Kinetic evidence for different mechanisms of acetylcholinesterase inhibition by (1R)- and (1S)-stereoisomers of isomalathion

Inhibition of acetylcholinesterase (AChE) by isomalathion has been assumed to proceed by expulsion of diethyl thiosuccinyl to produce O,S-dimethyl pho sphorylated AChE. If this assumption is correct, AChE inhibited by (1R)- or (1S)-isomalathions should reactivate at the same rate as AChE inhibited by configurationally equivalent (S)- or (R)-isoparathion methyl, respectively , which are expected to inhibit AChE by loss of 4-nitrophenoxyl to yield O, S-dimethyl phosphorylated AChEs. Previous work has shown that rat brain ACh E inhibited by (1R)-isomalathions reactivates at the same rate as the enzym e inhibited by (S)-isoparathion methyl. However, although rat brain AChE in hibited by (R)-isoparathion methyl reactivates at a measurable rate, the en zyme inhibited by (1S)-isomalathions is intractable to reactivation. This s urprising finding suggests the hypothesis that (1R)- and (1S)-stereoisomers of isomalathion inhibit AChE by different mechanisms, yielding enzymatic s pecies distinguishable by their postinhibitory kinetics. The present study was carried out to test this hypothesis by comparing kinetic constants of r eactivation (k(+3)) and aging (k(+4)) of hen brain AChE and bovine erythroc yte AChE inhibited by the four stereoisomers of isomalathion and the two st ereoisomers of isoparathion methyl. Both AChEs inhibited by either (1R,3R)- or (1R,3S)-isomalathion had comparable corresponding k(+3) values (spontan eous and oxime-mediated) to those of AChEs inhibited with (S)-isoparathion methyl. However, spontaneous and oxime-mediated k(+3) values comparable to those of (R)-isoparathion methyl could not be obtained for AChEs inhibited by (1S,3R)- and (1S,3S)-isomalathion. Comparison of k(+4) values for hen br ain AChE inhibited by each stereoisomer of isomalathion and isoparathion me thyl corroborated that only the (1S)-isomalathions failed to produce the ex pected O,S-dimethyl phosphoryl-conjugated enzymes. The results for (1R)-iso malathions suggest that the mechanism of inhibition of AChE by these isomer s is the expected one involving diethyl thiosuccinyl as the primary leaving group. In contrast, the results for (1S)-isomalathions are consistent with an alternative mechanism of inhibition by these isomers implicating loss o f thiomethyl as the primary leaving group. (C) 1999 Academic Press.