Immunosuppressive activity of an aqueous Viola tricolor herbal extract

Enhancing cyclotide bioproduction: harnessing biological synthesis methods and various expression systems for large-scale manufacturing

Peptide-based medications hold immense potential in addressing a wide range of human disorders and discomforts. However, their widespread utilization encounters two major challenges: preservation and production efficiency. Cyclotides, a class of ribosomally synthesized and post-translationally modified peptides (RiPPs), exhibit unique characteristics, such as a cyclic backbone and cystine knot, enhancing their stability and contributing to a wide range of pharmacological properties exhibited by these compounds. Cyclotides are efficient in the biomedical (e.g., antitumor, antidiabetic, antimicrobial, antiviral) and agrochemical fields by exhibiting activity against pests and plant diseases. Furthermore, their structural attributes make them suitable as molecular scaffolds for grafting and drug delivery. Notably, the mutated variant of kalata B1 cyclotide ([T20K] kalata B1) has recently entered phase 1 of human clinical trials for multiple sclerosis, building upon the success observed in animal trials. To enable large-scale production of cyclotides, it is crucial to further explore their remarkable structural and bioactive properties. This necessitates extensive research focused on enhancing the efficiency of the processes required for their production. This study provides a comprehensive review of the biological synthesis methods of cyclotides, with particular emphasis on various expression systems, namely bacteria, plants, yeast, and cell-free systems. By investigating these expression systems, it becomes possible to design production systems that are adaptable, economically viable, and efficient for generating active and pure cyclotides at an industrial scale. The advantages of biological synthesis over chemical synthesis are thoroughly explored, highlighting the potential of these expression systems in meeting the demands of large-scale cyclotide production.

Boosting the anti-tumor activity of natural killer cells by caripe 8 - A Carapichea ipecacuanha isolated cyclotide

Cyclotides are head-to-tail cyclized peptides with a unique cystine-knot motif. Their structure provides exceptional resistance against enzymatic, chemical, or thermal degradation compared to other peptides. Peptide-based therapeutics promise high specificity, selectivity and lower immunogenicity, making them safer alternatives to small molecules or large biologicals. Cyclotides were researched due to their anti-cancer properties by inducing apoptosis in tumor cells in the past, but the impact of cyclotides on cytotoxic immune cells was poorly studied. Natural Killer (NK) cells are cytotoxic innate lymphoid cells and play an important role in the defense against infected, stressed and transformed cells. NK cells do not need prior sensitization and act in an antigen independent manner, holding promising potential in the field of immunotherapy. To investigate the effect of immunomodulatory cyclotides on NK cells, we evaluated several peptide-enriched plant extracts on NK cell mediated cytotoxicity. We observed that the extract samples derived from Carapichea ipecacuanha (Brot.) L. Andersson augments the killing potential of mouse NK cells against different tumor targets in vitro. Subsequent isolation of cyclotides from C. ipecacuanha extracts led to the identification of a primary candidate that enhances cytotoxicity of both mouse and human NK cells. The augmented killing is facilitated by the increased degranulation capacity of NK cells. In addition, we noted a direct toxic effect of caripe 8 on tumor cells, suggesting a dual therapeutic potential in cancer treatment. This study offers novel insights how natural peptides can influence NK cell cytotoxicity. These pre-clinical findings hold significant promise for advancing current immunotherapeutic approaches.

Bioactive compounds, nanoparticles synthesis, health benefits and potential utilization of edible flowers for the development of functional dairy products: a review

The food sector faces difficulty meeting the expectations for high-quality food items with safe and clean perceptions in light of customers' increased concern and economic sanctions of synthetic and hazardous chemicals. Besides their widespread use as decoration, flowers are known to be consumed as a traditional food or a component of complementary therapy in many different civilizations worldwide. Because of their nutritional importance as a source of nutrients, proteins, essential amino acids, bioactive compounds, etc., many edible flowers can be viewed as a food source rather than just a delicacy or decoration. Polyphenols, flavonoids, and carotenoids are the phytochemicals that make up the bioactive components of edible flowers. These substances have anti-inflammatory, antibacterial, and antioxidant properties that can improve the nutritional profile of dairy products. Nanoparticles have become a cutting-edge strategy to make use of these advantages. In addition to encapsulating and protecting medicinal substances, nanoparticles made from edible flowers also enable regulated release, increasing bioavailability and durability. Numerous opportunities exist for the addition of edible flower- nanoparticles to dairy products. Their inclusion can add distinctive flavours, colours, and sensations, boosting the consumer's sensory perception. This review quotes the recent studies and discusses different aspects such as nanoparticle synthesis, quantification and characterization, health benefits, novel ingredient for the development of functional food, and the bioactive compounds for different varieties of edible flowers.Kindly check and confirm the edit made in the title.  The final title  is : "Bioactive compounds,nanoparticles synthesis, health benefits andpotential utilization of edible flowers for thedevelopment of functional dairy products: areview".

Graphical abstract

Immunosuppressive Cyclotides: A Promising Approach for Treating Autoimmune Diseases

The immune system maintains constant surveillance to prevent the infiltration of both endogenous and exogenous threats into host organisms. The process is regulated by effector immune cells that combat external pathogens and regulatory immune cells that inhibit excessive internal body inflammation, ultimately establishing a state of homeostasis within the body. Disruption to this process could lead to autoimmunity, which is often associated with the malfunction of both T cells and B cells with T cells playing a more major role. A number of therapeutic mediators for autoimmune diseases are available, from conventional disease-modifying drugs to biologic agents and small molecule inhibitors. Recently, ribosomally synthesized peptides, specifically cyclotides from plants are currently attracting more attention as potential autoimmune disease therapeutics due to their decreased toxicity compared to small molecules inhibitors as well as their remarkable stability against a number of factors. This review provides a concise overview of various cyclotides exhibiting immunomodulatory properties and their potential as therapeutic interventions for autoimmune diseases.

