Expression, crystallization, and three-dimensional structure of the catalytic domain of human plasma kallikrein

Unveiling therapeutic frontiers: DON/DRP-104 as innovative Plasma kallikrein inhibitors against carcinoma-associated hereditary angioedema shocks - a comprehensive molecular dynamics exploration

Human plasma kallikrein (PKa) is a member of the serine protease family and serves as a key mediator of the kallikrein-kinin system (KKS), which is known for its regulatory roles in inflammation, vasodilation, blood pressure, and coagulation. Genetic dysregulation of KKS leads to Hereditary Angioedema (HAE), which is characterized by spontaneous, painful swelling in various body regions. Importantly, HAE frequently coexists with various cancers. Despite substantial efforts towards the development of PKa inhibitors for HAE, there remains a need for bifunctional agents addressing both anti-cancer and anti-HAE aspects, especially against carcinoma-associated comorbid HAE conditions. Consequently, we investigated the therapeutic potential of the anti-glutamine prodrug, isopropyl(S)-2-((S)-2-acetamido-3-(1H-indol-3-yl)-propanamido)-6-diazo-5-oxo-hexanoate (DRP-104), and its active form, 6-Diazo-5-oxo-l-norleucine (DON), recognized for their anti-cancer properties, as novel PKa inhibitors. Utilizing structure-based in silico methods, we conducted a comparative analysis with berotralstat, a clinically approved HAE prophylactic, and sebetralstat, an investigational HAE therapeutic agent, in Phase 3 clinical trials. Inhibiting PKa with DON resulted in relatively heightened structural stability, rigidity, restricted protein folding, and solvent-accessible loop exposure, contributing to increased intra-atomic hydrogen bond formation. Conversely, PKa inhibition with DRP-104 induced restricted residue flexibility and significantly disrupted the critical SER195-HIS57 arrangement in the catalytic triad. Both DON and DRP-104, along with the reference drugs, induced strong cooperative intra-residue motion and bidirectional displacement in the PKa architecture. The results revealed favorable binding kinetics of DON/DRP-104, showing thermodynamic profiles that were either superior or comparable to those of the reference drugs. These findings support their consideration for clinical investigations into the management of carcinoma-associated HAE.

STAR-0215 is a Novel, Long-Acting Monoclonal Antibody Inhibitor of Plasma Kallikrein for the Potential Treatment of Hereditary Angioedema

Hereditary angioedema (HAE) is a rare autosomal dominant disorder caused by a deficiency in functional C1 esterase inhibitor, a serpin family protein that blocks the activity of plasma kallikrein. Insufficient inhibition of plasma kallikrein results in the overproduction of bradykinin, a vasoactive inflammatory mediator that produces both pain and unpredictable swelling during HAE attacks, with potentially life-threatening consequences. We describe the generation of STAR-0215, a humanized IgG1 antibody with a long circulating half-life (t1/2) that potently inhibits plasma kallikrein activity, with a >1000-fold lower affinity for prekallikrein and no measurable inhibitory activity against other serine proteases. The high specificity and inhibitory effect of STAR-0215 is demonstrated through a unique allosteric mechanism involving N-terminal catalytic domain binding, destabilization of the activation domain, and reversion of the active site to the inactive zymogen state. The YTE (M252Y/S254T/T256E) modified fragment crystallizable (Fc) domain of STAR-0215 enhances pH-dependent neonatal Fc receptor binding, resulting in a prolonged t1/2 in vivo (∼34 days in cynomolgus monkeys) compared with antibodies without this modification. A single subcutaneous dose of STAR-0215 (≥100 mg) was predicted to be active in patients for 3 months or longer, based on simulations using a minimal physiologically based pharmacokinetic model. These data indicate that STAR-0215, a highly potent and specific antibody against plasma kallikrein with extended t1/2, is a potential agent for long-term preventative HAE therapy administered every 3 months or less frequently. SIGNIFICANCE STATEMENT: STAR-0215 is a YTE-modified immunoglobulin G1 monoclonal antibody with a novel binding mechanism that specifically and potently inhibits the enzymatic activity of plasma kallikrein and prevents the generation of bradykinin. It has been designed to be a long-lasting prophylactic treatment to prevent attacks of HAE and to decrease the burden of disease and the burden of treatment for people with HAE.

Sebetralstat (KVD900): A Potent and Selective Small Molecule Plasma Kallikrein Inhibitor Featuring a Novel P1 Group as a Potential Oral On-Demand Treatment for Hereditary Angioedema

Hereditary angioedema (HAE) is a rare genetic disorder in which patients experience sudden onset of swelling in various locations of the body. HAE is associated with uncontrolled plasma kallikrein (PKa) enzyme activity and generation of the potent inflammatory mediator, bradykinin, resulting in episodic attacks of angioedema. Herein, we disclose the discovery and optimization of novel small molecule PKa inhibitors. Starting from molecules containing highly basic P1 groups, which typically bind to an aspartic acid residue (Asp189) in the serine protease S1 pocket, we identified novel P1 binding groups likely to have greater potential for oral-drug-like properties. The optimization of P4 and the central core together with the particularly favorable properties of 3-fluoro-4-methoxypyridine P1 led to the development of sebetralstat, a potent, selective, orally bioavailable PKa inhibitor in phase 3 for on-demand treatment of HAE attacks.

Berotralstat (BCX7353): Structure-Guided Design of a Potent, Selective, and Oral Plasma Kallikrein Inhibitor to Prevent Attacks of Hereditary Angioedema (HAE)

Hereditary angioedema (HAE) is a rare and potentially life-threatening disease that affects an estimated 1 in 50 000 individuals worldwide. Until recently, prophylactic HAE treatment options were limited to injectables, a burdensome administration route that has driven the need for an oral treatment. A substantial body of evidence has shown that potent and selective plasma kallikrein inhibitors that block the generation of bradykinin represent a promising approach for the treatment of HAE. Berotralstat (BCX7353, discovered by BioCryst Pharmaceuticals using a structure-guided drug design strategy) is a synthetic plasma kallikrein inhibitor that is potent and highly selective over other structurally related serine proteases. This once-daily, small-molecule drug is the first orally bioavailable prophylactic treatment for HAE attacks, having successfully completed a Phase III clinical trial (meeting its primary end point) and recently receiving the U.S. Food and Drug Administration's approval for the prophylactic treatment of HAE attacks in patients 12 years and older.

Crystal structure of inhibitor-bound human MSPL that can activate high pathogenic avian influenza

The structure of extracellular domain of MSPL and inhibitor complex helps to understand the TTSP functions, including TMPRSS2, and provides the insights of the infection of influenza and SARS-CoV.

