Structure/activity and molecular modeling studies of the lophotoxin family of irreversible nicotinic receptor antagonists

Lophotoxin is a small cyclic diterpene that irreversibly inhibits agonist binding to nicotinic acetylcholine receptors by reacting covalently with Tyr190 in the alpha-subunits of the receptor. Structure/activity and molecular modeling studies were undertaken to investigate the structural and conformational features responsible for this unique biological activity. A total of 18 naturally occurring and 7 chemically modified analogues were evaluated for their ability to inhibit the binding of [125I]-alpha-bungarotoxin to nicotinic acetylcholine receptors on membranes prepared from Torpedo electric organ. When the toxins were incubated with the receptor for short durations they did not slow the initial rate of binding of [125I]-alpha-bungarotoxin, suggesting that they have relatively low reversible affinity. However, their ability to inhibit the equilibrium binding of [125I]-alpha-bungarotoxin increased progressively with longer incubation times, consistent with an irreversible mechanism of action. Comparison of active and inactive analogues allowed identification of a conserved pharmacophore that appeared to be required for irreversible inhibition of the receptor. This pharmacophore contains lactone oxygens and an electron-deficient epoxide that may mimic the acetate oxygens and quaternary ammonium group of acetylcholine, respectively. Computer modeling of the toxins using molecular mechanics and dynamics revealed that the toxins have restricted conformational mobility, thus allowing identification of a minimum-energy conformation. The results allow speculation concerning the site of covalent reaction between Tyr190 and the toxins, the normal function of Tyr190 in binding acetylcholine, and the bound conformation of acetylcholine.