Edible Herbal Medicines as an Alternative to Common Medication for Sleep Disorders: A Review Article

Insomnia is repeated difficulty in falling asleep, maintaining sleep, or experiencing lowquality sleep, resulting in some form of daytime disturbance. Sleeping disorders cause daytime fatigue, mental confusion, and over-sensitivity due to insufficient recovery from a sound sleep. There are some drugs, such as benzodiazepines and anti-histaminic agents, which help to sleep induction and insomnia cure. However, the prolonged administration is unsuitable because of tolerance and dependence. Therefore, the researchers attempt to find new medicines with lesser adverse effects. Natural products have always been good sources for developing new therapeutics for managing diseases such as cancer, cardiovascular disease, diabetes, insomnia, and liver and renal problems. Ample research has justified the acceptable reason and relevance of the use of these herbs in the treatment of insomnia. It is worth noting that in this study, we looked into various Persian herbs in a clinical trial and in vivo to treat insomnia, such as Artemisia annua, Salvia reuterana, Viola tricolor, Passiflora incarnata, lettuce, and Capparis spinose. According to research, herb extracts and fractions, particularly n-butanol fractions with non-polar agents, impact the benzodiazepine receptors and have hypnotic properties. Also, alkaloids, glycosides, flavonoids, saponins, and tannins in practically every plant are mentioned making them the popular natural compounds to help with sleep disorders and promote calmness.

Medicinal Plant-derived Phytochemicals in Detoxification

The average worldwide human life expectancy is 70 years, with a significantly higher value in Western societies. Many modern diseases are not associated with premature mortality but with a decreased quality of life in aged patients and an excessive accumulation of various toxic compounds in the human body during life. Today, scientists are especially interested in finding compounds that can help increase a healthy lifespan by detoxifying the body. Phytotherapy with specific approaches is used in alternative medicine to remove toxins from the body. Worldwide, research is conducted to identify medicinal plant-derived molecules that, with few or no side effects, may protect the liver and other organs. This review provides updated information about the detoxification process, the traditional and modern use of the most effective medicinal plants, their active metabolites as detoxifying agents, and the mechanisms and pathways involved in the detoxification process. Among medicinal plants with substantial detoxifying properties, a major part belongs to the Asteraceae family (Silybum marianum, Cynara scolymus, Arctium lappa, Helichrysum species, Inula helenium, and Taraxacum officinale). The most widely used hepatoprotective phytocomponent is silymarin, a standardized extract from the Silybum marianum seeds containing a mixture of flavonolignans. Many polysaccharides, polyphenols, and terpenoids have a detoxifying effect. Overall, scientific data on medicinal plants used in phytotherapeutic practice worldwide provides an understanding and awareness of their efficacy in detoxification.

Exploring Immune Modulatory Effects of Cyclotide-Enriched Viola tricolor Preparations

Viola tricolor is a medicinal plant with documented application as an anti-inflammatory herb. The standard of care for the treatment of inflammatory bowel disease is immunosuppressive therapeutics or biologics, which often have undesired effects. We explored V. tricolor herbal preparations that are rich in an emerging class of phytochemicals with drug-like properties, so-called cyclotides. As an alternative to existing inflammatory bowel disease medications, cyclotides have immunomodulatory properties, and their intrinsic stability allows for application in the gastrointestinal tract, for instance, via oral administration. We optimized the isolation procedure to improve the yield of cyclotides and compared the cellular effects of violet-derived organic solvent-extracts, aqueous preparations, and an isolated cyclotide from this plant on primary human T lymphocytes and macrophages, i.e., cells that are crucial for the initiation and progression of inflammatory bowel disease. The hot water herbal decoctions have a stronger immunosuppressive activity towards proliferation, interferon-γ, and interleukin-21 secretion of primary human T cells than a DCM/MeOH cyclotide-enriched extract, and the isolated cyclotide kalata S appears as one of the active components responsible for the observed effects. This effect was increased by a longer boiling duration. In contrast, the DCM/MeOH cyclotide-enriched extract was more effective in reducing the levels of cytokines interleukin-6, interleukin-12, interleukin-23, tumor necrosis factor-α, and C - X-C motif chemokine ligand 10, secreted by human monocyte-derived macrophages. Defined cyclotide preparations of V. tricolor have promising pharmacological effects in modulating immune cell responses at the cytokine levels. This is important towards understanding the role of cyclotide-containing herbal drug preparations for future applications in immune disorders, such as inflammatory bowel disease.

A Systematic Review of Phytochemistry, Nutritional Composition, and Pharmacologic Application of Species of the Genus Viola in Noncommunicable Diseases (NCDs)

Viola L. is the largest genus of the Violaceae family with more than 500 species across the globe. The present extensive literature survey revealed Viola species to be a group of important nutritional and medicinal plants used for the ethnomedicinal treatment of noncommunicable diseases (NCDs) such as diabetes, asthma, lung diseases, and fatigue. Many plant species of this genus have also received scientific validation of their pharmacological activities including neuroprotective, immunomodulatory, anticancer, antihypertensive, antidyslipidemic, analgesic, antipyretic, diuretic, anti-inflammatory, anthelmintic, and antioxidant. Viola is highly rich in different natural products some of which have been isolated and identified in the past few decades; these include flavonoids terpenoids and phenylpropanoids of different pharmacological activities. The pharmacokinetics and clinical studies on this genus are lacking, and the present review is aimed at summarizing the current understanding of the ethnopharmacology, phytochemistry, nutritional composition, and pharmacological profile of medicinal plants from the Viola genus to reveal its therapeutic potentials, gaps, and subsequently open a new window for future pharmacological research.

Cyclotide Nanotubes as a Novel Potential Drug-Delivery System: Characterization and Biocompatibility

Cyclotides are a class of cyclic peptides that can be self-assembled. This study aimed to discover the properties of cyclotide nanotubes. We performed differential scanning calorimetric (DSC) to characterize their properties. Then, we incorporated the coumarin as a probe and identified the morphology of nanostructures. The stability of cyclotide nanotubes after 3 months of keeping at -20 °C was determined by field emission scanning electron microscopy (FESEM). The cytocompatibility of cyclotide nanotubes was evaluated on peripheral blood mononuclear cells. In vivo, studies were also conducted on female C57BL/6 mice by intraperitoneally administration of nanotubes at 5, 50, and 100 mg/kg doses. Blood sampling was done before and 24 h after nanotube administration and complete blood count tests were conducted. DSC thermogram showed that the cyclotide nanotubes were stable after heating until 200 °C. Fluorescence microscopy images proved that the self-assembled structures of cyclotide can encapsulate the coumarin. FESEM proved that these nanotubes were stable even after 3 months. The results of the cytotoxicity assay and in-vivo study confirmed that these novel prepared nanotubes were biocompatible. These results suggested that the cyclotide nanotubes could be considered as a new carrier in biological fields while they are biocompatible.

