Aspartic Protease Inhibitors Research Articles

Potent Inhibitors of Malarial Aspartic Proteases, the Plasmepsins, by Hydroformylation of Substituted 7‐Azanorbornenes

The increasing prevalence of multidrug-resistant strains of the malarial parasite Plasmodium falciparum requires the urgent development of new therapeutic agents with novel modes of action. The vacuolar malarial aspartic proteases plasmepsin (PM) I, II, and IV are involved in hemoglobin degradation and play a central role in the growth and maturation of the parasite in the human host. We report the structure-based design, synthesis, and in vitro evaluation of a new generation of PM inhibitors featuring a highly decorated 7-azabicyclo[2.2.1]heptane core. While this protonated central core addresses the catalytic Asp dyad, three substituents bind to the flap, the S1/S3, and the S1' pockets of the enzymes. A hydroformylation reaction is the key synthetic step for the introduction of the new vector reaching into the S1' pocket. The configuration of the racemic ligands was confirmed by extensive NMR and X-ray crystallographic analysis. In vitro biological assays revealed high potency of the new inhibitors against the three plasmepsins (IC(50) values down to 6 nM) and good selectivity towards the closely related human cathepsins D and E. The occupancy of the S1' pocket makes an essential contribution to the gain in binding affinity and selectivity, which is particularly large in the case of the PM IV enzyme. Designing non-peptidic ligands for PM II is a valid route to generate compounds that inhibit the entire family of vacuolar plasmepsins.

Secreted Aspartic Protease Cleavage of Candida albicans Msb2 Activates Cek1 MAPK Signaling Affecting Biofilm Formation and Oropharyngeal Candidiasis

Perception of external stimuli and generation of an appropriate response are crucial for host colonization by pathogens. In pathogenic fungi, mitogen activated protein kinase (MAPK) pathways regulate dimorphism, biofilm/mat formation, and virulence. Signaling mucins, characterized by a heavily glycosylated extracellular domain, a transmembrane domain, and a small cytoplasmic domain, are known to regulate various signaling pathways. In Candida albicans, the mucin Msb2 regulates the Cek1 MAPK pathway. We show here that Msb2 is localized to the yeast cell wall and is further enriched on hyphal surfaces. A msb2Δ/Δ strain formed normal hyphae but had biofilm defects. Cek1 (but not Mkc1) phosphorylation was absent in the msb2Δ/Δ mutant. The extracellular domain of Msb2 was shed in cells exposed to elevated temperature and carbon source limitation, concomitant with germination and Cek1 phosphorylation. Msb2 shedding occurred differentially in cells grown planktonically or on solid surfaces in the presence of cell wall and osmotic stressors. We further show that Msb2 shedding and Cek1 phosphorylation were inhibited by addition of Pepstatin A (PA), a selective inhibitor of aspartic proteases (Saps). Analysis of combinations of Sap protease mutants identified a sap8Δ/Δ mutant with reduced MAPK signaling along with defects in biofilm formation, thereby suggesting that Sap8 potentially serves as a major regulator of Msb2 processing. We further show that loss of either Msb2 (msb2Δ/Δ) or Sap8 (sap8Δ/Δ) resulted in higher C. albicans surface β-glucan exposure and msb2Δ/Δ showed attenuated virulence in a murine model of oral candidiasis. Thus, Sap-mediated proteolytic cleavage of Msb2 is required for activation of the Cek1 MAPK pathway in response to environmental cues including those that induce germination. Inhibition of Msb2 processing at the level of Saps may provide a means of attenuating MAPK signaling and reducing C. albicans virulence.

Bicyclic peptidomimetics targeting secreted aspartic protease 2 (SAP2) from Candida albicans reveal a constrained inhibitory chemotype

The in vitro screening of stereoisomeric bicyclic peptidomimetics towards SAP2 of Candida albicans revealed a constrained chemotype as aspartic protease inhibitor in the micromolar to nanomolar range. The results indicated that the acetal bridge may serve as a transition-state isostere, and that the right match between interactions with subsites and the orientation by hydrogen bonding with Gly85 is the main requisite for inhibitory activity. Molecular docking calculations suggested the bicyclic acetal scaffold to be capable of interacting with the two catalytic aspartic acids, thus resulting in good inhibitory activity with only two hydrophobic groups addressing the enzyme catalytic subsites.

