Inhibition of hen brain acetylcholinesterase and neurotoxic esterase by chlorpyrifos in vivo and kinetics of inhibition by chlorpyrifos oxon in vitro: Application to assessment of neuropathic risk*

Abstract

Chlorpyrifos (CPS; O,O-diethyl 3,5,6-trichloro-2-pyridyl phosphorothionate; Dursban) is a widely used broad-spectrum organophosphorus (OP) insecticide. Because some OP compounds can cause a sensory-motor distal axonopathy called OP compound-induced delayed neurotoxicity (OPIDN), CPS has been evaluated for this paralytic effect. Early studies of the neurotoxicity of CPS in young and adult hens reported reversible leg weakness but failed to detect OPIDN. More recently, a human case of mild OPIDN was reported to result from ingestion of a massive dose (about 300 mg/kg) in a suicide attempt. Subsequent experiments in adult hens (the currently accepted animal model of choice for studies of OPIDN) showed that doses of CPS in excess of the LD50 in atropine-treated animals inhibited brain neurotoxic esterase (NTE) and produced mild to moderate ataxia. Considering the extensive use of CPS and its demonstrated potential for causing OPIDN at supralethal doses, additional data are needed to enable quantitative estimates to be made of the neuropathic risk of this compound. Previous work has shown that the ability of OP insecticides to cause acute cholinergic toxicity versus OPIDN can be predicted from their relative tendency to inhibit the intended target, acetylcholinesterase (AChE), versus the putative neuropathic target, NTE, in brain tissue. The present study was designed to clarify the magnitude of neuropathic risk associated with CPS exposures by measuring hen brain AChE and NTE inhibition following dosing in vivo and determining the bimolecular rate constant of inhibition (ki) for each enzyme by the active metabolite, CPS oxon (CPO), in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Relative Inhibitory Potencies of Chlorpyrifos Oxon, Chlorpyrifos Methyl Oxon, and Mipafox for Acetylcholinesterase Versus Neuropathy Target Esterase

“…Using kinetic determinations of k i values obtained from linear kinetics, the corresponding RIP was found to be 107 (Richardson et al, 1993). In either case, it is clear that the RIP of CPO is >>1, indicating that CPS could not produce OPIDN at sublethal doses, consistent with experimental results in animals (Capodicasa et al, 1991;Richardson et al, 1993) and clinical data on humans (Lotti et al, 1986;Moretto & Lotti, 1998).…”

“…Nevertheless, as had been observed for CPO in previous studies using hen brain homogenates (Capodicasa et al, 1991;Richardson et al, 1993), it may be hypothesized that the RIP for its methyl homologue, CPMO, would also be >>1 and greater than the RIP for CPO. This contention arises from structure-activity studies showing that decreasing the carbon chain length of alkyl substituents attached to phosphorus tends to increase the RIP and decrease the neuropathic potential for a given homologous series of dialkyl phosphates (Makhaeva et al, 1998;Richardson, 1992).…”

“…First, the present work employed kinetic methods that are thought to be more valid than fixed-time methods for determinations of inhibitory potencies of OP compounds against esterases (Clothier et al, 1981;Fukuto, 1990; 1992; Richardson et al, 1993). Second, the kinetic measurements of the present work used more time points, inhibitor concentrations per experiment, and separate experiments than were done in previous determinations of k i values for CPO against hen brain AChE and NTE (Richardson et al, 1993). Third, the interference of extrinsic factors that may be present in whole tissue homogenates (Mortensen et al, 1998) was minimized by using microsomal preparations in the current investigation.…”

“…When pseudo-first-order kinetics are obtained and the apparent first-order rate constants are linearly related to inhibitor concentration, it is valid to calculate the fixed-time I 50 value from the relationship I 50 = (ln 2)/(k i × t), where I 50 is the inhibitor concentration that produces 50% inhibition of enzyme activity under specified conditions, and t is the time of preincubation of the inhibitor with the enzyme (Aldridge & Reiner, 1972). Moreover, under these kinetic conditions, experimentally determined I 50 values may be used to calculate RIPs (Richardson et al, 1993). When the RIP, calculated either as the [k i (AChE)/k i (NTE)] ratio or the [I 50 (NTE)/I 50 (AChE)] ratio, is greater than 1, the inhibitor has a greater inhibitory potency against AChE than NTE, and doses of the parent compound greater than the LD50 are required to produce OPIDN (Lotti & Johnson, 1978;Makhaeva et al, 1998;Richardson, 1992).…”

“…This contention arises from structure-activity studies showing that decreasing the carbon chain length of alkyl substituents attached to phosphorus tends to increase the RIP and decrease the neuropathic potential for a given homologous series of dialkyl phosphates (Makhaeva et al, 1998;Richardson, 1992). If the RIP data for CPMO and CPO were to support the prediction from structure-activity considerations, then this would indicate that the potential for CPMS to produce OPIDN would be even lower than that of CPS, which is already known to be incapable of producing OPIDN at sublethal doses (Capodicasa et al, 1991;Moretto & Lotti, 1998;Richardson, 1995;Richardson et al, 1993).…”