Abstract

Lophotoxin, a diterpene lactone paralytic toxin from gorgonian corals of the genus Lophogorgia, inhibits [125I]-alpha-toxin binding to surface nicotinic receptors of BC3H-1 cells by irreversible occupation of the primary agonist sites. In contrast, receptor-bearing membrane fragments or detergent-solubilized receptors prepared from BC3H-1 cells are not susceptible to lophotoxin block. Thus, lophotoxin inhibition requires intact cells. However, when intact cells were incubated with lophotoxin, subsequent membrane-fragment preparation or detergent solubilization of the receptors did not diminish lophotoxin occupation of [125I]-alpha-toxin-binding sites, indicating that lophotoxin binds very tightly to nicotinic receptors. These studies further demonstrate that both surface and nonsurface nicotinic receptors of BC3H-1 cells are susceptible to irreversible occupation by lophotoxin, indicating that the lipophilic toxin freely permeates intact cells. We also examined several structural analogs of lophotoxin, one of which was equipotent with lophotoxin for inhibition of [125I]-alpha-toxin binding to intact cells and, notably, also blocked alpha-toxin binding to detergent-extracted receptor. Furthermore, this active analog inhibited [125I]-alpha-toxin binding to receptor-rich membrane fragments prepared from Torpedo electric organ, a preparation in which lophotoxin was inactive. Structure-activity relationships exhibited by the lophotoxin congeners suggest mechanisms for covalent bonding to the receptor by way of a Michael addition or by Schiff base formation.