Selective loss of binding sites for the iodinated α‐neurotoxin I from Naja mossambica mossambica venom upon enzymatic deglycosylation of Torpedo electric organ membranes

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Abstract
Removal of asparagine-linked carbohydrate chains from Torpedo marmorata electric organ membranes was found to inhibit the binding of the iodinated .alpha.-neurotoxin I from Naja mossambica snake venom to its receptor. Optimal deglycosylation of membranes by endoglycosidase F resulted in a 55% inhibition of .alpha.-neurotoxin-I-saturable binding. Under these conditions, up to 70% of concanavalin A binding was also lost, indicating an efficient removal of mannose-rich carbohydrate chains. Saturation binding experiments at equilibrium on membranes incubated in the absence of endoglycosidase F indicated, when analyzed by Scatchard plots, the presence of two classes of high-affinity binding sites for .alpha.-neurotoxin I (Kd=9 pM and 68 pM respectively) with capacities of 24 and 14 pmol/mg membrane proteins, respectively. After endoglycosidase F treatment, only the former class of binding sites (Kd=11 pM) was recovered together with a 45% reduction in the number of total binding sites. Dissociation experiments further confirmed the presence of two types of toxin-receptor complexes in control membranes and the selective loss of the rapidly dissociating component upon deglycosylation. The binding of .alpha.-neurotoxin I to its receptor, deglycosylated or not, was totally inhibited by carbamoylcholine, d-tubocurarine or .alpha.-bungarotoxin. These findings show that the neurotoxin binding sites present on the acetylcholine receptor can be discriminated on the basis of their differential susceptibility to the removal of asparagine-linked carbohydrate chains.
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