Kinetic Characterization of High-Activity Mutants of Human Butyrylcholinesterase for Cocaine Metabolite Norcocaine

摘要

It has been known that cocaine produces the toxic and physiological effects through not only cocaine itself but also norcocaine formed from cocaine oxidation catalyzed by microsomal cytochrome P450 3A4 in the human liver. The catalytic parameters (kcat and KM) of human butyrylcholinesterase (BChE) and its three mutants (i.e. the A199S/S287G/A328W/Y332G, A199S/F227A/S287G/A328W/E441D, and A199S/F227A/S287G/A328W/Y332G mutants) for norcocaine have been characterized in the present study, for the first time, in comparison with those for cocaine. Based on the obtained kinetic data, wild-type human BChE has a significantly lower catalytic activity for norcocaine (kcat = 2.8 min⁻¹, KM = 15 μM, and kcat/KM = 1.87 × 10⁵ M⁻¹ min⁻¹) compared to its catalytic activity for (−)-cocaine. The BChE mutants examined in this study have considerably improved catalytic activities against both cocaine and norcocaine compared to the wild-type enzyme. Within the enzymes examined in this study, the A199S/F227A/S287G/A328W/Y332G mutant (CocH3) is identified as the most efficient enzyme for hydrolyzing both cocaine and norcocaine. CocH3 has a 1080-fold improved catalytic efficiency for norcocaine (kcat = 2610 min⁻¹, KM = 13 μM, and kcat/KM = 2.01 × 10⁸ M⁻¹ min⁻¹) and a 2020-fold improved catalytic efficiency for cocaine. It has been demonstrated that CocH3 as an exogenous enzyme can rapidly metabolize norcocaine, in addition to cocaine, in rats. Further kinetic modeling has suggested that CocH3 with an identical concentration as that of the endogenous BChE in human plasma can effectively eliminate both cocaine and norcocaine in a simplified kinetic model of cocaine abuse.