Crotoxin acidic subunit

One type of presynaptic toxin is composed of two subunits bound together. The basic subunit has phospholipase A activity, whereas the acidic subunit has no enzymic activity. Crotoxin is the first presynaptic toxin isolated from snake venom and is also one of the most well-studied presynaptic toxins. 

The role of the acidic subunit A is to guide the toxin to a specific site, then the basic subunit B functions as a presynaptic toxin (Hendon and Tu, 1979). Each subunit alone is relatively nontoxic, but combined, the toxicity is greatly enhanced (Trivedi et al., 1989). The undissociated crotoxin itself shows phospholipase A activity, indicating the active site of subunit B is not masked by subunit A (Radvanyi and Bon, 1984). Besides neurotoxicity, subunit B also has hemolytic activity. Subunit B attaches to many parts of the erythrocyte membranes (Jeng et al., 1978) and also on the postsynaptic membrane (Bon et al., 1979), in addition to the presynaptic binding site. 

Faure, G., Guillaume, J. L., Camoin, L., Saliou, B., and Bon, C. (1991). Multiplicity of acidic subunit isoforms of crotoxin, the phospholipase A2 neurotoxin from Crotalus durissus terrificus venom, results from post translational modifications. Biochemistry 30 8074-8083. 

Mojave toxin from Crotalus scutulatus is also structurally similar to crotoxin and is composed of two subunits (Bieber et al., 1975 Cate and Bieber, 1978). There are considerable amino acid sequence homologies between the two toxins (Aird et al., 1990). The amino acid sequence and spectroscopic properties of subunits of crotoxin are similar to other presynaptic phospholipases A from snake venoms (Aird et al., 1989a). 

Aird, S. D., Kaiser, 1.1., Lewis, R. V, and Kruggel, W. G. (1986). A complete amino acid sequence for the basic subunit of crotoxin. Arch. Biochem. Biophys. 249 296-300. 

Bouchier, C., Ducancel, R, Guignery-Frelat, G., Bon, C., Boulain, J.C., and Menez, A. (1988) Cloning and sequencing of cDNAs encoding the two subunits of crotoxin. Nucleic Acid Research, 16, 9050. 

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