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Animal-derived antimicrobial peptides are gaining increasing interest for their role in the innate immune system and for their potential applications in the antimicrobial field. Defining the factors that affect potency and selectivity is presently a major challenge to their effective and safe use. Since amidating the C-terminal carboxyl is one of the means of enhancing antimicrobial activity, we report here our comparative study of the solution structures of the antimicrobial peptide dermaseptin S3 and its amidated analogue. Circular dichroism measurements suggested that the peptides are basically found in an alpha-helical structure. In contrast, NMR measurements revealed the complete absence of alpha-helical elements in S3 and a single four-residue helix in the amidated analogue. Whereas the native peptide was found to be flexible, containing a hydrogen-bonded turn and bends, the amidated analogue exhibited a defined alpha-helix at the C-terminal region, causing the latter to be significantly elongated and more structured. Hence, although the increased potency in amidated antimicrobial peptides can be attributed to the increased overall positive charge, in this case, amidation has had additional effects beyond modifying the net positive charge. It has induced and/or stabilized a helical conformation, causing the amidated dermaseptin to be more rigid and more extended than its nonamidated analogue. The possible implications on the mode of action are discussed herein.

To better understand the structural requirements for selective cytotoxicity of antimicrobial peptides, seven dermaseptin S4 analogs were produced and investigated with respect to molecular organization in solution, binding properties to model phospholipid membranes, and cytotoxic properties. Native dermaseptin S4 displayed high aggregation in solution and high binding affinity. These properties correlated with high cytotoxicity. Yet, potency was progressively limited when facing cells whose plasma membrane was surrounded by increasingly complex barriers. Increasing the positive charge of the native peptide led to partial depolymerization that correlated with higher binding affinity and with virtually non-discriminative high cytotoxicity against all cell types. The C-terminal hydrophobic domain was found responsible for binding to membranes but not for their disruption. Truncations of the C terminus combined with increased positive charge of the N-terminal domain resulted in short peptides having similar binding affinity as the parent compound but displaying selective activity against microbes with reduced toxicity toward human red blood cells. Nuclear magnetic resonance-derived three-dimensional structures of three active derivatives enabled the delineation of a common amphipathic structure with a clear separation of two lobes of positive and negative electrostatic potential surfaces. Whereas the spatial positive electrostatic potential extended considerably beyond the peptide dimensions and was required for potency, selectivity was affected primarily by hydrophobicity. The usefulness of this approach for the design of potent and/or selective cytolytic peptides is discussed herein.

The antimicrobial activity of various naturally occurring microbicidal peptides was reported to result from their interaction with microbial membrane. In this study, we investigated the cytotoxicity of the hemolytic peptide dermaseptin S4 (DS4) and the nonhemolytic peptide dermaseptin S3 (DS3) toward human erythrocytes infected by the malaria parasite Plasmodium falciparum. Both DS4 and DS3 inhibited the parasite's ability to incorporate [3H]hypoxanthine. However, while DS4 was toxic toward both the parasite and the host erythrocyte, DS3 was toxic only toward the intraerythrocytic parasite. To gain insight into the mechanism of this selective cytotoxicity, we labeled the peptides with fluorescent probes and investigated their organization in solution and in membranes. In Plasmodium-infected cells, rhodamine-labeled peptides interacted directly with the intracellular parasite, in contrast to noninfected cells, where the peptides remained bound to the erythrocyte plasma membrane. Binding experiments to phospholipid membranes revealed that DS3 and DS4 had similar binding characteristics. Membrane permeation studies indicated that the peptides were equally potent in permeating phosphatidylserine/phosphatidylcholine vesicles, whereas DS4 was more permeative with phosphatidylcholine vesicles. In aqueous solutions, DS4 was found to be in a higher aggregation state. Nevertheless, both DS3 and DS4 spontaneously dissociated to monomers upon interaction with vesicles, albeit with different kinetics. In light of these results, we propose a mechanism by which dermaseptins permeate cells and affect intraerythrocytic parasites.