A protease inhibitor of the Kunitz family from skin secretions of the tomato frog, Dyscophus guineti (Microhylidae)

Cutaneous glands of the striped toad, Rhinella crucifer (Wied-Neuwied, 1821) (Amphibia: Bufonidae): Histological study and bioactivities of glandular secretions

This study investigated the morphology of Rhinella crucifer cutaneous glands, as well as the protein/peptide profiles and bioactivities of body gland secretions (BGS) and parotoid macrogland secretions (PS). The parotoid as well as dorsal and ventral skin fragments of male and female individuals were processed for histological analysis. The protein and peptide profiles of male and female gland secretions were evaluated. Male secretions were also assessed for proteolytic, trypsin inhibiting, hemagglutinating, hemolytic, antimicrobial, and anticoagulant activities. The R. crucifer skin structure presented protuberances that are clearly visible and formed by the integument, which has cutaneous glands throughout the body. An average of 438 and 333 glands were identified in males in females, respectively. No significant differences were observed in the distribution of glands across the body as well as for area and perimeter of glands. Differences were observed in protein composition between the PS and BGS from males and females, and secretions from animals collected from undisturbed and anthropogenically disturbed areas. Proteins with similarities to catalase and elongation factor 1-alpha were detected in the PS. Zymography revealed proteolytic activity in both male BGS and PS. Male BGS showed antibacterial activity against Enterococcus faecalis and Escherichia coli and anticoagulant activity, being able to prolong prothrombin time by 6.34-fold and activated partial thromboplastin time by 2.17-fold. Finally, male PS and BGS caused a maximum hemolysis degree of 1.4%. The data showed that the cutaneous secretions of R. crucifer are potentially promising for biotechnological prospecting.

Recurrent evolution of adhesive defence systems in amphibians by parallel shifts in gene expression

Natural selection can drive organisms to strikingly similar adaptive solutions, but the underlying molecular mechanisms often remain unknown. Several amphibians have independently evolved highly adhesive skin secretions (glues) that support a highly effective antipredator defence mechanism. Here we demonstrate that the glue of the Madagascan tomato frog, Dyscophus guineti, relies on two interacting proteins: a highly derived member of a widespread glycoprotein family and a galectin. Identification of homologous proteins in other amphibians reveals that these proteins attained a function in skin long before glues evolved. Yet, major elevations in their expression, besides structural changes in the glycoprotein (increasing its structural disorder and glycosylation), caused the independent rise of glues in at least two frog lineages. Besides providing a model for the chemical functioning of animal adhesive secretions, our findings highlight how recruiting ancient molecular templates may facilitate the recurrent evolution of functional innovations.

An Effective Modification Strategy to Build Multifunctional Peptides Based on a Trypsin Inhibitory Peptide of the Kunitz Family

Peptides with antimicrobial activity or protease inhibitory activity are potential candidates to supplement traditional antibiotics or cancer chemotherapies. However, the potential of many peptides are limited by drawbacks such as cytotoxicity or susceptibility to hydrolysis. Therefore, strategies to modify the structure of promising peptides may represent an effective approach for developing more promising clinical candidates. In this study, the mature peptide OSTI-1949, a Kunitz-type inhibitor from Odorrana schmackeri, and four designed analogues were successfully synthesised. In contrast to the parent peptide, the analogues showed impressive multi-functionality including antimicrobial, anticancer, and trypsin inhibitory activities. In terms of safety, there were no obvious changes observed in the haemolytic activity at the highest tested concentration, and the analogue OSTI-2461 showed an increase in activity against cancer cell lines without cytotoxicity to normal cells (HaCaT). In summary, through structural modification of a natural Kunitz-type peptide, the biological activity of analogues was improved whilst retaining low cytotoxicity. The strategy of helicity enhancement by forming an artificial α-helix and ß-sheet structure provides a promising way to develop original bioactive peptides for clinical therapeutics.

Structural, Functional, and Mutational Studies of a Potent Subtilisin Inhibitor from Budgett's Frog, Lepidobatrachus laevis

Subtilases play a significant role in microbial pathogen infections by degrading the host proteins. Subtilisin inhibitors are crucial in fighting against these harmful microorganisms. LL-TIL, from skin secretions of Lepidobatrachus laevis, is a cysteine-rich peptide belonging to the I8 family of inhibitors. Protease inhibitory assays demonstrated that LL-TIL acts as a slow-tight binding inhibitor of subtilisin Carlsberg and proteinase K with inhibition constants of 91 pM and 2.4 nM, respectively. The solution structures of LL-TIL and a mutant peptide reveal that they adopt a typical TIL-type fold with a canonical conformation of a reactive site loop (RSL). The structure of the LL-TIL-subtilisin complex and molecular dynamics (MD) simulations provided an in-depth view of the structural basis of inhibition. NMR relaxation data and molecular dynamics simulations indicated a rigid conformation of RSL, which does not alter significantly upon subtilisin binding. The energy calculation for subtilisin inhibition predicted Ile31 as the highest contributor to the binding energy, which was confirmed experimentally by site-directed mutagenesis. A chimeric mutant of LL-TIL broadened the inhibitory profile and attenuated subtilisin inhibition by 2 orders of magnitude. These results provide a template to engineer more specific and potent TIL-type subtilisin inhibitors.

Extraction and characterization of cyclic lipopeptides with antifungal and antioxidant activities from Bacillus amyloliquefaciens

AIMS

This study aimed to isolate active substances from metabolites of Bacillus amyloliquefaciens SJ100001 and examine their antifungal activity against Fusarium oxysporum (F. oxysporum) SJ300024 screened from the root-soil of cucumber wilt.

METHODS AND RESULTS

An active substance, anti-SJ300024, was obtained from the fermentation broth of strain SJ100001 by reversed-phase silica gel and gel chromatography, and further got its chemical structure as cyclic lipopeptide Epichlicin through nuclear magnetic resonance (NMR) and mass spectrometry (MS). In vitro experiments showed that Epichlicin had a better inhibitory rate (67.46%) against the strain SJ300024 than the commercially available fungicide hymexazol (45.1%) at the same concentration. The MTT assays proved that Epichlicin was non-cytotoxic, besides it also had good free radical scavenging ability and total reducing ability.