Infection of certain influenza viruses is triggered when its HA is cleaved by host cell proteases such as proprotein convertases and type II transmembrane serine proteases (TTSP). HA with a monobasic motif is cleaved by trypsin-like proteases, including TMPRSS2 and HAT, whereas the multibasic motif found in high pathogenicity avian influenza HA is cleaved by furin, PC5/6, or MSPL. MSPL belongs to the TMPRSS family and preferentially cleaves [R/K]-K-K-R↓ sequences. Here, we solved the crystal structure of the extracellular region of human MSPL in complex with an irreversible substrate-analog inhibitor. The structure revealed three domains clustered around the C-terminal α-helix of the SPD. The inhibitor structure and its putative model show that the P1-Arg inserts into the S1 pocket, whereas the P2-Lys and P4-Arg interacts with the Asp/Glu-rich 99-loop that is unique to MSPL. Based on the structure of MSPL, we also constructed a homology model of TMPRSS2, which is essential for the activation of the SARS-CoV-2 spike protein and infection. The model may provide the structural insight for the drug development for COVID-19.

Structural modeling and analysis of the SARS-CoV-2 cell entry inhibitor camostat bound to the trypsin-like protease TMPRSS2

The type II transmembrane serine protease TMPRSS2 facilitates the entry of coronaviruses, such as SARS-CoV-2, into host cells by cleaving the S₁/S₂ interface of the viral spike protein. Based on structural data derived from X-ray crystallographic data of related trypsin-like proteases, a homology model of TMPRSS2 is described and validated using the broad spectrum COVID-19 drug candidate camostat as a probe. Both active site recognition and catalytic function are examined using quantum mechanics/molecular mechanics molecular dynamic (QM/MM MD) simulations of camostat and its active metabolite, 4-(4-guanidinobenzoyloxy) phenylacetate (GBPA). Substrate binding is shown to be primarily stabilized through salt bridge formation between the shared guanidino pharmacophore and D435 in pocket A (flanking the catalytic S441). Based on the binding mode of GBPA, residues K342 and W461 have been identified as potential contacts involved in TMPRSS2 selective binding and activity. Additional data is reported that indicates the transition state structure is stabilized through H-bonding interactions with the backbone N–H groups within an oxyanion hole following bottom-side attack of the carbonyl by S441. This is supported by prior work on related serine proteases suggesting further strategies to exploit in the design of more potent inhibitors. Taken overall, the proposed structure along with the key contact sites and mechanistic features identified should prove highly advantageous to the design and rational development of safe and effective therapeutics that target TMPRSS2 and avoid inhibition of other trypsin-dependent processes.

Magnetic particles used in a new approach for designed protein crystallization

Designed protein crystallization using magnetic particles as additives in the crystallization of model case studies.

Plasma kallikrein structure reveals apple domain disc rotated conformation compared to factor XI

Essentials Zymogen PK is activated to PKa and cleaves substrates kininogen and FXII contributing to bradykinin generation. Monomeric PKa and dimeric homologue FXI utilize the N-terminal apple domains to recruit substrates. A high-resolution 1.3 Å structure of full-length PKa reveals an active conformation of the protease and apple domains. The PKa protease and four-apple domain disc organization is 180° rotated compared to FXI. SUMMARY: Background Plasma prekallikrein (PK) and factor XI (FXI) are apple domain-containing serine proteases that when activated to PKa and FXIa cleave substrates kininogen, factor XII, and factor IX, respectively, directing plasma coagulation, bradykinin release, inflammation, and thrombosis pathways. Objective To investigate the three-dimensional structure of full-length PKa and perform a comparison with FXI. Methods A series of recombinant full-length PKa and FXI constructs and variants were developed and the crystal structures determined. Results and conclusions A 1.3 Å structure of full-length PKa reveals the protease domain positioned above a disc-shaped assemblage of four apple domains in an active conformation. A comparison with the homologous FXI structure reveals the intramolecular disulfide and structural differences in the apple 4 domain that prevents dimer formation in PK as opposed to FXI. Two latchlike loops (LL1 and LL2) extend from the PKa protease domain to form interactions with the apple 1 and apple 3 domains, respectively. A major unexpected difference in the PKa structure compared to FXI is the 180° disc rotation of the apple domains relative to the protease domain. This results in a switched configuration of the latch loops such that LL2 interacts and buries portions of the apple 3 domain in the FXI zymogen whereas in PKa LL2 interacts with the apple 1 domain. Hydrogen-deuterium exchange mass spectrometry on plasma purified human PK and PKa determined that regions of the apple 3 domain have increased surface exposure in PKa compared to the zymogen PK, suggesting conformational change upon activation.

Structures of full-length plasma kallikrein bound to highly specific inhibitors describe a new mode of targeted inhibition

Plasma kallikrein (pKal) is a serine protease responsible for cleaving high-molecular-weight kininogen to produce the pro-inflammatory peptide, bradykinin. Unregulated pKal activity can lead to hereditary angioedema (HAE) following excess bradykinin release. HAE attacks can lead to a compromised airway that can be life threatening. As there are limited agents for prophylaxis of HAE attacks, there is a high unmet need for a therapeutic agent for regulating pKal with a high degree of specificity. Here we present crystal structures of both full-length and the protease domain of pKal, bound to two very distinct classes of small-molecule inhibitors: compound 1, and BCX4161. Both inhibitors demonstrate low nM inhibitory potency for pKal and varying specificity for related serine proteases. Compound 1 utilizes a surprising mode of interaction and upon binding results in a rearrangement of the binding pocket. Co-crystal structures of pKal describes why this class of small-molecule inhibitor is potent. Lack of conservation in surrounding residues explains the ∼10,000-fold specificity over structurally similar proteases, as shown by in vitro protease inhibition data. Structural information, combined with biochemical and enzymatic analyses, provides a novel scaffold for the design of targeted oral small molecule inhibitors of pKal for treatment of HAE and other diseases resulting from unregulated plasma kallikrein activity.

Structural studies of plasmin inhibition

Plasminogen (Plg) is the zymogen form of the serine protease plasmin (Plm), and it plays a crucial role in fibrinolysis as well as wound healing, immunity, tissue remodeling and inflammation. Binding to the targets via the lysine-binding sites allows for Plg activation by plasminogen activators (PAs) present on the same target. Cellular uptake of fibrin degradation products leads to apoptosis, which represents one of the pathways for cross-talk between fibrinolysis and tissue remodeling. Therapeutic manipulation of Plm activity plays a vital role in the treatments of a range of diseases, whereas Plm inhibitors are used in trauma and surgeries as antifibrinolytic agents. Plm inhibitors are also used in conditions such as angioedema, menorrhagia and melasma. Here, we review the rationale for the further development of new Plm inhibitors, with a particular focus on the structural studies of the active site inhibitors of Plm. We compare the binding mode of different classes of inhibitors and comment on how it relates to their efficacy, as well as possible future developments.

