Biochemical properties of purified cathepsin B from Schistosoma mansoni

Roles of Cysteine Proteases in Biology and Pathogenesis of Parasites

Cysteine proteases, also known as thiol proteases, are a class of nucleophilic proteolytic enzymes containing cysteine residues in the enzymatic domain. These proteases generally play a pivotal role in many biological reactions, such as catabolic functions and protein processing, in all living organisms. They specifically take part in many important biological processes, especially in the absorption of nutrients, invasion, virulence, and immune evasion of parasitic organisms from unicellular protozoa to multicellular helminths. They can also be used as parasite diagnostic antigens and targets for gene modification and chemotherapy, as well as vaccine candidates, due to their species and even life-cycle stage specificity. This article highlights current knowledge on parasitic cysteine protease types, biological functions, and their applications in immunodiagnosis and chemotherapy.

Exploring Sea Lice Vaccines against Early Stages of Infestation in Atlantic Salmon (Salmo salar)

The sea louse Caligus rogercresseyi genome has opened the opportunity to apply the reverse vaccinology strategy for identifying antigens with potential effects on lice development and its application in sea lice control. This study aimed to explore the efficacy of three sea lice vaccines against the early stage of infestation, assessing the transcriptome modulation of immunized Atlantic salmon. Therein, three experimental groups of Salmo salar (Atlantic salmon) were vaccinated with the recombinant proteins: Peritrophin (prototype A), Cathepsin (prototype B), and the mix of them (prototype C), respectively. Sea lice infestation was evaluated during chalimus I-II, the early-infective stages attached at 7-days post infestation. In parallel, head kidney and skin tissue samples were taken for mRNA Illumina sequencing. Relative expression analyses of genes were conducted to identify immune responses, iron transport, and stress responses associated with the tested vaccines during the early stages of sea lice infection. The vaccine prototypes A, B, and C reduced the parasite burden by 24, 44, and 52% compared with the control group. In addition, the RNA-Seq analysis exhibited a prototype-dependent transcriptome modulation. The high expression differences were observed in genes associated with metal ion binding, molecular processes, and energy production. The findings suggest a balance between the host’s inflammatory response and metabolic process in vaccinated fish, increasing their transcriptional activity, which can alter the early host–parasite interactions. This study uncovers molecular responses produced by three vaccine prototypes at the early stages of infestation, providing new knowledge for sea lice control in the salmon aquaculture.

Potential Molecular Mimicry Proteins Responsive to α-pinene in Bursaphelenchus xylophilus

: Bursaphelenchus xylophilus is a nematode species that has damaged pine trees worldwide, but its pathogenesis has not been fully characterized. α-pinene helps protect host species during the early B. xylophilus infection and colonization stages. In this study, we identified potential molecular mimicry proteins based on a comparative transcriptomic analysis of B. xylophilus. The expression levels of three genes encoding secreted B. xylophilus proteins were influenced by α-pinene. We cloned one gene encoding a thaumatin-like protein, Bx-tlp-2 (accession number MK000287), and another gene encoding a cysteine proteinase inhibitor, Bx-cpi (accession number MK000288). Additionally, α-pinene appeared to induce Bx-tlp-1 expression, but had the opposite effect on Bx-cpi expression. An analysis of the expression of the potential molecular mimicry proteins in B. xylophilus infecting pine trees revealed that the α-pinene content was consistent with the expression levels of Bx-tlp-1 (Bx-cpi) and Pm-tlp (Pm-cpi) over time. Thus, these genes likely have important roles contributing to the infection of pine species by B. xylophilus. The results of this study may be relevant for future investigations of the functions of Bx-tlp-1, Bx-tlp-2 and Bx-cpi, which may provide a point to explore the relationship between B. xylophilus and host pines.

