Toxicological sciences : an official journal of the Society of Toxicology
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Comparative Study
Comparison of chlorpyrifos-oxon and paraoxon acetylcholinesterase inhibition dynamics: potential role of a peripheral binding site.
The primary mechanism of action for organophosphorus (OP) insecticides, like chlorpyrifos and parathion, is to inhibit acetylcholinesterase (AChE) by their oxygenated metabolites (oxons), due to the phosphorylation of the serine hydroxyl group located in the active site of the molecule. The rate of phosphorylation is described by the bimolecular inhibitory rate constant (k(i)), which has been used for quantification of OP inhibitory capacity. It has been proposed that a peripheral binding site exists on the AChE molecule, which, when occupied, reduces the capacity of additional oxon molecules to phosphorylate the active site. ⋯ At low concentrations, the k(i) estimates were approximately similar for both CPO and PO (150-180 [two determinations] and 300 +/- 180 nM(-1)h(-1), respectively). This implies that, at low concentrations, both oxons exhibited similar inhibitory potency in contrast to the marked difference exhibited at higher concentrations. These results support the potential importance of a secondary peripheral binding site associated with AChE kinetics, particularly at low, environmentally relevant concentrations.
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This study was conducted to assess spinal safety of the cyclo-oxygenase inhibitor ketorolac in dogs and rats. Beagle dogs were prepared with lumbar intrathecal catheters and received continuous spinal infusions of 5 mg/ml ketorolac (N = 6), 0.5 mg/ml ketorolac (N = 8), or saline vehicle (N = 6) at 50 microl/h (1.2 ml/day) for 28 days. No systematic drug or dose-related changes were observed in motor function, heart rate, or blood pressure. ⋯ Bolus and continuous infusion of intrathecal ketorolac resulted in significant reduction of lumbar CSF PGE2 concentrations. In rats, with intrathecal catheters, four daily bolus deliveries of saline or ketorolac (5 mg/ml/10 microl) had no effect upon spinal histology or upon spinal cord blood flow. These data indicate that intrathecal ketorolac in two species at the dose/concentrations employed does not induce evident spinal pathology but diminishes spinal prostaglandin release.