Inhibition of XMRV and HIV-1 proteases by pepstatin A and acetyl-pepstatin.

The kinetic properties of two classical inhibitors of aspartic proteases (PRs), pepstatin A and acetyl-pepstatin, were compared in their interactions with HIV-1 and xenotropic murine leukemia virus related virus (XMRV) PRs. Both compounds are substantially weaker inhibitors of XMRV PR than of HIV-1 PR. Previous kinetic and structural studies characterized HIV-1 PR-acetyl-pepstatin and XMRV PR-pepstatin A complexes and suggested dramatically different binding modes. Interaction energies were calculated for the possible binding modes and suggested a strong preference for the one-inhibitor binding mode for HIV-1 PR-acetyl-pepstatin and the two-inhibitor binding mode for XMRV PR-pepstatin A interactions. Comparison of the molecular models suggested that in the case of XMRV PR the relatively unfavorable interactions at S3' and the favorable interactions at S4 and S4' sites with the statine residues may shift the ground state binding towards the two-inhibitor binding mode, whereas the single molecule ground state binding of statines to the HIV-1 PR appear to be more favorable. The preferred single molecular binding to HIV-1 PR allows the formation of the transition state complex, represented by substantially better binding constants. Intriguingly, the crystal structure of the complex of acetyl-pepstatin with XMRV PR has shown a mixed type of binding: the unusual binding mode of two molecules of the inhibitor to the enzyme, in a mode very similar to the previously determined complex with pepstatin A, together with the classical binding mode found for HIV-1 PR. The structure is thus in good agreement with the very similar interaction energies calculated for the two types of binding.

Inhibition of xyloglucanase from an alkalothermophilic Thermomonospora sp. by a peptidic aspartic protease inhibitor from Penicillium sp. VM24

A bifunctional inhibitor from Penicillium sp VM24 causing inactivation of xyloglucanase from Thermomonospora sp and an aspartic protease from Aspergillus saitoi was identified. Steady state kinetics studies of xyloglucanase and the inhibitor revealed an irreversible, non-competitive, two-step inhibition mechanism with IC50 and Ki values of 780 and 500nM respectively. The interaction of o-phthalaldehyde (OPTA)-labeled xyloglucanase with the inhibitor revealed that the inhibitor binds to the active site of the enzyme. Far- and near-UV spectrophotometric analysis suggests that the conformational changes induced in xyloglucanase by the inhibitor may be due to irreversible denaturation of enzyme. The bifunctional inhibitor may have potential as a biocontrol agent for the protection of plants against phytopathogenic fungi.

A contaminant trypsin-like activity from the timothy grass pollen is responsible for the conflicting enzymatic behavior of the major allergen Phl p 1

We intend to solve whether or not Phl p 1 can be regarded as a protease. A group reported that Phl p 1 has papain-like properties and later on, that this allergen resembles cathepsin B, while another one demonstrated that Phl p 1 lacks proteinase activity and suggested that the measured activity may rise either from a recombinant Phl p 1 contaminant or as a result of an incompletely purified natural allergen. A third group reported Phl p 1 to act by a non-proteolytic activity mechanism. We report the purification of the natural Phl p 1 by means of hydrophobic interaction, gel filtration and STI-Sepharose affinity chromatographies. The Phl p 1 purity was assessed by silver-stained SDS-PAGE and by ‘in-gel’ and ‘gel-free’ approaches associated to mass spectrometry analyses. The proteolytic activity was measured using Boc–Gln–Ala–Arg–AMC and Z-Phe–Arg–AMC as substrates. While amidolytic activity could be measured with Phl p 1 after rechromatography on gel filtration, it however completely disappeared after chromatography on STI-Sepharose. The contaminant activity co-eluting with Phl p 1 was not affected by cysteine proteases inhibitors and other thiol-blocking agents, by metalloproteases inhibitors and by aspartic proteases inhibitors. However, it was completely inhibited by low molecular weight and proteinaceous serine proteases inhibitors. TLCK, but not TPCK, inhibited the contaminant activity, showing a trypsin-like behavior. The pH and temperature optimum were 8.0 and 37°C, respectively. These data indicated that Phl p 1 is not a protease. The contaminant trypsin-like activity should be considered when Phl p 1 allergenicity is emphasized.

