Mechanism of activation of the gastric aspartic proteinases: pepsinogen, progastricsin and prochymosin

Mechanism of autocatalytic activation during proteasome assembly

Many large molecular machines are too elaborate to assemble spontaneously and are built through ordered pathways orchestrated by dedicated chaperones. During assembly of the core particle (CP) of the proteasome, where protein degradation occurs, its six active sites are simultaneously activated via cleavage of N-terminal propeptides. Such activation is autocatalytic and coupled to fusion of two half-CP intermediates, which protects cells by preventing activation until enclosure of the active sites within the CP interior. Here we uncover key mechanistic aspects of autocatalytic activation, which proceeds through alignment of the β5 and β2 catalytic triad residues, respectively, with these triads being misaligned before fusion. This mechanism contrasts with most other zymogens, in which catalytic centers are preformed. Our data also clarify the mechanism by which individual subunits can be added in a precise, temporally ordered manner. This work informs two decades-old mysteries in the proteasome field, with broader implications for protease biology and multisubunit complex assembly.

Pepsinogen C Interacts with IQGAP1 to Inhibit the Metastasis of Gastric Cancer Cells by Suppressing Rho-GTPase Pathway

AIM

This study systematically explored the biological effects and mechanisms of PGC on gastric cancer (GC) cells in vitro and in vivo.

METHOD

The critical biological roles of PGC in GC were assessed via EdU staining, Hoechst staining, flow cytometry, mouse models, CCK-8, wound healing, transwell, and sphere-forming assays. The interaction study with IQ-domain GTPase-activating protein 1 (IQGAP1) was used by Liquid chromatography-mass spectrometry co-immunoprecipitation, immunofluorescence staining, CHX-chase assay, MG132 assay, and qRT-PCR.

RESULTS

PGC inhibited the proliferation, viability, epithelial-mesenchymal transition, migration, invasion, and stemness of GC cells and promoted GC cell differentiation. PGC suppressed subcutaneous tumor growth and peritoneal dissemination in vivo. The interaction study found PGC inhibits GC cell migration and invasion by downregulating IQGAP1 protein and IQGAP1-mediated Rho-GTPase signaling suppression. In addition, PGC disrupts the stability of the IQGAP1 protein, promoting its degradation and significantly shortening its half-life. Moreover, the expression levels of PGC and IQGAP1 in GC tissues were significantly negatively correlated.

CONCLUSION

PGC may act as a tumor suppressor in the development and metastasis of GC. PGC can downregulate its interacting protein IQGAP1 and inhibit the Rho-GTPase pathway, thereby participating in the inhibition of GC cell migration and invasion.

Analysis of Some Biochemical Properties of Recombinant Siberian Roe Deer (Capreolus pygargus) Chymosin Obtained in the Mammalian Cell Culture (CHO-K1)

Structure of the chymosin gene of Siberian roe deer (Capreolus pygargus) was established for the first time and its exon/intron organization was determined. Coding part of the chymosin gene of C. pygargus was reconstructed by the Golden Gate method and obtained as a DNA clone. Comparative sequence analysis of the roe deer, cow, and one-humped camel prochymosins revealed a number of amino acid substitutions at the sites forming the substrate-binding cavity of the enzyme and affecting the S4 and S1' + S3' specificity subsites. Integration vector pIP1 was used to construct a plasmid pIP1-Cap in order to express recombinant roe deer prochymosin gene in CHO-K1 cells. CHO-K1-CYM-Cap pool cells were obtained, allowing synthesis and secretion of recombinant prochymosin into the culture fluid. As a result of zymogen activation, a recombinant roe deer chymosin was obtained and its total milk-clotting activity was estimated to be 468.4 ± 11.1 IMCU/ml. Yield of the recombinant roe deer chymosin was 500 mg/liter or ≈468,000 IMCU/liter, which exceeds the yields of genetically engineered chymosins in most of the expression systems used. Basic biochemical properties of the obtained enzyme were compared with the commercial preparations of recombinant chymosins from one-humped camel (Camelus dromedarius) and cow (Bos taurus). Specific milk-clotting activity of the recombinant chymosin of C. pygargus was 938 ± 22 IMCU/mg, which was comparable to that of the reference enzymes. Non-specific proteolytic activity of the recombinant roe deer chymosin was 1.4-4.5 times higher than that of the cow and camel enzymes. In terms of coagulation specificity, recombinant chymosin of C. pygargus occupied an intermediate position between the genetically engineered analogs of B. taurus and C. dromedarius chymosins. Thermostability threshold of the recombinant roe deer chymosin was 55°C. At 60°C, the enzyme retained <1% of its initial milk-clotting activity, and its complete thermal inactivation was observed at 65°C.

A multi-tasking stomach: functional coexistence of acid-peptic digestion and defensive body inflation in three distantly related vertebrate lineages

Puffer and porcupine fishes (families Diodontidae and Tetraodontidae, order Tetradontiformes) are known for their extraordinary ability to triple their body size by swallowing and retaining large amounts of seawater in their accommodating stomachs. This inflation mechanism provides a defence to predation; however, it is associated with the secondary loss of the stomach's digestive function. Ingestion of alkaline seawater during inflation would make acidification inefficient (a potential driver for the loss of gastric digestion), paralleled by the loss of acid-peptic genes. We tested the hypothesis of stomach inflation as a driver for the convergent evolution of stomach loss by investigating the gastric phenotype and genotype of four distantly related stomach inflating gnathostomes: sargassum fish, swellshark, bearded goby and the pygmy leatherjacket. Strikingly, unlike in the puffer/porcupine fishes, we found no evidence for the loss of stomach function in sargassum fish, swellshark and bearded goby. Only the pygmy leatherjacket (Monochanthidae, Tetraodontiformes) lacked the gastric phenotype and genotype. In conclusion, ingestion of seawater for inflation, associated with loss of gastric acid secretion, is restricted to the Tetraodontiformes and is not a selective pressure for gastric loss in other reported gastric inflating fishes.

Follow the Metaplasia: Characteristics and Oncogenic Implications of Metaplasia's Pattern of Spread Throughout the Stomach

The human stomach functions as both a digestive and innate immune organ. Its main product, acid, rapidly breaks down ingested products and equally serves as a highly effective microbial filter. The gastric epithelium has evolved mechanisms to appropriately handle the myriad of injurious substances, both exogenous and endogenous, to maintain the epithelial barrier and restore homeostasis. The most significant chronic insult that the stomach must face is Helicobacter pylori (Hp), a stomach-adapted bacterium that can colonize the stomach and induce chronic inflammatory and pre-neoplastic changes. The progression from chronic inflammation to dysplasia relies on the decades-long interplay between this oncobacterium and its gastric host. This review summarizes the functional and molecular regionalization of the stomach at homeostasis and details how chronic inflammation can lead to characteristic alterations in these developmental demarcations, both at the topographic and glandular levels. More importantly, this review illustrates our current understanding of the epithelial mechanisms that underlie the pre-malignant gastric landscape, how Hp adapts to and exploits these changes, and the clinical implications of identifying these changes in order to stratify patients at risk of developing gastric cancer, a leading cause of cancer-related deaths worldwide.