CONCLUSIONS

Epichlicin isolated from strain SJ100001 can effectively control F. oxysporum SJ300024 screened from the root-soil of cucumber wilt.

SIGNIFICANCE AND IMPACT OF THE STUDY

Epichlicin may be used as an environmentally friendly and efficient biocontrol agent for controlling Fusarium wilt of cucumber and reducing crop losses. More importantly, the non-cytotoxicity of Epichlicin can avoid harm to consumers. Additionally, Epichlicin has broad application prospects in medicine due to its antioxidant properties.

Engineering and Structural Insights of a Novel BBI-like Protease Inhibitor Livisin from the Frog Skin Secretion

The Bowman–Birk protease inhibitor (BBI) family is a prototype group found mainly in plants, particularly grasses and legumes, which have been subjected to decades of study. Recently, the discovery of attenuated peptides containing the canonical Bowman–Birk protease inhibitory motif has been detected in the skin secretions of amphibians, mainly from Ranidae family members. The roles of these peptides in amphibian defense have been proposed to work cooperatively with antimicrobial peptides and reduce peptide degradation. A novel trypsin inhibitory peptide, named livisin, was found in the skin secretion of the green cascade frog, Odorrana livida. The cDNA encoding the precursor of livisin was cloned, and the predicted mature peptide was characterized. The mature peptide was found to act as a potent inhibitor against several serine proteases. A comparative activity study among the native peptide and its engineered analogs was performed, and the influence of the P₁ and P_2′ positions, as well as the C-terminal amidation on the structure–activity relationship for livisin, was illustrated. The findings demonstrated that livisin might serve as a potential drug discovery/development tool.

Putative Antimicrobial Peptides of the Posterior Salivary Glands from the Cephalopod Octopus vulgaris Revealed by Exploring a Composite Protein Database

Cephalopods, successful predators, can use a mixture of substances to subdue their prey, becoming interesting sources of bioactive compounds. In addition to neurotoxins and enzymes, the presence of antimicrobial compounds has been reported. Recently, the transcriptome and the whole proteome of the Octopus vulgaris salivary apparatus were released, but the role of some compounds—e.g., histones, antimicrobial peptides (AMPs), and toxins—remains unclear. Herein, we profiled the proteome of the posterior salivary glands (PSGs) of O. vulgaris using two sample preparation protocols combined with a shotgun-proteomics approach. Protein identification was performed against a composite database comprising data from the UniProtKB, all transcriptomes available from the cephalopods’ PSGs, and a comprehensive non-redundant AMPs database. Out of the 10,075 proteins clustered in 1868 protein groups, 90 clusters corresponded to venom protein toxin families. Additionally, we detected putative AMPs clustered with histones previously found as abundant proteins in the saliva of O. vulgaris. Some of these histones, such as H2A and H2B, are involved in systemic inflammatory responses and their antimicrobial effects have been demonstrated. These results not only confirm the production of enzymes and toxins by the O. vulgaris PSGs but also suggest their involvement in the first line of defense against microbes.

A New Family of Diverse Skin Peptides from the Microhylid Frog Genus Phrynomantis

A wide range of frogs produce skin poisons composed of bioactive peptides for defence against pathogens, parasites and predators. While several frog families have been thoroughly screened for skin-secreted peptides, others, like the Microhylidae, have remained mostly unexplored. Previous studies of microhylids found no evidence of peptide secretion, suggesting that this defence adaptation was evolutionarily lost. We conducted transcriptome analyses of the skins of Phrynomantis bifasciatus and Phrynomantis microps, two African microhylid species long suspected to be poisonous. Our analyses reveal 17 evolutionary related transcripts that diversified from to those of cytolytic peptides found in other frog families. The 19 peptides predicted to be processed from these transcripts, named phrynomantins, show a striking structural diversity that is distinct from any previously identified frog skin peptide. Functional analyses of five phrynomantins confirm the loss of a cytolytic function and the absence of insecticidal or proinflammatory activity, suggesting that they represent an evolutionary transition to a new, yet unknown function. Our study shows that peptides have been retained in the defence poison of at least one microhylid lineage and encourages research on similarly understudied taxa to further elucidate the diversity and evolution of skin defence molecules.

A Novel Kunitzin-Like Trypsin Inhibitor Isolated from Defensive Skin Secretion of Odorrana versabilis

Protease inhibitors that were identified from amphibian skin secretions with low molecular weights and potent inhibitory activity were thought to be potential candidates for novel peptide drugs. Here, a novel peptide with trypsin inhibitory activity was found in the skin secretion of the Chinese bamboo leaf odorous frog, Odorrana versabilis. Based on the sequence alignments of sequencing results, the novel peptide (ALKYPFRCKAAFC) was named as Kunitzin-OV. The synthetic replicate of Kunitzin-OV was subjected to a series of functional assays, and it exhibited a trypsin inhibitory activity with a Ki value of 3.042 µM, whereas, when Lys-9 at P1 position was substituted by Phe, trypsin inhibitory activity was undetected and the chymotrypsin inhibitory activity was optimized with a Ki value of 2.874 µM. However, its protease-binding loop was catabolized by trypsin during the trypsin cleavage test. In conclusion, Kunizin-OV is a novel peptide with trypsin inhibitory activity as a member of kunitzins, which is a non-typical Kunitz-like trypsin inhibitor with a highly conserved reactive site (K-A) and quite a short sequence.

Identification of Arenin, a Novel Kunitz-Like Polypeptide from the Skin Secretions of Dryophytes arenicolor

Amphibian skin secretions are enriched with complex cocktails of bioactive molecules such as proteins, peptides, biogenic amines, alkaloids guanidine derivatives, steroids and other minor components spanning a wide spectrum of pharmacological actions exploited for centuries in folk medicine. This study presents evidence on the protein profile of the skin secretions of the canyon tree frog, Dryophytes arenicolor. At the same time, it presents the reverse-phase liquid chromatography isolation, mass spectrometry characterization and identification at mRNA level of a novel 58 amino acids Kunitz-like polypeptide from the skin secretions of Dryophytes arenicolor, arenin. Cell viability assays performed on HDFa, CaCo2 and MCF7 cells cultured with different concentrations of arenin showed a discrete effect at low concentrations (2, 4, 8 and 16 µg/mL) suggesting a multi-target interaction in a hormetic-like dose-response. Further work is required to investigate the mechanisms underlying the variable effect on cell viability produced by different concentrations of arenin.