Purification and characterization of two cysteine proteinase inhibitors from silkworm, Bombyx mori

Cysteine proteinase inhibitors from silkworm are selective inhibitors with low molecular weight and regulate cathepsin L-like cysteine proteinase activity, thus, affecting silkworm metamorphosis. In a previous study, two cysteine proteinase inhibitors, BCPI and BmCPI, were identified in the silkworm genome. To characterize these inhibitors, we expressed and purified them in an Escherichia coli system and analyzed their structure and inhibitory activity in vitro. Both inhibitors showed strong tolerance to high temperature. Their CD spectra revealed that their secondary structures could be recovered by a gradual decrease in temperature. Compared to BCPI, BmCPI exhibited weak inhibitory activity toward cathepsin L. BCPI activity was significantly decreased when its C-terminus was truncated, whereas BmCPI activity increased considerably when the C-terminus tail of BCPI was attached to BmCPI. Additionally, the inhibitory activity of BCPI was strongly reduced if R31 was mutated to A31. In summary, two cysteine proteinase inhibitors from silkworm were characterized in the present study, which facilitates an understanding of the interaction mechanism between cysteine proteinase and its inhibitors in the silkworm.

Comparing Proteolytic Fingerprints of Antigen-Presenting Cells during Allergen Processing

Endolysosomal processing has a critical influence on immunogenicity as well as immune polarization of protein antigens. In industrialized countries, allergies affect around 25% of the population. For the rational design of protein-based allergy therapeutics for immunotherapy, a good knowledge of T cell-reactive regions on allergens is required. Thus, we sought to analyze endolysosomal degradation patterns of inhalant allergens. Four major allergens from ragweed, birch, as well as house dust mites were produced as recombinant proteins. Endolysosomal proteases were purified by differential centrifugation from dendritic cells, macrophages, and B cells, and combined with allergens for proteolytic processing. Thereafter, endolysosomal proteolysis was monitored by protein gel electrophoresis and mass spectrometry. We found that the overall proteolytic activity of specific endolysosomal fractions differed substantially, whereas the degradation patterns of the four model allergens obtained with the different proteases were extremely similar. Moreover, previously identified T cell epitopes were assigned to endolysosomal peptides and indeed showed a good overlap with known T cell epitopes for all four candidate allergens. Thus, we propose that the degradome assay can be used as a predictor to determine antigenic peptides as potential T cell epitopes, which will help in the rational design of protein-based allergy vaccine candidates.

Pepsin is a positive regulator of Ac-cathB-2 involved in the rat gut penetration of Angiostrongylus cantonensis

Background

Angiostrongyliasis caused by the rat lungworm, Angiostrongylus cantonensis (A. cantonensis), has globally spread from the traditional epidemic areas. The small intestine is the site where the third-stage larvae (L3) of this worm entered the host body, and parasite proteases are involved in this process. Ac-cathB-2, a cathepsin B-like cysteine of A. cantonensis, was formerly isolated from the insoluble part of fragmentised Escherichia coli without activity. The unplanned low activity of prokaryotic expression proteins and difficulties in genetic modification hindered understanding the function of this protein.

Methods

The recombinant Ac-cathB-2 was expressed and harvested from 293 T cells and the enzymatic property and the effects of processing on the activity of the recombinant protease were investigated in vitro. The expression of Ac-cathB-2 in response to external stimulation was assessed, and the function of this protease during host gut penetration was observed by using antiserum for inhibition.

Results

Of the life-cycle stages studied, L3 expressed the highest level of Ac-cathB-2 gene and released the corresponding gene product from the body. The expression of this gene was rapidly upregulated after incubating L3 in small intestine homogenate of rat. Recombinant Ac-cathB-2 was harvested from 293 T cell culture medium. This protease was activated by pepsin-HCl and the enabled Ac-cathB-2 could subsequently digest laminin and fibronectin readily. Moreover, the small intestine isolated from rat was disrupted after incubating with the activated Ac-cathB-2, resulting in the detachment of epithelial cells. Antiserum treatment inhibited the hydrolytic ability of recombinant Ac-cathB-2 by 82.7 %, and also reduced the tissue penetration of activated L3 by 41.2 %. Additionally, pre-incubation of L3 with artificial gastric acid increased the number of penetrating larvae by 53.2 %, and this alteration could be partly blocked by antiserum treatment.

Conclusion

We believe that Ac-cathB-2 from A. cantonensis might help the worm to penetrate the rat gut, because the protease was able to degrade the tissue components of host. Nevertheless, our results further indicated that host pepsin played a beneficial role in this process by cleaving Ac-cathB-2 for activation. Thus, Ac-cathB-2 may probably represent an important target for the control of A. cantonensis infection.

