A SINGLE AMINO-ACID SUBSTITUTION, GLY117HIS, CONFERS PHOSPHOTRIESTERASE (ORGANOPHOSPHORUS ACID ANHYDRIDE HYDROLASE) ACTIVITY ON HUMAN BUTYRYLCHOLINESTERASE

The G117H mutant of human butyrylcholinesterase (EC 3.1.1.8) was expre ssed in Chinese hamster ovary cells. Substitution of Gly 117 with His to make the G117H mutant endowed butyrylcholinesterase with the abilit y to catalyze the hydrolysis of organophosphate esters, G117H was stil l able to hydrolyze butyrylthiocholine, benzoylcholine, and o-nitrophe nyl butyrate, but in addition it had acquired the ability to hydrolyze the antiglaucoma drug echothiophate and the pesticide paraoxon. Wild- type butyrylcholinesterase was irreversibly inhibited by echothiophate and paraoxon, but G117H regained 100% activity within 2-3 min followi ng reaction with these compounds. On a polyacrylamide gel, the same ba nds that stained for activity with butyrylthiocholine also stained for activity with echothiophate. G117H is the only enzyme known that hydr olyzes echothiophate. Diethoxyphosphorylated G117H aged with a half-ti me of 5.5 h, a rate 600 times slower than the rate of hydrolysis. Echo thiophate and paraoxon were hydrolyzed with the same k(cat) of 0.75 mi n(-1). This calculates to a rate acceleration of 100 000-fold for hydr olysis of echothiophate and paraoxon by the G117H mutant compared to t he nonenzymatic rate.