Novel Kazal-type proteinase inhibitors from the skin secretion of the Splendid leaf frog, Cruziohyla calcarifer

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18 novel Kazal proteins were identified in skin secretions of Cruziohyla calcarifer.

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CCKPs share the C-X₍₇₎-C-X_(6,7)-C-X_(6,7)-Y-X₍₃₎-C-X₍₂₎-C-X_(15-21)-C pattern.

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Trypsin and chymotrypsin inhibitory activity was proposed for 5 types of CCKPs.

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CCKP-1 has trypsin inhibitory activity and molecular mass of [M+H]⁺ = 5926.43 Da.

Peptidase inhibitors have an important role controlling a variety of biological processes. Here, we employed a peptidomic approach including molecular cloning, tandem mass spectrometry and enzymatic assays to reveal 7 Kazal-type proteinase inhibitors (CCKPs) (18 variants) in the skin secretion of the unexplored frog, Cruziohyla calcarifer. All 18 proteins shared the Kazal pattern C-X₍₇₎-C-X_(6,7)-C-X_(6,7)-Y-X₍₃₎-C-X₍₂₎-C-X_(15-21)-C and 3 disulphide bridges. Based on structural comparative analysis, we deemed trypsin and chymotrypsin inhibitory activity in CCKP-1, 4 and CCKP 2, 5, 7, respectively. These peptidase inhibitors presumably play a role to control the balance between other functional peptides produced in the amphibian skin secretions.

Protease Inhibitors as Ad-hoc Antibiotics

Background:

Proteases are important enzymes that can degrade proteins and are found in animals, plants, bacteria, fungi and viruses. The action of proteases can be controlled by Protease Inhibitors (PIs), chemical or proteinaceous in nature that can block the active site of protease. Since the step catalyzed by proteases may play important role in life cycle of microbes, hindering the action of proteases by PIs may act as therapeutic intervention for microbial infection.

Material and Methods:

A thorough study was performed and wide range of literature was surveyed to confirm our results of PIs showing antibacterial activity.

Results:

PIs have shown to be effective drugs against bacterial pathogens, pathogenic viruses- Human Immunodeficiency Virus (HIV), Herpes virus, Hepatitis Virus. PIs have recently been investigated for controlling protozoan parasites. Clinical value of proteases and their inhibitors has been studied inHelicobacter pyloriwhich is the etiologic agent of gastritis.

Conclusion:

This review is intended to highlight the role of PIs in the Battle against Microbial Pathogens.

pLR-HL: A Novel Amphibian Bowman-Birk-type Trypsin Inhibitor from the Skin Secretion of the Broad-folded Frog, Hylarana latouchii

In this study, we report a novel heptadecapeptide (LIGGCWTKSIPPKPCLV) of the pLR/ranacyclin family, named pLR-HL, whose structure was deduced from its biosynthetic precursor-encoding cDNA cloned from the skin secretion-derived cDNA library of the broad-folded frog, Hylarana latouchii, by employing a 'shotgun' cloning technique. It contains a disulphide loop between Cys(5) and Cys(15) which is consistent with Bowman-Birk-type protease inhibitors. The primary structure of pLR-HL deduced from the cDNA sequence was confirmed by fractionating the skin secretion using reverse-phase HPLC and subsequent analysis using MALDI-TOF mass spectrometry and LC/MS/MS fragmentation sequencing. On the basis of the establishment of unequivocal amino acid sequence, a synthetic replicate was synthesized by solid-phase Fmoc chemistry, and it displayed a moderately potent trypsin inhibition with a Ki of 143 nm. The substitution of Lys-8 by Phe (Phe(8) -pLR-HL) resulted in abolition of trypsin inhibition but generation of modest inhibition on chymotrypsin with a Ki of 2.141 μm. Additionally, both the disulphide loops of pLR-HL and Phe(8) -pLR-HL were synthesized and tested. Both of the catalytic loops retained similar inhibitory potencies towards trypsin or chymotrypsin in comparison with the original intact molecules. Thus, the replacement of reactive site residues could alter the specificity of these protease inhibitors, while the canonical reactive loop alone can independently constitute biologically active moiety.

Biological characterization of compounds from Rhinella schneideri poison that act on the complement system

Background

The skin secretions of toads of the family Bufonidae contain biogenic amines, alkaloids, steroids (bufotoxins), bufodienolides (bufogenin), peptides and proteins. The poison of Rhinella schneideri, formerly classified as Bufo paracnemis, presents components that act on different biological systems, including the complement system. The aim of this study was to isolate and examine the activity of Rhinella schneideri poison (RsP) components on the complement system.

Methods

The components active on the complement system were purified in three chromatographic steps, using a combination of cation-exchange, anion-exchange and gel filtration chromatography. The resulting fractions were analyzed by SDS-PAGE and screened for their activity in the hemolytic assay of the classical/lectin complement pathways. Fractions active on the complement system were also assessed for their ability to generate C3 fragments evaluated by two dimensional immunoelectrophoresis assay, C3a and C5a by neutrophil chemotaxis assay and SC5b-9 complex by ELISA assay.

Results

The fractionation protocol was able to isolate the component S5 from the RsP, as demonstrated by SDS-PAGE and the RP-FPLC profile. S5 is a protein of about 6000 Da, while S2 presents components of higher molecular mass (40,000 to 50,000 Da). Fractions S2 and S5 attenuated the hemolytic activity of the classical/lectin pathways after preincubation with normal human serum. Both components stimulated complement-dependent neutrophil chemotaxis and the production of C3 fragments, as shown by two-dimensional immunoelectrophoresis. S2 showed a higher capacity to generate the SC5b-9 complex than the other fractions. This action was observed after the exposure of normal human serum to the fractions.

Conclusions

This is the first study to examine the activity of RsP components on the complement system. Fractions S2 and S5 reduced the complement hemolytic activity, stimulated complement-dependent neutrophil chemotaxis and stimulated the production of C3 fragments, indicating that they were able to activate the complement cascade. Furthermore, fraction S2 was also able to generate the SC5b-9 complex. These components may be useful tools for studying dysfunction of the complement cascade.

