Bioactivity of a synthetic peptide derived from a defensin from the tick Ornithodoros savignyi Audouin (1827)

Abstract

With the emergence of drug resistant bacteria, the need for new antimicrobial agents is growing. Antimicrobial peptides (AMPs) are natural antibiotics which have a broad spectrum of activity and also due to a great diversity in structure and chemical formula, it is less likely that bacteria will develop resistance to these agents. In addition to their antimicrobial roles, AMPs and/or their derivatives display immunomodulatory functions and thus have the potential for a wide range of medical applications. Ticks are rich sources of bioactive molecules which may serve as templates for the development of multifunctional peptides. The purpose of this study was to evaluate a synthetic peptide (Os_C) derived from the carboxy-terminus of Ornithodoros savignyi defensin isoform 2 (OsDef2). In the derived peptide (Os_C) the cysteine residues were omitted, an extra tryptophan was added to the N-terminus and the C-terminus was amidated. Os_C was evaluated in terms of its antibacterial, cytotoxic and antioxidant activities as well as nitric oxide scavenging ability. Os_C showed strong antibacterial activity against Gram-negative Escherichia coli and Pseudomonas aeruginosa with a minimal bactericidal concentration (MBC) of 0.40 _M. The MBC for the peptide against Gram-positive Bacillus subtilus and Staphyloccocus aureus was 0.81 _M and 12.8 _M, respectively. Circular dichroism (CD) analysis showed that Os_C maintained a predominantly random coil structure in water and 505% trifluoroethanol (TFE), whereas in 25 mM sodium dodecylsulfate (SDS) it adopted a more _-helical structure. Os_C showed no significant cytotoxicity when evaluated against human erythrocytes, mouse fibroblasts (Sc-1 cell line) and human colon cancer cells (Caco-2 cell line). Oxygen radical absorbance capacity (ORAC) and trolox equivalence antioxidant capacity (TEAC) assays were used to determine its antioxidant potential and in both of the assays, Os_C showed strong antioxidant activity when compared to equal concentrations of glutathione. Intracellular oxidant scavenging capacity was confirmed in the Caco-2 cell line using the 2Ô,7Ô-dichlorfluorescein-diacetate (DCFH-DA) assay and indicated that Os_C, at a concentration of 12.5 _M, was able to offer approximately 20% protection to Caco-2 cells. Reactive nitrogen species may also lead to nitrosative stress and nitric oxide (NO) scavenging activity would be beneficial in preventing the consequence of this effect. NO scavengers may also serve as anti-inflammatory agents. The Griess reaction was used to measure the NO scavenging capacity of Os_C and showed that at a concentration of 12.5 _M, approximately 89% of NO was scavenged. Overall Os_C was found to possess potent antimicrobial, antioxidant and NO scavenging activities, while no toxicity of the peptide to mammalian cells was observed. Os_C thus shows potential as a multifunctional peptide and further research into each of the peptideÔs bioactivities is needed to determine its full potential.