Title : Aging of cholinesterases phosphylated by tabun proceeds through O-dealkylation
Abstract :
- Human butyrylcholinesterase ( hBChE ) hydrolyzes or scavenges a wide range of toxic esters, including heroin , cocaine, carbamate pesticides, organophosphorus pesticides, and nerve agents
- Organophosphates (OPs) exert their acute toxicity through inhibition of acetylcholinesterase ( AChE ) by phosphorylation of the catalytic serine
- Phosphylated cholinesterase ( ChE ) can undergo a spontaneous, time-dependent process called "aging", during which the OP- ChE conjugate is dealkylated
- This leads to irreversible inhibition of the enzyme
- The inhibition of ChEs by tabun and the subsequent aging reaction are of particular interest, because tabun- ChE conjugates display an extraordinary resistance toward most current oxime reactivators
- We investigated the structural basis of oxime resistance for phosphoramidated ChE conjugates by determining the crystal structures of the non-aged and aged forms of hBChE inhibited by tabun, and by updating the refinement of non-aged and aged tabun-inhibited mouse AChE ( mAChE )
- Structures for non-aged and aged tabun- hBChE were refined to 2.3 and 2.1 A, respectively
- The refined structures of aged ChE conjugates clearly show that the aging reaction proceeds through O-dealkylation of the P(R) enantiomer of tabun
- After dealkylation, the negatively charged oxygen forms a strong salt bridge with protonated His438N epsilon2 that prevents reactivation
- Mass spectrometric analysis of the aged tabun-inhibited hBChE showed that both the dimethylamine and ethoxy side chains were missing from the phosphorus
- Loss of the ethoxy is consistent with the crystallography results
- Loss of the dimethylamine is consistent with acid-catalyzed deamidation during the preparation of the aged adduct for mass spectrometry
- The reported 3D data will help in the design of new oximes capable of reactivating tabun- ChE conjugates