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Alpha-toxin

Alpha-toxin is a protein of 42.5 kDa (370 amino acids) secreted by the Gram-positive anaerobe Clostridium perfringens. Alpha-toxin is a zinc-dependent phospholipase C [EC3.1.4.3] (Saint-Joanis et al, 1989; Tso and Siebel, 1989; Leslie et al, 1989; Okabe et al, 1989; Titball et al, 1991) that hydrolyzes phosphatidylcholine, lecithin, sphingomyelin in eukaryotic cell membranes (Rood, 1998). Calcium ions have been shown to be required for the specific binding of toxin to membranes prior to phospholipid cleavage (Naylor et al, 1999).

The toxin is essential for the virulence of Clostridium perfringens type A and one of the most toxic extracellular proteins produced by this organism (Awad et al, 2000; Ellemor et al, 1999; Stevens et al, 1988).

The secreted toxin has the ability to lyse erythrocytes from various species. It is a key virulence determinant in gas gangrene (clostridial myonecrosis). The enzymatic and hemolytic activities are the result of the presence of a single active site in the protein and are essential for lethality in vivo and for mediating platelet aggregation in vitro or platelet aggregation in vivo. The N-domain (residues 1-250) of the alpha-toxin possesses PLC activity but does not bind to rabbit erythrocytes and lyse them. The C-domain (residues 251-370) is required for binding to erythrocytes and the hemolytic and enzymatic activities of the N-domain. Amino acid positions essential for the various functions of the toxin have been determined by mutation analysis.

Expression of Alpha-toxin in the infected host causes tissue necrosis, inhibition of the influx of polymorphonuclear leukocytes into the lesion, and thrombosis formation, with alpha-toxin mutants of Clostridium perfringens being avirulent in a mouse myonecrosis model. Alpha-toxin can cause cell activation of inflammatory cells, and massive accumulation of neutrophils at the margins of the infected areas. The toxin has been shown to cause an increase in vascular permeability. Mast cells release histamine in response to the toxin.

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