Cathepsin Gene Family Reveals Transcriptome Patterns Related to the Infective Stages of the Salmon Louse Caligus rogercresseyi

Cathepsins are proteases involved in the ability of parasites to overcome and/or modulate host defenses so as to complete their own lifecycle. However, the mechanisms underlying this ability of cathepsins are still poorly understood. One excellent model for identifying and exploring the molecular functions of cathepsins is the marine ectoparasitic copepod Caligus rogercresseyi that currently affects the Chilean salmon industry. Using high-throughput transcriptome sequencing, 56 cathepsin-like sequences were found distributed in five cysteine protease groups (B, F, L, Z, and S) as well as in an aspartic protease group (D). Ontogenic transcriptome analysis evidenced that L cathepsins were the most abundant during the lifecycle, while cathepsins B and K were mostly expressed in the larval stages and adult females, thus suggesting participation in the molting processes and embryonic development, respectively. Interestingly, a variety of cathepsins from groups Z, L, D, B, K, and S were upregulated in the infective stage of copepodid, corroborating the complexity of the processes involved in the parasitic success of this copepod. Putative functional roles of cathepsins were conjectured based on the differential expressions found and on roles previously described in other phylogenetically related species. Moreover, 140 single nucleotide polymorphisms (SNP) were identified in transcripts annotated for cysteine and aspartic proteases located into untranslated regions, or the coding region. This study reports for the first time the presence of cathepsin-like genes and differential expressions throughout a copepod lifecycle. The identification of cathepsins together with functional validations represents a valuable strategy for pinpointing target molecules that could be used in the development of new delousing drugs or vaccines against C. rogercresseyi.

Molecular cloning and characterization of a cathepsin B from Angiostrongylus cantonensis

Cysteine proteases, a superfamily of hydrolytic enzymes, have numerous functions in parasites. Here, we reported the cloning and characterization of a cDNA encoding a cathepsin B (AcCPB) from Angiostrongylus cantonensis fourth-stage larvae cDNA library. The deduced amino acid sequence analysis indicated AcCPB is related to other cathepsin B family members with an overall conserved architecture. AcCPB is evolutionarily more close to other parasitic nematode cathepsin B than the ones from hosts, sharing 43-53% similarities to the homologues from other organisms. Real-time quantitative PCR analysis revealed that AcCPB was expressed significantly higher in the fourth-stage larvae (L4) and the fifth-stage larvae (L5) than that in the third-stage larvae (L3) and adult worms (Aw). Unexpectedly, AcCPB was expressed at a higher level in L4 and L5 derived from mice than the larvae at the same stages derived from rats. The protease activity of recombinant AcCPB (rAcCPB) expressed in Escherichia coli showed high thermostability and acidic pH optima. The role in ovalbumin digestion and enzyme activity of rAcCPB could be evidently inhibited by cystatin from A.cantonensis. Furthermore, we found rAcCPB increased the expression levels of CD40, MHC II, and CD80 on LPS-stimulated dendritic cells (DCs). In this study, we provided the first experimental evidence for the expression of cathepsin B in A.cantonensis. Besides its highly specific expression in the stages of L4 and L5 when the worms cause dysfunction of the blood-brain barrier of hosts, AcCPB displayed different expression profiles in non-permissive host- and permissive host-derived larval stages and was involved in the maturation of DCs, suggesting a potential role in the central nervous system invasion and the immunoregulation during parasite-host interactions.

