Cyclotides are a family of ultra-stable, head-to-tail cyclic mini-proteins from plants, with each member comprising about 30 amino acid residues. Their stability derives from 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 the pharmaceutical industry as stable peptide-based scaffolds for unstable drugs, and also as medicinal agents, due to the wide range of their inherent pharmacological activities. However, there is a lack of fundamental toxicological studies on this type of compound. The current study determined the possible DNA-damaging effects of three cyclotides, i.e., cycloviolacin O2, vaby D, and kalata B1, in human lymphoma cells by use of the alkaline version of the comet assay. The three cyclotides induced massive DNA fragmentation at lethal concentrations. At a sub-lethal 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 micro-autoradiography 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, both at 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 and targets still remain unknown, the current study has unequivocally demonstrated that cyclotides also must have other dose-dependent modes of action.
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