Dietary Diversification and Specialization in Neotropical Bats Facilitated by Early Molecular Evolution

Dietary adaptation is a major feature of phenotypic and ecological diversification, yet the genetic basis of dietary shifts is poorly understood. Among mammals, Neotropical leaf-nosed bats (family Phyllostomidae) show unmatched diversity in diet; from a putative insectivorous ancestor, phyllostomids have radiated to specialize on diverse food sources including blood, nectar, and fruit. To assess whether dietary diversification in this group was accompanied by molecular adaptations for changing metabolic demands, we sequenced 89 transcriptomes across 58 species and combined these with published data to compare ∼13,000 protein coding genes across 66 species. We tested for positive selection on focal lineages, including those inferred to have undergone dietary shifts. Unexpectedly, we found a broad signature of positive selection in the ancestral phyllostomid branch, spanning genes implicated in the metabolism of all major macronutrients, yet few positively selected genes at the inferred switch to plantivory. Branches corresponding to blood- and nectar-based diets showed selection in loci underpinning nitrogenous waste excretion and glycolysis, respectively. Intriguingly, patterns of selection in metabolism genes were mirrored by those in loci implicated in craniofacial remodeling, a trait previously linked to phyllostomid dietary specialization. Finally, we show that the null model of the widely-used branch-site test is likely to be misspecified, with the implication that the test is too conservative and probably under-reports true cases of positive selection. Our findings point to a complex picture of adaptive radiation, in which the evolution of new dietary specializations has been facilitated by early adaptations combined with the generation of new genetic variation.

Physiological and pathophysiological roles of acidic mammalian chitinase (CHIA) in multiple organs

Acidic mammalian chitinase (CHIA) belongs to the 18-glycosidase family and is expressed in epithelial cells and certain immune cells (such as neutrophils and macrophages) in various organs. Under physiological conditions, as a hydrolase, CHIA can degrade chitin-containing pathogens, participate in Type 2 helper T (Th2)-mediated inflammation, and enhance innate and adaptive immunity to pathogen invasion. Under pathological conditions, such as rhinitis, ocular conjunctivitis, asthma, chronic atrophic gastritis, type 2 diabetes, and pulmonary interstitial fibrosis, CHIA expression is significantly changed. In addition, studies have shown that CHIA has an anti-apoptotic effect, promotes epithelial cell proliferation and maintains organ integrity, and these effects are not related to chitinase degradation. CHIA can also be used as a biomolecular marker in diseases such as chronic atrophic gastritis, dry eye, and acute kidney damage caused by sepsis. Analysis of the authoritative TCGA database shows that CHIA expression in gastric adenocarcinoma, liver cancer, renal clear cell carcinoma and other tumors is significantly downregulated compared with that in normal tissues, but the specific mechanism is unclear. This review is based on all surveys conducted to date and summarizes the expression patterns and functional diversity of CHIA in various organs. Understanding the physiological and pathophysiological relevance of CHIA in multiple organs opens new possibilities for disease treatment.

Gastroesophageal Reflux Disease and Foregut Dysmotility in Children with Intestinal Failure

Gastrointestinal dysmotility is a common problem in a subgroup of children with intestinal failure (IF), including short bowel syndrome (SBS) and pediatric intestinal pseudo-obstruction (PIPO). It contributes significantly to the increased morbidity and decreased quality of life in this patient population. Impaired gastrointestinal (GI) motility in IF arises from either loss of GI function due to the primary disorder (e.g., neuropathic or myopathic disorder in the PIPO syndrome) and/or a critical reduction in gut mass. Abnormalities of the anatomy, enteric hormone secretion and neural supply in IF can result in rapid transit, ineffective antegrade peristalsis, delayed gastric emptying or gastroesophageal reflux. Understanding the underlying pathophysiologic mechanism(s) of the enteric dysmotility in IF helps us to plan an appropriate diagnostic workup and apply individually tailored nutritional and pharmacological management, which might ultimately lead to an overall improvement in the quality of life and increase in enteral tolerance. In this review, we have focused on the pathogenesis of GI dysmotility in children with IF, as well as the management and treatment options.

Infant acid suppression use is associated with the development of eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is an esophageal allergic inflammatory disorder often presenting with infant/toddler gastroesophageal reflux symptoms refractory to treatment, including acid suppression trials with histamine H2 antagonists and proton pump inhibitors. We propose to evaluate the impact of infant acid suppressant exposure in EoE. Geisinger's pediatric EoE cases were matched to controls (1:5 EoE case control ratio) using age, race, sex, and ages at other diagnoses of asthma, eczema, and environmental allergies, totaling 526 EoE cases and 2,630 controls. Comparisons between EoE cases and matched controls were tested with regard to rates of acid suppression use with H2 antagonists and PPIs during infancy. Our analyses found the use of acid suppression in infancy was positively associated with EoE: PPI (5.7% EoE cases vs. 1.6% controls; P < 0.0001), H2 antagonists (8.8% EoE cases vs. 4.5% controls; P < 0.0001). Additionally, analysis of EoE cases using acid suppression during infancy indicated a likelihood for the diagnosis with EoE at an earlier age. Early acid suppression use in infants is significantly associated with the diagnosis of EoE in childhood in this well-matched retrospective cohort study. The potential link warrants additional investigation. Our study further reinforces the evidence-based stewardship of acid suppressant use, especially in our most vulnerable populations.

Role of metaplasia during gastric regeneration

Spasmolytic polypeptide/trefoil factor 2 (TFF2)-expressing metaplasia (SPEM) is a mucous-secreting reparative lineage that emerges at the ulcer margin in response to gastric injury. Under conditions of chronic inflammation with parietal cell loss, SPEM has been found to emerge and evolve into neoplasia. Cluster-of-differentiation gene 44 (CD44) is known to coordinate normal and metaplastic epithelial cell proliferation. In particular, CD44 variant isoform 9 (CD44v9) associates with the cystine-glutamate transporter xCT, stabilizes the protein, and provides defense against reactive oxygen species (ROS). xCT stabilization by CD44v9 leads to defense against ROS by cystine uptake, glutathione (GSH) synthesis, and maintenance of the redox balance within the intracellular environment. Furthermore, p38 signaling is a known downstream ROS target, leading to diminished cell proliferation and migration, two vital processes of gastric epithelial repair. CD44v9 emerges during repair of the gastric epithelium after injury, where it is coexpressed with other markers of SPEM. The regulatory mechanisms for the emergence of CD44v9 and the role of CD44v9 during the process of gastric epithelial regeneration are largely unknown. Inflammation and M2 macrophage infiltration have recently been demonstrated to play key roles in the induction of SPEM after injury. The following review proposes new insights into the functional role of metaplasia in the process of gastric regeneration in response to ulceration. Our insights are extrapolated from documented studies reporting oxyntic atrophy and SPEM development and our current unpublished findings using the acetic acid-induced gastric injury model.

