Chemical modification of methionines in a cobra venom cytotoxin differentiates between lytic and binding domains.
Cytotoxin-III from Naja naja atra (CTX) was chemically modified at either or both of its two methionine residues: Over 50% oxidation of methionine-26 occurred with a 1:1 molar ratio of chloramine-T:methionine; at a 5:1 molar ratio, methionine-26 was almost completely oxidized, while methionine-24 was modified only 26%; at a 10:1 molar ratio, both methionines were completely oxidized. Each oxidized derivative demonstrated a lower toxicity toward T-cells than toward heart cells. Conversely, binding to heart cells was affected more than binding to T-cells. Cyanogen bromide cleaved native CTX at both methionines, excising phenyl-alanine-25 and methionine-26 and converting methionine-24 to homoserine lactone. This treatment of CTX eliminated cytotoxicity toward both heart and T-cells, but had only a modest effect upon T-cell binding, as had 50% oxidation of methionine-26, suggesting that CTX lytic and binding regions may be distinct. A selective loss in heart cell binding following oxidation of methionine-24 further suggests that different parts of CTX may interact with the two types of target cells. Perturbation of the relatively flat hydrophobic surface of the CTX' triple-stranded beta-sheet could result from the introduction of negative charge due to methionine-24 oxidation. Alternatively, amino acid side chain participation in a CTX binding domain may be altered by the potential formation of a new hydrogen bond between tyrosine-51 and methionine-24 sulfoxide, as revealed by computer modeling of the completely oxidized CTX derivative.
Stevens-Truss
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Department of Medicinal & Biological Chemistry, College of Pharmacy, University of Toledo, Ohio 43606, USA.
Hinman
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D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D004179:Cobra Cardiotoxin Proteins / Q000737:chemistry*; D003039:Cobra Neurotoxin Proteins / Q000737:chemistry*; D017815:Elapidae; D006321:Heart / Q000187:drug effects; D006801:Humans; D008715:Methionine / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009206:Myocardium / Q000166:cytology; D051381:Rats; D017421:Sequence Analysis; D013601:T-Lymphocytes / Q000187:drug effects
D004179:Cobra Cardiotoxin Proteins; D003039:Cobra Neurotoxin Proteins; D008715:Methionine