DIFFERENTIAL-EFFECTS OF PERIPHERAL SITE LIGANDS ON TORPEDO AND CHICKEN ACETYLCHOLINESTERASE

Comparison of the effect of three 'peripheral' site ligands, propidium , d-tubocurarine, and gallamine, on acetylcholinesterase (acetylcholin e hydrolase; EC 3.1.1.7) of Torpedo and chicken shows that all three a re substantially more effective inhibitors of the Torpedo enzyme than of the chicken enzyme. In contrast, edrophonium, which is directed to the ''anionic'' subsite of the active site, inhibits the chicken and T orpedo enzymes equally effectively. Two bisquaternary ligands, decamet honium and 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromid e, which are believed to bridge the anionic subsite of the active site and the ''peripheral'' anionic site, are much weaker inhibitors of th e chicken enzyme than of Torpedo acetylcholinesterase, whereas the sho rter bisquaternary ligand hexamethonium inhibits the two enzymes simil arly. The concentration dependence of activity towards the natural sub strate acetylcholine is almost identical for the two enzymes, whereas substrate inhibition of chicken acetylcholinesterase is somewhat weake r than that of the Torpedo enzyme. The experimental data can be ration alized on the basis of the three-dimensional structure of the Torpedo enzyme and alignment of the chicken and Torpedo sequences; it is sugge sted that the absence, in the chicken enzyme, of two aromatic residues , Tyr-70 and Trp-279, that contribute to the peripheral site of Torped o acetylcholinesterase is responsible for the differential effects of peripheral site ligands on the two enzymes.