Total synthesis of grassystatin A, a probe for cathepsin E function

The linear depsipeptide grassystatin A, a valuable probe for the study of cathepsin E function, has been synthesized by a [4+6] strategy. It exhibited specific inhibitory activity against cathepsin E with an IC50 value of 0.8nM. Our studies indicated that inhibition of cathepsin E did not have an impact on ovalbumin antigen processing and peptide presentation, unique from studies of other aspartic protease inhibitors.

Design, Synthesis and Evaluation of Unique 2,4,5-triaryl Imidazole Derivatives as Novel Potent Aspartic Protease Inhibitors

The 2,4,5-triaryl imidazole derivatives (API) were designed, screened and characterized kinetically & thermodynamically against Pepsin and their activity was also tested on the in silico platform. The docking studies of API with Pepsin show that these are novel and unique inhibitors of Aspartic protease. Drug like properties of these compounds were validated in silico based on Lipinski's rule of Five by calculating ClogP, LogS, H-bond acceptors, H-Bond donors, rotational bonds, PSA, PB and BBB values. The Et/Ki and Et/Km values of API show that they follow the Michaelis-Menten kinetics. The binding of inhibitors with proteases was explained by using Van't Hoff plot and thermodynamic parameters viz. free energy (ΔG), Entropy (ΔS) and Enthalpy (ΔH). The Van't Hoff analysis showed that the value of Ki decreases with increase in temperature and the binding of the inhibitor are entropically driven. API act as new potent aspartic protease inhibitors with Ki, for Pepsin, ranges from 3.7 µM to 16.7 µM. Strong hydrophobic groups at C-4 & C-5 position in API favor binding of inhibitors with Pepsin. Experiments also showed that among C-2 aryl substituted imidazole, a 4-substitution on aryl ring is preferred and less polar substituent makes the molecule more active whereas polar substituents at 2-position on C-2 aryl ring makes the molecule less active. The docking studies of API with Pepsin further intensify and validate our results.

Cathepsin E induces itch-related response through the production of endothelin-1 in mice

This study investigated the pruritogenic potency of cathepsin E, an aspartic protease, and its mechanisms in mice. An intradermal injection of cathepsin E to the rostral back elicited scratching, an itch-associated response, of the injection site. This action was inhibited by the aspartic protease inhibitor pepstatin A, the endothelin ETA receptor antagonist BQ-123, and the opioid receptor antagonists naltrexone and naloxone, but not by the H1 histamine receptor antagonist terfenadine, the proteinase-activated receptor-2 antagonist FSLLRY-NH2, or mast cell deficiency. Pepstatin A inhibited scratching induced by intradermal injection of the mast-cell degranulator compound 48/80, but not by tryptase, a mast-cell mediator. An intradermal injection of cathepsin E increased endothelin-1 levels in the skin at the injection site. Preproendothelin-1 mRNA was present in primary cultures of keratinocytes, and immunohistochemistry using an antibody recognizing endothelin-1 and big-endothelin-1 revealed immunoreactivity in the epidermis, especially in the prickle and granular cell layers, but not in the basal cell layer. These results suggest that cathepsin E is an endogenous itch inducer, and that its action is mediated at least in part by the production of endothelin-1 in the epidermis.

Antimalarial Activity of the Anticancer Histone Deacetylase Inhibitor SB939

Histone deacetylase (HDAC) enzymes posttranslationally modify lysines on histone and nonhistone proteins and play crucial roles in epigenetic regulation and other important cellular processes. HDAC inhibitors (e.g., suberoylanilide hydroxamic acid [SAHA; also known as vorinostat]) are used clinically to treat some cancers and are under investigation for use against many other diseases. Development of new HDAC inhibitors for noncancer indications has the potential to be accelerated by piggybacking onto cancer studies, as several HDAC inhibitors have undergone or are undergoing clinical trials. One such compound, SB939, is a new orally active hydroxamate-based HDAC inhibitor with an improved pharmacokinetic profile compared to that of SAHA. In this study, the in vitro and in vivo antiplasmodial activities of SB939 were investigated. SB939 was found to be a potent inhibitor of the growth of Plasmodium falciparum asexual-stage parasites in vitro (50% inhibitory concentration [IC(50)], 100 to 200 nM), causing hyperacetylation of parasite histone and nonhistone proteins. In combination with the aspartic protease inhibitor lopinavir, SB939 displayed additive activity. SB939 also potently inhibited the in vitro growth of exoerythrocytic-stage Plasmodium parasites in liver cells (IC(50), ~150 nM), suggesting that inhibitor targeting to multiple malaria parasite life cycle stages may be possible. In an experimental in vivo murine model of cerebral malaria, orally administered SB939 significantly inhibited P. berghei ANKA parasite growth, preventing development of cerebral malaria-like symptoms. These results identify SB939 as a potent new antimalarial HDAC inhibitor and underscore the potential of investigating next-generation anticancer HDAC inhibitors as prospective new drug leads for treatment of malaria.

