INHIBITION OF ACETYLCHOLINESTERASE AND BUTYRYLCHOLINESTERASE BY CHLORPYRIFOS-OXON

Phosphorothionate insecticides such as parathion (O,O-diethyl O-p-nitr ophenyl phosphorothioate) and chlorpyrifos (CPS; O,O-diethyl O-3,5,6-t richloro-2-pyridyl phosphorothioate; Dursban) are metabolically conver ted by oxidative desulfuration into paraoxon and chlorpyrifos-oxon (CP O). The insecticidal action of chlorpyrifos stems from inhibition of a cetylcholinesterase (AChE) by CPO, resulting in severe cholinergic tox icity. Sensory peripheral neuropathy was observed in people exposed en vironmentally to chlorpyrifos sprayed in confined areas. We have exami ned the kinetics of inhibition of AChE and butyrylcholinesterase (BChE ) by paraoxon and CPD. The bimolecular rate constants (k(i)) for inhib ition by paraoxon of recombinant human (rH) AChE, recombinant mouse (r M) AChE, and fetal bovine serum (FBS) AChE were 7.0, 4.0, and 3.2 x 10 (5) M-1 min(-1). The k(i) values for the inhibition by CPO of rH AChE, fetal bovine serum AChE, human RBC AChE, Torpedo AChE, and recombinan t mouse (rM) AChE were 9.3, 2.2, 3.8, 8.0, and 5.1 x 10(6) M-1 min(-1) , respectively. Inhibition of human serum BChE, rH BChE, and rM BChE b y CPO yielded k(i) values of 1.65, 1.67, and 0.78 x 10(9) M-1 min(-1), respectively. The k(i) values obtained for BChE from various species were 160- to 750-fold larger than those of AChE from parallel sources. Inhibition of the single-site mutant A(328)Y of rH BChE by CPO displa yed a 21-fold lower rate than that of wild-type rH BChE (k(i), 1.9 x 1 0(7) vs 1.67 x 10(9) M-1 min(-1)). The double mutant of acyl pocket re sidues of rH AChE, F295L/F297V, was inhibited by CPO with a 150-fold l arger k(i) than wild type (1.5 x 10(9) vs 1.0 x 10(7) M-1 min(-1)). Th e increased rate obtained with the double mutant displaying characteri stics of the BChE active center provides a rationale for higher effica cy of CPO scavenging by BChE, compared with AChE. BIOCHEM PHARMACOL 56 ;3:293-299, 1998. (C) 1998 Elsevier Science Inc.