Schistosome asparaginyl endopeptidase (legumain) is not essential for cathepsin B1 activation in vivo

Schistosomes are parasitic platyhelminths that constitute an important public health problem. Adult parasites live in the vasculature of their vertebrate hosts where they consume blood. Ingested blood proteins are degraded by a proteolytic cascade. One of the best characterized schistosome proteases is cathepsin B1 (SmCB1 or Sm31). This protein is synthesized as a large 38 kDa precursor form which is proteolytically cleaved to yield a mature, active 31 kDa enzyme. A second schistosome protease--the asparaginyl endopeptidase SmAE (also known as Sm32, or schistosome legumain), has been proposed to proteolytically convert the 38 kDa precursor SmCB1 into its mature form. Recombinant activated SmAE has been shown to trans-process SmCB1 into the mature, catalytic form in vitro. In the present study, our aim was to test the hypothesis that in vivo SmAE likewise processes SmCB1 into its active form. To do this, expression of the SmAE gene was suppressed in adult Schistosoma mansoni using RNA interference (RNAi). The results of these experiments show that, even in the absence of detectable SmAE protein, SmCB1 is fully processed and active and support the assertion that SmAE is not essential to activate SmCB1 in vivo. The data indicate that our original hypothesis is incorrect and that SmAE is not pivotal in the in vivo conversion of cathepsin B1 into its mature, active form.

Proteases in egg, miracidium and adult of Fasciola gigantica. Characterization of serine and cysteine proteases from adult

Proteolytic activity of 0-12 day old eggs, miracidium and adult worm of Fasciola gigantica was assessed and proteases were partially purified by DEAE-Sepharose and CM-cellulose columns. Four forms of protease were separated, PIa, PIb, PIc and PII. Purifications were completed for PIc and PII using Sephacryl S-200 chromatography. A number of natural and synthetic proteins were tested as substrates for F. gigantica PIc and PII. The two proteases had moderate activity levels toward azoalbumin and casein compared to azocasein, while gelatin, hemoglobin, albumin and fibrin had very low affinity toward the two enzymes. Amidolytic substrates are more specific to protease activity. PIc had higher affinity toward BAPNA-HCl (N-benzoyl-arginine-p-nitroanilide-HCl) and BTPNA-HCl (N-benzoyl-tyrosine-p-nitroanilide-HCl) at pH 8.0 indicating that the enzyme was a serine protease. However, PII had higher affinity toward BAPNA at pH 6.5 in the presence of sulfhydryl groups (beta-mercaptoethanol) indicating that the enzyme was a cysteine protease. The effect of specific protease inhibitors on these enzymes was studied. The results confirmed that proteases PIc and PII could be serine and cysteine proteases, respectively. The molecular weights of F. gigantica PIc and PII were 60,000 and 25,000, respectively. F. gigantica PIc and PII had pH optima at 7.5 and 5.5 and K(M) of 2 and 5 mg azocasein/mL, respectively. For amidolytic substrates, PIc had K(M) of 0.3 mM BAPNA/mL and 0.5 mM BTPNA/mL at pH 8.0 and PII had K(M) of 0.6 mM BAPNA/mL at pH 6.5 with reducing agent. F. gigantica PIc and PII had the same optimum temperature at 50 degrees C and were stable up to 40 degrees C. All examined metal cations tested had inhibitory effects toward the two enzymes. From substrate specificity and protease inhibitor studies, PIc and PII could be designated as serine PIc and cysteine PII, respectively.

SmCB2, a novel tegumental cathepsin B from adult Schistosoma mansoni

Papain-like cysteine endopeptidases have been recognized as potential targets for chemotherapy and serodiagnostic reagents in infections with the human parasitic helminth Schistosoma. A novel cathepsin B endopeptidase from adult S. mansoni has been isolated and characterized. The enzyme is termed SmCB2 to distinguish it from the first recorded schistosome cathepsin B, SmCB1, also known as Sm31. A rapid and convenient protocol involving anion exchange and affinity chromatography is described for the isolation of SmCB1 and SmCB2 from the same parasite starting material. SmCB2 has been functionally expressed in and purified from Pichia pastoris. Both native and recombinant SmCB2 migrate similarly (33 kDa) by SDS-PAGE. Both display strict acidic pH activity profiles and similar K(m) and k(cat) for dipeptidyl amidomethylcoumarin substrates. We conclude that the recombinant enzyme is properly folded. The S(2) subsite specificity of recombinant SmCB2 exhibits the preferences Phe>Leu>Val>>Arg. By immunoblotting with anti-SmCB2 IgG, a 33 kDa protein was identified in soluble extracts of male schistosomes. By immunohistochemistry, SmCB2 was localized in the tegumental tubercles and parenchyma of males with less product being visualized in the parenchyma of females. The enzyme may be lysosomal and function at the host parasite-interface.

