| Summary: | This study reports the isolation of two novel cysteine-rich antibacterial peptides, turgencin A and turgencin B, along with their oxidized derivatives, from the Arctic marine colonial ascidian . The peptides are post-translationally modified, containing six cysteines with an unusual disulfide connectivity of Cys-Cys, Cys-Cys, and Cys-Cys and an amidated C-terminus. Furthermore, the peptides contain methionine residues resulting in the isolation of peptides with different degrees of oxidation. The most potent peptide, turgencin A with one oxidized methionine, displayed antimicrobial activity against both Gram-negative and Gram-positive bacteria with a minimum inhibitory concentration (MIC) as low as 0.4 µM against selected bacterial strains. In addition, the peptide inhibited the growth of the melanoma cancer cell line A2058 (IC = 1.4 µM) and the human fibroblast cell line MRC-5 (IC = 4.8 µM). The results from this study show that natural peptides isolated from marine tunicates have the potential to be promising drug leads.
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