Activation mechanism of plasmepsins, pepsin-like aspartic proteases from Plasmodium, follows a unique trans-activation pathway

Plasmodium parasites that cause malaria produce plasmepsins (PMs), pepsin-like aspartic proteases that are important antimalarial drug targets due to their role in host hemoglobin degradation. The enzymes are synthesized as inactive zymogens (pro-PMs), and the mechanism of their conversion to the active, mature forms has not been clearly elucidated. Our structural investigations of vacuolar pro-PMs with truncated prosegment (pro-tPMs) reveal that the formation of the S-shaped dimer is their innate property. Further structural studies, biochemical analysis, and molecular dynamics simulations indicate that disruption of the Tyr-Asp loop (121p-4), coordinated with the movement of the loop L1 (237-247) and helix H2 (101p-113p), is responsible for the extension of the pro-mature region (harboring the cleavage site). Consequently, under acidic pH conditions, these structural changes result in the dissociation of the dimers to monomers and the protonation of the residues in the prosegment prompts its unfolding. Subsequently, we demonstrated that the active site of the monomeric pro-tPMs with the unfolded prosegment is accessible for peptide substrate binding; in contrast, the active site is blocked in folded prosegment form of pro-tPMs. Thus, we propose a novel mechanism of auto-activation of vacuolar pro-tPMs that under acidic conditions can form a catalytically competent active site. One monomer cleaves the prosegment of the other one through a trans-activation process, resulting in formation of mature enzyme. As a result, once a mature enzyme is generated, it leads to the complete conversion of all the inactive pro-tPMs to their mature form. DATABASE: Atomic coordinates and structure factors have been submitted in the Protein Data Bank (PDB) under the PDB IDs 6KUB, 6KUC, and 6KUD.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Molecular ontogeny of the stomach in the catshark Scyliorhinus canicula

The origin of extracellular digestion in metazoans was accompanied by structural and physiological alterations of the gut. These adaptations culminated in the differentiation of a novel digestive structure in jawed vertebrates, the stomach. Specific endoderm/mesenchyme signalling is required for stomach differentiation, involving the growth and transcription factors: 1) Shh and Bmp4, required for stomach outgrowth; 2) Barx1, Sfrps and Sox2, required for gastric epithelium development and 3) Cdx1 and Cdx2, involved in intestinal versus gastric identity. Thus, modulation of endoderm/mesenchyme signalling emerges as a plausible mechanism linked to the origin of the stomach. In order to gain insight into the ancient mechanisms capable of generating this structure in jawed vertebrates, we characterised the development of the gut in the catshark Scyliorhinus canicula. As chondrichthyans, these animals retained plesiomorphic features of jawed vertebrates, including a well-differentiated stomach. We identified a clear molecular regionalization of their embryonic gut, characterised by the expression of barx1 and sox2 in the prospective stomach region and expression of cdx1 and cdx2 in the prospective intestine. Furthermore, we show that gastric gland development occurs close to hatching, accompanied by the onset of gastric proton pump activity. Our findings favour a scenario in which the developmental mechanisms involved in the origin of the stomach were present in the common ancestor of chondrichthyans and osteichthyans.

Identification of Novel Cryptic Multifunctional Antimicrobial Peptides from the Human Stomach Enabled by a Computational-Experimental Platform

Novel approaches are needed to combat antibiotic resistance. Here, we describe a computational-experimental framework for the discovery of novel cryptic antimicrobial peptides (AMPs). The computational platform, based on previously validated antimicrobial scoring functions, indicated the activation peptide of pepsin A, the main human stomach protease, and its N- and C-terminal halves as antimicrobial peptides. The three peptides from pepsinogen A3 isoform were prepared in a recombinant form using a fusion carrier specifically developed to express toxic peptides in Escherichia coli. Recombinant pepsinogen A3-derived peptides proved to be wide-spectrum antimicrobial agents with MIC values in the range 1.56-50 μM (1.56-12.5 μM for the whole activation peptide). Moreover, the activation peptide was bactericidal at pH 3.5 for relevant foodborne pathogens, suggesting that this new class of previously unexplored AMPs may contribute to microbial surveillance within the human stomach. The peptides showed no toxicity toward human cells and exhibited anti-infective activity in vivo, reducing by up to 4 orders of magnitude the bacterial load in a mouse skin infection model. These peptides thus represent a promising new class of antibiotics. We envision that computationally guided data mining approaches such as the one described here will lead to the discovery of antibiotics from previously unexplored sources.

Novel Structural Mechanism of Allosteric Regulation of Aspartic Peptidases via an Evolutionarily Conserved Exosite

Pepsin-family aspartic peptidases are biosynthesized as inactive zymogens in which the propeptide blocks the active site until its proteolytic removal upon enzyme activation. Here, we describe a novel dual regulatory function for the propeptide using a set of crystal structures of the parasite cathepsin D IrCD1. In the IrCD1 zymogen, intramolecular autoinhibition by the intact propeptide is mediated by an evolutionarily conserved exosite on the enzyme core. After activation, the mature enzyme employs the same exosite to rebind a small fragment derived from the cleaved propeptide. This fragment functions as an effective natural inhibitor of mature IrCD1 that operates in a pH-dependent manner through a unique allosteric inhibition mechanism. The study uncovers the propeptide-binding exosite as a target for the regulation of pepsin-family aspartic peptidases and defines the structural requirements for exosite inhibition.

Chitin digestibility is dependent on feeding behaviors, which determine acidic chitinase mRNA levels in mammalian and poultry stomachs

Chitin, a polymer of N-acetyl-D-glucosamine (GlcNAc), functions as a major structural component in chitin-containing organism including crustaceans, insects and fungi. Recently, we reported that acidic chitinase (Chia) is highly expressed in mouse, chicken and pig stomach tissues and that it can digest chitin in the respective gastrointestinal tracts (GIT). In this study, we focus on major livestock and domestic animals and show that the levels of Chia mRNA in their stomach tissues are governed by the feeding behavior. Chia mRNA levels were significantly lower in the bovine (herbivores) and dog (carnivores) stomach than those in mouse, pig and chicken (omnivores). Consistent with the mRNA levels, Chia protein was very low in bovine stomach. In addition, the chitinolytic activity of E. coli-expressed bovine and dog Chia enzymes were moderately but significantly lower compared with those of the omnivorous Chia enzymes. Recombinant bovine and dog Chia enzymes can degrade chitin substrates under the artificial GIT conditions. Furthermore, genomes of some herbivorous animals such as rabbit and guinea pig do not contain functional Chia genes. These results indicate that feeding behavior affects Chia expression levels as well as chitinolytic activity of the enzyme, and determines chitin digestibility in the particular animals.

Spontaneous pepsin C-catalyzed activation of human pepsinogen C in transgenic rice cell suspension culture: Production and characterization of human pepsin C

A human pepsinogen C (hPGC) gene was synthesized with rice-optimized codon usage and cloned into a rice expression vector containing the promoter, signal peptide, and terminator derived from the rice α-amylase 3D (Ramy3D) gene. In addition, a 6-His tag was added to the 3' end of the synthetic hPGC gene for easy purification. The plant expression vector was introduced into rice calli (Oryza sativa L. cv. Dongjin) mediated by Agrobacterium tumefaciens. The integration of the hPGC gene into the chromosome of the transgenic rice callus and hPGC expression in transgenic rice cell suspensions was verified via genomic DNA polymerase chain reaction amplification and Northern blot analysis. Western blot analysis indicated both hPGC and its mature form, human pepsin C, with masses of 42- and 36-kDa in the culture medium under sugar starvation conditions. Human pepsin C was purified from the culture medium using a Ni-NTA agarose column and the NH₂-terminal 5-residue sequences were verified by amino acid sequencing. The hydrolyzing activity of human pepsin C was confirmed using bovine hemoglobin as a substrate. The optimum pH and temperature for pepsin activity were 2.0 and 40°C, respectively.

