Enzymatic kinetic parameters for polyfluorinated alkyl phosphate hydrolysis by alkaline phosphatase

The hydrolysis kinetics of three polyfluorinated alkyl phosphate monoesters (monoPAPs), differing in fluorinated chain length, were measured using bovine intestinal alkaline phosphatase to catalyze the reaction. Kinetic values were also measured for analogous hydrogenated phosphate monoesters to elucidate the effects of the fluorinated chain on the rate of enzymatic hydrolysis. Michaelis constants (Km) were obtained by a competition kinetics technique in the presence of p-nitrophenyl phosphate (PNPP) using UV-vis spectroscopy. Compared with Km (PNPP), Michaelis constants for monoPAPs ranged from 0.9 to 2.1 compared with hydrogenated phosphates, which ranged from 4.0 to 13.0. Apparent bimolecular rate constants (kcat/Km) were determined by monitoring rates of product alcohol formation at low substrate concentrations using gas chromatographymass spectrometry. The experimental values for kcat/Km averaged as 1.1?x?107 M-1s-1 for monoPAPs compared with 3.8?x?105 M-1s-1 for hexyl phosphate. This suggests that the electron-withdrawing nature of the fluorinated chain enhanced the alcohol leaving group ability. The results were used in a simple model to suggest that monoPAPs in a typical mammalian digestive tract would hydrolyze in approximately 100?s, supporting a previous study that showed its absence after a dosing study in rats. Environ. Toxicol. Chem. 2012; 31: 19661971. (c) 2012 SETAC