Hemoglobin-degrading, aspartic proteases of blood-feeding parasites: substrate specificity revealed by homology models

Blood-feeding parasites, including schistosomes, hookworms, and malaria parasites, employ aspartic proteases to make initial or early cleavages in ingested host hemoglobin. To better understand the substrate affinity of these aspartic proteases, sequences were aligned with and/or three-dimensional, molecular models were constructed of the cathepsin D-like aspartic proteases of schistosomes and hookworms and of plasmepsins of Plasmodium falciparum and Plasmodium vivax, using the structure of human cathepsin D bound to the inhibitor pepstatin as the template. The catalytic subsites S5 through S4' were determined for the modeled parasite proteases. Subsequently, the crystal structure of mouse renin complexed with the nonapeptidyl inhibitor t-butyl-CO-His-Pro-Phe-His-Leu [CHOHCH(2)]Leu-Tyr-Tyr-Ser- NH(2) (CH-66) was used to build homology models of the hemoglobin-degrading peptidases docked with a series of octapeptide substrates. The modeled octapeptides included representative sites in hemoglobin known to be cleaved by both Schistosoma japonicum cathepsin D and human cathepsin D, as well as sites cleaved by one but not the other of these enzymes. The peptidase-octapeptide substrate models revealed that differences in cleavage sites were generally attributable to the influence of a single amino acid change among the P5 to P4' residues that would either enhance or diminish the enzymatic affinity. The difference in cleavage sites appeared to be more profound than might be expected from sequence differences in the enzymes and hemoglobins. The findings support the notion that selective inhibitors of the hemoglobin-degrading peptidases of blood-feeding parasites at large could be developed as novel anti-parasitic agents.

Vaccine efficacy of recombinant cathepsin D aspartic protease from Schistosoma japonicum

Mice were vaccinated with recombinant Schistosoma japonicum cathepsin D aspartic protease, expressed in both insect cells and bacteria, in order to evaluate the vaccine efficacy of the schistosome protease. Mean total worm burdens were significantly reduced in vaccinated mice by 21-38%, and significant reductions in female worm burdens were also recorded (22-40%). Vaccination did not reduce fecundity; rather, we recorded increased egg output per female worm in vaccinated animals, suggesting a crowding effect. Vaccinated mice developed high levels of antibodies (predominantly IgG1, IgG2a and IgG2b isotypes), but there was no correlation between antibody levels and protective efficacy. Immune sera from vaccinated mice did not inhibit the in vitro degradation of human haemoglobin by the recombinant protease, and passive transfer of serum or antibodies from vaccinated animals, before and after parasite challenge, did not significantly reduce worm or egg burdens in recipient animals. These results suggest that antibodies may not play a key role in the protective effect elicited, and that protection may be due to a combination of humoral and cell-mediated responses

Proteolysis of human hemoglobin by schistosome cathepsin D

Schistosomes feed on human blood. They employ proteases to degrade hemoglobin from ingested erythrocytes, using the residues released for amino acid metabolism. However, the identity and the role of the participating protease(s) are unclear and controversial. Confocal microscopy localized schistosomal cathepsin D to the parasite gastrodermis, and revealed elevated protease expression in females. At sub-cellular level, cathepsin D was localized to superficial digestive vacuoles of the gut and to cisternae of the gastrodermal rough endoplasmic reticulum. Schistosome cathepsin D, expressed in insect cells, autoactivated at pH 3.6 to a approximately 40 kDa form that cleaved the substrates o-aminobenzoyl-Ile-Glu-Phe-nitroPhe-Arg-leu-NH(2) and hemoglobin. The NH(2)-terminal residues of mature cathepsin D of Schistosoma japonicum and Schistosoma mansoni were Asn1 and Gly1, respectively, revealing that the proregion peptide was comprised of 35 residues. The proteases cleaved hemoglobin at pH 2.5--4.6, releasing numerous fragments. S. Japonicum cathepsin D cleaved at 13 sites, S. mansoni cathepsin D at 15 sites. Early cleavage sites were alpha Phe33-Leu34 and beta Phe41-Phe42, while others included alpha Leu109-Ala-110 and beta Leu14-Trp15, demonstrating a preference for bulky hydrophobic residues at P1 and P1'. Most of the schistosomal cathepsin D cleavage sites were discrete from those of human cathepsin D. The gastrodermal location, elevated expression in females, acidic pH optima, similar substrate preferences in two species, and the discrete substrate preferences compared with human cathepsin D together provide compelling support for the hypothesis that schistosomal cathepsin D plays an integral role in hemoglobin proteolysis, and might be selectively targeted by drugs based on protease inhibition.

