Butyrylcholinesterase GII7H/EI97Q is a somanase

We reported previously on the design and expression of a human butyrylcholinesterase (BuChE GII7N) that reactivates spontaneously to catalyze hydrolysis of the nerve agents, sarin and VX [JferAtwri734,(IM5)]. Unfortunately, GII7H is irreversibly inhibited by the most toxic nerve agent, soman (CD). To overcome this limitation, we designed, expressed and partially purified a double mutant (Gl I7H/EI97Q) based upon the hypothesis that removal of the acid at EI97 would slow the so-called "aging" reaction of GD sufficiently to allow the new HI 17 to catalyze reactivation. For comparison, we also made the single mutant EI97Q. All three enzymes retained esterase activity with K_m values for butyrylthiocholine (BuSCh) of 0.05±O.OI mM (GII7H),0.078±0.005 mM (E197Q) and 028±0.02 mM (GII7H/EI97Q) at pH 7.5,25°C ;[wildtype (WT) K_m=0.025±0.006 mM]. Like WT, substrate activation was observed with K_m values of 0.8,23 and 120 mM for GII7H, EI97Q and G117H/E197Q, respectively, compared with 0.9 for WT. All were slowly inhibited by GD. For example, apparent It, values at pH 7.5, 25°C for the CD P(-)C(-) isomer were: > 10s M^-1sec^-1 (WT); 7,600 ±400 M^-1sec^-1(GII7H);3,600±300 M W(EI97Q); and6IO± 40 HV(GII7H/EI97Q). Following inhibition and subsequent removal of excess GD by gel filtration, Gl I7H/EI97Q underwent spontaneous reactivation, with approximate k; values of 11.4 × 10^-5 and 210 × 10^-5 sec^-1 for the P(-)C(-) and P()C(+) GD stereoisomen, respectively, at pH 7.5, 25°C. No recovery of activity was observed for WT, GII7H or E197Q inhibited by GD. Measurement of free inhibitor using a separate, indirect assay confirmed that GII7H/EI97Q reactivation resulted in destruction of GD. We conclude that G117H/E197Q is a cholinesterase with novel somanase activity.