Native and Engineered Cyclic Disulfide-Rich Peptides as Drug Leads

Bioactive peptides are a highly abundant and diverse group of molecules that exhibit a wide range of structural and functional variation. Despite their immense therapeutic potential, bioactive peptides have been traditionally perceived as poor drug candidates, largely due to intrinsic shortcomings that reflect their endogenous heritage, i.e., short biological half-lives and poor cell permeability. In this review, we examine the utility of molecular engineering to insert bioactive sequences into constrained scaffolds with desired pharmaceutical properties. Applying lessons learnt from nature, we focus on molecular grafting of cyclic disulfide-rich scaffolds (naturally derived or engineered), shown to be intrinsically stable and amenable to sequence modifications, and their utility as privileged frameworks in drug design.

Prosopis strombulifera aqueous extract reduces T cell response and ameliorates type I diabetes in NOD mice

Background

New products with tolerogenic properties on T cell response are necessary to improve current efficacy, cost and side effects of immunosuppressants. Prosopis strombulifera aqueous extract (PsAE) have reported cytotoxic, antitumoral, antiatherogenic and antileishmanial activities, containing phytochemicals with immune-related activities. Despite these, there are no previous studies with respect to PsAE suppressive properties over T cell proliferation and their function.

Goal

Because of previous antecedents, this study aims to evaluate the effect of PsAE on T cell activation, proliferation, cytokine production, and to investigate its effect over an in vivo model of type 1 diabetes (T1D).

Experimental procedure

Splenocytes and sorted CD4⁺/CD8⁺ from wild type C57BL/6 mice were cultured to determine activation, IFN-γ release and T-cell proliferation after polyclonal stimulation. NOD (non-obese diabetic) mice were used to study the effects of orally administered extract on glycemia, insulitis stages and perforin-1 (PRF-1)/granzyme-B (GRZ-B) expression.

Results

In primary cultures, the plant extract impairs T cell activation, decreases IFN-γ release, and reduces proliferation after different polyclonal stimuli. In vivo, PsAE improves NOD mice glycemic levels and T1D progression by diminution of pancreas insulitis and reduction of PRF-1 and GRZ-B mRNA expression. To our knowledge, this is the first report characterizing the therapeutic properties of PsAE on T cell activation.

Conclusion

The current work provides evidence about in vitro and in vivo immunosuppressive effects of PsAE and promotes this plant extract as a complementary and alternative treatment in autoimmune T-cell mediated diseases as T1D.

Ipecac root extracts and isolated circular peptides differentially suppress inflammatory immune response characterised by proliferation, activation and degranulation capacity of human lymphocytes in vitro

Circular peptides are attractive lead compounds for drug development; this study investigates the immunomodulatory effects of defined root powder extracts and isolated peptides (called cyclotides) from Carapichea ipecacuanha (Brot.) L. Andersson ('ipecac'). Changes in the viability, proliferation and function of activated human primary T cells were analysed using flow cytometry-based assays. Three distinct peptide-enriched extracts of pulverised ipecac root material were prepared via C₁₈ solid-phase extraction and analysed by reversed-phase HPLC and mass spectrometry. These extracts induced caspase 3/7 dependent apoptosis, thus leading to a suppressed proliferation of activated T cells and a reduction of the number of cells in the G2 phase. Furthermore, the stimulated T cells had a lower activation potential and a reduced degranulation capacity after treatment with ipecac extracts. Six different cyclotides were isolated from C. ipecacuanha and an T cell proliferation inhibiting effect was determined. Furthermore, the degranulation capacity of the T cells was diminished specifically by some cyclotides. In contrast to kalata B1 and its analog T20K, secretion of IL-2 and IFN- γ was not affected by any of the caripe cyclotides. The findings add to our increased understanding of the immunomodulating effects of cyclotides, and may provide a basis for the use of ipecac extracts for immunomodulation in conditions associated with an exessive immune responses.

Immunomodulatory effects of cyclotides isolated from Viola odorata in an experimental autoimmune encephalomyelitis animal model of multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system that causes chronic inflammation. Cyclotides are small plant proteins with a wide range of biological activity, making them a target for researchers to investigate. This study was conducted to investigate the possible effects of cyclotide-rich fractions from Viola odorata as an immunomodulatory agent in an experimental autoimmune encephalomyelitis (EAE) model of MS.

METHODS

At room temperature, the plant materials were subjected to maceration in methanol: dichloromethane (1:1; v/v) for 3 days. The extraction was repeated 3 times, and the final concentrated extract was partitioned 3 times by 1/2 volume of double-distilled water. The aqueous phases were separated and freeze-dried. Finally, the crude extract was fractionated by C18 silicagel using vacuum liquid chromatography, with mobile phases of 30%, 50% and 80% of ethanol: water, respectively. The 50%, and 80% fractions were analyzed by HPLC and MALDI-TOF analysis and administrated intraperitoneally to forty-five female C57BL/6 EAE-induced mice, at 5, 25, and 50 mg/kg doses. After 28 days, the animals were evaluated using EAE clinical scoring which was done every 3 days, cytokine levels, and myelination level.

RESULTS

The results confirmed the presence of cyclotides in V. odorata based on their retention time and the composition of mobile phase in HPLC and the molecular weight of the peaks in MALDI-TOF analysis. It was observed that cyclotides, especially in the 80% fraction group at the dose of 50 mg/kg significantly reduced the clinical scores, inflammation, and demyelination in EAE mice compared with the normal saline group (P<0.05), and the results of this group were comparable with fingolimod (P>0.05).

CONCLUSION

It could be concluded that V. odorata is a rich source of cyclotides which they could be extracted by an easily available process and also, they could be used as immunomodulatory agents in MS, with similar effects to fingolimod.