Structures of human plasma β-factor XIIa cocrystallized with potent inhibitors

Activated factor XIIa (FXIIa) is a serine protease that has received a great deal of interest in recent years as a potential target for the development of new antithrombotics. Despite the strong interest in obtaining structural information, only the structure of the FXIIa catalytic domain in its zymogen conformation is available. In this work, reproducible experimental conditions found for the crystallization of human plasma β-FXIIa and crystal growth optimization have led to determination of the first structure of the active form of the enzyme. Two crystal structures of human plasma β-FXIIa complexed with small molecule inhibitors are presented herein. The first is the noncovalent inhibitor benzamidine. The second is an aminoisoquinoline containing a boronic acid-reactive group that targets the catalytic serine. Both benzamidine and the aminoisoquinoline bind in a canonical fashion typical of synthetic serine protease inhibitors, and the protease domain adopts a typical chymotrypsin-like serine protease active conformation. This novel structural data explains the basis of the FXII activation, provides insights into the enzymatic properties of β-FXIIa, and is a great aid toward the further design of protease inhibitors for human FXIIa.

Plasma kallikrein cleaves and inactivates apelin-17: Palmitoyl- and PEG-extended apelin-17 analogs as metabolically stable blood pressure-lowering agents

Apelins are human peptide hormones with various physiological activities, including the moderation of cardiovascular, renal, metabolic and neurological function. Their potency is dependent on and limited by proteolytic degradation in the circulatory system. Here we identify human plasma kallikrein (KLKB1) as a protease that cleaves the first three N-terminal amino acids (KFR) of apelin-17. The cleavage kinetics are similar to neprilysin (NEP), which cleaves within the critical 'RPRL'-motif thereby inactivating apelin. The resulting C-terminal 14-mer after KLKB1 cleavage has much lower biological activity, and the presence of its N-terminal basic arginine seems to negate the blood pressure lowering effect. Based on C-terminally engineered apelin analogs (A2), resistant to angiotensin converting enzyme 2 (ACE2), attachment of an N-terminal C16 fatty acid chain (PALMitoylation) or polyethylene glycol chain (PEGylation) minimizes KLKB1 cleavage of the 17-mers, thereby extending plasma half-life while fully retaining biological activity. The N-terminally PEGylated apelin-17(A2) is a highly protease resistant analog, with excellent apelin receptor activation and pronounced blood pressure lowering effect.

A disease-causing novel missense mutation in the ST14 gene underlies autosomal recessive ichthyosis with hypotrichosis syndrome in a consanguineous family

Autosomal recessive ichthyosis with hypotrichosis (ARIH; MIM 602400) syndrome is characterized by diffused congenital ichthyosis and generalized non-scarring hypotrichosis. The underlying genetic cause of ARIH syndrome has been associated with sequence variants of the gene ST14, encoding type II transmembrane serine protease matriptase, which maps to chromosome 11q24.3. The current report aimed to investigate the clinical features and genetic cause of ARIH syndrome in a large consanguineous family of Pakistani origin. The technique of homozygosity mapping with highly polymorphic microsatellite markers was employed to establish linkage within the family. Sanger sequencing of exons and intron-exon boundaries of ST14 was performed to identify the potential pathogenic sequence variants, followed by structural analysis of the mutated protein. Linkage was established to chromosome 11q24.3, comprising the gene ST14. Sequence analysis led to the identification of a novel homozygous missense variant (c.1315G>A, p.Gly439Ser) in the ST14 gene that co-segregated with the disease phenotype in all affected members. Homology modelling and molecular docking analysis of ST14 with wild-type TMEFF1 protein was performed which revealed that glycine at position 439 is crucial for maintaining normal structural confirmation and interaction with the EGF domain of TMEFF1 protein. Taken together, the data strongly advocate this ST14 variant as the underlying genetic cause of ARIH syndrome in this first reported affected family from Pakistan. Moreover, the present study adds to the spectrum of mutations in the ST14 gene, implicating them in the pathogenesis of ARIH syndrome.

iTRAQ based investigation of plasma proteins in HIV infected and HIV/HBV coinfected patients - C9 and KLK are related to HIV/HBV coinfection

OBJECTIVES

Human immunodeficiency virus (HIV) and hepatitis B virus (HBV) share similar routes of transmission, and rapid progression of hepatic and immunodeficiency diseases has been observed in coinfected individuals. Our main objective was to investigate the molecular mechanism of HIV/HBV coinfections.

METHODS

We selected HIV infected and HIV/HBV coinfected patients with and without Highly Active Antiretroviral Therapy (HAART). Low abundance proteins enriched using a multiple affinity removal system (MARS) were labeled with isobaric tags for relative and absolute quantitation (iTRAQ) kits and analyzed using liquid chromatography-mass spectrometry (LC-MS). The differential proteins were analyzed by Gene Ontology (GO) database.

RESULTS

A total of 41 differential proteins were found in HIV/HBV coinfected patients as compared to HIV mono-infected patients with or without HAART treatment, including 7 common HBV-regulated proteins. The proteins involved in complement and coagulation pathways were significantly enriched, including plasma kallikrein (KLK) and complement component C9 (C9). C9 and KLK were verified to be down-regulated in HIV/HBV coinfected patients through ELISA analysis.

CONCLUSION

The present iTRAQ based proteomic analyses identified 7 proteins that are related to HIV/HBV coinfection. HBV might influence hepatic and immune functions by deregulating complement and coagulation pathways. C9 and KLK could potentially be used as targets for the treatment of HIV/HBV coinfections.

