Stereospecificity in the enzymatic hydrolysis of cyclosarin (GF)

Enzymatic catalysis is one means of accelerating the rate of hydrolysis of G-type organophosphorus nerve agents. Here, the stereospecificity of the catalysis of cyclosarin (GF, O-cyclohexyl methylphosphonofluoridate) hydrolysis by several enzymes was investigated. Stereospecificity was not evident at 3 mM GF but was evident at 0.5 mM GF. The differential effect was apparently due to fluoride-catalyzed racemization of the substrate. Alteromonas sp. JD6.5 organophosphorus acid anhydrolase (OPAA), Alteromonas haloplanktis OPAA and the wild-type phosphotriesterase (PTE) enzymes were all found to catalyze preferentially the hydrolysis of the (+)GF isomer, as determined by GC analysis of the remaining unreacted (−)GF isomer. Acetylcholinesterase inhibition experiments showed the purified (−)GF isomer to be approximately twice as toxic as the racemic mixture. One PTE mutant, H254G/H259W/L303T, was found to reverse the native PTE stereospecificity and preferentially catalyze the hydrolysis of the (−)GF isomer, as shown by its complementation of Alteromonas sp. JD6.5 OPAA and by GC analysis of the remaining (+)GF isomer. This procedure also permitted the individual preparation of either of the two GF isomers by enzymatic degradation followed by extraction of the remaining isomer.