Slow-tight binding inhibition of pepsin by an aspartic protease inhibitor from Streptomyces sp. MBR04

The present article reports a low molecular weight aspartic protease inhibitor from a Streptomyces sp. MBR04 exhibiting a two-step inhibition mechanism against pepsin. The kinetic interactions revealed a reversible, competitive, slow-tight binding inhibition with an IC50 and Ki values of 4.5nM and 4nM respectively. The conformational changes induced upon inhibitor binding to pepsin was monitored by far and near UV analysis, demonstrated that the inhibitor binds to the active site and causes inactivation. Chemical modification of the inhibitor with WRK and TNBS abolished the antiproteolytic activity of the inhibitor.

A low-molecular-mass aspartic protease inhibitor from a novel Penicillium sp.: implications in combating fungal infections

A low-molecular-mass aspartic protease inhibitor was isolated from a novel Penicillium sp. The inhibitor was purified to homogeneity, as shown by reversed-phase HPLC and SDS-PAGE. The M(r) of the inhibitor was 1585 and the amino acid composition showed the presence of D, D, D, E, A, K, L, Y, H, I and W residues. The steady-state kinetic interactions of Aspergillus saitoi aspartic protease with the inhibitor revealed the reversible, competitive, time-dependent tight-binding nature of the inhibitor, with IC(50) and K(i) values of 1.8 and 0.85 µM, respectively. Fluorescence spectroscopy and circular dichroism analysis showed that inactivation of the enzyme was due to binding of the inhibitor to the active site. The inhibitor was found to inhibit mycelial growth and spore germination of Aspergillus fumigatus and Aspergillus niger in vitro with MIC values of 1.65 and 0.30 µg ml(-1), respectively. This study will potentially open the way towards the development of a tight-binding peptidic inhibitor against fungal aspartic proteases to combat human fungal infections.

Synthesis and molecular modelling studies of novel carbapeptide analogs for inhibition of HIV-1 protease

Novel glycopeptides containing amino acids such as valine and alanine were designed, synthesized and tested for inhibition of the wild type C-SA HIV-1 protease enzyme. The incorporation of dipeptide sequences Val-Ala/Ala-Val to the sugar B-amino acid at two side chain positions resulted in a series of nine novel compounds. Compounds 3a, 3b, 3c, 3d, 4a, 4b and 5 displayed significant activities against the HIV protease enzyme. The glycopeptides are orders of magnitude less toxic to human MT-4 cells than lopinavir. Computational results were in good agreement with the experimental HIV-PR activity. The sugar hydroxyl group at the C3 position interacts with the enzymatic Asp25/Asp25′ residues. The docked position of the inhibitor is preserved during MD simulations and at least five hydrogen bond forms between the inhibitor and the enzymatic pocket. The results provide a platform for the progress of more effective carbohydrate supported inhibitors of HIV-1 and other aspartic proteases.

Purification of a Novel Pepsin Inhibitor from Coriolus versicolor and its Biochemical Properties

A novel pepsin inhibitor was isolated from Coriolus versicolor. The purification was carried out by a 2-step ultrafiltration followed by DEAE-52 and Mono Q ion-exchange chromatography. SDS-PAGE and gel filtration chromatography analysis showed that the isolated inhibitor was a 22.3 kDa protein with a single subunit. Heat stability of this inhibitor was estimated and only 7% of its inhibitory activity lost after treatment at 98 °C. The inhibitor was more specific against pepsin than several other proteases. The dissociation constant (K(i)) and concentration required for 50% pepsin inhibition (IC50) were 5.84 × 10(-5) M and 26.26 μg/mL, respectively. Apparent decrease of α-helix and increase of random coil were observed in the circular dichroism spectra of pepsin when an equimolar amount of the inhibitor was added. The inhibition mechanism of this inhibitor differs from the reported aspartic protease inhibitors, according to the secondary structure and the kinetic studies of this inhibitor.