Discovery of the cyclotide caripe 11 as a ligand of the cholecystokinin-2 receptor

The cholecystokinin-2 receptor (CCK2R) is a G protein-coupled receptor (GPCR) that is expressed in peripheral tissues and the central nervous system and constitutes a promising target for drug development in several diseases, such as gastrointestinal cancer. The search for ligands of this receptor over the past years mainly resulted in the discovery of a set of distinct synthetic small molecule chemicals. Here, we carried out a pharmacological screening of cyclotide-containing plant extracts using HEK293 cells transiently-expressing mouse CCK2R, and inositol phosphate (IP1) production as a readout. Our data demonstrated that cyclotide-enriched plant extracts from Oldenlandia affinis, Viola tricolor and Carapichea ipecacuanha activate the CCK2R as measured by the production of IP1. These findings prompted the isolation of a representative cyclotide, namely caripe 11 from C. ipecacuanha for detailed pharmacological analysis. Caripe 11 is a partial agonist of the CCK2R (Emax = 71%) with a moderate potency of 8.5 µM, in comparison to the endogenous full agonist cholecystokinin-8 (CCK-8; EC50 = 11.5 nM). The partial agonism of caripe 11 is further characterized by an increase on basal activity (at low concentrations) and a dextral-shift of the potency of CCK-8 (at higher concentrations) following its co-incubation with the cyclotide. Therefore, cyclotides such as caripe 11 may be explored in the future for the design and development of cyclotide-based ligands or imaging probes targeting the CCK2R and related peptide GPCRs.

In vitro Inhibition of HIV-1 by Cyclotide-Enriched Extracts of Viola tricolor

Since viral infectious diseases continue to be a global health threat, new antiviral drugs are urgently needed. A unique class of therapeutic compounds are antimicrobial peptides (AMPs). They can be found in humans, bacteria and plants. Plants express a wide variety of such defense peptides as part of their innate immune system to protect from invading pathogens. Cyclotides are non-classical AMPs that share a similar structure. Their unique topology consists of a circular peptide backbone and disulfide bonds. In previous studies they have been attributed to a wide range of biological activities. To identify novel cyclotides with antiviral activity, we established a library of plant extracts largely consisting of cyclotide-rich species and screened them as inhibitors of HIV-1 infection. Subsequent extraction and fractionation revealed four cyclotide-containing subfractions from Viola tricolor with antiviral activity. These subfractions inhibited HIV-1 infection with IC₅₀ values between 0.6 and 11.2 μg/ml, and selectivity indices of up to 8.1. The identification and characterization of antiviral cyclotides and the determination of the antiviral mechanisms may allow to develop novel agents to combat viral infections. Therefore, cyclotides represent a natural source of bioactive molecules with prospects for development as therapeutics.

Anti-inflammatory, Analgesic, and Cytotoxic Effects of The Phytexponent: A Polyherbal Formulation

The Phytexponent is used to treat pain and inflammation in complementary and alternative medicine practices; however, empirical data supporting its pharmacological efficacy and safety is scanty, hence the present study. We used the carrageenan-induced paw oedema and the acetic acid-induced writhing techniques to determine the anti-inflammatory and analgesic efficacies, respectively, of the Phytexponent in Swiss albino mice models. The 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay technique was used to investigate the in vitro cytotoxic effects of the Phytexponent in the Vero E6 cell line. The Phytexponent exerted significant (P < .05) anti-inflammatory effects in the carrageenan-induced paw oedema mouse model in a dose- and time-dependent manner, with significantly higher efficacy at 250 mg/Kg BW, than indomethacin (4 mg/Kg BW), in the first, second, and third hour (P < .05). Besides, the Phytexponent significantly reduced the acetic acid-induced writhing frequency in mice (P < .05), in a dose-dependent manner, depicting its analgesic efficacy. Notably, the Phytexponent (at doses: 125 mg/Kg BW and 250 mg/Kg BW) exhibited significantly higher analgesic efficacy than the Indomethacin (P<.05). Moreover, the Phytexponent was not cytotoxic to Vero E6 cells (CC₅₀ >1000 µg/ml) compared to cyclophosphamide (CC₅₀ = 2.48 µg/ml). Thus, the Phytexponent has significant in vivo anti-inflammatory and analgesic efficacy in mice models and is not cytotoxic to Vero E6 cell line, depicting its therapeutic potential upon further empirical investigation.

Interaction of cyclotide Kalata B1 protein with model cellular membranes of varied electrostatics

A uni-molecular layer of lipids at air-water interface mimicking one of the leaflets of the cellular membrane provides a simple model to understand the interaction of any foreign molecules with the membrane. Here, the interactions of protein Kalata B1 (KB1) of cyclotide family with the phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG), and 1,2-distearoyl-sn-glycero-3-ethylphosphocholine chloride salt (DSEPC) have been investigated. The addition of KB1 induces a change in pressure of the lipid monolayers. The characteristic time of the change in pressure is found to be dependent on the electrostatic nature of the lipid. Even though the protein is weakly surface active, it is capable of modifying the phase behavior and elastic properties of lipid monolayers with differences in their strength and nature making the layers more floppy. The KB1-lipid interaction has been quantified by calculating the excess Gibb's free energy of interaction and the 1-anilino-8-naphthalenesulfonate (ANS) binding studies. The interaction with zwitterionic DPPC and negatively charged DPPG lipids are found to be thermodynamically favorable whereas the protein shows a weaker response to positively charged DSEPC lipid. Therefore, the long ranged electrostatic is the initial driving force for the KB1 to recognize and subsequently attach to a cellular membrane. Thereafter, the hydrophobic region of the protein may penetrate into the hydrophobic core of the membrane via specific amino acid residues.

Importance of the Cyclic Cystine Knot Structural Motif for Immunosuppressive Effects of Cyclotides

The cyclotide T20K inhibits the proliferation of human immune cells and is currently in clinical trials for multiple sclerosis. Here, we provide novel functional data and mechanistic insights into structure-activity relationships of T20K. Analogs with partial or complete reduction of the cystine knot had loss of function in proliferation experiments. Similarly, an acyclic analog of T20K was inactive in lymphocyte bioassays. The lack of activity of non-native peptide analogs appears to be associated with the ability of cyclotides to interact with and penetrate cell membranes, since cellular uptake studies demonstrated fast fractional transfer only of the native peptide into the cytosol of human immune cells. Therefore, structural differences between cyclic and linear native folded peptides were investigated by NMR to elucidate structure-activity relationships. Acyclic T20K had a less rigid backbone and considerable structural changes in loops 1 and 6 compared to the native cyclic T20K, supporting the idea that the cyclic cystine knot motif is a unique bioactive scaffold. This study provides evidence that this structural motif in cyclotides governs bioactivity, interactions with and transport across biological membranes, and the structural integrity of these peptides. These observations could be useful to understand the structure-activity of other cystine knot proteins due to the structural conservation of the cystine knot motif across evolution and to provide guidance for the design of novel cyclic cysteine-stabilized molecules.

