Synergism of the two subunits of crotoxin.

Crotoxin, a potent neurotoxin from the venom of Crotalus durissus terrificus, is composed of an acidic subunit which is non-toxic and enzymatically inactive and a basic subunit which possesses a phospholipase activity and low toxicity. It is shown that crotoxin very efficiently blocks the cholinergic post-synaptic response of the isolated electroplaque from Electrophorus electricus and of cellular microsacs fromTorpedo marmorata. This post-synpatic effects was investigated by studying the binding of crotoxin and its isolated subunits to acetylcholine receptor-rich membranes from Torpedo and by analysing the relationship between its catalytic activity and its pharmacological effects on Electrophorus electroplaque. The mechanism of action of crotoxin could be divided into two distinct steps: a quasi irreversible binding step, which has no blocking action by itself, and a catalytic step, which irreversibly inhibits the postsynaptic response. These results suggest that the non-enzymatic subunit of crotoxin enhances the pharmacological efficiency of the phospholipase by preventing its adsorption to non-saturable binding sites, restricting its binding to specific critical target sites. These sites are distinct from the cholinergic receptor sites, but probably closely related to them, as suggested by their approximately equal number and by the fact that after crotoxin action the receptor appears to be blocked in a molecular form very similar to the desensitized state. The mechanism proposed explains in simple terms the synergistic action of the two subunits of crotoxin at the level of the cholinergic receptor-ionophore assembly.