First serine protease inhibitor isolated from Rhinella schneideri poison

BACKGROUND

Toad secretions are a source of molecules with potential biotechnological application on a wide spectrum of diseases. Toads from the Rhinella family have two kinds of poisonous glands, namely granular and mucous glands. Rhinella schneideri toads produce granular secretions that comprise a great number of molecules, including serine proteases inhibitors. Serine proteases, such as trypsin, chymotrypsin and elastase, are enzymes that have a serine amino acid into its catalytic site and can be found in a large number of vertebrate species and pathogenic microorganisms. Therefore, the present work aims to purify a serine protease inhibitor from Rhinella schneideri granular secretions.

FINDINGS

This study presents the protocol used to purify a serine protease inhibitor from the Rhinella schneideri poison. The granular secretion was submitted to dialysis in order to separate the low molecular weight compounds, which were submitted to a reversed phase-fast protein liquid chromatography fractionation step in a C2C18 column. The major fractions were tested over trypsin, chymotrypsin and elastase through colorimetric assay. The inhibition tests were performed with the enzyme in absence (positive control) and presence of fractions, denatured enzyme (negative control) and the respective chromogenic substrate. Rs20 was the compound with the major inhibitory activity over chymotrypsin, inducing a delay in the formation of the chromogenic enzymatic product. The structure characterization of Rs20 was performed by high resolution electronspray ionization-mass spectrometry (HRESI-MS) and gas chromatography coupled with mass spectrometry (GC-MS). HRESI showed an intense signal suggesting the presence of bufadienolide with less than 10 ppm error. In addition, it was observed a low intense signal at m/z 399 that could be lithocholic acid, a biosynthetic precursor of bufadienolide. Finally, GC-MS analysis applying NIST library identification reinforced this hypothesis.

CONCLUSIONS

The current study have isolated and partially characterized the function and structure of the first bufadienolide with inhibitory action over chymotrypsin.

A Novel Trypsin Inhibitor-Like Cysteine-Rich Peptide from the Frog Lepidobatrachus laevis Containing Proteinase-Inhibiting Activity

Various bio-active substances in amphibian skins play important roles in survival of the amphibians. Many protease inhibitor peptides have been identified from amphibian skins, which are supposed to negatively modulate the activity of proteases to avoid premature degradation or release of skin peptides, or to inhibit extracellular proteases produced by invading bacteria. However, there is no information on the proteinase inhibitors from the frog Lepidobatrachus laevis which is unique in South America. In this work, a cDNA encoding a novel trypsin inhibitor-like (TIL) cysteine-rich peptide was identified from the skin cDNA library of L. laevis. The 240-bp coding region encodes an 80-amino acid residue precursor protein containing 10 half-cysteines. By sequence comparison and signal peptide prediction, the precursor was predicted to release a 55-amino acid mature peptide with amino acid sequence, IRCPKDKIYKFCGSPCPPSCKDLTPNCIAVCKKGCFCRDGTVDNNHGKCVKKENC. The mature peptide was named LL-TIL. LL-TIL shares significant domain similarity with the peptides from the TIL supper family. Antimicrobial and trypsin-inhibitory abilities of recombinant LL-TIL were tested. Recombinant LL-TIL showed no antimicrobial activity, while it had trypsin-inhibiting activity with a Ki of 16.5178 μM. These results suggested there was TIL peptide with proteinase-inhibiting activity in the skin of frog L. laevis. To the best of our knowledge, this is the first report of TIL peptide from frog skin.

Atomic resolution crystal structure of HV-BBI protease inhibitor from amphibian skin in complex with bovine trypsin

Protease inhibitors of the Bowman-Birk (BBI) family are commonly found in plants and animals where they play a protective role against invading pathogens. Here, we report an atomic resolution (1Å) crystal structure of a peptide inhibitor isolated from a skin secretion of a Chinese bamboo odorous frog Huia versabilis (HV-BBI) in complex with trypsin. HV-BBI shares significant similarities in sequence with a previously described inhibitor from a diskless-fingered odorous frog Odorrana graham (ORB). However, the latter is characterized by more than a 16,000 fold higher Ki against trypsin than HV-BBI. Comparative analysis of trypsin cocrystal structures of HV-BBI and ORB and additionally that of Sunflower Trypsin Inhibitor (SFTI-1) together with accessory information on the affinities of inhibitor variants allowed us to pinpoint the inhibitor moiety responsible for the observed large difference in activity and also to define the extent of modifications permissible within the common protease-binding loop scaffold of BBI inhibitors. We suggest that modifications outside of the inhibitory loop permit the evolution of specificity toward different enzymes characterized by trypsin-like specificity.

The diversity and evolution of anuran skin peptides

Amphibians exhibit various, characteristic adaptations related to their "incomplete" shift from the aquatic to the terrestrial habitat. In particular, the integument was subject to a number of specialized modifications during the evolution of these animals. In this review, we place special emphasis on endogenous host-defence skin peptides from the cuteanous granular glands anuran amphibians (frogs and toads). The overview on the two broad groups of neuroactive and antimicrobial peptides (AMPs) goes beyond a simple itemization in that we provide a new perspective into the evolution and function of anuran AMPs. Briefly, these cationic, amphipathic and α-helical peptides are traditionally viewed as being part of the innate immune system, protecting the moist skin against invading microorganisms through their cytolytic action. However, the complete record of anuran species investigated to date suggests that AMPs are distributed sporadically (i.e., non-universally) across Anura. Together with the intriguing observation that virtually all anurans known to produce neuropeptides in their granular glands also co-secrete cytolytic peptides, we call the traditional role for AMPs as being purely antimicrobial into question and present an alternative scenario. We hypothesize AMPs to assist neuroactive peptides in their antipredator role through their cytolytic action increasing the delivery of the latter to the endocrine and nervous system of the predator. Thus, AMPs are more accurately viewed as cytolysins and their contribution to the immune system is better regarded as an accessory benefit.