Plasma Kallikrein Inhibitors in Cardiovascular Disease: An Innovative Therapeutic Approach

Plasma prekallikrein is the liver-derived precursor of the trypsin-like serine protease plasma kallikrein, and circulates in plasma bound to high molecular weight kininogen. Plasma prekallikrein is activated to plasma kallikrein by activated factor XII or prolylcarboxypeptidase. Plasma kallikrein regulates the activity of multiple proteolytic cascades in the cardiovascular system such as the intrinsic pathway of coagulation, the kallikrein-kinin system, the fibrinolytic system, the renin-angiotensin system, and the complement pathways. As such, plasma kallikrein plays a central role in the pathogenesis of thrombosis, inflammation, and blood pressure regulation. Under physiological conditions, plasma kallikrein serves as a cardioprotective enzyme. However, its increased plasma concentration or hyperactivity perpetuates cardiovascular disease (CVD). In this article, we review the biochemistry and cell biology of plasma kallikrein and summarize data from preclinical and clinical studies that have established important functions of this serine protease in CVD states. Finally, we propose plasma kallikrein inhibitors as a novel class of drugs with potential therapeutic applications in the treatment of CVDs.

Effects of Glycosylation on the Enzymatic Activity and Mechanisms of Proteases

Posttranslational modifications are an important feature of most proteases in higher organisms, such as the conversion of inactive zymogens into active proteases. To date, little information is available on the role of glycosylation and functional implications for secreted proteases. Besides a stabilizing effect and protection against proteolysis, several proteases show a significant influence of glycosylation on the catalytic activity. Glycans can alter the substrate recognition, the specificity and binding affinity, as well as the turnover rates. However, there is currently no known general pattern, since glycosylation can have both stimulating and inhibiting effects on activity. Thus, a comparative analysis of individual cases with sufficient enzyme kinetic and structural data is a first approach to describe mechanistic principles that govern the effects of glycosylation on the function of proteases. The understanding of glycan functions becomes highly significant in proteomic and glycomic studies, which demonstrated that cancer-associated proteases, such as kallikrein-related peptidase 3, exhibit strongly altered glycosylation patterns in pathological cases. Such findings can contribute to a variety of future biomedical applications.

A Single Glycan at the 99-Loop of Human Kallikrein-related Peptidase 2 Regulates Activation and Enzymatic Activity

Human kallikrein-related peptidase 2 (KLK2) is a key serine protease in semen liquefaction and prostate cancer together with KLK3/prostate-specific antigen. In order to decipher the function of its potential N-glycosylation site, we produced pro-KLK2 in Leishmania tarentolae cells and compared it with its non-glycosylated counterpart from Escherichia coli expression. Mass spectrometry revealed that Asn-95 carries a core glycan, consisting of two GlcNAc and three hexoses. Autocatalytic activation was retarded in glyco-pro-KLK2, whereas the activated glyco-form exhibited an increased proteolytic resistance. The specificity patterns obtained by the PICS (proteomic identification of protease cleavage sites) method are similar for both KLK2 variants, with a major preference for P1-Arg. However, glycosylation changes the enzymatic activity of KLK2 in a drastically substrate-dependent manner. Although glyco-KLK2 has a considerably lower catalytic efficiency than glycan-free KLK2 toward peptidic substrates with P2-Phe, the situation was reverted toward protein substrates, such as glyco-pro-KLK2 itself. These findings can be rationalized by the glycan-carrying 99-loop that prefers to cover the active site like a lid. By contrast, the non-glycosylated 99-loop seems to favor a wide open conformation, which mostly increases the apparent affinity for the substrates (i.e. by a reduction of K_m). Also, the cleavage pattern and kinetics in autolytic inactivation of both KLK2 variants can be explained by a shift of the target sites due to the presence of the glycan. These striking effects of glycosylation pave the way to a deeper understanding of kallikrein-related peptidase biology and pathology.

Design of Specific Serine Protease Inhibitors Based on a Versatile Peptide Scaffold: Conversion of a Urokinase Inhibitor to a Plasma Kallikrein Inhibitor

All serine proteases hydrolyze peptide bonds by the same basic mechanism and have very similar active sites, in spite of the fact that individual proteases have different physiological functions. We here report a strategy for designing high-affinity and high-specificity serine protease inhibitors using a versatile peptide scaffold, a 10-mer peptide, mupain-1 (CPAYSRYLDC). Mupain-1 was previously reported as a specific inhibitor of murine urokinase-type plasminogen activator (Ki = 0.55 μM) without measurable affinity to plasma kallikrein (Ki > 1000 μM). On the basis of a structure-based rational design, we substituted five residues of mupain-1 and converted it to a potent plasma kallikrein inhibitor (Ki = 0.014 μM). X-ray crystal structure analysis showed that the new peptide was able to adapt a new set of enzyme surface interactions by a slightly changed backbone conformation. Thus, with an appropriate re-engineering, mupain-1 can be redesigned to specific inhibitors of other serine proteases.

Site-specific N-glycosylation analysis of human factor XI: Identification of a noncanonical NXC glycosite

Human factor XI (hFXI) is a 160-kDa disulphide-linked homodimer zymogen involved in the coagulation cascade. Its deficiency results in bleeding diathesis referred to as hemophilia C. hFXI bears five N-glycosylation consensus sites per monomer, N72 , N108 , N335 on the heavy chain and N432 , N473 on the light chain. This study reports the first in-depth glycosylation analysis of hFXI based on advanced MS approaches. Hydrophilic interaction LC and MS characterization and quantification of the N-glycans showed that the two major forms are complex biantennary mono-α2,6-sialylated (A2 S1 , 20%) and bis-α2,6-sialylated structures (A2 S2 , 66%). Minor triantennary structures (A3 S3 F, ∼1.5%; A3 S3 , ∼2%) were also identified. MS analyses of intact hFXI revealed full occupation of two of the three heavy-chain glycosites and almost full-site occupancy of the light chain. Analysis of hFXI glycopeptides by LC-MS/MS enabled site-specific glycan profiling and occupancy. It was evidenced that N335 was not glycosylated and that N72 and N108 were fully occupied, whereas N432 and N473 were occupied at about 92 and 95%, respectively. We also identified a new glycosite of the noncanonical format NXC at N145 , occupied at around 5%. These data provide valuable structural information useful to understand the potential roles of N-glycosylation on hFXI function and could serve as a structural reference.

Structure of BbKI, a disulfide-free plasma kallikrein inhibitor

A serine protease inhibitor from Bauhinia bauhinioides (BbKI) belongs to the Kunitz family of plant inhibitors, which are common in plant seeds. BbKI does not contain any disulfides, unlike most other members of this family. It is a potent inhibitor of plasma kallikrein, in addition to other serine proteases, and thus exhibits antithrombotic activity. A high-resolution crystal structure of recombinantly expressed BbKI was determined (at 1.4 Å resolution) and was compared with the structures of other members of the family. Modeling of a complex of BbKI with plasma kallikrein indicates that changes in the local structure of the reactive loop that includes the specificity-determining Arg64 are necessary in order to explain the tight binding. An R64A mutant of BbKI was found to be a weaker inhibitor of plasma kallikrein, but was much more potent against plasmin, suggesting that this mutant may be useful for preventing the breakup of fibrin and maintaining clot stability, thus preventing excessive bleeding.