Proteinases and associated genes of parasitic helminths

Many parasites have deployed proteinases to accomplish some of the tasks imposed by a parasitic life style, including tissue penetration, digestion of host tissue for nutrition and evasion of host immune responses. Information on proteinases from trematodes, cestodes and nematode parasites is reviewed, concentrating on those worms of major medical and economical importance. Their biochemical characterization is discussed, along with their putative biological roles and, where available, their associated genes. For example, proteinases expressed by the various stages of the schistosome life-cycle, in particular the well-characterized cercarial elastase which is involved in the penetration of the host skin and the variety of proteinases, such as cathepsin B (Sm31), cathepsin L1, cathepsin L2, cathepsin D, cathepsin C and legumain (Sm32), which are believed to be involved in the catabolism of host haemoglobin. The various endo- and exoproteinases of Fasciola hepatica, the causative agent of liver fluke disease, are reviewed, and recent reports of how these enzymes have been successfully employed in cocktail vaccines are discussed. The various proteinases of cestodes and of the diverse superfamilies of parasitic nematodes are detailed, with special attention being given to those parasites for which most is known, including species of Taenia, Echinococcus, Spirometra, Necator, Acylostoma and Haemonchus. By far the largest number of papers in the literature and entries to the sequence data bases dealing with proteinases of parasitic helminths report on enzymes belonging to the papain superfamily of cysteine proteinases. Accordingly, the final section of the review is devoted to a phylogenetic analysis of this superfamily using over 150 published sequences. This analysis shows that the papain superfamily can be divided into two major branches. Branch A contains the cathepin Bs, the cathepsin Cs and a novel family termed cathepsin Xs, while Branch B contains the cruzipains, cathepsin Ls, papain-like and aleurain/cathepsin H-like proteinases. The relationships of the helminth proteinases, and similar proteinases from protozoan parasites and other organisms, within these groups are discussed.

Recombinant expression and localization of Schistosoma mansoni cathepsin L1 support its role in the degradation of host hemoglobin

Cysteine proteinases expressed by schistosomes appear to play key roles in the digestion of host hemoglobin, the principal source of amino acid nutrients utilized by these parasites. We have shown previously that the predominant cysteine proteinase activity in soluble extracts and excretory/secretory (ES) products of adults of Schistosoma mansoni and S. japonicum is cathepsin L-like in its substrate specificity. However, biochemical analysis of the cathepsin L activity in extracts and ES products of schistosomes has been complicated by the presence of at least two distinct forms of schistosome cathepsin L, termed SmCL1 and SmCL2. We now report the purification and enzyme characteristics of active, recombinant SmCL1 which was obtained by transforming Saccharomyces cerevisiae with an expression plasmid encoding the preproenzyme of SmCL1. Recombinant SmCL1 was secreted by the transformed yeast into the culture media from which it was purified by gel filtration and ion-exchange chromatography. The purified enzyme exhibited substrate specificity against synthetic peptidyl substrates (e.g., Boc-Val-Leu-Lys-NHMec and Z-Phe-Arg-NHMec; kcat/Km = 17.25 and 6.24 mM-1 s-1, respectively) and against gelatin and hemoglobin, characteristic of cathepsin L. Immunoblot analysis using antiserum raised against recombinant SmCL1 demonstrated that native SmCL1 of 33 kDa was present in ES products and soluble extracts of S. mansoni. Using this antiserum and thin tissue sections, we localized the native SmCL1 to the gastrodermis and to the tegument of adult schistosomes. Recombinant SmCL1 was capable of degrading human hemoglobin at pH 4.0 to 4.5 but not higher, suggesting that denaturation of hemoglobin by low pH, as found in the cecum of the adult schistosome, may be necessary for its catalysis by cathepsin L and other gut-associated proteinases. Together, these results support a role for SmCL1 in the degradation of host hemoglobin within the gut of the schistosome.

