Paralithocins, Antimicrobial Peptides with Unusual Disulfide Connectivity from the Red King Crab, Paralithodes camtschaticus

Abstract

As part of an ongoing exploration of marine invertebrates as a source of new antimicrobial peptides, hemocyte extracts from the red king crab, <i>Paralithodes camtschaticus</i>, were studied. Three cationic cysteine (Cys)-rich peptides, named paralithocins 1–3, were isolated by bioassay-guided purification, and their amino acid sequences determined by Edman degradation and expressed sequences tag analysis. Disulfide bond mapping was performed by high-resolution tandem mass spectrometry. The peptides (38–51 amino acids in length) share a unique Cys motif composed of eight Cys, forming four disulfide bridges with a bond connectivity of (Cys relative position) Cys1–Cys8, Cys2–Cys6, Cys3–Cys5, and Cys4–Cys7, a disulfide arrangement that has not been previously reported among antimicrobial peptides. Thus, paralithocins 1–3 may be assigned to a previously unknown family of antimicrobial peptides within the group of Cys-rich antimicrobial peptides. Although none of the isolated peptides displayed antimicrobial activity against the target strains <i>Escherichia coli, Pseudomonas aeruginosa</i>, or <i>Staphylococcus aureus</i>, they inhibited the growth of several marine bacterial strains with minimal inhibitory concentrations in the 12.5–100 μM range. These findings corroborate the hypothesis that marine organisms are a valuable source for discovering bioactive peptides with new structural motifs.