IN vitro protection of acetylcholinesterase and butyrylcholinesterase by tetrahydroaminoacridine : Comparison with physostigmine

Abstract

The protective action of 1,2,3,4-tetrahydro-9-aminoacridine (THA) against the long-lasting inactivation of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) brought about by diisopropylnuorophosphate (DFP) and physostigmine, as well as by neostigmine in the case of AChE only, was evaluated by a dilution technique using Electrophorus electricus AChE and horse serum BuChE as target enzymes. In parallel experiments, the ability of physostigmine itself to protect these enzymes from DFP was evaluated and compared with that of THA. THA pretreatment was seen to prevent in a dose-dependent manner the inhibition of both AChE and BuChE. However, it was appreciably more potent towards AChE than towards BuChE. THA mean ec 50 values for protecting AChE against 10, 40 and 100 μM DFP were 0.04, 0.16 and 0.45 μM, respectively; against 1 μM physostigmine the value was 1.8 μM and against 1.2 μm neostigmine it was 3.0 μM. The THA mean ec 50 value for protecting BuChE against 3 μM physostigmine was 0.55 μM and the values for protecting against 3, 10 and 40 μM DFP were 1.5, 3 and >10 μM, respectively. The protective action of THA was time independent: recovery of the maximal enzymic activity was immediate upon dilution. Unlike THA, the protective action of physostigmine developed progressively after dilution and was maximal within 3–4 (AChE) or 6–8 hr (BuChE). Under our experimental conditions, 0.3 μM physostigmine protected approximately 70% of AChE from 40 μM DFP and 5 μM physostigmine protected 9 and 47% of BuChE from 40 and 3 μM DFP, respectively. The results of this work suggest that THA exerts its protective action by shielding the active site of AChE and BuChE from the attack of the inactivating agents on account of its higher enzymic affinity, whereas the protective action of physostigmine against DFP takes advantage also of the carbamylation of the enzyme. These results are in line with the hypothesis that protection of AChE is the primary mechanism responsible for the antidotal action of THA against organophosphorus poisoning.