TISSUE-SPECIFIC EFFECTS OF CHLORPYRIFOS ON CARBOXYLESTERASE AND CHOLINESTERASE ACTIVITY IN ADULT RATS - AN IN-VITRO AND IN-VIVO COMPARISON

Organophosphate (OF) pesticides can bind to carboxylesterase (CaE), wh ich may lower the concentration of OPs at the target site enzyme, acet ylcholinesterase (ChE). It is unclear from the literature whether it i s the CaE's affinity for the OP and/or the number of CaE molecules whi ch is the dominant factor in determining the protective potential of C aE. We undertook a detailed, in vitro and in vivo survey of both CaE a nd ChE to ascertain if in vitro sensitivity of CaE and ChE predicted t he pattern of inhibition seen after in vivo dosing with chlorpyrifos ( CPF; 80 mg/ kg, p.o.) in male or female adult Long-Evans rats. For the brain, the in vitro sensitivity to CPF-oxon did predict the in vivo p atterns of inhibition: In vitro, brain ChE was approximately 25 times more sensitive to the active metabolite, CPF-oxon, than brain CaE, and in vivo brain ChE was more inhibited than brain CaE. In contrast, the in vitro sensitivity of plasma ChE and CaE did not correlate well wit h the in vivo pattern of inhibition: In vitro, plasma ChE was approxim ately 6.5 times less sensitive to CPF-oxon than plasma CaE, but in viv o, plasma ChE was more inhibited than CaE. In order to understand the role of CaE in protecting the brain ChE from inhibition by CPF-oxon in vitro, adult rat striatal tissue was incubated in the presence and ab sence of adult rat liver tissue and IC(50)s of CPF-oxon were determine d. The increase in the striatal CPF-oxon IC50 value noted for ChE in t he presence of liver suggested that CaE was binding the CPF-oxon and l imiting its access to ChE. Male liver CaE, which has the same affinity for binding CPF-oxon as female liver CaE but has twice as many bindin g sites, caused a greater increase in the striatal CPF-oxon IC50 than female liver, suggesting that the number of binding sites does play a role in the detoxification potential of a tissue. In summary, we found that (1) there are tissue and gender-related differences for basal Ch E and CaE activity; (2) the in vitro sensitivity of CaE or ChE to CPF- oxon is highly tissue-specific; (3) the pattern of ChE and CaE inhibit ion after in vivo dosing with CPF is not necessarily predictable from the in vitro IC50 for these same enzymes, and (4) the number of CaE mo lecules may play a role in modifying the toxicity of CPF.