlorpyrifos-oxon, therefore, was due to fewer extrinsic factors present than in adult plasma. We concluded, "These data show clearly that IC50 values from a crude homogenate do not measure the true sensitivity of AChE to the inhibitor."
These same issues would also hold for measuring inhibitor sensitivity using a Ki value in crude homogenates as has been done in the Kousba et al. paper. Although Kousba and co-workers were aware of our 1998 publication, they did not heed our cautionary note regarding extrinsic factors affecting sensitivity to chlorpyrifos-oxon. Further, despite their considering possible complications in the Discussion, those caveats are not reflected in their Abstract.
Therefore, the authors' conclusion regarding differential sensitivity of rat brain acetylcholinesterase to chlorpyrifos-oxon should be held in abeyance until they demonstrate that immunoprecipitated acetylcholinesterase from neonatal and adult rat brain are differentially inhibited by chlorpyrifos-oxon. Alternatively, a simple way to rule out the influence of extrinsic factors is to repeat their studies using varying concentrations of brain homogenate (see Fig. 2 of Mortensen et. al, 1998). If the Ki or IC50 is constant across all tissue concentrations, then extrinsic factors are likely not an issue.
NOTES
Disclaimer: This has been reviewed by the National Health and Environmental Effects Research Laboratory, USEPA, and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Agency nor does mention of trade names or commercial products constitute endorsement or recommendation for use.
REFERENCES
Kousba AA, Poet TS, Timchack C. Age-related brain cholinesterase inhibition kinetics following IN VITRO incubation with chlorpyrifos-oxon and diazinon-axon. Toxicol. Sci. (2007) 95(1):147–155.
Mortensen SR, Brimijoin S, Hooper MJ, Padilla S. Comparison of the in vitro sensitivity of rat acetylcholinesterase to chlorpyrifos-oxon: What do tissue IC50 values represent? Toxicol. Appl. Pharmacol. (1998) 148:46–49.[CrossRef][Web of Science][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||