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During the last decade there has been increasing interest in small circular proteins found in plants of the violet family (Violaceae). These so-called cyclotides consist of a circular chain of approximately 30 amino acids, including six cysteines forming three disulfide bonds, arranged in a cyclic cystine knot (CCK) motif. In this study we map the occurrence and distribution of cyclotides throughout the Violaceae. Plant material was obtained from herbarium sheets containing samples up to 200 years of age. Even the oldest specimens contained cyclotides in the preserved leaves, with no degradation products observable, confirming their place as one of the most stable proteins in nature. Over 200 samples covering 17 of the 23-31 genera in Violaceae were analyzed, and cyclotides were positively identified in 150 species. Each species contained a unique set of between one and 25 cyclotides, with many exclusive to individual plant species. We estimate the number of different cyclotides in the Violaceae to be 5000-25,000, and propose that cyclotides are ubiquitous among all Violaceae species. Twelve new cyclotides from six phylogenetically dispersed genera were sequenced. Furthermore, the first glycosylated derivatives of cyclotides were identified and characterized, further increasing the diversity and complexity of this unique protein family.

Cyclotides are a family of ultra stable, head-to-tail cyclic plant mini-proteins with each member comprising about 30 amino acid residues. Their stability is associated with the unique structural topology where the cyclic backbone and two disulfide bonds make up an embedded ring which is knotted by a third disulfide bond. The cyclotides find potential applications in drug industry as a drug scaffolds for unstable drugs and also as medicinal agents due to the wide range of inherent pharmacological activities they possess. However, there is a lack of fundamental toxicological studies on these classes of compounds. The current study determined a possible DNA damaging effect of three cyclotides, i.e., cycloviolacin O2, vaby D, and kalata B1 in human lymphoma cells using the alkaline version of the comet assay. The three cyclotides induced massive DNA fragmentation at lethal concentrations. At a sublethal concentration, cycloviolacin O2 and vaby D gave a bell shaped dose-response curve for their DNA-damaging effect. Kalata B1 caused no significant DNA damage at sub cytotoxic concentrations. Single cell microautoradiography was carried out on tritium labeled cycloviolacin O2 in order to understand the mechanism behind the dose-response curve. The results revealed that the peptide is taken up into the cell, at both cytotoxic and at low concentrations. Most biological effects of the cyclotides have been taken to follow from the disruption of cell membranes, but even if the intracellular mechanisms/targets still remain unknown, the current study has unequivocally demonstrated that these compounds also must have other dose-dependent modes of action.

As part of ongoing explorations of the structural diversity of cyclotides, the cyclotide content of a native violet of the East African highlands, Viola abyssinica (which grows at altitudes up to 3400 m), was studied. Six new cyclotides, vaby A-E (1-5) and varv E (6), were isolated and characterized by employing HPLC and MS techniques and quantitative amino acid analysis. Cyclotides 1-5 were found to have new sequences, and 1-3 have a further novel feature in their sequences, an alanine moiety in loop 2. Two of the cyclotides (1 and 4) also exhibited cytotoxic properties in a flourometric microculture cytotoxicity assay. The findings corroborate the hypothesis that investigating the cyclotide contents of violets growing in diverse environments is a promising approach for extending our knowledge of both the structural and biological diversity of cyclotides.

Production and release of proteases is among the strategies by which pathogenic bacteria increase their virulence. Moreover, these bacteria are also known to produce a cocktail of enzymes to degrade and inhibit antimicrobial peptides. Cyclotides, on the other hand, are circular plant mini-proteins having a unique cyclic topology (termed as the cyclic cystine knot, CCK) which endows them with exceptional stability. Their ultra-stable framework has attracted interest in drug design as a scaffold to otherwise susceptible drugs including antibacterial peptides. We have determined the stability of three peptides with the CCK motif, i.e., the cyclotides cycloviolacin O2, kalata B1 and MCoTI-II in elastase and aureolysin and also in the supernatant cultures of P. aeruginosa and S. aureus. The cyclotides have resisted enzymatic degradation both from the specific proteases and the bacterial cultures, with MCoTI-II being the least stable of the three. The cyclotides find application in drug engineering as their stable framework can be used as a scaffold or as a template for unstable drugs. The findings of the current study further support this idea; the cyclotide structure can be utilized as a novel strategy in enhancing the stability of antibacterial peptides.

Cyclotides are gene-encoded plant mini-proteins that contain a unique circular and cystine knotted amide backbone. Because of that ultra stable scaffold and the ability to harness a wide variety of sequences and biological activities within the scaffold, cyclotides find interesting potential applications for drug discovery and in agriculture. However, some fundamental knowledge is still missing to exploit these plant compounds including finding the optimal process of their extraction from plant material. In the current work, the extraction parameters solvent type, time of extraction, number of re-macerations and the plant material to solvent ratio have been compared using the sweet violet (Viola odorata L.) as a model plant. That species is a well-characterized and rich source of cyclotides that contains prototypic cyclotides with different chemical and physical properties. We found that hydroalcoholic solutions of medium polarity give good yield of the cyclotide cocktail. In conclusion, single maceration with 50 % MeOH for 6 h at a plant material to solvent ratio of 0.5: 10 (g/mL) represents an optimum extraction method.