The Role of the Lys628 (192) Residue of the Complement Protease, C1s, in Interacting with Peptide and Protein Substrates

The C1s protease of the classical complement pathway propagates the initial activation of this pathway of the system by cleaving and thereby activating the C4 and C2 complement components. This facilitates the formation of the classical pathway C3 convertase (C4bC2a). C1s has a Lys residue located at position 628 (192 in chymotrypsin numbering) of the SP domain that has the potential to partially occlude the S2–S2′ positions of the active site. The 192 residue of serine proteases generally plays an important role in interactions with substrates. We therefore investigated the role of Lys628 (192) in interactions with C4 by altering the Lys residue to either a Gln (found in many other serine proteases) or an Ala residue. The mutant enzymes had altered specificity profiles for a combinatorial peptide substrate library, suggesting that this residue does influence the active site specificity of the protease. Generally, the K628Q mutant had greater activity than wild type enzyme against peptide substrates, while the K628A residue had lowered activity, although this was not always the case. Against peptide substrates containing physiological substrate sequences, the K628Q mutant once again had generally higher activity, but the activity of the wild type and mutant enzymes against a C4 P4–P4′ substrate were similar. Interestingly, alteration of the K628 residue in C1s had a marked effect on the cleavage of C4, reducing cleavage efficiency for both mutants about fivefold. This indicates that this residue plays a different role in cleaving protein versus peptide substrates and that the Lys residue found in the wild type enzyme plays an important role in interacting with the C4 substrate. Understanding the basis of the interaction between C1s and its physiological substrates is likely to lead to insights that can be used to design efficient inhibitors of the enzyme for use in treating diseases caused by inflammation as result of over-activity of the classical complement pathway.

Characterization of the prostate-specific antigen (PSA) catalytic mechanism: a pre-steady-state and steady-state study

Prostate-specific antigen (PSA), an enzyme of 30 kDa grouped in the kallikrein family is synthesized to high levels by normal and malignant prostate epithelial cells. Therefore, it is the main biomarker currently used for early diagnosis of prostate cancer. Here, presteady-state and steady-state kinetics of the PSA-catalyzed hydrolysis of the fluorogenic substrate Mu-His-Ser-Ser-Lys-Leu-Gln-AMC (spanning from pH 6.5 to pH 9.0, at 37.0°C) are reported. Steady-state kinetics display at every pH value a peculiar feature, represented by an initial "burst" phase of the fluorescence signal before steady-state conditions are taking place. This behavior, which has been already observed in other members of the kallikrein family, suggests the occurrence of a proteolytic mechanism wherefore the acylation step is faster than the deacylation process. This feature allows to detect the acyl intermediate, where the newly formed C-terminal carboxylic acid of the cleaved substrate forms an ester bond with the -OH group of the Ser195 catalytic residue, whereas the AMC product has been already released. Therefore, the pH-dependence of the two enzymatic steps (i.e., acylation and deacylation) has been separately characterized, allowing the determination of pKa values. On this basis, possible residues are tentatively identified in PSA, which might regulate these two steps by interacting with the two portions of the substrate.

Inhibition of plasma kallikrein by a highly specific active site blocking antibody

Plasma kallikrein (pKal) proteolytically cleaves high molecular weight kininogen to generate the potent vasodilator and the pro-inflammatory peptide, bradykinin. pKal activity is tightly regulated in healthy individuals by the serpin C1-inhibitor, but individuals with hereditary angioedema (HAE) are deficient in C1-inhibitor and consequently exhibit excessive bradykinin generation that in turn causes debilitating and potentially fatal swelling attacks. To develop a potential therapeutic agent for HAE and other pKal-mediated disorders, we used phage display to discover a fully human IgG1 monoclonal antibody (DX-2930) against pKal. In vitro experiments demonstrated that DX-2930 potently inhibits active pKal (K_i = 0.120 ± 0.005 nm) but does not target either the zymogen (prekallikrein) or any other serine protease tested. These findings are supported by a 2.1-Å resolution crystal structure of pKal complexed to a DX-2930 Fab construct, which establishes that the pKal active site is fully occluded by the antibody. DX-2930 injected subcutaneously into cynomolgus monkeys exhibited a long half-life (t_½ ∼12.5 days) and blocked high molecular weight kininogen proteolysis in activated plasma in a dose- and time-dependent manner. Furthermore, subcutaneous DX-2930 reduced carrageenan-induced paw edema in rats. A potent and long acting inhibitor of pKal activity could be an effective treatment option for pKal-mediated diseases, such as HAE.

Elapid snake venom analyses show the specificity of the peptide composition at the level of genera Naja and Notechis

Elapid snake venom is a highly valuable, but till now mainly unexplored, source of pharmacologically important peptides. We analyzed the peptide fractions with molecular masses up to 10 kDa of two elapid snake venoms—that of the African cobra, N. m. mossambica (genus Naja), and the Peninsula tiger snake, N. scutatus, from Kangaroo Island (genus Notechis). A combination of chromatographic methods was used to isolate the peptides, which were characterized by combining complimentary mass spectrometric techniques. Comparative analysis of the peptide compositions of two venoms showed specificity at the genus level. Three-finger (3-F) cytotoxins, bradykinin-potentiating peptides (BPPs) and a bradykinin inhibitor were isolated from the Naja venom. 3-F neurotoxins, Kunitz/basic pancreatic trypsin inhibitor (BPTI)-type inhibitors and a natriuretic peptide were identified in the N. venom. The inhibiting activity of the peptides was confirmed in vitro with a selected array of proteases. Cytotoxin 1 (P01467) from the Naja venom might be involved in the disturbance of cellular processes by inhibiting the cell 20S-proteasome. A high degree of similarity between BPPs from elapid and viperid snake venoms was observed, suggesting that these molecules play a key role in snake venoms and also indicating that these peptides were recruited into the snake venom prior to the evolutionary divergence of the snakes.

