Comparison of active sites of butyrylcholinesterase and acetylcholinesterase based on inhibition by geometric isomers of benzene-di-N-substituted carbamates.

We have reported that benzene-1,2-, 1,3-, and 1,4-di-N-substituted carbamates (1-15) are characterized as the conformationally constrained inhibitors of acetylcholinesterase and mimic gauche, eclipsed, and anti-conformations of acetylcholine, respectively (J Biochem Mol Toxicol 2007;21:348-353). We further report the inhibition of butyrylcholinesterase by these inhibitors. Carbamates 1-15 are also characterized as the pseudosubstrate inhibitors of butyrylcholinesterase as in the acetylcholinesterase catalysis. Benzene-1,4-di-N-n-hexylcarbamate (12) and benzene-1,4-di-N-n-octylcarbamate (13) are the two most potent inhibitors of butyrylcholinesterase among inhibitors 1-15. These two para compounds, with the angle of 180 degrees between two C(benzene)--O bonds, mimic the preferable anti C--O/C--N conformers for the choline ethylene backbone of butyrylcholine during the butyrylcholinesterase catalysis. The second n-hexylcarbamyl or n-octylcarbamyl moiety of inhibitors 12 and 13 is proposed to bind tightly to the peripheral anionic site of butyrylcholinesterase from molecular modeling. Butyrylcholinesterase prefers para-carbamates to ortho- and meta-carbamates, whereas acetylcholinesterase prefers para- and meta-carbamates to ortho-carbamates. This result implies that the anionic site of butyrylcholinesterase is relatively smaller than that of acetylcholinesterase because meta-carbamates, which may bind to the anionic sites of both enzymes, are not potent inhibitors of butyrylcholinesterase.