Protease inhibitors from marine venomous animals and their counterparts in terrestrial venomous animals

The Kunitz-type protease inhibitors are the best-characterized family of serine protease inhibitors, probably due to their abundance in several organisms. These inhibitors consist of a chain of ~60 amino acid residues stabilized by three disulfide bridges, and was first observed in the bovine pancreatic trypsin inhibitor (BPTI)-like protease inhibitors, which strongly inhibit trypsin and chymotrypsin. In this review we present the protease inhibitors (PIs) described to date from marine venomous animals, such as from sea anemone extracts and Conus venom, as well as their counterparts in terrestrial venomous animals, such as snakes, scorpions, spiders, Anurans, and Hymenopterans. More emphasis was given to the Kunitz-type inhibitors, once they are found in all these organisms. Their biological sources, specificity against different proteases, and other molecular blanks (being also K⁺ channel blockers) are presented, followed by their molecular diversity. Whereas sea anemone, snakes and other venomous animals present mainly Kunitz-type inhibitors, PIs from Anurans present the major variety in structure length and number of Cys residues, with at least six distinguishable classes. A representative alignment of PIs from these venomous animals shows that, despite eventual differences in Cys assignment, the key-residues for the protease inhibitory activity in all of them occupy similar positions in primary sequence. The key-residues for the K⁺ channel blocking activity was also compared.

Inhibitory and antimicrobial activities of OGTI and HV-BBI peptides, fragments and analogs derived from amphibian skin

A series of linear and cyclic fragments and analogs of two peptides (OGTI and HV-BBI) isolated from skin secretions of frogs were synthesized by the solid-phase method. Their inhibitory activity against several serine proteinases: bovine β-trypsin, bovine α-chymotypsin, human leukocyte elastase and cathepsin G from human neutrophils, was investigated together with evaluation of their antimicrobial activities against Gram-negative bacteria (Escherichia coli) and Gram-positive species isolated from patients (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus sp., Streptococcus sp.). The cytotoxicity of the selected peptides toward an immortal human skin fibroblast cell line was also determined. Three peptides: HV-BBI, its truncated fragment HV-BBI(3-18) and its analog [Phe(8)]HV-BBI can be considered as bifunctional compounds with inhibitory as well as antibacterial properties. OGTI, although it did not display trypsin inhibitory activity as previously reported in the literature, exerted antimicrobial activity toward S. epidermidis. In addition, under our experimental conditions, this peptide did not show cytotoxicity.

A novel Kazal-type trypsin inhibitor from the skin secretion of the Central American red-eyed leaf frog, Agalychnis callidryas

The chemical complexity of the defensive skin secretion of the red-eyed leaf frog, (Agalychnis callidryas), has not been elucidated in detail. During a systematic study of the skin secretion peptidomes of phyllomedusine frogs, we discovered a novel Kazal-type protein with potent trypsin inhibitory activity (Ki = 1.9 nM) that displays the highest degree of structural similarity with Kazal proteins from bony fishes. The protein was located in reverse-phase HPLC fractions following a screen of such for trypsin inhibition and subsequent partial Edman degradation of the peak active fraction derived the sequence: ATKPR-QYIVL-PRILRPV-GT. The molecular mass of the major component in this fraction was established by MALDI-TOF MS as 5893.09 Da. This partial sequence (assuming blank cycles to be Cys residues) was used to design a degenerate primer pool that was employed successfully in RACE-PCR to clone homologous precursor-encoding cDNA that encoded a mature Kazal protein of 52 amino acid residues with a computed molecular mass of 5892.82 Da. The protein was named A. callidryas Kazal trypsin inhibitor (ACKTI). BLAST analysis revealed that ACKTI contained a canonical Kazal motif (C-x(7)-C-x(6)-Y-x(3)-C-x(2,3)-C). This novel amphibian skin Kazal trypsin inhibitor adds to the spectrum of trypsin inhibitors of Kunitz- and Bowman Birk-type reported from this amphibian source.

Functional peptidomics of amphibian skin secretion: A novel Kunitz-type chymotrypsin inhibitor from the African hyperoliid frog, Kassina senegalensis

Amphibian skin secretions are, for the most part, complex peptidomes. While many peptide components have been biologically- and structurally-characterised into discrete "families", some of which are analogues of endogenous vertebrate regulatory peptides, a substantial number are of unique structure and unknown function. Among the components of these secretory peptidomes is an array of protease inhibitors. Inhibitors of trypsin are of widespread occurrence in different taxa and are representative of many established structural classes, including Kunitz, Kazal and Bowman-Birk. However, few protease inhibitors with activity against other specific proteases have been described from this source. Here we report for the first time, the isolation and structural characterisation of an inhibitor of chymotrypsin of Kunitz-type from the skin secretion of the African hyperoliid frog, Kassina senegalensis. To this end, we employed a functional peptidomic approach. This scheme involves fractionation of the peptidome, functional end-point screening, structural characterisation of resultant actives followed by molecular cloning of biosynthetic precursor-encoding cDNA(s). The novel mature and active polypeptide identified consisted of 62 amino acid residues (average molecular mass 6776.24 Da), of which 6 were positionally-conserved cysteines. The P(1) position within the active site was occupied by a phenylalanyl residue. Bioinformatic analysis of the sequence using BLAST, revealed a structural similarity to Kunitz-type chymotrypsin inhibitors from other organisms, ranging from silkworms to snakes.

Structural diversity and species distribution of host-defense peptides in frog skin secretions

Cationic peptides that adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many frog species. These peptides often display cytolytic activities against bacteria and fungi consistent with the idea that they play a role in the host's system of defense against pathogenic microorganisms, but their importance in the survival strategy of the animal is not clearly understood. Despite the common misconception that antimicrobial peptides are synthesized in the skins of all anurans, the species distribution is sporadic, suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.

A Kunitz proteinase inhibitor from corms of Xanthosoma blandum with bactericidal activity

Bacterial infections directly affect the world's population, and this situation has been aggravated by indiscriminate use of antimicrobial agents, which can generate resistant microorganisms. In this report, an initial screening of proteins with antibacterial activity from corms of 15 species of the Xanthosoma genus was conducted. Since Xanthosoma blandum corms showed enhanced activity toward bacteria, a novel protein with bactericidal activity was isolated from this particular species. Edman degradation was used for protein N-termini determination; the primary structure showed similarities with Kunitz inhibitors, and this protein was named Xb-KTI. This protein was further challenged against serine proteinases from different sources, showing clear inhibitory activities. Otherwise, no hemolytic activity was observed for Xb-KTI. The results demonstrate the biotechnological potential of Xb-KTI, the first proteinase inhibitor with antimicrobial activity described in the Xanthosoma genus.