Antibacterial Activity of Medicinal Plants and Their Constituents in the Context of Skin and Wound Infections, Considering European Legislation and Folk Medicine-A Review

Bacterial infections of skin and wounds may seriously decrease the quality of life and even cause death in some patients. One of the largest concerns in their treatment is the growing antimicrobial resistance of bacterial infectious agents and the spread of resistant strains not only in the hospitals but also in the community. This trend encourages researchers to seek for new effective and safe therapeutical agents. The pharmaceutical industry, focusing mainly on libraries of synthetic compounds as a drug discovery source, is often failing in the battle with bacteria. In contrast, many of the natural compounds, and/or the whole and complex plants extracts, are effective in this field, inactivating the resistant bacterial strains or decreasing their virulence. Natural products act comprehensively; many of them have not only antibacterial, but also anti-inflammatory effects and may support tissue regeneration and wound healing. The European legislative is in the field of natural products medicinal use formed by European Medicines Agency (EMA), based on the scientific work of its Committee on Herbal Medicinal Products (HMPC). HMPC establishes EU monographs covering the therapeutic uses and safe conditions for herbal substances and preparations, mostly based on folk medicine, but including data from scientific research. In this review, the medicinal plants and their active constituents recommended by EMA for skin disorders are discussed in terms of their antibacterial effect. The source of information about these plant products in the review is represented by research articles listed in scientific databases (Science Direct, PubMed, Scopus, Web of Science, etc.) published in recent years.

Cyclotides Isolated From Violet Plants of Cameroon Are Inhibitors of Human Prolyl Oligopeptidase

Traditional medicine and the use of herbal remedies are well established in the African health care system. For instance, Violaceae plants are used for antimicrobial or anti-inflammatory applications in folk medicine. This study describes the phytochemical analysis and bioactivity screening of four species of the violet tribe Allexis found in Cameroon. Allexis cauliflora, Allexis obanensis, Allexis batangae and Allexis zygomorpha were evaluated for the expression of circular peptides (cyclotides) by mass spectrometry. The unique cyclic cystine-rich motif was identified in several peptides of all four species. Knowing that members of this peptide family are protease inhibitors, the plant extracts were evaluated for the inhibition of human prolyl oligopeptidase (POP). Since all four species inhibited POP activity, a bioactivity-guided fractionation approach was performed to isolate peptide inhibitors. These novel cyclotides, alca 1 and alca 2 exhibited IC₅₀ values of 8.5 and 4.4 µM, respectively. To obtain their amino acid sequence information, combinatorial enzymatic proteolysis was performed. The proteolytic fragments were evaluated in MS/MS fragmentation experiments and the full-length amino acid sequences were obtained by de novo annotation of fragment ions. In summary, this study identified inhibitors of the human protease POP, which is a drug target for inflammatory or neurodegenerative disorders.

Cyclotides from Brazilian Palicourea sessilis and Their Effects on Human Lymphocytes

Cyclotides are plant-derived peptides found within five families of flowering plants (Violaceae, Rubiaceae, Fabaceae, Solanaceae, and Poaceae) that have a cyclic backbone and six conserved cysteine residues linked by disulfide bonds. Their presence within the Violaceae species seems ubiquitous, yet not all members of other families produce these macrocyclic peptides. The genus Palicourea Aubl. (Rubiaceae) contains hundreds of neotropical species of shrubs and small trees; however, only a few cyclotides have been discovered hitherto. Herein, five previously uncharacterized Möbius cyclotides within Palicourea sessilis and their pharmacological activities are described. Cyclotides were isolated from leaves and stems of this plant and identified as pase A-E, as well as the known peptide kalata S. Cyclotides were de novo sequenced by MALDI-TOF/TOF mass spectrometry, and their structures were solved by NMR spectroscopy. Because some cyclotides have been reported to modulate immune cells, pase A-D were assayed for cell proliferation of human primary activated T lymphocytes, and the results showed a dose-dependent antiproliferative function. The toxicity on other nonimmune cells was also assessed. This study reveals that pase cyclotides have potential for applications as immunosuppressants and in immune-related disorders.

Bioactive Peptides and Dietary Polyphenols: Two Sides of the Same Coin

The call for health-promoting nutraceuticals and functional foods containing bioactive compounds is growing. Among the great diversity of functional phytochemicals, polyphenols and, more recently, bioactive peptides have stood out as functional compounds. The amount of an ingested nutrient able to reach the bloodstream and exert the biological activity is a critical factor, and is affected by several factors, such as food components and food processing. This can lead to unclaimed interactions and/or reactions between bioactive compounds, which is particularly important for these bioactive compounds, since some polyphenols are widely known for their ability to interact and/or precipitate proteins/peptides. This review focuses on this important topic, addressing how these interactions could affect molecules digestion, absorption, metabolism and (biological)function. At the end, it is evidenced that further research is needed to understand the true effect of polyphenol-bioactive peptide interactions on overall health outcomes.

Ex vivo and in vivo studies of Viola tricolor Linn. as potential cardio protective and hypotensive agent: Inhibition of voltage-gated Ca++ ion channels

Viola tricolor Linn. is used as cardio-protective and anti-hypertensive agent in traditional medicine. Current study objective was to evaluate cardio-protective and hypotensive effects of Viola tricolor L. in vitro and in vivo studies. Viola tricolor L. crude extract (Vt.Cr) and its fractions (Aqueous and organic) were tested at rabbit atria and aorta coupled to Power Lab Data Acquisition System for cardio depressant and vasorelaxant effects in vitro whereas in vivo Blood Pressure was checked by invasive method in normotensive ketamine-diazepam anesthetized rats. Isoproterenol was employed for acute myocardial infarction (AMI) and left ventricular hypertrophy (LVH) development and cardioprotective effects of Vt.Cr were evaluated hemodynamically and histopathologically. Vt.Cr and its fractions decreased heart rate and contractile force in paired atria and relaxed Phenylephrine (1 µM) and K⁺ (80 mM) stimulated contractions in aorta possibly mediated through Voltage dependent L-type calcium channels blockage supported by in vivo hypotensive action. In LVH, Vt.Cr lowered Angiotensin Converting Enzymes and renin, increased cyclic Guanosine Monophosphate and nitric oxide levels, decreased cardiomyocytes size and fibrosis attributed to Gallic acid as detected by High Performance Liquid Chromatography. Partial positive results were seen hemodynamically and histologically in AMI Viola tricolor L. showed vasorelaxant, cardio-relaxant, hypotensive, and cardio protective effect validating traditional practice in cardiovascular disorders.

