TY - JOUR
T1 - Comparison of the in vitro sensitivity of rat acetylcholinesterase to chlorpyrifos-oxon
T2 - What do tissue IC50 values represent?
AU - Mortensen, S. R.
AU - Brimijoin, S.
AU - Hooper, M. J.
AU - Padilla, S.
N1 - Funding Information:
The authors thank Sridhar Basavaraju for performing the IC50 experiments with the recombinant AChE, Dr. Palmer Taylor for the kindly supplying the mouse recombinant AChE, and Dr. Sushmita Chanda for performing the IC50 determinations in the presence and absence of iso-OMPA. This project was supported by DowElanco (Indianapolis, IN) and Cooperative Agreement CT902813 between Clemson University and the U.S. Environmental Protection Agency. The research in this article has been reviewed by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, 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.
PY - 1998/1
Y1 - 1998/1
N2 - The toxicological literature is replete with studies which have attempted to correlate differences in in vivo sensitivity to anticholinesterases with a common in vitro measure: acetylcholinesterase (AChE) IC50 values. Generally, it is assumed that these IC50 values reflect the intrinsic sensitivity of the AChE molecule to the inhibitor. Our goal was to ascertain whether differences in AChE sensitivity to an organophosphate (i.e., IC50 values) are due to varying properties of the enzyme molecule (i.e., present assumption) or to extrinsic factors. Tissue samples were obtained from immature and adult Long-Evans rats. AChE IC50 values were determined by incubating tissue homogenates with chlorpyrifos-oxon (active metabolite of chlorpyrifos, a common organophosphate insecticide) for 30 min at 26°C, and then measuring residual AChE activity. The following IC50 values were noted for postnatal day 4 and adult animals, respectively: brain, 10 nM for both ages; liver, 96 and 527 nM; plasma, 18 and 326 nM. Thus, the 'apparent' sensitivity of AChE was prone to vary dramatically with age and tissue type. In contrast, when AChE was isolated from the same tissues by immunoprecipitation, there were no age- or tissue-related differences (IC50 ~ 3 nM in every case). These data show clearly that IC50 values from a crude homogenate do not measure the true sensitivity of AChE to the inhibitor. Presumably, for chlorpyrifos-oxon, at least, the tissue 1C50 values depend greatly on a tissue's propensity to sequester or hydrolyze chlorpyrifos-oxon.
AB - The toxicological literature is replete with studies which have attempted to correlate differences in in vivo sensitivity to anticholinesterases with a common in vitro measure: acetylcholinesterase (AChE) IC50 values. Generally, it is assumed that these IC50 values reflect the intrinsic sensitivity of the AChE molecule to the inhibitor. Our goal was to ascertain whether differences in AChE sensitivity to an organophosphate (i.e., IC50 values) are due to varying properties of the enzyme molecule (i.e., present assumption) or to extrinsic factors. Tissue samples were obtained from immature and adult Long-Evans rats. AChE IC50 values were determined by incubating tissue homogenates with chlorpyrifos-oxon (active metabolite of chlorpyrifos, a common organophosphate insecticide) for 30 min at 26°C, and then measuring residual AChE activity. The following IC50 values were noted for postnatal day 4 and adult animals, respectively: brain, 10 nM for both ages; liver, 96 and 527 nM; plasma, 18 and 326 nM. Thus, the 'apparent' sensitivity of AChE was prone to vary dramatically with age and tissue type. In contrast, when AChE was isolated from the same tissues by immunoprecipitation, there were no age- or tissue-related differences (IC50 ~ 3 nM in every case). These data show clearly that IC50 values from a crude homogenate do not measure the true sensitivity of AChE to the inhibitor. Presumably, for chlorpyrifos-oxon, at least, the tissue 1C50 values depend greatly on a tissue's propensity to sequester or hydrolyze chlorpyrifos-oxon.
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U2 - 10.1006/taap.1997.8287
DO - 10.1006/taap.1997.8287
M3 - Article
C2 - 9465262
AN - SCOPUS:0031881999
SN - 0041-008X
VL - 148
SP - 46
EP - 49
JO - Toxicology and Applied Pharmacology
JF - Toxicology and Applied Pharmacology
IS - 1
ER -