Bakry N M, el-Rashidy A H, Eldefrawi A T, Eldefrawi M E
Department of Pharmacology and Experimental Therapeutics, University of Maryland, School of Medicine, Baltimore 21201.
J Biochem Toxicol. 1988 Winter;3:235-59. doi: 10.1002/jbt.2570030404.
Four nerve agents and one therapeutic organophosphate (OP) anticholinesterase (anti-ChE) bind to acetylcholine (ACh) receptors, inhibit or modulate binding of radioactive ligands to these receptors, and modify events regulated by them. The affinity of nicotinic (n) ACh receptors of Torpedo electric organs and most muscarinic (m) ACh receptors of rat brain and N1E-115 neuroblastoma cultures for the OP compounds was usually two to three orders of magnitude lower than concentrations required to inhibit 50% (IC-50) of ACh-esterase activity. However, a small population of m-ACh receptors had an affinity as high as that of ACh-esterase for the OP compound. This population is identified by its high-affinity [3H]-cis-methyldioxolane ([3H]-CD) binding. Although sarin, soman, and tabun had no effect, (O-ethyl S[2-(diisopropylamino)ethyl)] methyl phosphonothionate (VX) and echothiophate inhibited competitively the binding of [3H]-quinuclidinyl benzilate ([3H]-QNB) and [3H]-pirenzepine ([3H]-PZ) to m-ACh receptors. However, VX was more potent than echothiophate in inhibiting this binding and 50-fold more potent in inhibiting carbamylcholine (carb)-stimulated [3H]-cGMP synthesis in N1E-115 neuroblastoma cells--both acting as m receptor antagonist. All five OPs inhibited [3H]-CD binding, with IC-50s of 3, 10, 40, 100, and 800 nM for VX, soman, sarin, echothiophate, and tabun, respectively. The OP anticholinesterases also bound to allosteric sites on the n-ACh receptor (identified by inhibition of [3H]-phencyclidine binding), but some bound as well to the receptor's recognition site (identified by inhibition of [125I]-alpha-bungarotoxin binding). Soman and echothiophate in micromolar concentrations acted as partial agonists of the n-ACh receptor and induced receptor desensitization. On the other hand, VX acted as an open channel blocker of the activated receptor and also enhanced receptor desensitization. It is suggested that the toxicity of OP anticholinesterases may include their action on n-ACh as well as m-ACh receptors if their concentrations in circulation rise above micromolar levels. At nanomolar concentrations their toxicity is due mainly to their inhibition of ACh-esterase. However, at these low concentrations, many OP anticholinesterases (eg, VX and soman) may affect a small population of m-ACh receptors, which have a high affinity for CD. Such effects on m-ACh receptors may play an important role in the toxicity of certain OP compounds.
四种神经毒剂和一种治疗用有机磷酸酯（OP）抗胆碱酯酶（anti-ChE）可与乙酰胆碱（ACh）受体结合，抑制或调节放射性配体与这些受体的结合，并改变由它们调节的事件。电鳐电器官的烟碱型（n）ACh受体以及大鼠脑和N1E-115神经母细胞瘤培养物中的大多数毒蕈碱型（m）ACh受体对OP化合物的亲和力通常比抑制50%（IC-50）ACh酯酶活性所需的浓度低两到三个数量级。然而，一小部分m-ACh受体对OP化合物的亲和力与ACh酯酶一样高。这一群体通过其高亲和力的[3H]-顺式甲基二氧戊环（[3H]-CD）结合来识别。虽然沙林、梭曼和塔崩没有作用，但（O-乙基S[2-（二异丙基氨基）乙基]甲基硫代磷酸酯（VX）和依可碘酯竞争性抑制[3H]-喹核醇基苯甲酸酯（[3H]-QNB）和[3H]-哌仑西平（[3H]-PZ）与m-ACh受体的结合。然而，VX在抑制这种结合方面比依可碘酯更有效，在抑制N1E-115神经母细胞瘤细胞中氨甲酰胆碱（carb）刺激的[3H]-cGMP合成方面效力高50倍——两者均作为m受体拮抗剂。所有五种OP均抑制[3H]-CD结合，VX、梭曼、沙林、依可碘酯和塔崩的IC-50分别为3、10、40、100和800 nM。OP抗胆碱酯酶也与n-ACh受体上的变构位点结合（通过抑制[3H]-苯环利定结合来识别），但有些也与受体的识别位点结合（通过抑制[125I]-α-银环蛇毒素结合来识别）。微摩尔浓度的梭曼和依可碘酯作为n-ACh受体的部分激动剂并诱导受体脱敏。另一方面，VX作为活化受体的开放通道阻滞剂并增强受体脱敏。有人提出，如果OP抗胆碱酯酶在循环中的浓度升高到微摩尔水平以上，其毒性可能包括它们对n-ACh以及m-ACh受体的作用。在纳摩尔浓度下，它们的毒性主要归因于对ACh酯酶的抑制。然而，在这些低浓度下，许多OP抗胆碱酯酶（如VX和梭曼）可能会影响一小部分对CD具有高亲和力的m-ACh受体。这种对m-ACh受体的影响可能在某些OP化合物的毒性中起重要作用。