The contribution of skin antimicrobial peptides to the system of innate immunity in anurans

Cationic peptides with the propensity to adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many species of anurans (frogs and toads). These peptides frequently display cytolytic activities against a range of pathogenic bacteria and fungi consistent with the idea that they play a role in the host's system of innate immunity. However, the importance of the peptides in the survival strategy of the animal is not clearly understood. It is a common misconception that antimicrobial peptides are synthesized in the skins of all anurans. In fact, the species distribution is sporadic suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. Although growth inhibitory activity against the chytrid fungus Batrachochytrium dendrobatidis, responsible for anuran population declines worldwide, has been demonstrated in vitro, the ability of frog skin antimicrobial peptides to protect the animal in the wild appears to be limited and there is no clear correlation between their production by a species and its resistance to fatal chytridiomycosis. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.

Isolation and characterization of a trypsin inhibitor from the skin secretions of Kaloula pulchra hainana

Amphibian skin secretions contain many bioactive compounds. A trypsin inhibitor termed KPHTI was purified from the skin secretions of frog Kaloula pulchra hainana by successive ion-exchange and gel-filtration chromatography. KPHTI is a single chain glycoprotein, with an apparent molecular weight of 23 kDa in SDS-PAGE. It is a competitive inhibitor and effectively inhibits trypsin catalytic activity on peptide substrate with the inhibitor constant (K(i)) value of 27 nM. KPHTI shows no inhibitory effect on chymotrypsin, thrombin, elastase, and subtilisin. The N-terminal sequence of KPHTI is DHEVTS, which shows no similarity with other known trypsin inhibitors. DTT apparently affected the inhibitory activity of KPHTI. But it was not sensitive to temperature and pH range, which suggested that it possessed stable trypsin inhibitory activity in natural environment, and maybe play an important role in against predators.

Antimicrobial action of histone H2B in Escherichia coli: evidence for membrane translocation and DNA-binding of a histone H2B fragment after proteolytic cleavage by outer membrane proteinase T

Previous studies have led to the isolation of histone H2B with antibacterial properties from an extract of the skin of the Schlegel's green tree frog Rhacophorus schlegelii and it is now demonstrated that the intact peptide is released into norepinephrine-stimulated skin secretions. In order to investigate the mechanism of action of this peptide, a maltose-binding protein (MBP)-fused histone H2B (MBP-H2B) conjugate was prepared and subjected to antimicrobial assay. The fusion protein showed bacteriostatic activity against Escherichia coli strain JCM5491 with a minimum inhibitory concentration of 11 microM. The lysate prepared from JCM5491 cells was capable of fragmenting MBP-H2B within the histone H2B region, but the lysate from the outer membrane proteinase T (OmpT) gene-deleted BL21(DE3) cells was not. FITC-labeled MBP-H2B (FITC-MBP-H2B) penetrated into the bacterial cell membrane of JCM5491 and ompT-transformed BL21(DE3) cells, but not into ompT-deleted BL21(DE3) cells. Gel retardation assay using MBP-H2B-deletion mutants indicated that MBP-H2B bound to DNA at a site within the N-terminal region of histone H2B. Consequently, it is proposed that the antimicrobial action of histone H2B involves, at least in part, penetration of an OmpT-produced N-terminal histone H2B fragment into the bacterial cell membrane with subsequent inhibition of cell functions.

HV-BBI--a novel amphibian skin Bowman-Birk-like trypsin inhibitor

Here we describe the isolation of a novel C-terminally amidated octadecapeptide--SVIGCWTKSIPPRPCFVK-amide--that contains a disulphide loop between Cys(5) and Cys(15) that is consistent with a Bowman-Birk type protease inhibitor, from the skin secretion of the Chinese Bamboo odorous frog, Huia versabilis. Named HV-BBI, the peptide is encoded by a single precursor of 62 amino acid residues whose primary structure was deduced from cloned skin cDNA. The precursor exhibits the typical organization of that encoding an amphibian skin peptide with a highly-conserved signal peptide, an intervening acidic amino acid residue-rich domain and a single HV-BBI-encoding domain located towards the C-terminus. A synthetic replicate of HV-BBI, with the wild-type K (Lys-8) residue in the presumed P1 position, was found to be a potent inhibitor of trypsin with a K(i) just slightly less than 19 nM. Substitution at this site with R (Arg) resulted in a significant reduction in potency (K(i) 57 nM), whereas replacement of K with F (Phe) resulted in the complete abolition of trypsin inhibitory activity. Thus, HV-BBI is a potent inhibitor of trypsin and the lysyl (K) residue that occupies the P1 position appears to be optimal for potency of action against this protease.

A small trypsin inhibitor from the frog of Odorrana grahami

A novel peptide inhibitor (OGTI) of serine protease with a molecular weight of 1949.8, was purified from the skin secretion of the frog, Odorrana grahami. Of the tested serine proteases, OGTI only inhibited the hydrolysis activity of trypsin on synthetic chromogenic substrate. This precursor deduced from the cDNA sequence is composed of 70 amino acid residues. The mature OGTI contains 17 amino acid residues including a six-residue loop disulfided by two half-cysteines (AVNIPFKVHFRCKAAFC). In addition to its unique six-residue loop, the overall structure and precursor of OGTI are different from those of other serine protease inhibitors. It is also one of the smallest serine protease inhibitors ever found.

A serine proteinase inhibitor from frog eggs with bacteriostatic activity

By Sephadex G-50 gel filtration, Resource Q anionic exchange and C4 reversed phase liquid high performance liquid chromatography, a proteinase inhibitor protein (Ranaserpin) was identified and purified from the eggs of the odour frog, Rana grahami. The protein displayed a single band adjacent to the molecular weight marker of 14.4 kDa analyzed by SDS-PAGE. The inhibitor protein homogeneity and its molecular weight were confirmed again by MALDI-TOF mass spectrometry analysis. The MALDI-TOF mass spectrum analysis gave this inhibitor protein an m/z of 14422.26 that was matched well with the result from SDS-PAGE. This protein is a serine proteinase inhibitor targeting multiple proteinases including trypsin, elastase, and subtilisin. Ranaserpin inhibited the proteolytic activities of trypsin, elastase, and subtilisin. It has an inhibitory constant (K(i)) of 6.2 x 10(-8) M, 2.7 x 10(-7) M and 2.2 x 10(-8) M for trypsin, elastase, and subtilisin, respectively. This serine proteinase inhibitor exhibited bacteriostatic effect on Gram-positive bacteria Bacillus subtilis (ATCC 6633). It was suggested that ranaserpin might act as a defensive role in resistance to invasion of pests or pathogens. This is the first report of serine proteinase inhibitor and its direct defensive role from amphibian eggs.