Structure of plasma and tissue kallikreins

The kallikrein kinin system (KKS) consists of serine proteases involved in the production of peptides called kinins, principally bradykinin and Lys-bradykinin (kallidin). The KKS contributes to a variety of physiological processes including inflammation, blood pressure control and coagulation. Here we review the protein structural data available for these serine proteases and examine the molecular mechanisms of zymogen activation and substrate recognition focusing on plasma kallikrein (PK) and tissue kallikrein (KLK1) cleavage of kininogens. PK circulates as a zymogen bound to high-molecular-weight kininogen (HK). PK is activated by coagulation factor XIIa and then cleaves HK to generate bradykinin and factor XII to generate further XIIa.A structure has been described for the activated PK protease domain in complex with the inhibitor benzamidine. Kallikrein-related peptidases (KLKs) have a distinct domain structure and exist as a family of 15 genes which are differentially expressed in many tissues and the central nervous system.They cleave a wide variety of substrates including low-molecular-weight kininogen (LK) and matrix proteins. Crystal structures are available for KLK1, 3, 4, 5, 6 and 7 activated protease domains typically in complex with S1 pocket inhibitors. A substrate mimetic complex is described for KLK3 which provides insight into substrate recognition. A zymogen crystal structure determined for KLK6 reveals a closed S1 pocket and a novel mechanism of zymogen activation. Overall these structures have proved highly informative in understanding the molecular mechanisms of the KKS and provide templates to design inhibitors for treatment of a variety of diseases.

The structure of human microplasmin in complex with textilinin-1, an aprotinin-like inhibitor from the Australian brown snake

Textilinin-1 is a Kunitz-type serine protease inhibitor from Australian brown snake venom. Its ability to potently and specifically inhibit human plasmin (K_i = 0.44 nM) makes it a potential therapeutic drug as a systemic anti-bleeding agent. The crystal structures of the human microplasmin-textilinin-1 and the trypsin-textilinin-1 complexes have been determined to 2.78 Å and 1.64 Å resolution respectively, and show that textilinin-1 binds to trypsin in a canonical mode but to microplasmin in an atypical mode with the catalytic histidine of microplasmin rotated out of the active site. The space vacated by the histidine side-chain in this complex is partially occupied by a water molecule. In the structure of microplasminogen the χ₁ dihedral angle of the side-chain of the catalytic histidine is rotated by 67° from its “active” position in the catalytic triad, as exemplified by its location when microplasmin is bound to streptokinase. However, when textilinin-1 binds to microplasmin the χ₁ dihedral angle of this amino acid residue changes by −157° (i.e. in the opposite rotation direction compared to microplasminogen). The unusual mode of interaction between textilinin-1 and plasmin explains textilinin-1′s selectivity for human plasmin over plasma kallikrein. This difference can be exploited in future drug design efforts.

Prekallikrein deficiency presenting as recurrent cerebrovascular accident: case report and review of the literature

We report the case of a woman with history of hypertension and hyperlipidemia presenting with recurrent episodes consistent clinically with cerebrovascular accidents (CVA), and MRI changes suggestive of ischemia versus vasculitis as their cause. No anatomical neurological, rheumatic, cardioembolic, or arteriosclerotic etiologies could be determined by extensive workup. Incidentally, the patient was found to have prolonged activated Partial Thromboplastin Time (aPTT) and a normal Prothrombin Time (PT); further testing revealed a prekallikrein deficiency. Since no other cause for the CVAs was established, and other prothrombotic states were ruled out, it is proposed that they are clinical manifestations derived from the prekallikrein deficiency, which in a patient with known cardiovascular risk factors could lead to thrombotic complications such as stroke.

Optimized production of scygonadin in Pichia pastoris and analysis of its antimicrobial and antiviral activities

The crab antimicrobial peptide scygonadin is confirmed to have antimicrobial activity against bacteria and it is probably associated with the reproductive immunity in Scylla paramamosain. To obtain large quantity of scygonadin for further biological assays, a 306 bp cDNA sequence encoding the mature peptide of scygonadin was cloned into a secretion vector of pPIC9K, and a high-level of the recombinant scygonadin was achieved in Pichia pastoris. The optimal expression condition was determined as incubation with 0.5% methanol for 48 h at 28 °C under pH 6.0, and a total of 70 mg scygonadin was expressed in 1L culture medium. The recombinant product was purified and 97% pure scygonadin was obtained using immobilized metal affinity chromatography with a yield of 46 mg/L. The recombinant scygonadin was confirmed using SDS-PAGE analysis and MS-fingerprinting. P. pastoris-derived scygonadin exhibited relatively higher antimicrobial activities against bacteria than Escherichia coli-derived scygonadin. The antimicrobial activity of the recombinant scygonadin against pathogenic Aeromonas hydrophila showed salt resistant and the killing kinetics of A. hydrophila was time dependent. Besides, the antiviral assay demonstrated that scygonadin could interfere with white spot syndrome virus (WSSV) replication in vitro-cultured crayfish haematopoietic (Hpt) cells. Taken together, this is the first report on the heterologous expression of scygonadin in P. pastoris, and P. pastoris is an effective expression system for producing large quantities of biological active scygonadin for both research and agricultural application.

Cleavage of kininogen and subsequent bradykinin release by the complement component: mannose-binding lectin-associated serine protease (MASP)-1

Bradykinin (BK), generated from high-molecular-weight kininogen (HK) is the major mediator of swelling attacks in hereditary angioedema (HAE), a disease associated with C1-inhibitor deficiency. Plasma kallikrein, activated by factor XIIa, is responsible for most of HK cleavage. However other proteases, which activate during episodes of angioedema, might also contribute to BK production. The lectin pathway of the complement system activates after infection and oxidative stress on endothelial cells generating active serine proteases: MASP-1 and MASP-2. Our aim was to study whether activated MASPs are able to digest HK to release BK. Initially we were trying to find potential new substrates of MASP-1 in human plasma by differential gel electrophoresis, and we identified kininogen cleavage products by this proteomic approach. As a control, MASP-2 was included in the study in addition to MASP-1 and kallikrein. The proteolytic cleavage of HK by MASPs was followed by SDS-PAGE, and BK release was detected by HPLC. We showed that MASP-1 was able to cleave HK resulting in BK production. MASP-2 could also cleave HK but could not release BK. The cleavage pattern of MASPs is similar but not strictly identical to that of kallikrein. The catalytic efficiency of HK cleavage by a recombinant version of MASP-1 and MASP-2 was about 4.0×10² and 2.7×10² M⁻¹s⁻¹, respectively. C1-inhibitor, the major inhibitor of factor XIIa and kallikrein, also prevented the cleavage of HK by MASPs. In all, a new factor XII- and kallikrein-independent mechanism of bradykinin production by MASP-1 was demonstrated, which may contribute to the pro-inflammatory effect of the lectin pathway of complement and to the elevated bradykinin levels in HAE patients.