Cathepsin D is released after severe tissue trauma in vivo and is capable of generating C5a in vitro

In response to severe tissue trauma several danger sensing and signalling cascades are activated, including the complement and the apoptosis systems. In polytrauma patients, both the early activation of the complement cascade with an excessive generation of the potent anaphylatoxin C5a and the induction of apoptosis have been shown to modulate the post-traumatic immune response. However, little is known about a direct interaction between the complement and apoptosis systems after severe tissue trauma. Therefore the focus of the present study was to elucidate the interplay between the central complement component C5 and the pro-apoptotic aspartic protease cathepsin D.In vivo, the cathepsin D plasma concentration of multiple injured patients was markedly increased when compared to healthy volunteers. In vitro incubation of C5 with cathepsin D resulted in a concentration- and time-dependent generation of C5a, which was inhibited by the aspartate protease inhibitor pepstatin A. Immunoblotting and sequencing analysis indicated that the C5 cleavage product represents the native form of human C5a, also exhibiting chemotactic activity for human neutrophils.In conclusion, these data show for the first time that cathepsin D is increased in plasma early after severe tissue injury. Furthermore, the results provide in vitro evidence of cleavage of C5 by an aspartic protease with subsequent generation of functional C5a, which represents a new path of complement activation.

Multiple effects of pepstatin A on Trypanosoma cruzi epimastigote forms

Herein, we have aimed to explore the effects of pepstatin A, a powerful aspartic protease inhibitor, on Trypanosoma cruzi, the etiologic agent of Chagas' disease. Pepstatin A arrested the proliferation of epimastigotes of T. cruzi (clone Dm28c, TcI lineage), in both dose- and time-dependent manner. The IC(50) value was calculated to be 36.2 μM after 96 h of parasite-drug contact. The parasite treatment with pepstatin A resulted in significant morphological alterations, including parasites becoming round in shape, reduction (≈25%) of the parasite size, and parasites presenting parts or the whole flagellum detached from the cell body. Cell lysis was not observed, resulting in a trypanostatic effect. The treatment of different T. cruzi strains, belonging to distinct phylogenetic lineages, with pepstatin A at 36.2 μM resulted in growth inhibition as follows: 28% to Y (TcII), 45% to CL Brener (TcII), 45.4% to 4167 (Z3), and 26.4% to 3663 (Z3) strains. The hydrolysis of a cathepsin D fluorogenic substrate (7-methoxycoumarin-4-acetyl-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(DNP)-D: -Arg-amide) by T. cruzi epimastigote extract was inhibited (≈65%) by pepstatin A at 10 μM, suggesting that an aspartic protease may be the intracellular target of this inhibitor. Curiously, pepstatin A induced an increase of 54% and 98%, respectively, in the surface expression of gp63- and calpain-related molecules in epimastigotes, but not in the cruzipain level, as well as stimulated the epimastigote-to-trypomastigote differentiation in a dose-dependent manner. However, approximately 45% of the trypomastigotes had their flagellum detached from the cell body. These results contribute to understand the possible role of aspartic proteases in the physiology of T. cruzi cells, adding new in vitro insights into the possibility of exploiting aspartic protease as promising targets to treat Chagas' disease.

Predicting functional residues of the Solanum lycopersicum aspartic protease inhibitor (SLAPI) by combining sequence and structural analysis with molecular docking

The Solanum lycopersicum aspartic protease inhibitor (SLAPI), which belongs to the STI-Kunitz family, is an effective inhibitor of the aspartic proteases human cathepsin D and Saccharomyces proteinase A. However, in contrast with the large number of studies on the inhibition mechanism of the serine proteases by the STI-Kunitz inhibitors, the structural aspects of the inhibition mechanism of aspartic proteases from this family of inhibitors are poorly understood. In the present study, we have combined sequence and structural analysis methods with protein-protein docking to gain a better understanding of the SLAPI inhibition mechanism of the proteinase A. The results suggest that: i) SLAPI loop L9 may be involved in the inhibitor interaction with the proteinase A´s active site, and ii) the residues I144, V148, L149, P151, F152 and R154 are implicated in the difference in the potency shown previously by SLAPI and another STI-Kunitz inhibitor isolated from Solanum tuberosum to inhibit proteinase A. These results will be useful in the design of site directed mutagenesis experiments to understand more thoroughly the aspartic protease inhibition mechanism of SLAPI and other related STI-Kunitz inhibitors.