Two serine protease inhibitors from the skin secretions of the toad, Bombina microdeladigitora

Two serine protease inhibitors (named BMSI 1 and BMSI 2, respectively) were identified from the skin secretions of the toad, Bombina microdeladigitora. The cDNAs encoding BMSIs were cloned from a cDNA library prepared from the toad skin. The deduced complete amino acid sequences of BMSIs indicate that mature BMSI 1 and BMSI 2 are composed of 60 amino acids including 10 half-cystines to form 5 disulfide bridges. A FASTA search in the databanks revealed that BMSIs exhibit sequence similarity with other serine protease inhibitors from amphibians of the genus Bombina. BMSI 1 potently inhibited trypsin and thrombin with a K(i) value of 0.02 microM and 0.15 microM, respectively. Sequence analysis revealed that all serine protease inhibitors from five amphibians of the genus Bombina share highly conserved primary structures.

Isolation and preliminary characterization of a 22-kDa protein with trypsin inhibitory activity from toad Bufo andrewsi skin

A novel trypsin inhibitor termed BATI was purified to homogeneity from the skin extracts of toad Bufo andrewsi by successive ion-exchange, gel-filtration and reverse-phase chromatography. BATI is basic single chain glycoprotein, with apparent molecular weight of 22 kDa in SDS-PAGE. BATI is a thermal stable competitive inhibitor and effectively inhibits trypsin's catalytic activity on peptide substrate with the inhibitor constant (K(i)) value of 14 nM and shows no inhibitory effect on chymotrypsin, thrombin and elastase. The N-terminal sequence of BATI is EKDSITD, which shows no similarity with other known trypsin inhibitors.

Purification and characterization of an irreversible serine protease inhibitor from skin secretions of Bufo andrewsi

Amphibian skin secretions contain many bioactive compounds. In the present work, an irreversible serine protease inhibitor, termed baserpin, was purified for the first time from the skin secretions of toad Bufo andrewsi by successive ion-exchange and gel-filtration chromatography. Baserpin is a single chain glycoprotein, with an apparent molecular weight of about 60 kDa in SDS-PAGE. Baserpin is an irreversible inhibitor and effectively inhibits the catalytic activity of trypsin, chymotrypsin and elastase. SDS-stable baserpin-trypsin complex could be seen in SDS-PAGE indicates that it possibly belongs to the serpin superfamily. According to the association rates determined, baserpin is a potent inhibitor of bovine trypsin (4.6 x 10(6) M(-1) s(-1)), bovine chymotrypsin (8.9 x 10(6) M(-1) s(-1)) and porcine elastase (6.8 x 10(6) M(-1) s(-1)), whereas it shows no inhibitory effect on thrombin. The N-terminal sequence of baserpin is HTQYPDILIAKPXDK, which shows no similarity with other known serine protease inhibitors.

Pentadactylin: an antimicrobial peptide from the skin secretions of the South American bullfrog Leptodactylus pentadactylus

Norepinephrine-stimulated skin secretions were obtained from male specimens of the South American bullfrog, Leptodactylus pentadactylus and shown to contain two peptides that inhibited the growth of microorganisms. The primary structure of a previously undescribed peptide, termed pentadactylin, was established as Gly-Leu-Leu-Asp-Thr-Leu-Lys-Gly-Ala-Ala-Lys-Asn-Val-Val-Gly-Ser-Leu-Ala-Ser-Lys-Val-Met-Glu-Lys-Leu.NH2. The second peptide, which differs from pentadactylin by eight amino acid residues, is identical to fallaxin previously isolated from skin secretions of the Caribbean mountain chicken frog L. fallax. Pentadactylin inhibited the growth of reference strains of both Gram-negative bacteria (Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Streptococcus group B) but potencies were relatively low (MIC values in the range 25-200 microM). The peptide showed very low hemolytic activity against human erythrocytes (LD50>400 microM).

A Kazal prolyl endopeptidase inhibitor isolated from the skin of Phyllomedusa sauvagii

Searching for bioactive peptides, we analyzed acidic extracts of Phyllomedusa sauvagii skin and found two new proteins, PSKP-1 and PSKP-2, of 6.7 and 6.6 kDa, respectively, which, by sequence homology, belong to the Kazal family of serine protease inhibitors. PSKP-1 and PSKP-2 exhibit the unprecedented feature of having proline at P(1) and P(2) positions. A gene encoding PSKP-1 was synthesized and expressed in Escherichia coli. Recombinant PSKP-1 was purified from inclusion bodies, oxidatively refolded to the native state, and characterized by chemical, hydrodynamic and optical studies. PSKP-1 shows inhibitory activity against a serum prolyl endopeptidase, but is unable to inhibit trypsin, chymotrypsin, V8 protease, or proteinase K. In addition, PSKP-1 can be rendered active against trypsin by active-site site-specific mutagenesis, has bactericidal activity, and induces agglutination of red cells at micromolar concentrations. PSKP-1 might protect P. sauvagii teguments from microbial invasion, by acting as an inhibitor of an as-yet unidentified prolyl endopeptidase or directly as a microbicidal compound.

