Binding proteins on synaptic membranes for crotoxin and taipoxin, two phospholipases A2 with neurotoxicity

Crotoxin and taipoxin are both neurotoxic phospholipases A2 capable of affecting the presynaptic activity to bring about ultimate blockade of synaptic transmission. The enzymatic activity has generally been considered to be necessary but not sufficient for the blockade. Since many phospholipases A2 with comparable or even higher enzymatic activity are not toxic, it has been postulated that the difference lies in the affinity of binding to the presynaptic membrane. In confirmation of this proposition, we and others have previously shown that iodinated crotoxin and taipoxin bind specifically with high affinity to the isolated synaptic membrane fraction from guinea-pig brain, whereas specific binding is not detected with the nontoxic pancreatic phospholipase A2. Experiments based on photoaffinity labeling and simple chemical cross-linking techniques have led to the identification of three polypeptides preferentially present in neuronal membranes as (subunits of) the binding protein(s) for crotoxin and/or taipoxin. Some, but not all, other toxic phospholipases A2 also appear to be ligands for the three polypeptides. We now report studies on partial purification of these polypeptides using affinity chromatography and other techniques. In order to learn the normal physiological roles played by the toxin-binding proteins, the phospholipase-independent effects of the toxins on the synaptosomes have been sought. We have found that under Ca(2+)-free condition, taipoxin or crotoxin inhibits with IC50 of 20-1000 nM the Na(+)-dependent uptake of norepinephrine, dopamine and serotonin by the synaptosomes. In contrast, choline uptake is not affected.(ABSTRACT TRUNCATED AT 250 WORDS)