Pepsinogen C expression, regulation and its relationship with cancer

Pepsinogen C (PGC) belongs to the aspartic protease family and is secreted by gastric chief cells. PGC could be activated to pepsin C and digests polypeptides and amino acids, but as a zymogen PGC’s functions is unclear. In normal physiological conditions, PGC is initially detected in the late embryonic stage and is mainly expressed in gastric mucosa. The in situ expression of PGC in gastric mucosa is decreased considerably in the process of superficial gastritis → atrophic gastritis → gastric cancer (GC), proving that PGC is a comparatively ideal negative marker of GC. Serum PGC, and PGA levels and the PGA/PGC ratio have satisfactory sensitivity, specificity and price–quality ratio for predicting high GC risk. Ectopic PGC expression is significantly increased in prostate cancer, breast cancer, ovary cancer and endometrial cancer. In those sex-related cancers high level PGC expression indicates better prognosis and longer survival. The regulation of PGC expression involves genetic and epigenetic alteration of the encoding PGC gene, hormones modulation and interactions between PGC with other transcription factors and protein kinases. More and more research evidence hinted that PGC has strong correlation with cancer. In the systematic review, we respectively elaborate the structure, potential physiological functions, expression characteristics and regulation of PGC, and especially focus on the relationship between PGC expression and cancer to highlight the role of PGC in the tumorigenesis and its application value in clinical practice.

Protein Corona-Induced Modification of Silver Nanoparticle Aggregation in Simulated Gastric Fluid

Due to their widespread incorporation into a range of biomedical and consumer products, the ingestion of silver nanoparticles (AgNPs) is of considerable concern to human health. However, the extent to which AgNPs will be modified within the gastric compartment of the gastrointestinal tract is still poorly understood. Studies have yet to fully evaluate the extent of physicochemical changes to AgNPs in the presence of biological macromolecules, such as pepsin, the most abundant protein in the stomach, or the influence of AgNPs on protein structure and activity. Herein, AgNPs of two different sizes and surface coatings (20 and 110 nm, citrate or polyvinylpyrrolidone) were added to simulated gastric fluid (SGF) with or without porcine pepsin at three pHs (2.0, 3.5, and 5.0), representing a range of values between preprandial (fasted) and postprandial (fed) conditions. Rapid increases in diameter were observed for all AgNPs, with a greater increase in diameter in the presence of pepsin, indicating that pepsin facilitated AgNPs aggregation. AgNPs interaction with pepsin only minimally reduced the protein's proteolytic functioning capability, with the greatest inhibitory effect caused by smaller (20 nm) particles of both coatings. No changes in pepsin secondary structural elements were observed for the different AgNPs, even at high particle concentrations. This research highlights the size-dependent kinetics of nanoparticle aggregation or dissolution from interaction with biological elements such as proteins in the gastrointestinal tract. Further, these results demonstrate that, in addition to mass, knowing the chemical form and aggregation state of nanoparticles is critical when evaluating toxicological effects from nanoparticle exposure in the body.

Acidic mammalian chitinase is a proteases-resistant glycosidase in mouse digestive system

Chitinases are enzymes that hydrolyze chitin, a polymer of β-1, 4-linked N-acetyl-D-glucosamine (GlcNAc). Chitin has long been considered as a source of dietary fiber that is not digested in the mammalian digestive system. Here, we provide evidence that acidic mammalian chitinase (AMCase) can function as a major digestive enzyme that constitutively degrades chitin substrates and produces (GlcNAc)₂ fragments in the mouse gastrointestinal environment. AMCase was resistant to endogenous pepsin C digestion and remained active in the mouse stomach extract at pH 2.0. The AMCase mRNA levels were much higher than those of four major gastric proteins and two housekeeping genes and comparable to the level of pepsinogen C in the mouse stomach tissues. Furthermore, AMCase was expressed in the gastric pepsinogen-synthesizing chief cells. The enzyme was also stable and active in the presence of trypsin and chymotrypsin at pH 7.6, where pepsin C was completely degraded. Mouse AMCase degraded polymeric colloidal and crystalline chitin substrates in the gastrointestinal environments in presence of the proteolytic enzymes. Thus, AMCase can function as a protease-resistant major glycosidase under the conditions of stomach and intestine and degrade chitin substrates to produce (GlcNAc)₂, a source of carbon, nitrogen and energy.

Procathepsin E is highly abundant but minimally active in pancreatic ductal adenocarcinoma tumors

The cathepsin family of lysosomal proteases is increasingly being recognized for their altered expression in cancer and role in facilitating tumor progression. The aspartyl protease cathepsin E is overexpressed in several cancers and has been investigated as a biomarker for pancreatic ductal adenocarcinoma (PDAC). Here we show that cathepsin E expression in mouse PDAC tumors is increased by more than 400-fold when compared to healthy pancreatic tissue. Cathepsin E accumulates over the course of disease progression and accounts for more than 3% of the tumor protein in mice with end-stage disease. Through immunoblot analysis we determined that only procathepsin E exists in mouse PDAC tumors and cell lines derived from these tumors. By decreasing the pH, this procathepsion E is converted to the mature form, resulting in an increase in proteolytic activity. Although active site inhibitors can bind procathepsin E, treatment of PDAC mice with the aspartyl protease inhibitor ritonavir did not decrease tumor burden. Lastly, we used multiplex substrate profiling by mass spectrometry to identify two synthetic peptides that are hydrolyzed by procathepsin E near neutral pH. This work represents a comprehensive analysis of procathepsin E in PDAC and could facilitate the development of improved biomarkers for disease detection.

Purification and molecular cloning of aspartic proteinases from the stomach of adult Japanese fire belly newts, Cynops pyrrhogaster

Six aspartic proteinase precursors, a pro-cathepsin E (ProCatE) and five pepsinogens (Pgs), were purified from the stomach of adult newts (Cynops pyrrhogaster). On sodium dodecylsulfate-polyacrylamide gel electrophoresis, the molecular weights of the Pgs and active enzymes were 37-38 kDa and 31-34 kDa, respectively. The purified ProCatE was a dimer whose subunits were connected by a disulphide bond. cDNA cloning by polymerase chain reaction and subsequent phylogenetic analysis revealed that three of the purified Pgs were classified as PgA and the remaining two were classified as PgBC belonging to C-type Pg. Our results suggest that PgBC is one of the major constituents of acid protease in the urodele stomach. We hypothesize that PgBC is an amphibian-specific Pg that diverged during its evolutional lineage. PgBC was purified and characterized for the first time. The purified urodele pepsin A was completely inhibited by equal molar units of pepstatin A. Conversely, the urodele pepsin BC had low sensitivity to pepstatin A. In acidic condition, the activation rates of newt pepsin A and BC were similar to those of mammalian pepsin A and C1, respectively. Our results suggest that the enzymological characters that distinguish A- and C-type pepsins appear to be conserved in mammals and amphibians.

Characterizing Protease Specificity: How Many Substrates Do We Need?