Gas chromatography-mass spectrometry analysis, phytochemical screening and antiprotozoal effects of the methanolic Viola tricolor and acetonic Laurus nobilis extracts

Background

The antiprotozoal and antioxidant activities of Viola tricolor and Laurus nobilis have been reported recently. Thus, the existing study pursued to assess the growth inhibition effect of methanolic extract of V. tricolor (MEVT) and acetonic extract of L. nobilis (AELN) against five Babesia parasites and Theileria equi in vitro and in vivo.

Results

MEVT and AELN suppressed Babesia bovis, B. bigemina, B. divergens, B. caballi, and T. equi growth at half-maximal inhibitory concentration (IC₅₀) values of 75.7 ± 2.6, 43.3 ± 1.8, 67.6 ± 2.8, 48 ± 3.8, 54 ± 2.1 μg/mL, and 86.6 ± 8.2, 33.3 ± 5.1, 62.2 ± 3.3, 34.5 ± 7.5 and 82.2 ± 9.3 μg/mL, respectively. Qualitative phytochemical estimation revealed that both extracts containing multiple bioactive constituents and significant amounts of flavonoids and phenols. The toxicity assay revealed that MEVT and AELN affected the mouse embryonic fibroblast (NIH/3 T3) and Madin–Darby bovine kidney (MDBK) cell viability with half-maximum effective concentrations (EC₅₀) of 930 ± 29.9, 1260 ± 18.9 μg/mL, and 573.7 ± 12.4, 831 ± 19.9 μg/mL, respectively, while human foreskin fibroblasts (HFF) cell viability was not influenced even at 1500 μg/mL. The in vivo experiment revealed that the oral administration of MEVT and AELN prohibited B. microti multiplication in mice by 35.1 and 56.1%, respectively.

Conclusions

These analyses indicate the prospects of MEVT and AELN as good candidates for isolating new anti-protozoal compounds which could assist in the development of new drug molecules with new drug targets.

Cyclotides: From Structure to Function

This Review explores the class of plant-derived macrocyclic peptides called cyclotides. We include an account of their discovery, characterization, and distribution in the plant kingdom as well as a detailed analysis of their sequences and structures, biosynthesis and chemical synthesis, biological functions, and applications. These macrocyclic peptides are around 30 amino acids in size and are characterized by their head-to-tail cyclic backbone and cystine knot motif, which render them to be exceptionally stable, with resistance to thermal or enzymatic degradation. Routes to their chemical synthesis have been developed over the past two decades, and this capability has facilitated a wide range of mutagenesis and structure-activity relationship studies. In turn, these studies have both led to an increased understanding of their mechanisms of action as well as facilitated a range of applications in agriculture and medicine, as ecofriendly crop protection agents, and as drug leads or scaffolds for pharmaceutical design. Our overall objective in this Review is to provide readers with a comprehensive overview of cyclotides that we hope will stimulate further work on this fascinating family of peptides.

Reporting a Transcript from Iranian Viola Tricolor, Which May Encode a Novel Cyclotide-Like Precursor: Molecular and in silico Studies

The cyclotides are the largest known family of cyclic proteins, which are found in several plant families including Violaceae. They are circular bioactive peptides consisting of 28-37 amino acids, which possess a cyclic cystine knot (CCK) motif and could be useful in biotechnology and drug design as scaffolds for peptide-based drugs. This study describes our finding of a potentially novel gene transcript from the petals of the Iranian Viola tricolor (V. tricolor) flowers. This study is based on the cDNA screening method employed for isolation of cyclotide precursor genes and in silico analysis. Our study resulted in the finding of a novel cyclotide-like precursor from V. tricolor, which is documented in the NCBI by GenBank accession number: KP065812. The in silico analysis revealed that there are lots of similar sequences in many other plant families and they all exhibit some different features from previously discovered cyclotide precursors. The differences occur particularly in the main cyclotide domain that exists without the usual CCK structure. All of these hypothetical precursors have a conserved ER-signal sequence, a Cysteine (C)-rich sequence forming two zinc finger motifs and a cyclotide-like region containing several conserved elements including two highly conserved C residues. In conclusion, using the cDNA screening method we found a potentially new cyclotide-like precursor gene and in silico studies revealed its significant characteristics that may open up a new research line on the distribution and evolution of cyclotides.

Anticonvulsant Activity of Viola tricolor against Seizures Induced by Pentylenetetrazol and Maximal Electroshock in Mice

BACKGROUND

Recently, there has been much more interest in the use of medicinal plants in search of novel therapies for human neurodegenerative diseases such as epilepsy. In the present study, we investigated the anticonvulsant effects of Viola tricolor (V. tricolor) on seizure models induced by pentylenetetrazol (PTZ) and maximal electroshock stimulation (MES).

METHODS

Totally, 260 mice were divided into 26 groups (n=10). Thirty minutes after treatment with the hydroalcoholic extract of V. tricolor (VHE 100, 200, and 400 mg/kg) and its ethyl acetate (EAF 50, 100, and 200 mg/kg) and n-butanol (NBF 50, 100, and 200 mg/kg) fractions as well as diazepam (3 mg/kg), seizure was induced by PTZ (100 mg/kg) or by MES (50 Hz, 1 s and 50 mA). Analysis was performed via ANOVA with the Tukey-Kramer post-hoc test using GraphPad Prism 6.01 (La Jolla, CA).

RESULTS

The VHE (400 mg/kg) significantly enhanced latency to the first generalized tonic-clonic seizures (GTCs) induced by PTZ in comparison to the control group (P<0.001). All 3 concentrations of the EAF (50, 100, and 200 mg/kg) significantly prolonged the latency of PTZ-induced seizures compared to the control group. Additionally, all the concentrations of the NBF (50, 100, and 200 mg/kg) made a significant increment in GTCs latency induced by PTZ in comparison to the control group. On the other hand, all the concentrations of the VHE, EAF, and NBF significantly reduced the incidence of hind-limb tonic extension (HLTE) induced by MES, when compared to the control group.

CONCLUSION

The present study showed that V. tricolor and its ethyl acetate and n-butanol fractions possessed anticonvulsant effects as confirmed by the prolongation of latency to the first GTCs induced by PTZ and decrement in the incidence of HLTE induced by MES.

