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Molecular mechanism of cardiotoxin action on axonal membranes
Insights
Cardiotoxin from Naja mossambica mossambica selectively deactivates axonal sodium-potassium adenosine triphosphatase. This toxin binds to the membrane lipid phase, causing irreversible enzyme deactivation, unlike cobra neurotoxins.
Area of Science:
- Biochemistry
- Neuroscience
- Toxicology
Background:
- Snake venom cardiotoxins, despite sequence homology with neurotoxins, exhibit distinct mechanisms of action.
- Understanding cardiotoxin's interaction with neuronal membranes is crucial for elucidating its toxic effects.
Purpose of the Study:
- To investigate the mechanism by which cardiotoxin from Naja mossambica mossambica deactivates axonal sodium-potassium adenosine triphosphatase.
- To characterize the binding kinetics and interactions of cardiotoxin with neuronal membranes.
Main Methods:
- Monitoring the deactivation of sodium-potassium adenosine triphosphatase activity.
- Direct binding assays using a tritiated cardiotoxin derivative.
- Investigating the effects of Ca2+ and dibucaine on cardiotoxin binding.
Main Results:
- Cardiotoxin selectively deactivates axonal sodium-potassium adenosine triphosphatase, with no effect on tetrodotoxin binding or acetylcholinesterase activity.
- Maximal binding capacity suggests cardiotoxin associates with the membrane's lipid phase.
- Cardiotoxin binding involves rapid, reversible association with lipids, followed by irreversible membrane structural rearrangement and enzyme deactivation.
Conclusions:
- Cardiotoxin inhibits sodium-potassium adenosine triphosphatase by binding to the axonal membrane's lipid phase, a mechanism distinct from neurotoxin action.
- The enzyme, once solubilized, becomes resistant to cardiotoxin-induced deactivation, indicating a membrane-dependent effect.
- High Ca2+ or dibucaine concentrations inhibit cardiotoxin binding to the membrane.
Abstract:
Cardiotoxin isolated from Naja mossambica mossambica selectively deactivates the sodium-potassium activated adenosine triphosphatase of axonal membranes. Tetrodotoxin binding and acetylcholinesterase activities are unaffected by cardiotoxin treatment. The details of association of cardiotoxin with the axonal membrane were studied by following the deactivation of the sodium-potassium activated adenosine triphosphatase and by direct binding measurements with a tritiated derivative of the native cardiotoxin. The maximal binding capacity of the membrane is 42-50 nmol of cardiotoxin/mg of membrane protein. The high amount of binding suggests association of the toxin with the lipid phase of the membrane. It has been shown that cardiotoxin first associates rapidly and reversibly to membrane lipids, then, in a second step, it induces a rearrangement of the membrane structure which produces and irreversible deactivation of the sodium-potassium activated adenosine triphosphatase. Solubilization of the membrane-bound ATPase with Lubrol WX gives an active enzyme species that is resistant to cardiotoxin-induced deactivation. Cardiotoxin binding to the membrane is prevented by high concentrations of Ca 2+ and dibucaine. Although cardiotoxins and neurotoxins of cobra venom have large sequence homologies, their mode of action on membranes is very different. The cardiotoxin seems to bind to the lipid phase of the axonal membrane and inhibits the sodium-potassium activated adenosine triphosphatase, whereas the neurotoxin associates with a protein receptor in the post-synaptic membrane and blocks acetylcholine transmission.