Characterisation of Plasmodium falciparum aspartic protease inhibition by piperidine derivatives

Piperidine derivatives are reported to exhibit a variety of pharmacological activities. In this article, synthesis and aspartic protease inhibitory activity of three nitrophenacyl derivatives of N-methyl-4-hydroxy piperidine are reported. Enzyme assays showed that the attachment of a nitro group in the benzene ring plays an important role in the inhibition of plasmepsin-II of Plasmodium falciparum. The compound 1-methyl-1-(4’-nitrophenacyl)-4-hydroxypiperidinium bromide (3), consisting of a nitro group at the para position, was the most active at the concentration of 1.0 µM. The activity of the compounds was evaluated through the observed orientation and diagrammatic representation of nitrophenacyl derivatives of 4-hydroxy piperidine.

Shewasin A, an active pepsin homolog from the bacterium Shewanella amazonensis

The view has been widely held that pepsin-like aspartic proteinases are found only in eukaryotes, and not in bacteria. However, a recent bioinformatics search [Rawlings ND & Bateman A (2009) BMC Genomics10, 437] revealed that, in seven of ∼ 1000 completely sequenced bacterial genomes, genes were present encoding polypeptides that displayed the requisite hallmark sequence motifs of pepsin-like aspartic proteinases. The implications of this theoretical observation prompted us to generate biochemical data to validate this finding experimentally. The aspartic proteinase gene from one of the seven identified bacterial species, Shewanella amazonensis, was expressed in Escherichia coli. The recombinant protein, termed shewasin A, was produced in soluble form, purified to homogeneity, and shown to display properties remarkably similar to those of pepsin-like aspartic proteinases. Shewasin A was maximally active at acidic pH values, cleaving a substrate that has been widely used for assessment of the proteolytic activity of other aspartic proteinases, and displayed a clear preference for cleaving peptide bonds between hydrophobic residues in the P1*P1' positions of the substrate. It was completely inhibited by the general inhibitor of aspartic proteinases, pepstatin, and mutation of one of the catalytic Asp residues (in the Asp-Thr-Gly motif of the N-terminal domain) resulted in complete loss of enzymatic activity. It can thus be concluded unequivocally that this Shewanella gene encodes an active pepsin-like aspartic proteinase. It is now beyond doubt that pepsin-like aspartic proteinases are not confined to eukaryotes, but are encoded within some species of bacteria. The distinctions between the bacterial and eukaryotic polypeptides are discussed and their evolutionary relationships are outlined.

Purification and characterization of a novel aspartic protease from basidiomycetous yeast Cryptococcus sp. S-2

An aspartic protease (Cap1) was purified from basidiomycetous yeast Cryptococcus sp. S-2 (FERM ABP-10961) using HiTrap DEAE FF column and HiTrap Q HP column chromatography with azocasein as a substrate. Cap1 has a molecular mass of 34 kDa on SDS-PAGE. It was stable up to 50°C with maximum activity at 30°C. Maximum proteolytic activity was observed at pH 5.0. Cap1 was stable in the pH range 3.0-7.0. Its enzyme activity was strongly inhibited by pepstatin A, an inhibitor of aspartic proteases, indicating that Cap1 is an aspartic protease. Cap1 hydrolyzed protein substrates, including BSA, hemoglobin, α-casein, β-casein, and κ-casein. It showed activity on synthetic substrates, such as MOCAc-Gly-Lys-Pro-Ile-Leu-Phe-Phe-Arg-Leu-Lys(Dnp)-D-Arg-NH₂ and MOCAc-Ala-Pro-Ala-Lys-Phe-Phe-Arg-Leu-Lys(Dnp)-NH₂. Hydrolysis of the oxidized insulin B chain followed by amino acid sequencing analysis of the cleavage products revealed that 9 of its 30 peptide bonds were hydrolyzed by Cap1. This result was similar to that observed with pig pepsin A and human pepsin A. Cap1 also exhibited milk-clotting activity. We cloned the cDNA of CAP1 gene, which contained a 1254 bp open reading frame encoding a protein of 417 amino acid residues. Homology search in the NCBI database revealed that the amino acid sequence of Cap1 showed less than 39% identity to other known proteins. Therefore, we proposed that Cap1 is a novel aspartic protease.