Organophosphorus (OP)
nerve agents are potent toxins that inhibit
cholinesterases and produce a rapid and lethal
cholinergic crisis. Development of
protein-based
therapeutics is being pursued with the goal of preventing
nerve agent toxicity and protecting against the long-term side effects of these agents. The
drug-metabolizing
enzyme human
carboxylesterase 1 (hCE1) is a candidate
protein-based therapeutic because of its similarity in structure and function to the
cholinesterase targets of
nerve agent poisoning. However, the ability of wild-type hCE1 to process the G-type
nerve agents sarin and
cyclosarin has not been determined. We report the crystal structure of hCE1 in complex with the
nerve agent cyclosarin. We further use stereoselective
nerve agent analogs to establish that hCE1 exhibits a 1700- and 2900-fold preference for the P(R) enantiomers of analogs of
soman and
cyclosarin, respectively, and a 5-fold preference for the P(S) isomer of a
sarin analog. Finally, we show that for
enzyme inhibited by racemic mixtures of bona fide
nerve agents, hCE1 spontaneously reactivates in the presence of
sarin but not
soman or
cyclosarin. The addition of the neutral
oxime 2,3-butanedione monoxime increases the rate of reactivation of hCE1 from
sarin inhibition by more than 60-fold but has no effect on reactivation with the other agents examined. Taken together, these data demonstrate that hCE1 is only reactivated after inhibition with the more toxic P(S) isomer of
sarin. These results provide important insights toward the long-term goal of designing novel forms of hCE1 to act as
protein-based
therapeutics for
nerve agent detoxification.