Calculation of cleavage entropies allows to quantify, map and compare protease substrate specificity by an information entropy based approach. The metric intrinsically depends on the number of experimentally determined substrates (data points). Thus a statistical analysis of its numerical stability is crucial to estimate the systematic error made by estimating specificity based on a limited number of substrates. In this contribution, we show the mathematical basis for estimating the uncertainty in cleavage entropies. Sets of cleavage entropies are calculated using experimental cleavage data and modeled extreme cases. By analyzing the underlying mathematics and applying statistical tools, a linear dependence of the metric in respect to 1/n was found. This allows us to extrapolate the values to an infinite number of samples and to estimate the errors. Analyzing the errors, a minimum number of 30 substrates was found to be necessary to characterize substrate specificity, in terms of amino acid variability, for a protease (S4-S4’) with an uncertainty of 5 percent. Therefore, we encourage experimental researchers in the protease field to record specificity profiles of novel proteases aiming to identify at least 30 peptide substrates of maximum sequence diversity. We expect a full characterization of protease specificity helpful to rationalize biological functions of proteases and to assist rational drug design.

Foldase and inhibitor functionalities of the pepsinogen prosegment are encoded within discrete segments of the 44 residue domain

Pepsin is initially produced as the zymogen pepsinogen, containing a 44 residue prosegment (PS) domain. When folded without the PS, pepsin forms a thermodynamically stable denatured state (refolded pepsin, Rp). To guide native folding, the PS binds to Rp, stabilizes the folding transition state, and binds tightly to native pepsin (Np), thereby driving the folding equilibrium to favor the native state. It is unknown whether these functionalities of the PS are encoded within the entire sequence or within discrete segments. PS residues 1p-29p correspond to a highly conserved region in pepsin-like aspartic proteases and we hypothesized that this segment is critical to PS-catalyzed folding. This notion was tested in the present study by characterizing the ability of various truncated PS peptides to bind Rp, catalyze folding from Rp to Np, and to inhibit Np. Four PS truncations were examined, corresponding to PS residues 1p-16p (PS1-16), 1p-29p (PS1-29), 17p-44p (PS17-44) and 30p-44p (PS30-44). The three PS functionalities could be ascribed primarily to discrete regions within the highly conserved motif: 1p-16p dictated Rp binding, 17p-29p dictated Np binding/inhibition, while the entire 1p-29p dictated transition state binding/catalyzing folding. Conversely, PS30-44 played no obvious role in PS-catalyzed folding; it is hypothesized that this more variable region may serve as a linker between PS1-29 and the mature domain. The high sequence conservation of PS1-29 and its role in catalyzing pepsin folding strongly suggest that there is a conserved PS-catalyzed folding mechanism shared by pepsin-like aspartic proteases with this motif.

Functional properties of the catalytic domain of mouse acidic mammalian chitinase expressed in Escherichia coli

Mouse acidic mammalian chitinase (AMCase) plays important physiological roles in defense and nutrition. AMCase is composed of an N-terminal catalytic domain (CatD) and a C-terminal chitin-binding domain (CBD). We expressed CatD of mouse AMCase as a recombinant fusion protein with Protein A and V5-His in Escherichia coli (Protein A-CatD-V5-His), evaluated its functional properties and compared them to the full-length AMCase (Protein A-AMCase-V5-His). Under our experimental conditions, the chitinolytic activity of both proteins against 4-nitrophenyl N,N'-diacetyl-β-d-chitobioside was equivalent with regard to their specific enzymatic activities, optimal pH and temperature as well as to the pH and temperature stability. CatD bound to chitin beads and cleaved the N-acetylglucosamine hexamer, colloidal and crystalline chitin as well as the shrimp shell, and released primarily N,N'-diacetylchitobiose fragments at pH 2.0. These results indicate that the primary structure of CatD is sufficient to form a proper tertiary structure required for chitinolytic activity, recognize chitin substrates and degrade them in the absence of a CBD. Our recombinant proteins can be used for further studies evaluating pathophysiological roles of AMCase in different diseases.

Pharmacological and alimentary alteration of the gastric barrier

The gastric barrier contains several lines of defence which protect the epithelium from harmful microbes and toxins. Pre-mucosal defence mechanisms include secreted acid (HCl 0.1 mmol/L) and pepsin, which are capable of denaturing tissue. A tightly adherent mucous layer provides the next line of defence, and physically separates any potentially hazardous substance in the lumen from the mucosal surface. Apical secretion of HCO3(-) maintains a non-acidic microenvironment at the mucosal surface. Membrane-bound phospholipids repel soluble toxins, and sulphydryls scavenge reactive oxygen species. However, when noxious agents overwhelm these mechanisms, the epithelium is damaged. Herein, we discuss the pathological and physiological basis for several disease states which are associated with a breakdown in one or more components of the gastric barrier, including: Helicobacter pylori-associated gastritis, atrophic gastritis, stress-related mucosal disease, age-related gastropathy and portal hypertensive gastropathy. The effect of non-steroidal anti-inflammatory drugs and proton pump inhibitors on the gastric mucosa, is explored. Finally, we outline the alterations in mucosal defence caused by alcohol, caffeine, minerals and vitamins.

Matriptase autoactivation is tightly regulated by the cellular chemical environments

The ability of cells to rapidly detect and react to alterations in their chemical environment, such as pH, ionic strength and redox potential, is essential for cell function and survival. We present here evidence that cells can respond to such environmental alterations by rapid induction of matriptase autoactivation. Specifically, we show that matriptase autoactivation can occur spontaneously at physiological pH, and is significantly enhanced by acidic pH, both in a cell-free system and in living cells. The acid-accelerated autoactivation can be attenuated by chloride, a property that may be part of a safety mechanism to prevent unregulated matriptase autoactivation. Additionally, the thio-redox balance of the environment also modulates matriptase autoactivation. Using the cell-free system, we show that matriptase autoactivation is suppressed by cytosolic reductive factors, with this cytosolic suppression being reverted by the addition of oxidizing agents. In living cells, we observed rapid induction of matriptase autoactivation upon exposure to toxic metal ions known to induce oxidative stress, including CoCl₂ and CdCl₂. The metal-induced matriptase autoactivation is suppressed by N-acetylcysteine, supporting the putative role of altered cellular redox state in metal induced matriptase autoactivation. Furthermore, matriptase knockdown rendered cells more susceptible to CdCl₂-induced cell death compared to control cells. This observation implies that the metal-induced matriptase autoactivation confers cells with the ability to survive exposure to toxic metals and/or oxidative stress. Our results suggest that matriptase can act as a cellular sensor of the chemical environment of the cell that allows the cell to respond to and protect itself from changes in the chemical milieu.