Network approach for capturing ligand-induced subtle global changes in protein structures

Ligand-induced conformational changes in proteins are of immense functional relevance. It is a major challenge to elucidate the network of amino acids that are responsible for the percolation of ligand-induced conformational changes to distal regions in the protein from a global perspective. Functionally important subtle conformational changes (at the level of side-chain noncovalent interactions) upon ligand binding or as a result of environmental variations are also elusive in conventional studies such as those using root-mean-square deviations (r.m.s.d.s). In this article, the network representation of protein structures and their analyses provides an efficient tool to capture these variations (both drastic and subtle) in atomistic detail in a global milieu. A generalized graph theoretical metric, using network parameters such as cliques and/or communities, is used to determine similarities or differences between structures in a rigorous manner. The ligand-induced global rewiring in the protein structures is also quantified in terms of network parameters. Thus, a judicious use of graph theory in the context of protein structures can provide meaningful insights into global structural reorganizations upon perturbation and can also be helpful for rigorous structural comparison. Data sets for the present study include high-resolution crystal structures of serine proteases from the S1A family and are probed to quantify the ligand-induced subtle structural variations.

A network-based multi-target computational estimation scheme for anticoagulant activities of compounds

Background

Traditional virtual screening method pays more attention on predicted binding affinity between drug molecule and target related to a certain disease instead of phenotypic data of drug molecule against disease system, as is often less effective on discovery of the drug which is used to treat many types of complex diseases. Virtual screening against a complex disease by general network estimation has become feasible with the development of network biology and system biology. More effective methods of computational estimation for the whole efficacy of a compound in a complex disease system are needed, given the distinct weightiness of the different target in a biological process and the standpoint that partial inhibition of several targets can be more efficient than the complete inhibition of a single target.

Methodology

We developed a novel approach by integrating the affinity predictions from multi-target docking studies with biological network efficiency analysis to estimate the anticoagulant activities of compounds. From results of network efficiency calculation for human clotting cascade, factor Xa and thrombin were identified as the two most fragile enzymes, while the catalytic reaction mediated by complex IXa:VIIIa and the formation of the complex VIIIa:IXa were recognized as the two most fragile biological matter in the human clotting cascade system. Furthermore, the method which combined network efficiency with molecular docking scores was applied to estimate the anticoagulant activities of a serial of argatroban intermediates and eight natural products respectively. The better correlation (r = 0.671) between the experimental data and the decrease of the network deficiency suggests that the approach could be a promising computational systems biology tool to aid identification of anticoagulant activities of compounds in drug discovery.

Conclusions

This article proposes a network-based multi-target computational estimation method for anticoagulant activities of compounds by combining network efficiency analysis with scoring function from molecular docking.

Engineering kunitz domain 1 (KD1) of human tissue factor pathway inhibitor-2 to selectively inhibit fibrinolysis: properties of KD1-L17R variant

Tissue factor pathway inhibitor-2 (TFPI-2) inhibits factor XIa, plasma kallikrein, and factor VIIa/tissue factor; accordingly, it has been proposed for use as an anticoagulant. Full-length TFPI-2 or its isolated first Kunitz domain (KD1) also inhibits plasmin; therefore, it has been proposed for use as an antifibrinolytic agent. However, the anticoagulant properties of TFPI-2 or KD1 would diminish its antifibrinolytic function. In this study, structure-based investigations and analysis of the serine protease profiles revealed that coagulation enzymes prefer a hydrophobic residue at the P2' position in their substrates/inhibitors, whereas plasmin prefers a positively charged arginine residue at the corresponding position in its substrates/inhibitors. Based upon this observation, we changed the P2' residue Leu-17 in KD1 to Arg (KD1-L17R) and compared its inhibitory properties with wild-type KD1 (KD1-WT). Both WT and KD1-L17R were expressed in Escherichia coli, folded, and purified to homogeneity. N-terminal sequences and mass spectra confirmed proper expression of KD1-WT and KD1-L17R. Compared with KD1-WT, the KD1-L17R did not inhibit factor XIa, plasma kallikrein, or factor VIIa/tissue factor. Furthermore, KD1-L17R inhibited plasmin with ∼6-fold increased affinity and effectively prevented plasma clot fibrinolysis induced by tissue plasminogen activator. Similarly, in a mouse liver laceration bleeding model, KD1-L17R was ∼8-fold more effective than KD1-WT in preventing blood loss. Importantly, in this bleeding model, KD1-L17R was equally or more effective than aprotinin or tranexamic acid, which have been used as antifibrinolytic agents to prevent blood loss during major surgery/trauma. Furthermore, as compared with aprotinin, renal toxicity was not observed with KD1-L17R.

Analysis of an engineered plasma kallikrein inhibitor and its effect on contact activation

Engineering of protein-protein interactions is used to enhance the affinity or specificity of proteins, such as antibodies or protease inhibitors, for their targets. However, fully diversifying all residues in a protein-protein interface is often unfeasible. Therefore, we limited our phage library for the serine protease inhibitor ecotin by restricting it to only tetranomial diversity and then targeted all 20 amino acid residues involved in protein recognition. This resulted in a high-affinity and highly specific plasma kallikrein inhibitor, ecotin-Pkal. To validate this approach we dissected the energetic contributions of each wild type (wt) or mutated surface loop to the binding of either plasma kallikrein (PKal) or membrane-type serine protease 1. The analysis demonstrated that a mutation in one loop has opposing effects depending on the sequence of surrounding loops. This finding stresses the cooperative nature of loop-loop interactions and justifies targeting multiple loops with a limited diversity. In contrast to ecotin wt, the specific loop combination of ecotin-Pkal discriminates the subtle structural differences between the active enzymes, PKal and Factor XIIa, and their respective zymogen forms. We used ecotin-Pkal to specifically inhibit contact activation of human plasma at the level mediated by plasma kallikrein.

Human plasma kallikrein-kinin system: physiological and biochemical parameters

The plasma kallikrein-kinin system (KKS) plays a critical role in human physiology. The KKS encompasses coagulation factor XII (FXII), the complex of prekallikrein (PK) and high molecular weight kininogen (HK). The conversion of plasma prekallikrein to kallikrein by the activated FXII and in response to numerous different stimuli leads to the generation of bradykinin (BK) and activated HK (HKa, an antiangiogenic peptide). BK is a proinflammatory peptide, a pain mediator and potent vasodilator, leading to robust accumulation of fluid in the interstitium. Systemic production of BK, HKa with the interplay between BK bound-BK receptors and the soluble form of HKa are key to angiogenesis and hemodynamics. KKS has been implicated in the pathogenesis of inflammation, hypertension, endotoxemia, and coagulopathy. In all these cases increased BK levels is the hallmark. In some cases, the persistent production of BK due to the deficiency of the blood protein C1-inhibitor, which controls FXII, is detrimental to the survival of the patients with hereditary angioedema (HAE). In others, the inability of angiotensin converting enzyme (ACE) to degrade BK leads to elevated BK levels and edema in patients on ACE inhibitors. Thus, the mechanisms that interfere with BK liberation or degradation would lead to blood pressure dysfunction. In contrast, anti-kallikrein treatment could have adverse effects in hemodynamic changes induced by vasoconstrictor agents. Genetic models of kallikrein deficiency are needed to evaluate the quantitative role of kallikrein and to validate whether strategies designed to activate or inhibit kallikrein may be important for regulating whole-body BK sensitivity.

