Crotoxin subunits

Hendon, R. A., and Tu, A. T. (1979). The role of crotoxin subunits in tropical rattlesnake neurotoxic action. 

The synergistic behavior of crotoxin subunits was further demonstrated by studying the differential stability of crotoxin isoforms differing in their toxicity and their enzymatic activity (Faure et al. 1993). More toxic isoforms are less enzymatically active, block neuromuscular transmission of chick biventer cervicis more efficiently, are more stable and dissociate more slowly than less toxic complexes. Therefore, less toxic crotoxin isoforms, because of their lower stability, are more likely to dissociate en route before reaching their target, CA being no more able to prevent CB non specific binding. 

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. 

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). 

Crotoxin s neurotoxic action is very similar to P-bungarotoxin (P-btx), but has some difference. For instance, crotoxin and its subunit B have a postsynaptic effect, whereas p-btx has no such activity (Bon et al., 1979). 

In P-btx, the A chain is the one that has phospholipase A activity The nomenclature is somewhat opposite that of crotoxin or Mojave toxin in crotoxin or Mojave toxin, subunit B is the one showing phospholipase A activity. Oxidation of methionine at the 6 and 8 positions lowered the toxicity without affecting antigenicity. Moreover, the NH2-terminal region of the A chain plays a crucial role in maintaining functional activity (Chang and Yang, 1988). 

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. 

Aird, S. D., Steadman, B. L., Middaugh, C. R., and Kaiser, 1.1. (1989a). Comparative spectroscopic studies of four crotoxin homologies and their subunits. Biochim. Biophys. Acta 997 211-218. 

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. 

THE CROTOXIN COMPLEX, A STRIKING MODEL OF COMPLEMENTARITY BETWEEN TWO SUBUNITS... 

IMMUNOCHEMICAL STUDIES OF CROTOXIN AND ITS SUBUNITS MECHANISM OF NEUTRALIZATION OF CROTOXIN BY ANTIBODIES AND SEARCH FOR A TOXIC SITE ... 

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. 

Curin-Serbec, V., Delot, E., Faure, G., Saliou, B. Gubensek, F., Bon, C. and Choumet, V. (1993) Antipeptide antibodies directed to the C-terminal part of ammodytoxin A react with the PLA2 subunit of crotoxin and neutralize its pharmacological activity. Toxicon 32, 1337-1348. 

Since subunit A did not bind to the membranes in the end of the equilibrium binding assay, obviously the dissociation of the crotoxin complex at some point is an important step in the action of the toxin. Hendon and Tu (28) converted crotoxin complex into a non-dis-sociable conjugate by cross-linking the two subunits with a bifunctional cross-linker, dimethyl suberimidate. This conversion abolished the lethality of crotoxin but did not significantly reduce its PLA2 activity. When we used the non-dissociable crotoxin in binding... 

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