Actinidin from kiwifruit (Actinidia deliciosacv. Hayward) increases the digestion and rate of gastric emptying of meat proteins in the growing pig

The present study aimed to investigate the effect of dietary actinidin on the kinetics of gastric digestion of beef muscle proteins and on the rate of stomach emptying in growing pigs. For this purpose, 120 pigs (mean body weight 28 (sd2·9) kg) were fed beef muscle protein-based diets containing either actinidin (fresh green kiwifruit pulp or gold kiwifruit pulp supplemented with purified actinidin) or no actinidin (fresh gold kiwifruit pulp or green kiwifruit pulp with inactivated actinidin). Additionally, fifteen pigs were fed with a protein-free diet to determine the endogenous protein flow. Pigs were euthanised at exactly 0·5, 1, 3, 5 and 7 h postprandially (n6 per time point for each kiwifruit diet andn3 for protein-free diet). Stomach chyme was collected for measuring gastric retention, actinidin activity, individual beef muscle protein digestion based on SDS–PAGE and the degree of hydrolysis based on the appearance of free amino groups. The stomach emptying of DM and N was faster when actinidin was present in the diet (P< 0·05): the half gastric emptying time of DM was 137v. 172 min ( ± 7·4 min pooled standard error) for the diets with and without actinidin, respectively. The presence of dietary actinidin in the stomach chyme increased the digestion of beef muscle protein (P< 0·05) and, more specifically, those proteins with a high molecular weight (>34 kDa;P< 0·05). In conclusion, dietary actinidin fed in the form of fresh green kiwifruit increased the rate of gastric emptying and the digestion of several beef muscle proteins.

Protein A-mouse acidic mammalian chitinase-V5-His expressed in periplasmic space of Escherichia coli possesses chitinase functions comparable to CHO-expressed protein

Acidic mammalian chitinase (AMCase) has been shown to be associated with asthma in mouse models, allergic inflammation and food processing. Here, we describe an E. coli-expression system that allows for the periplasmic production of active AMCase fused to Protein A at the N-terminus and V5 epitope and (His)6 tag (V5-His) at the C-terminus (Protein A-AMCase-V5-His) in E. coli. The mouse AMCase cDNA was cloned into the vector pEZZ18, which is an expression vector containing the Staphylococcus Protein A promoter, with the signal sequence and truncated form of Protein A for extracellular expression in E. coli. Most of the Protein A-AMCase-V5-His was present in the periplasmic space with chitinolytic activity, which was measured using a chromogenic substrate, 4-nitrophenyl N,N'-diacetyl-β-D-chitobioside. The Protein A-AMCase-V5-His was purified from periplasmic fractions using an IgG Sepharose column followed by a Ni Sepharose chromatography. The recombinant protein showed a robust peak of activity with a maximum observed activity at pH 2.0, where an optimal temperature was 54°C. When this protein was preincubated between pH 1.0 and pH 11.0 on ice for 1 h, full chitinolytic activity was retained. This protein was also heat-stable till 54°C, both at pH 2.0 and 7.0. The chitinolytic activity of the recombinant AMCase against 4-nitrophenyl N,N'-diacetyl-β-D-chitobioside was comparable to the CHO-expressed AMCase. Furthermore, the recombinant AMCase bound to chitin beads, cleaved colloidal chitin and released mainly N,N'-diacetylchitobiose fragments. Thus, the E. coli-expressed Protein A-mouse AMCase-V5-His fusion protein possesses chitinase functions comparable to the CHO-expressed AMCase. This recombinant protein can be used to elucidate detailed biomedical functions of the mouse AMCase.

Use of a molybdenum(VI) complex as artificial protease in protein photocleavage

In this study, a molybdenum(VI) peroxo α-amino acid complex, MoO(O2)2(α-leucine) (H2O), was prepared and used as an artificial protease for site-specific cleavage of porcine pepsin, a model protein. Cleavage of pepsin by MoO(O2)2(α-leucine) (H2O) was achieved under photochemical conditions at room temperature and pH 7.0. The reaction was activated by irradiation of the MoO(O2)2(α-leucine) (H2O)-protein mixture by UV light (320 and 340nm) for up to 30min. No cleavage was observed in the absence of MoO(O2)2(α-leucine) (H2O) or the light. The photocleavage yield increased with irradiation time. The cleaved fragments were sequencable, and the cleavage site was assigned to Leu(112)-Tyr(113). The cleavage reaction was quenched by ethanol. Therefore, hydroxyl radicals may be involved in the reaction and responsible for the cleavage of the protein. This is the first demonstration of the successful photocleavage of proteins by a molybdenum complex. This observation can provide a new approach for the photochemical footprinting of metal binding sites on proteins.

Properties and applications of phytepsins from thistle flowers

Aqueous extracts of thistle flowers from the genus Cynara-Cardueae tribe Cass. (Cynareae Less.), Asteraceae Dumortier-are traditionally used in the Mediterranean region for production of artisanal cheeses. This is because of the presence of aspartic proteases (APs) with the ability to coagulate milk. Plant APs, collectively known as phytepsins (EC 3.4.23.40), are bilobed endopeptidases present in an ample variety of plant species with activity mainly at acidic pHs, and have two aspartic residues located on each side of a catalytic cleft that are responsible for catalysis. The cleavage of the scissile peptide-bond occurs primarily between residues with large hydrophobic side-chains. Even when aspartylendopeptidase activity in plants is normally present at relatively low levels overall, the flowers of several species of the Cardueae tribe possess APs with extremely high specific activities in certain tissues. For this reason, in the last two decades, APs present in thistle flowers have been the subject of intensive study. Present here is a compilation of work that summarizes the known chemical and biological properties of these proteases, as well as their biomedical and biotechnological applications.

Quantification of Chitinase mRNA Levels in Human and Mouse Tissues by Real-Time PCR: Species-Specific Expression of Acidic Mammalian Chitinase in Stomach Tissues

Chitinase hydrolyzes chitin, which is an N-acetyl-D-glucosamine polymer that is present in a wide range of organisms, including insects, parasites and fungi. Although mammals do not contain any endogenous chitin, humans and mice express two active chitinases, chitotriosidase (Chit1) and acidic mammalian chitinase (AMCase). Because the level of expression of these chitinases is increased in many inflammatory conditions, including Gaucher disease and mouse models of asthma, both chitinases may play important roles in the pathophysiologies of these and other diseases. We recently established a quantitative PCR system using a single standard DNA and showed that AMCase mRNA is synthesized at extraordinarily high levels in mouse stomach tissues. In this study, we applied this methodology to the quantification of chitinase mRNAs in human tissues and found that both chitinase mRNAs were widely expressed in normal human tissues. Chit1 mRNA was highly expressed in the human lung, whereas AMCase mRNA was not overexpressed in normal human stomach tissues. The levels of these mRNAs in human tissues were significantly lower than the levels of housekeeping genes. Because the AMCase expression levels were quite different between the human and mouse stomach tissues, we developed a quantitative PCR system to compare the mRNA levels between human and mouse tissues using a human-mouse hybrid standard DNA. Our analysis showed that Chit1 mRNA is expressed at similar levels in normal human and mouse lung. In contrast, the AMCase expression level in human stomach was significantly lower than that expression level observed in mouse stomach. These mRNA differences between human and mouse stomach tissues were reflecting differences in the chitinolytic activities and levels of protein expression. Thus, the expression level of the AMCase in the stomach is species-specific.

