Hydrolysis of acetylthiocholine iodide and reactivation of phoxim-inhibited acetylcholinesterase by pralidoxime chloride, obidoxime chloride and trimedoxime

The hydrolysis of acetylthiocholine iodide (ATCh) by pralidoxime chloride (2-PAM Cl), trimedoxime (TMB4) and obidoxime chlpride (LÜH6) was studied at pH 5.8–8.0 and incubation temperature from 5 to 40°C in vitro. Significant ATCh hydrolysis by 2-PAM Cl, TMB4 and LÜH6 was found, with the exceptions of those at pH 7.0, 6.2 and 5.8 at 5°C and those at pH 6.2 and 5.8 at 15°C. The hydrolysis by TMB4 and LÜH6 was significantly stronger than that by 2-PAM Cl. The hydrolysis increased with increasing pH, incubation temperature and three oxime or ATCh concentration. Significant hydrolysis of ATCh by the three oximes could be found when the terminal concentration of oxime was higher than 0.01 mM at pH 7.0 and 7.4 at 30 and 37°C. However, no hydrolysis of natural substrate (acetylcholine iodide) by the three oximes was found when very high terminal concentrations of oximes were used. In addition, the three oximes displayed an extraordinary efficiency in the reactivation of phoxim-inhibited acetylcholinesterase (AChE) from fish (Carassius auratus) or rabbit (Oryctolagus cuniculus domestic) brain in vitro. Parallel to the level of ATCh hydrolysis by the oximes, TMB4 and LÜH6 displayed significantly higher reactivation efficiency than 2-PAM Cl to phoxim-inhibited AChE. And, the extent of reactivation by 2-PAM Cl was also lower than the other two. Plausible antidotal actions of the oximes against organophosphate poisoning AChE and erroneously high estimation of AChE activity by the Ellman method were discussed.