Free amino acids in Viola tricolor in relation to different habitat conditions

The purpose of this study was to establish the free amino acids profile of Viola tricolor collected from different habitats in Poland. Viola tricolor (heartsease) is a very popular plant found worldwide, classified both as weed and medicinal plant. Based on a validated method, the following nineteen free amino acids were analyzed using liquid chromatography-electrospray ionization coupled to a triple quadrupole mass spectrometer (LC-ESI-MS/MS):alanine, glycine, leucine, valine, isoleucine, proline, phenylalanine, tryptophan, tyrosine, serine, threonine, methionine, asparagine, glutamine, lysine, arginine, histidine, aspartic acid, glutamic acid. The total free amino acids (TAA) ranged from 9938.0 to 11393.8 mg/kg of fresh weight. The variability of the investigated amino acids with respect to different habitat conditions was statistically assessed using the method of discriminant and cluster analysis. Alanine, valine, glutamine and aspartic acid were the most abundant free amino acids present in both localizations. The ratio of total essential amino acids (EAA) to TAA was 0.27 and 0.11 in Zagródki and Wrocław, respectively. Discriminant analysis has demonstrated that the investigated habitats significantly differentiated the free amino acids content of Viola tricolor. Only methionine showed a similar concentration in both Viola tricolor populations.

Inhibition of Breast Cancer Cell Migration by Cyclotides Isolated from Pombalia calceolaria

Two new bracelet cyclotides from roots of Pombalia calceolaria with potential anticancer activity have been characterized in this work. The cyclotides Poca A and B (1 and 2) and the previously known CyO4 (3) were de novo sequenced by MALDI-TOF/TOF mass spectrometry (MS). The MS² spectra were examined and the amino acid sequences were determined. The purified peptides were tested for their cytotoxicity and effects on cell migration of MDA-MB-231, a triple-negative breast cancer cell line. The isolated cyclotides reduced the number of cancer cells by more than 80% at 20 μM, and the concentration-related cytotoxic responses were observed with IC₅₀ values of 1.8, 2.7, and 9.8 μM for Poca A (1), Poca B (2), and CyO4 (3), respectively. Additionally, the inhibition of cell migration (wound-healing assay) exhibited that CyO4 (3) presents an interesting activity profile, in being able to inhibit cell migration (50%) at a subtoxic concentration (2 μM). The distribution of these cyclotides in the roots was analyzed by MALDI imaging, demonstrating that all three compounds are present in the phloem and cortical parenchyma regions.

Russian traditional medicine in dermatology

The use of herbal remedies for various medical issues is becoming increasingly commonplace in all fields of medicine, and dermatology is no exception. This review focuses on traditional dermatologic herbal remedies, commonly used in Russia, as the rich array of 11 different plant zones has resulted in a great variety of medicinal plants. Herbal remedies warrant deeper investigation and research, especially due to their active substance content, which may interfere with or reinforce the effect of modern medications, something that medical professionals should be aware of when prescribing treatments. Although there are a great number of traditional herbal treatments in Russia, only the most commonly used and known treatments and applications will be described as an introduction to the field, which has had many books of varying quality written about it. The preparation and application of treatments for vitiligo, pyodermas, parasitic and infectious skin diseases, acne, dermatitides, rosacea, hyperpigmentation, rhytides, psoriasis, and hyperhidrosis are discussed.

Cyclotides Isolated from an Ipecac Root Extract Antagonize the Corticotropin Releasing Factor Type 1 Receptor

Cyclotides are plant derived, cystine-knot stabilized peptides characterized by their natural abundance, sequence variability and structural plasticity. They are abundantly expressed in Rubiaceae, Psychotrieae in particular. Previously the cyclotide kalata B7 was identified to modulate the human oxytocin and vasopressin G protein-coupled receptors (GPCRs), providing molecular validation of the plants' uterotonic properties and further establishing cyclotides as valuable source for GPCR ligand design. In this study we screened a cyclotide extract derived from the root powder of the South American medicinal plant ipecac (Carapichea ipecacuanha) for its GPCR modulating activity of the corticotropin-releasing factor type 1 receptor (CRF₁R). We identified and characterized seven novel cyclotides. One cyclotide, caripe 8, isolated from the most active fraction, was further analyzed and found to antagonize the CRF₁R. A nanomolar concentration of this cyclotide (260 nM) reduced CRF potency by ∼4.5-fold. In contrast, caripe 8 did not inhibit forskolin-, or vasopressin-stimulated cAMP responses at the vasopressin V₂ receptor, suggesting a CRF₁R-specific mode-of-action. These results in conjunction with our previous findings establish cyclotides as modulators of both classes A and B GPCRs. Given the diversity of cyclotides, our data point to other cyclotide-GPCR interactions as potentially important sources of drug-like molecules.

Ribosomally-synthesised cyclic peptides from plants as drug leads and pharmaceutical scaffolds

Owing to their exceptional stability and favourable pharmacokinetic properties, plant-derived cyclic peptides have recently attracted significant attention in the field of peptide-based drug design. This article describes the three major classes of ribosomally-synthesised plant peptides - the cyclotides, the PawS-derived peptides and the orbitides - and reviews their applications as leads or scaffolds in drug design. These ribosomally-produced peptides have a range of biological activities, including anti-HIV, cytotoxic and immunomodulatory activity. In addition, recent interest has focused on their use as scaffolds to stabilise bioactive peptide sequences, thereby enhancing their biopharmaceutical properties. There are now more than 30 published papers on such 'grafting' applications, most of which have been reported only in the last few years, and several such studies have reported in vivo activity of orally delivered cyclic peptides. In this article, we describe approaches to the synthesis of cyclic peptides and their pharmaceutically-grafted derivatives as well as outlining their biosynthetic routes. Finally, we describe possible bioproduction routes for pharmaceutically active cyclic peptides, involving plants and plant suspension cultures.

Understanding the Diversity and Distribution of Cyclotides from Plants of Varied Genetic Origin

Cyclotides are a large family of naturally occurring plant-derived macrocyclic cystine-knot peptides, with more than 400 having been identified in species from the Violaceae, Rubiaceae, Cucurbitaceae, Fabaceae, and Solanaceae families. Nevertheless, their specialized distribution within the plant kingdom remains poorly understood. In this study, the diversity of cyclotides was explored through the screening of 197 plants belonging to 43 different families. In total, 28 cyclotides were sequenced from 15 plant species, one of which belonged to the Rubiaceae and 14 to the Violaceae. Every Violaceae species screened contained cyclotides, but they were only sparsely represented in Rubiaceae and nonexistent in other families. The study thus supports the hypothesis that cyclotides are ubiquitous in the Violaceae, and it adds to the list of plants found to express kalata S and cycloviolacin O12. Finally, previous studies suggested the existence of cyclotide isoforms with either an Asn or an Asp at the C-terminal processing site of the cyclotide domain within the precursor proteins. Here we found that despite the discovery of a few cyclotides genuinely containing an Asp in loop 6 as evidenced by gene sequencing, deamidation of Asn during enzymatic digestion resulted in the artifactual presence of Asp isoforms. This result is consistent with studies suggesting that peptides can undergo deamidation after being subjected to external factors, including pH, temperature, and enzymatic digestion.