Pepsin is nitrated in the rat stomach, acquiring antiulcerogenic activity: a novel interaction between dietary nitrate and gut proteins

Dietary nitrate is reduced to nitrite and nitric oxide ((•)NO) in the gut, producing reactive species able to nitrate proteins and lipids. We investigated intragastric production of (•)NO and nitrating agents in vivo by examining selective nitration of pepsinogen and pepsin. We further addressed the functional impact of nitration on peptic activity by evaluating the progression of secretagogue-induced ulcers. Pepsinogen nitration was assessed in healthy and diclofenac-induced ulcerated rat stomachs. Both groups were fed nitrite or water by oral gavage. Protein nitration was studied by immunofluorescence and immunoprecipitation. In parallel experiments, pentagastrin was administered to rats and nitrite was then instilled intragastrically. (•)NO levels were measured before and after nitrite administration by chemiluminescence. Macroscopic damage was assessed and nitrated pepsin was examined in the margin of ulcers. Protein nitration was detected physiologically in the stomach of healthy animals. Nitrite had a dual effect on intragastric nitration: overall nitration was decreased under physiological conditions but enhanced by acute inflammation. Pepsin and pepsinogen were also nitrated via a nitrite-dependent pathway. Nitration of both pepsin and its zymogen led to decreased peptic activity in response to classical substrates (e.g., collagen). Under conditions of acute ulceration, nitrite-dependent pepsin nitration prevented the development of gastric ulcers. Dietary nitrite generates nitrating agents in the stomach in vivo, markedly decreasing peptic activity. Under inflammatory and ulcerogenic conditions pepsin nitration attenuates the progression of gastric ulceration. These results suggest that dietary nitrite-dependent nitration of pepsin may have a novel antiulcerogenic effect in vivo.

Autoproteolytic Activation of a Symbiosis-regulated Truffle Phospholipase A2

Fungal phospholipases are members of the fungal/bacterial group XIV secreted phospholipases A(2) (sPLA(2)s). TbSP1, the sPLA(2) primarily addressed in this study, is up-regulated by nutrient deprivation and is preferentially expressed in the symbiotic stage of the ectomycorrhizal fungus Tuber borchii. A peculiar feature of this phospholipase and of its ortholog from the black truffle Tuber melanosporum is the presence of a 54-amino acid sequence of unknown functional significance, interposed between the signal peptide and the start of the conserved catalytic core of the enzyme. X-ray diffraction analysis of a recombinant TbSP1 form corresponding to the secreted protein previously identified in T. borchii mycelia revealed a structure comprising the five α-helices that form the phospholipase catalytic module but lacking the N-terminal 54 amino acids. This finding led to a series of functional studies that showed that TbSP1, as well as its T. melanosporum ortholog, is a self-processing pro-phospholipase A(2), whose phospholipase activity increases up to 80-fold following autoproteolytic removal of the N-terminal peptide. Proteolytic cleavage occurs within a serine-rich, intrinsically flexible region of TbSP1, does not involve the phospholipase active site, and proceeds via an intermolecular mechanism. Autoproteolytic activation, which also takes place at the surface of nutrient-starved, sPLA(2) overexpressing hyphae, may strengthen and further control the effects of phospholipase up-regulation in response to nutrient deprivation, also in the context of symbiosis establishment and mycorrhiza formation.

Chitinase mRNA levels by quantitative PCR using the single standard DNA: acidic mammalian chitinase is a major transcript in the mouse stomach

Chitinases hydrolyze the β-1-4 glycosidic bonds of chitin, a major structural component of fungi, crustaceans and insects. Although mammals do not produce chitin or its synthase, they express two active chitinases, chitotriosidase (Chit1) and acidic mammalian chitinase (AMCase). These mammalian chitinases have attracted considerable attention due to their increased expression in individuals with a number of pathological conditions, including Gaucher disease, Alzheimer's disease and asthma. However, the contribution of these enzymes to the pathophysiology of these diseases remains to be determined. The quantification of the Chit1 and AMCase mRNA levels and the comparison of those levels with the levels of well-known reference genes can generate useful and biomedically relevant information. In the beginning, we established a quantitative real-time PCR system that uses standard DNA produced by ligating the cDNA fragments of the target genes. This system enabled us to quantify and compare the expression levels of the chitinases and the reference genes on the same scale. We found that AMCase mRNA is synthesized at extraordinarily high levels in the mouse stomach. The level of this mRNA in the mouse stomach was 7- to 10-fold higher than the levels of the housekeeping genes and was comparable to that the level of the mRNA for pepsinogen C (progastricsin), a major component of the gastric mucosa. Thus, AMCase mRNA is a major transcript in mouse stomach, suggesting that AMCase functions as a digestive enzyme that breaks down polymeric chitin and as part of the host defense against chitin-containing pathogens in the gastric contents. Our methodology is applicable to the quantification of mRNAs for multiple genes across multiple specimens using the same scale.

The evolution of pepsinogen C genes in vertebrates: duplication, loss and functional diversification

Background

Aspartic proteases comprise a large group of enzymes involved in peptide proteolysis. This collection includes prominent enzymes globally categorized as pepsins, which are derived from pepsinogen precursors. Pepsins are involved in gastric digestion, a hallmark of vertebrate physiology. An important member among the pepsinogens is pepsinogen C (Pgc). A particular aspect of Pgc is its apparent single copy status, which contrasts with the numerous gene copies found for example in pepsinogen A (Pga). Although gene sequences with similarity to Pgc have been described in some vertebrate groups, no exhaustive evolutionary framework has been considered so far.

Methodology/Principal Findings

By combining phylogenetics and genomic analysis, we find an unexpected Pgc diversity in the vertebrate sub-phylum. We were able to reconstruct gene duplication timings relative to the divergence of major vertebrate clades. Before tetrapod divergence, a single Pgc gene tandemly expanded to produce two gene lineages (Pgbc and Pgc2). These have been differentially retained in various classes. Accordingly, we find Pgc2 in sauropsids, amphibians and marsupials, but not in eutherian mammals. Pgbc was retained in amphibians, but duplicated in the ancestor of amniotes giving rise to Pgb and Pgc1. The latter was retained in mammals and probably in reptiles and marsupials but not in birds. Pgb was kept in all of the amniote clade with independent episodes of loss in some mammalian species. Lineage specific expansions of Pgc2 and Pgbc have also occurred in marsupials and amphibians respectively. We find that teleost and tetrapod Pgc genes reside in distinct genomic regions hinting at a possible translocation.

Conclusions

We conclude that the repertoire of Pgc genes is larger than previously reported, and that tandem duplications have modelled the history of Pgc genes. We hypothesize that gene expansion lead to functional divergence in tetrapods, coincident with the invasion of terrestrial habitats.

Selective cleavage of pepsin by molybdenum metallopeptidase

In this study, the cleavage of protein by molybdenum cluster is reported for the first time. The protein target used is porcine pepsin. The data presented in this study show that pepsin is cleaved to at least three fragments with molecular weights of ∼23, ∼19 and ∼16 kDa when the mixture of the protein and ammonium heptamolybdate tetrahydrate ((NH(4))(6)Mo(7)O(24)·4H(2)O) was incubated at 37°C for 24h. No self cleavage of pepsin occurs at 37 °C, 24h indicating that the reaction is mediated by the metal ions. N-terminal sequencing of the peptide fragments indicated three cleavage sites of pepsin between Leu 112-Tyr 113, Leu 166-Leu 167 and Leu 178-Asn 179. The cleavage reaction occurs after incubation of the mixture of pepsin and (NH(4))(6)Mo(7)O(24)·4H(2)O) only for 2h. However, the specificity of the cleavage decreases when incubation time is longer than 48 h. The mechanism for cleavage of pepsin is expected to be hydrolytic chemistry of the amide bonds in the protein backbone.

