Jianmongkol S, Marable B R, Berkman C E, Talley T T, Thompson C M, Richardson R J
Department of Environmental & Industrial Health, The University of Michigan, Ann Arbor, Michigan, 48109, USA.
Toxicol Appl Pharmacol. 1999 Feb 15;155(1):43-53. doi: 10.1006/taap.1998.8608.
Inhibition of acetylcholinesterase (AChE) by isomalathion has been assumed to proceed by expulsion of diethyl thiosuccinyl to produce O, S-dimethyl phosphorylated 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 AChE inhibited by (1R)-isomalathions reactivates at the same rate as the enzyme inhibited by (S)-isoparathion methyl. However, although rat brain AChE inhibited by (R)-isoparathion methyl reactivates at a measurable rate, the enzyme inhibited by (1S)-isomalathions is intractable to reactivation. This surprising finding suggests the hypothesis that (1R)- and (1S)-stereoisomers of isomalathion inhibit AChE by different mechanisms, yielding enzymatic species distinguishable by their postinhibitory kinetics. The present study was carried out to test this hypothesis by comparing kinetic constants of reactivation (k+3) and aging (k+4) of hen brain AChE and bovine erythrocyte AChE inhibited by the four stereoisomers of isomalathion and the two stereoisomers of isoparathion methyl. Both AChEs inhibited by either (1R,3R)- or (1R,3S)-isomalathion had comparable corresponding k+3 values (spontaneous 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 brain AChE inhibited by each stereoisomer of isomalathion and isoparathion methyl corroborated that only the (1S)-isomalathions failed to produce the expected O,S-dimethyl phosphoryl-conjugated enzymes. The results for (1R)-isomalathions suggest that the mechanism of inhibition of AChE by these isomers 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 of thiomethyl as the primary leaving group.
异马拉硫磷对乙酰胆碱酯酶（AChE）的抑制作用被认为是通过排出二乙基硫代琥珀酰基来产生O，S - 二甲基磷酸化的AChE。如果这一假设正确，那么被（1R） - 或（1S） - 异马拉硫磷抑制的AChE重新激活的速率，应该分别与被构型等效的（S） - 或（R） - 甲基异对硫磷抑制的AChE相同，预计后者通过失去4 - 硝基苯氧基来抑制AChE，从而产生O，S - 二甲基磷酸化的AChE。先前的研究表明，被（1R） - 异马拉硫磷抑制的大鼠脑AChE与被（S） - 甲基异对硫磷抑制的酶以相同的速率重新激活。然而，尽管被（R） - 甲基异对硫磷抑制的大鼠脑AChE能以可测量的速率重新激活，但被（1S） - 异马拉硫磷抑制的酶却难以重新激活。这一惊人发现提出了一个假设，即异马拉硫磷的（1R） - 和（1S） - 立体异构体通过不同机制抑制AChE，产生的酶种类可通过其抑制后的动力学加以区分。本研究旨在通过比较被异马拉硫磷的四种立体异构体和甲基异对硫磷的两种立体异构体抑制的鸡脑AChE和牛红细胞AChE的重新激活（k + 3）和老化（k + 4）动力学常数来验证这一假设。被（1R，3R） - 或（1R，3S） - 异马拉硫磷抑制的两种AChE，其相应的k + 3值（自发和肟介导的）与被（S） - 甲基异对硫磷抑制的AChE相当。然而，对于被（1S，3R） - 和（1S，3S） - 异马拉硫磷抑制的AChE，无法获得与被（R） - 甲基异对硫磷相当的自发和肟介导的k + 3值。比较被异马拉硫磷和甲基异对硫磷的各立体异构体抑制的鸡脑AChE的k + 4值证实，只有（1S） - 异马拉硫磷未能产生预期的O，S - 二甲基磷酰共轭酶。（1R） - 异马拉硫磷的结果表明，这些异构体抑制AChE的机制是预期的机制之一，即以二乙基硫代琥珀酰基作为主要离去基团。相反，（1S） - 异马拉硫磷的结果与这些异构体的另一种抑制机制一致，即意味着以硫甲基的损失作为主要离去基团。