Synthesis and Protein Engineering Applications of Cyclotides

Cyclotides are a group of plant-derived peptides with a head-to-tail cyclized backbone that is stabilized by three knotted disulfide bonds. Their exceptional stability and tolerance for residue substitutions have led to interest in their application as drug design scaffolds. To date, chemical synthesis has been the dominant methodology for producing cyclotides and their analogues. Native chemical ligation is the most common strategy to generate the cyclic backbone and has been highly successful at producing a wide range of cyclotides for studies of structure–activity relationships. Both this and other chemical approaches require a specific linker at the C-terminus and typically involve a non-directed folding (disulfide oxidation) regimen, which can sometimes be a limiting factor in final yields. Following the recent discovery of enzymes involved in peptide cyclization in planta, site-specific and highly efficient enzymatic ligations have been used for synthetic cyclotide backbone cyclization. In this review, chemical synthesis strategies and approaches involving cyclization via enzymes for the production of cyclotides are described.

Discovery, structure, function, and applications of cyclotides: circular proteins from plants

Cyclotides are plant-derived cyclic peptides that have a head-to-tail cyclic backbone and three conserved disulphide bonds that form a cyclic cystine knot motif. They occur in plants from the Violaceae, Rubiaceae, Cucurbitaceae, Fabaceae, and Solanaceae families, typically with 10-100 cyclotides in a given plant species, in a wide range of tissues, including flowers, leaves, stems, and roots. Some cyclotides are expressed in large amounts (up to 1g kg(-1) wet plant weight) and their natural function appears to be to protect plants from pests or pathogens. This article provides a brief overview of their discovery, distribution in plants, and applications. In particular, their exceptional stability has led to their use as peptide-based scaffolds in drug design applications. They also have potential as natural 'ecofriendly' insecticides, and as protein engineering frameworks.

Microwave-assisted extraction of cyclotides from Viola ignobilis

Cyclotides are an interesting family of circular plant peptides. Their unique three-dimensional structure, comprising a head-to-tail circular backbone chain and three disulfide bonds, confers them stability against thermal, chemical, and enzymatic degradation. Their unique stability under extreme conditions creates an idea about the possibility of using harsh extraction methods such as microwave-assisted extraction (MAE) without affecting their structures. MAE has been introduced as a potent extraction method for extraction of natural compounds, but it is seldom used for peptide and protein extraction. In this work, microwave irradiation was applied to the extraction of cyclotides. The procedure was performed in various steps using a microwave instrument under different conditions. High-performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) results show stability of cyclotide structures on microwave radiation. The influential parameters, including time, temperature, and the ratio of solvents that are affecting the MAE potency, were optimized. Optimal conditions were obtained at 20 min of irradiation time, 1200 W of system power in 60 °C, and methanol/water at the ratio of 90:10 (v/v) as solvent. The comparison of MAE results with maceration extraction shows that there are similarities between cyclotide sequences and extraction yields.

Peptidomics of Circular Cysteine-Rich Plant Peptides: Analysis of the Diversity of Cyclotides from Viola tricolor by Transcriptome and Proteome Mining

Cyclotides are plant-derived mini proteins. They are genetically encoded as precursor proteins that become post-translationally modified to yield circular cystine-knotted molecules. Because of this structural topology cyclotides resist enzymatic degradation in biological fluids, and hence they are considered as promising lead molecules for pharmaceutical applications. Despite ongoing efforts to discover novel cyclotides and analyze their biodiversity, it is not clear how many individual peptides a single plant specimen can express. Therefore, we investigated the transcriptome and cyclotide peptidome of Viola tricolor. Transcriptome mining enabled the characterization of cyclotide precursor architecture and processing sites important for biosynthesis of mature peptides. The cyclotide peptidome was explored by mass spectrometry and bottom-up proteomics using the extracted peptide sequences as queries for database searching. In total 164 cyclotides were discovered by nucleic acid and peptide analysis in V. tricolor. Therefore, violaceous plants at a global scale may be the source to as many as 150 000 individual cyclotides. Encompassing the diversity of V. tricolor as a combinatorial library of bioactive peptides, this commercially available medicinal herb may be a suitable starting point for future bioactivity-guided screening studies.

Inhibition of Human Prolyl Oligopeptidase Activity by the Cyclotide Psysol 2 Isolated from Psychotria solitudinum

Cyclotides are head-to-tail cyclized peptides comprising a stabilizing cystine-knot motif. To date, they are well known for their diverse bioactivities such as anti-HIV and immunosuppressive properties. Yet little is known about specific molecular mechanisms, in particular the interaction of cyclotides with cellular protein targets. Native and synthetic cyclotide-like peptides from Momordica plants are potent and selective inhibitors of different serine-type proteinases such as trypsin, chymotrypsin, matriptase, and tryptase-beta. This study describes the bioactivity-guided isolation of a cyclotide from Psychotria solitudinum as an inhibitor of another serine-type protease, namely, the human prolyl oligopeptidase (POP). Analysis of the inhibitory potency of Psychotria extracts and subsequent fractionation by liquid chromatography yielded the isolated peptide psysol 2 (1), which exhibited an IC50 of 25 μM. In addition the prototypical cyclotide kalata B1 inhibited POP activity with an IC50 of 5.6 μM. The inhibitory activity appeared to be selective for POP, since neither psysol 2 nor kalata B1 were able to inhibit the proteolytic activity of trypsin or chymotrypsin. The enzyme POP is well known for its role in memory and learning processes, and it is currently being considered as a promising therapeutic target for the cognitive deficits associated with several psychiatric and neurodegenerative diseases, such as schizophrenia and Parkinson's disease. In the context of discovery and development of POP inhibitors with beneficial ADME properties, cyclotides may be suitable starting points considering their stability in biological fluids and possible oral bioavailability.