Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.1.4.3 (phospholipase C)
 18,461 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
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PMID:Structure-activity relationships for the irreversible blockade of nicotinic receptor agonist sites by lophotoxin and congeneric diterpene lactones. 405 23

Lophotoxin, a cyclic diterpenoid isolated from coral, irreversibly inactivates the nicotinic acetylcholine receptor on intact BC3H-1 cells. Inactivation can be prevented by simultaneous incubation of lophotoxin with nicotinic agonists and competitive antagonists but not by noncompetitive antagonists such as dibucaine. Analysis of lophotoxin inhibition of carbamylcholine-elicited 22Na+ permeability, KG/KGmax, in relation to the number of sites blocked, y, reveals a function showing greater curvature than the parabolic function kG/kGmax = (1 - y)2 found for cobra alpha-toxin inhibition of 22Na+ permeability. This relationship is consistent with lophotoxin not binding randomly to the two primary agonist-antagonist sites but rather exhibiting a preference for one of the two sites. Binding of lophotoxin to a single site per receptor oligomer is sufficient to render the receptor nonfunctional. A comparison of the concentration dependencies for occupation by competitive antagonists reveals a shift to lower antagonist concentrations and an increase in Hill coefficient to approach unity after partial occupation by lophotoxin. Competitive antagonists are known to bind to two sites of unequal affinity on the receptor, and the preferential site of lophotoxin inactivation is the site of lower affinity for the competitive antagonists. In the case of agonists fractional inactivation of sites by lophotoxin results in a loss of the positive cooperativity and a shift of the concentration dependence for carbamylcholine binding to higher agonist concentrations. Similar behavior is observed for cobra alpha-toxin inactivation, but a comparison of concentration dependencies for agonist binding following partial occupation by lophotoxin and cobra alpha-toxin reveals that lophotoxin blockade yields residual sites with a lower affinity and Hill coefficients for agonists closer to unity. These shifts in agonist and antagonist binding profiles are also in accord with preferential inactivation of one of the two agonist sites by lophotoxin. The findings indicate that the site of lower affinity for antagonists may possess the higher affinity for agonists.
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PMID:Lophotoxin irreversibly inactivates the nicotinic acetylcholine receptor by preferential association at one of the two primary agonist sites. 614 93