A protein with antimicrobial activity in the skin of Schlegel’s green tree frog Rhacophorus schlegelii (Rhacophoridae) identified as histone H2B

An extract of the skin of Schlegel's green tree frog, Rhacophorus schlegelii (Anura: Rhacophoridae), contained a protein that inhibited the growth of the Gram-negative bacterium Escherichia coli but was inactive against the Gram-positive bacterium Staphylococcus aureus. The protein was purified to near homogeneity by reverse-phase HPLC and amino acid sequence analysis of the products of an endoproteinase Glu-C digest identified the protein as histone H2B. The complete primary structure of the 125 amino acid residue Rhacophorus histone H2B was determined by nucleotide sequence analysis of a cloned cDNA encoding the protein. Mass spectrometry demonstrated that the protein isolated from the skin was not post-translationally modified. Histone fragments with antimicrobial activity were not identified in the Rhacophorus skin extract nor were cationic, alpha-helical antimicrobial peptides of the kind isolated from the skins of several other frog families. The data provide further evidence that histones play a role in the defense against microorganisms.

Identification and molecular cloning of novel trypsin inhibitor analogs from the dermal venom of the Oriental fire-bellied toad (Bombina orientalis) and the European yellow-bellied toad (Bombina variegata)

The structural diversity of polypeptides in amphibian skin secretion probably reflects different roles in dermal regulation or in defense against predators. Here we report the structures of two novel trypsin inhibitor analogs, BOTI and BVTI, from the dermal venom of the toads, Bombina orientalis and Bombina variegata. Cloning of their respective precursors was achieved from lyophilized venom cDNA libraries for the first time. Amino acid alignment revealed that both deduced peptides, consisting of 60 amino acid residues, including 10 cysteines and the reactive center motif, -CDKKC-, can be affirmed as structural homologs of the trypsin inhibitor from Bombina bombina skin.

A novel trypsin inhibitor from Peltophorum dubium seeds, with lectin-like properties, triggers rat lymphoma cell apoptosis

A trypsin inhibitor (PDTI) was isolated from Peltophorum dubium seeds by affinity chromatography on a thyroglobulin-agarose or a trypsin-agarose column. In both cases, SDS-PAGE showed two bands of M(r) 20,000 and 22,000, which could not be resolved. Their amino-terminal sequences were identical and similar to that of Kunitz-type soybean trypsin inhibitor (SBTI). Mass spectrometry analysis of tryptic digests of both bands showed 16 coincident peaks, suggesting that they are closely related proteins. The K(i)s for trypsin and chymotrypsin inhibitory activity of PDTI were 1.6 x 10(-7) and 1.3 x 10(-5)M, respectively. Lectin-like activity of PDTI and SBTI, detected by hemagglutination of rabbit erythrocytes, was inhibited by sialic acid-containing compounds. PDTI and SBTI caused apoptosis of Nb2 rat lymphoma cells, demonstrated by decrease of viability, DNA hypodiploidy, DNA fragmentation, and caspase-3-like activity. They had no effect on normal mouse splenocytes or lymphocytes, whereas they caused apoptosis of concanavalin A-stimulated mouse lymphocytes.

Antimicrobial peptides and protease inhibitors in the skin secretions of the crawfish frog, Rana areolata

The dorsal skin of the crawfish frog, Rana areolata, is associated with numerous prominent granular glands. Proteomic analysis of electrically stimulated skin secretions from these glands enabled the identification and characterization of eight peptides with antimicrobial and hemolytic activity belonging to the previously identified brevinin-1, temporin-1, palustrin-2, palustrin-3, esculentin-1 (two peptides), and ranatuerin-2 (two peptides) families. The primary structures of the peptides were consistent with a close phylogenetic relationship between R. areolata and the pickerel frog, Rana palustris. Three structurally related cationic, cysteine-containing peptides were identified that show sequence similarity to peptide Leucine-Arginine, a peptide with immunomodulatory and histamine-releasing properties from the skin of the northern leopard frog, Rana pipiens. The skin secretions contained a 61-amino-acid-residue peptide that inhibited porcine trypsin and possessed a 10-cysteine-residue motif that is characteristic of a protease inhibitor previously isolated from the parasitic nematode, Ascaris suum. A 48-amino-acid-residue protein containing eight cysteine residues in the whey acidic protein (WAP) motif, characteristic of elafin (skin-derived antileukoproteinase) and secretory leukocyte protease inhibitor, was also isolated. The data suggest that protease inhibitors in skin secretions may play a role complementary to cationic, amphipathic alpha-helical peptides in protecting anurans from invasions by microorganisms.

Identification and cloning of a trypsin inhibitor from skin secretions of Chinese red-belly toad Bombina maxima

A novel trypsin inhibitor was identified and purified from skin secretions of Chinese red-belly toad Bombina maxima. The partial N-terminal 29 amino acid residues of the peptide, named BMTI, were determined by automated Edman degradation. This allowed the cloning of a full-length cDNA encoding BMTI from a cDNA library prepared from the toad skin. The deduced complete amino acid sequence of BMTI indicates that mature BMTI is composed of 60 amino acids. A FASTA search in the databanks revealed that BMTI exhibits 81.7% sequence identity with BSTI, a trypsin/thrombin inhibitor from European toad Bombina bombina skin secretions. Sequence differences between BMTI and BSTI were due to 11 substitutions at positions 2, 9, 25, 27, 36-37, 39, 41-42, 50 and 56. BMTI potently inhibited trypsin with a K(i) value of 0.06 microM, similar to that of BSTI. However, unlike BSTI, which also inhibited thrombin with a K(i) value of 1 microM, no inhibitory effect of BMTI on thrombin was observed under the assay conditions.

Pseudin-2: an antimicrobial peptide with low hemolytic activity from the skin of the paradoxical frog

Four structurally related peptides (pseudins 1-4) with antimicrobial activity were isolated from an extract of the skin of the paradoxical frog Pseudis paradoxa (Pseudidae). Pseudin-2 (GLNALKKVFQGIHEAIKLINNHVQ) was the most abundant peptide (22 nmol/g tissue) and also the most potent (minimum inhibitory concentrations, MIC = 2.5 microM against Escherichia coli, 80 microM against Staphylococcus aureus, and 130 microM against Candida albicans). The concentration of pseudin-2 producing 50% hemolysis of human erythrocytes was >300 microM. Circular dichroism studies showed that the pseudins belong to the class of cationic, amphipathic alpha-helical antimicrobial peptides but their amino acid sequences are not similar to any previously characterized peptides from frog skin. The pseudins do, however, show sequence similarity with a region at the C-terminus of DEFT, a death effector domain-containing protein expressed in mammalian testicular germ cells that is involved in the regulation of apoptosis.