S1′ and S2′ subsite specificities of human plasma kallikrein and tissue kallikrein 1 for the hydrolysis of peptides derived from the bradykinin domain of human kininogen

The S1′ and S2′ subsite specificities of human tissue kallikrein 1 (KLK1) and human plasma kallikrein (HPK) were examined with the peptide series Abz-GFSPFRXSRIQ-EDDnp and Abz-GFSPFRSXRIQ-EDDnp [X=natural amino acids or S(PO3H2)]. KLK1 efficiently hydrolyzed most of the peptides except those containing negatively charged amino acids at P1′ and P2′ positions. Abz-GFSPFRSSRIQ-EDDnp, as in human kininogen, is the best substrate for KLK1 and exclusively cleaved the R-S bond. All other peptides were cleaved also at the F-R bond. The synthetic human kininogen segment Abz-MISLMKRPPGFSPFRS390S391RI-NH2was hydrolyzed by KLK1 first at R-S and then at M-K bonds, releasing Lys-bradykinin. In the S390and S391phosphorylated analogs, this order of hydrolysis was inverted due to the higher resistance of the R-S bond. Abz-MISLMKRPPG-FSPFRSS(PO3H2)391RI-NH2was hydrolyzed by KLK1 at M-K and mainly at the F-R bond, releasing des-(Arg9)-Lys-Bk which is a B1 receptor agonist. HPK cleaved all the peptides at R and showed restricted specificity for S in the S1′ subsite, with lower specificity for the S2′ subsite. Abz-MISLMKRPPGFSPFRSSRI-NH2was efficiently hydrolyzed by HPK under bradykinin release, while the analogs containing S(PO3H2) were poorly hydrolyzed. In conclusion, S1′ and S2′ subsite specificities of KLK1 and HPK showed peculiarities that were observed with substrates containing the amino acid sequence of human kininogen.

Expression of tissue and plasma kallikreins and kinin B1 and B2 receptors in lung cancer

Tissue kallikrein (hK1) and plasma kallikrein (PK, hKB1) are serine proteases that produce biologically active kinin peptides from endogenous kininogen substrates. There is evidence linking the kallikreins and the mitogenic kinin peptides to carcinogenesis. The aim of this study was to investigate the expression of tissue prokallikrein (pro-hK1), plasma prekallikrein (PPK, pre-hKB1) and kinin B1 and B2 receptor proteins in different subtypes of lung cancer. Immunohistochemistry, using specific antibodies, was performed on archived normal lung sections and sections from adenocarcinomas, squamous cell carcinomas, large cell carcinomas, small cell carcinomas and carcinoid tumours of the lung. Immunoperoxidase labelling was visualised by brightfield microscopy and immunofluorescence labelling by confocal microscopy. Extensive cytoplasmic expression of pro-hK1 and PPK was observed, which was similar in small cell and non-small cell tumours. However, nuclear labelling for the kallikreins was absent or limited. The kinin B1 and B2 receptors were highly expressed in the cytoplasm of all tumour types and in the nuclei of non-small cell tumours. Further studies are required to assess the functional significance of the expression of hK1, PK and kinin receptors in lung tumours, and whether any of these proteins may be potential biomarkers for specific subtypes of lung carcinoma.

Structure of human prostasin, a target for the regulation of hypertension

Prostasin (also called channel activating protease-1 (CAP1)) is an extracellular serine protease implicated in the modulation of fluid and electrolyte regulation via proteolysis of the epithelial sodium channel. Several disease states, particularly hypertension, can be affected by modulation of epithelial sodium channel activity. Thus, understanding the biochemical function of prostasin and developing specific agents to inhibit its activity could have a significant impact on a widespread disease. We report the expression of the prostasin proenzyme in Escherichia coli as insoluble inclusion bodies, refolding and activating via proteolytic removal of the N-terminal propeptide. The refolded and activated enzyme was shown to be pure and monomeric, with kinetic characteristics very similar to prostasin expressed from eukaryotic systems. Active prostasin was crystallized, and the structure was determined to 1.45 A resolution. These apoprotein crystals were soaked with nafamostat, allowing the structure of the inhibited acyl-enzyme intermediate structure to be determined to 2.0 A resolution. Comparison of the inhibited and apoprotein forms of prostasin suggest a mechanism of regulation through stabilization of a loop which interferes with substrate recognition.

Novel expression of kallikreins, kallikrein-related peptidases and kinin receptors in human pleural mesothelioma

Malignant mesothelioma is an aggressive cancer of the pleura that is causally related to exposure to asbestos fibres. The kallikrein serine proteases [tissue (hK1) and plasma (hKB1) kallikreins, and kallikrein-related peptidases (KRP/hK2-15)] and the mitogenic kinin peptides may have a role in tumourigenesis. However, it is not known whether hK1, hKB1, KRP/hK proteins or kinin receptors are expressed in pleural mesotheliomas. The expression of hK1, hKB1, KRP/hK2, 5, 6, 7, 8 and 9, and kinin B(1) and B(2) receptors was assessed in archived selected normal tissue and mesothelioma tumour sections by immunoperoxidase and immunofluorescence labelling. hK1, hKB1 and kinin B(1) and B(2) receptors were expressed in malignant cells of the epithelioid and sarcomatoid components of biphasic mesothelioma tumour cells. The percentage of cells with cytoplasmic and nuclear labelling and the intensity of labelling were similar for hK1, hKB1 and the kinin receptors. KRP/hK2, 6, 8 and 9 were also expressed in the cytoplasm and nuclei of mesothelioma cells, whereas KRP/hK5 and hK7 showed predominantly cytoplasmic localisation. This is a first report, but further studies are required to determine whether these proteins have a functional role in the pathogenesis of mesothelioma and/or may be potential biomarkers for pleural mesothelioma.