Mastoparan B is a cationic, amphiphilic tetradecapeptide (LKLKSIVSWAKKVL-CONH2) isolated from the venom of the hornet Vespa basalis. Intravenous injection of the peptide into rats caused a profound depression of blood pressure and cardiac function, which was inhibited by cyproheptadine, reserpine and multiple doses of compound 48 80, but not by diphenhydramine and cromolyn. Mastoparan from Paravespula lewisii showed little cardiovascular inihibitory activity. A synthetic mastoparan B analog in which lysine at position 2 was replaced by asparagine showed a marked decrease in the cardiovascular depressor effects, while replacing lysine at position 4, 11 or 12 with leucine did not cause a significant reduction in these effects. Replacing lysine at position 12 with leucine even caused a more sustained depressor effect. However, the analog in which lysines at positions 11 and 12 were replaced by leucine lost its cardiovascular inhibitory activity. Replacing tryptophan at position 9 with phenylalanine in mastoparan B did not affect its activity. It is concluded that mastoparan B is involved in the cardiovascular disturbances induced by the hornet venom. Lysine at position 2 is a critical residue for the cardiovascular effects of mastoparan B. A peptide molecule with two lysine residues, one located close to the amino terminus and the other near the carboxyl end of the peptide, appears to be the optimal structure for eliciting the cardiovascular depressor effects.
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