Diagnostic significance of Schistosoma mansoni proteins Sm31 and Sm32 in human schistosomiasis in an endemic area in Egypt

We performed a series of ELISAs to evaluate the diagnostic significance of two Schistosoma mansoni proteins, Sm31 (cysteine proteinase, cathepsin B) and Sm32 (asparaginyl endopeptidase). Our study populations were chosen from two villages in an endemic area close to Alexandria. Using fusion proteins MS2-Sm31 and MS2-Sm32 as antigens, 70% and 78.9%, respectively, of patient sera from 134 parasitologically confirmed cases reacted positively. The percentage of seropositivity increased to 84.5% when parasite-derived proteins Sm31 and Sm32 were used. The serum levels of antibodies to these two proteins in recombinant or native forms do not correlate with intensity of infection and hence are detected even when egg counts are low, which makes proteins Sm31 and Sm32 useful antigens in the identification of S. mansoni infected cases, particularly in endemic areas in Egypt.

Proteolytic degradation of host hemoglobin by schistosomes

Schistosomes acquire amino acids for growth, development, and reproduction by catabolizing hemoglobin obtained from ingested host erythrocytes. While the biochemical pathway(s) involved has not been determined definitively, a number of proteases including schistosome legumain and cathepsin L-, D-, B- and C-like enzymes have been ascribed roles in the degradation of hemoglobin to diffusible peptides. Transcripts encoding these schistosome proteases, which appear to be expressed in the gastrodermis and cecum of the schistosome, have been reported. Because these enzymes are candidate targets at which to direct novel anti-schistosomal therapies, the comparative biochemistry of these and their counterpart mammalian proteases is now the focus of research in a number of laboratories. This paper reviews reports dating from 40 years ago to the present on how schistosomes digest host-derived hemoglobin, and interprets apparent anomalies in some earlier compared to later reports, the latter having benefited from the availability of PCR and gene cloning technologies. More specifically, the review concentrates on five proteolytic enzymes, and their associated genes, which have been ascribed key roles in the pathway of hemoglobin degradation.

The in vitro uptake and incorporation of hemoglobin by adult Haemonchus-contortus

The incorporation of radioactivity from [3H]leucine-labeled hemoglobin (Hb) into adult Haemonchus contortus proteins was investigated. Further, the role of previously described cysteine proteases present in intestinal tissue and excretory/secretory products of H. contortus was assessed in the breakdown of Hb. A cell lysate preparation (predominantly Hb) was obtained from reticulocytes metabolically labeled, in vitro, with [3H]leucine. Following 24-h incubation in the presence of [3H]Hb, adult H. contortus incorporated radioactivity. The presence of the protein synthesis inhibitor puromycin (200 micrograms ml-1) reduced incorporation by 72%, indicating that this process was dependent on protein synthesis. The specific cysteine protease inhibitor Z-phe-ala-FMK (PAF) at 0.1 mM had no effect on incorporation of radioactivity; however, the breakdown of Hbg in the culture medium was reduced by 50%. In contrast, PAF at 1.0 mM caused a 78% reduction in incorporated radioactivity. Parasite viability was also decreased by 1.0 mM PAF, and thus the reduction of incorporation of radioactivity may not be due to specific enzyme inhibition. The serine protease inhibitor 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF) at 1.0 mM caused a 40% reduction in incorporation of radioactivity; the aspartic protease inhibitor pepstatin (1 mM) was without effect. Adult H. contortus also incorporated radioactivity from [3H]leucine-labeled intact reticulocytes. This incorporation was inhibited-by 1.0 mM PAF and AEBSF in a manner similar to that for the cell lysate preparation. These data indicate that adult H. contortus degrade Hbg and incorporate the radioactivity into their macromolecules. The specific action of the endogenous cysteine protease in the digestion of Hbg could not be demonstrated unequivocally. However, the hypothesis that the secreted cysteine protease functions in extracorporeal digestion was supported.