The oxidation of methionine and its effect of the properties of cardiotoxin VII1 from Naja melanoleuca venom.

Methionine residues 24 and 26 of cardiotoxin VII1 from Naja melanoleuca were oxidised to sulphoxides using N-chlorosuccinimide at pH 8.5. The number of equivalents of oxidant required for complete oxidation suggested that the methionine side-chains existed in a relatively "exposed" conformational state in cardiotoxin. The oxidised cardiotoxin was devoid of lethality. It was also non-haemolytic, both on its own and in the presence of phospholipase A2. However, it was still able to precipitate with anti-cardiotoxin antibody. CD studies indicated that the polypeptide backbone conformation was intact in the oxidised cardiotoxin but some perturbation of tyrosine residues was evident. The possibility of a direct or indirect involvement of the methionine residues in the biological activity of the cardiotoxin is discussed.