Kinetic studies of the hydrolysis of organophosphate insecticides by phosphotriesterase

Abstract

The bacterial phosphotriesterase (PTE) form Pseudomonas diminuta catalyzes the detoxification of a wide variety of organophosphate insecticides. Although the use of these agents is essential in increasing crops yields, they frequently act as nerve poisons by blocking mammalian acetylcholinesterase (AChE). AChE is an enzyme responsible for the degradation of acetylcholine. The catalytic activities toward twenty-two organophosphate insecticides, which are currently in agricultural use in the U.S.A., were measured. The kinetic parameters for these insecticides were determined. Some of these insecticidal substrates such as propetamphos and profenofos are chiral compounds. The PTE specificity toward these chiral substrates has been assessed. By examining the enzymatic hydrolysis of these racemic mixtures, it was determined spectroscopically that PTE hydrolyzes both enantiomers at different rates. The enantiomeric preference of the enzymatic hydrolysis for propetamphos is 46:1 and 13:1 for profenofos. The relative stereochemistry of the enzymatic hydrolysis of both substrates, however, remains unclear. For propetamphos, this ambiguity is due to the failure of developing a method using ³¹P NMR spectroscopy to visualize the individual peaks of the two stereoisomer components of the racemic product.