Removal of hepatitis C virus-infected cells by a zymogenized bacterial toxin

Hepatitis C virus (HCV) infection is a major cause of chronic liver disease and has become a global health threat. No HCV vaccine is currently available and treatment with antiviral therapy is associated with adverse side effects. Moreover, there is no preventive therapy for recurrent hepatitis C post liver transplantation. The NS3 serine protease is necessary for HCV replication and represents a prime target for developing anti HCV therapies. Recently we described a therapeutic approach for eradication of HCV infected cells that is based on protein delivery of two NS3 protease-activatable recombinant toxins we named "zymoxins". These toxins were inactivated by fusion to rationally designed inhibitory peptides via NS3-cleavable linkers. Once delivered to cells where NS3 protease is present, the inhibitory peptide is removed resulting in re-activation of cytotoxic activity. The zymoxins we described suffered from two limitations: they required high levels of protease for activation and had basal activities in the un-activated form that resulted in a narrow potential therapeutic window. Here, we present a solution that overcame the major limitations of the "first generation zymoxins" by converting MazF ribonuclease, the toxic component of the E. coli chromosomal MazEF toxin-antitoxin system, into an NS3-activated zymoxin that is introduced to cells by means of gene delivery. We constructed an expression cassette that encodes for a single polypeptide that incorporates both the toxin and a fragment of its potent natural antidote, MazE, linked via an NS3-cleavable linker. While covalently paired to its inhibitor, the ribonuclease is well tolerated when expressed in naïve, healthy cells. In contrast, activating proteolysis that is induced by even low levels of NS3, results in an eradication of NS3 expressing model cells and HCV infected cells. Zymoxins may thus become a valuable tool in eradicating cells infected by intracellular pathogens that express intracellular proteases.

Engineered toxins "zymoxins" are activated by the HCV NS3 protease by removal of an inhibitory protein domain

The synthesis of inactive enzyme precursors, also known as "zymogens," serves as a mechanism for regulating the execution of selected catalytic activities in a desirable time and/or site. Zymogens are usually activated by proteolytic cleavage. Many viruses encode proteases that execute key proteolytic steps of the viral life cycle. Here, we describe a proof of concept for a therapeutic approach to fighting viral infections through eradication of virally infected cells exclusively, thus limiting virus production and spread. Using the hepatitis C virus (HCV) as a model, we designed two HCV NS3 protease-activated "zymogenized" chimeric toxins (which we denote "zymoxins"). In these recombinant constructs, the bacterial and plant toxins diphtheria toxin A (DTA) and Ricin A chain (RTA), respectively, were fused to rationally designed inhibitor peptides/domains via an HCV NS3 protease-cleavable linker. The above toxins were then fused to the binding and translocation domains of Pseudomonas exotoxin A in order to enable translocation into the mammalian cells cytoplasm. We show that these toxins exhibit NS3 cleavage dependent increase in enzymatic activity upon NS3 protease cleavage in vitro. Moreover, a higher level of cytotoxicity was observed when zymoxins were applied to NS3 expressing cells or to HCV infected cells, demonstrating a potential therapeutic window. The increase in toxin activity correlated with NS3 protease activity in the treated cells, thus the therapeutic window was larger in cells expressing recombinant NS3 than in HCV infected cells. This suggests that the "zymoxin" approach may be most appropriate for application to life-threatening acute infections where much higher levels of the activating protease would be expected.

Isolation, biochemical characterization, and molecular modeling of American lobster digestive cathepsin D1

An aspartic proteinase was isolated from American lobster gastric fluid. The purified cathepsin D runs as a single band on native-PAGE displaying proteolytic activity on a zymogram at pH 3.0, with an isoelectric point of 4.7. Appearance of the protein in SDS-PAGE, depended on the conditions of the gel electrophoresis. SDS treatment by itself was not able to fully unfold the protein. Thus, in SDS-PAGE the protein appeared to be heterogeneous. A few minute of boiling the sample in the presence of SDS was necessary to fully denature the protein that then run in the gel as a single band of ~50 kDa. The protein sequence of lobster cathepsin D1, as deduced from its mRNA sequence, lacks a 'polyproline loop' and β-hairpin, which are characteristic of some of its structural homologues. A comparison of amino acid sequences of digestive and non-digestive cathepsin D-like enzymes from invertebrates showed that most cathepsin D enzymes involved in food digestion, lack the polyproline loop, whereas all non-digestive cathepsin Ds, including the American lobster cathepsin D2 paralog, contain the polyproline loop. We propose that the absence or presence of this loop may be characteristic of digestive and non-digestive aspartic proteinases, respectively.

An examination of the proteolytic activity for bovine pregnancy-associated glycoproteins 2 and 12

The pregnancy-associated glycoproteins (PAGs) represent a complex group of putative aspartic peptidases expressed exclusively in the placentas of species in the Artiodactyla order. The ruminant PAGs segregate into two classes: the 'ancient' and 'modern' PAGs. Some of the modern PAGs possess alterations in the catalytic center that are predicted to preclude their ability to act as peptidases. The ancient ruminant PAGs in contrast are thought to be peptidases, although no proteolytic activity has been described for these members. The aim of the present study was to investigate (1) if the ancient bovine PAGs (PAG-2 and PAG-12) have proteolytic activity, and (2) if there are any differences in activity between these two closely related members. Recombinant bovine PAG-2 and PAG-12 were expressed in a baculovirus expression system and the purified proteins were analyzed for proteolytic activity against a synthetic fluorescent cathepsin D/E substrate. Both proteins exhibited proteolytic activity with acidic pH optima. The k(cat)/K(m) for bovine PAG-2 was 2.7x10(5) m(-1) s(-1) and for boPAG-12 it was 6.8x10(4) m(-1) s(-1). The enzymes were inhibited by pepstatin A with a K(i) of 0.56 and 7.5 nm for boPAG-2 and boPAG-12, respectively. This is the first report describing proteolytic activity in PAGs from ruminant ungulates.

Vinegar decreases allergenic response in lentil and egg food allergy

BACKGROUND

Food allergy results from an atypical response of the mucosal immune system to orally consumed allergens. Antacid medication inhibits the digestion of dietary proteins and causes food allergy. A decrease of the gastric pH might enhance the function of digestion and reduce the risk of food allergy.

OBJECTIVE

To test a possible decrease in the allergenicity of powerful food allergens (egg, chicken, lentils) with the addition of vinegar during the cooking process.

METHODS

We included seven patients who suffered from anaphylaxis due to egg, chicken and lentils. We added vinegar to egg, chicken and lentil processed extracts used for skin prick tests (SPT) and compared the wheal areas obtained with the same extracts sources and the same way but without vinegar addition. Immunodetection was performed with the different processed extracts and patients' sera. Only one patient consented food challenge with vinegar-marinated-chicken.

RESULTS

Wheal areas were significantly minor with the food extract with vinegar. Immunodetection showed a decrease of the response with vinegar processed extracts.

CONCLUSIONS

Vinegar addition during the cooking process may decrease lentil and chicken allergenicity.