Properties of the acid thiol proteinase from Schistosoma mansoni adults

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.

Cysteine protease inhibitors block schistosome hemoglobin degradation in vitro and decrease worm burden and egg production in vivo

Schistosome parasites utilize hemoglobin as a major protein source for their metabolism. Degradation of hemoglobin has been hypothesized to be mediated by both cysteine and aspartyl proteases secreted into the lumen of the parasite intestine. We now show that two distinct types of irreversible cysteine protease-specific inhibitors both arrest schistosome hemoglobin degradation in vitro. Arrest of hemoglobin degradation is followed by death of developing schistosomula 1 week later. Schistosome infected mice treated by a dose of 2 mg inhibitor per day for 1 week early in infection, and 2 weeks at the time of egg production, showed a significant reduction in worm burden, hepatomegaly, and the number of eggs produced per female worm. Histopathology showed a minimal immune response to those eggs which were produced, consistent with a delay in egg production relative to untreated infections. By tagging the inhibitor with biotin, specific cysteine protease targets were identified in extracts of schistosome worms.

Digestion of haemoglobin by schistosomes: 35 years on

Adult schistosomes obtain nutrients by digesting haemoglobin, which they obtain from ingested host red blood cells. Here John Dalton, Angela Smith, Karen Clough and Paul Brindley argue that a cathepsin L proteinase recently identified in their laboratory as the predominant cysteine proteinase activity of Schistosoma mansoni may play the leading role in haemoglobin digestion. They discuss the importance of elucidating the roles of both cathepsin B and cathepsin L in the digestion of haemoglobin, since both should be considered important targets to which novel schistosomicidal agents could be directed.

Characterization of a cysteine proteinase from adult worms of Paragonimus westermani

Paragonimus westermani, the lung fluke, is known to migrate to the pulmonary tissue of mammalian hosts and causes pathological changes in the lungs. An acidic thiol-dependent proteinase with a molecular weight of approximately 20,000 daltons was purified to homogeneity using ion-exchange chromatography and gel filtration chromatography. On SDS-PAGE, the molecular weight of the enzyme was 17,500 daltons. Isoelectric point was 6.45. The enzyme was similar to the acidic cysteine proteinase of vertebrates in the properties of pH 5.5 for at least two days when stored at 4 degrees C. The cysteine proteinase was capable of degrading collagen and hemoglobin. Sera of patients with paragonimiasis and mice infected with P. westermani reacted in immunoblots with the partially purified proteinase. This result suggested that the cysteine proteinase of P. westermani may play a role in migration in tissues, and in acquisition of nutrients by parasites from the host. It is also potentially an antigen for the serodiagnosis of paragonimiasis.

Expression and partial characterization of a cathepsin B-like enzyme (Sm31) and a proposed 'haemoglobinase' (Sm32) from Schistosoma mansoni

Schistosoma mansoni protein Sm31 is a cysteine proteinase similar to mammalian lysosomal cathepsin B, proposed to be a key enzyme in schistosome metabolism. Protein Sm32 has been identified as a putative cysteine proteinase termed a 'haemoglobinase'. Since neither Sm31 nor Sm32 have been completely purified, some controversy of the nature of the 'true' digestive enzyme still exists. By incubating a radiolabelled cysteine-proteinase active-site-directed synthetic inhibitor with total S. mansoni proteins, the target of inhibition was Sm31 and not Sm32. The selectivity and irreversibility of inactivation make affinity labelling an invaluable tool for exploring key differences among closely related enzymes and also for studying proteinase activity in a cellular environment. In order to confirm these results, we expressed the complete cDNA sequences of Sm31 and Sm32 in insect cells and analysed the recombinant gene products for proteolytic activities. Cell extracts containing S. mansoni cathepsin B, but not those expressing 'haemoglobinase', were demonstrated to cleave a synthetic substrate benzyloxycarbonyl-arginylarginylaminomethylcoumarin in fluorescence assays. Our findings confirm previous assertions that a cysteine proteinase resembling cathepsin B is the haemoglobinase involved in digestion of host proteins. Thus, the original proposal that Sm32 is a cysteine proteinase has not been verified, and its function remains unknown.

Partial purification and characterization of cysteine proteinases from various developmental stages of Paragonimus westermani

1. During development of Paragonimus westermani, larvae develop during migration within the host, and adult worms feed on pulmonary tissues, causing significant pathology in the mammalian host. In this report acidic extracts of various developmental stages (metacercariae and worms at one, two and three months of development) were examined for cysteine proteinase activity. 2. A soluble thiol-dependent proteinase activity with a native molecular weight of approximately 20,000 was isolated and partially purified. 3. The enzymes purified from the various developmental stages of the parasite had maximal activity at acidic pH and showed inhibitor susceptibilities similar to the vertebrate acidic cysteine proteinases. 4. Enzymatic activity was stable at pH 5.0 for at least two days when stored at 4 degrees C. 5. It is suggested that these enzymes may be involved in the nutrition of these parasites and/or during penetration and lysis of the tissues.

The relative roles of the intestines and external surfaces in the nutrition of monogeneans, digeneans and nematodes

Several major groups of parasitic helminths (Monogenea, Digenea and Nematoda) possess two surfaces that are potentially absorptive in nature. These are an external surface, a tegument in the platyhelminths and cuticle in the nematodes, and the intestine. This paper discusses the relative contributions of these absorptive surfaces in the nutrition of these parasitic helminths. There are many factors that determine the availability of, and a parasite's ability to absorb nutrients via either of these surfaces, and this review discusses individually some of the more important morphological, physiological and environmental factors affecting the potential nutritional roles of these surfaces. It is clear from such a summary of previous studies that the intestines and teguments (cuticles) of helminth parasites can each serve an important nutritional role. However, insufficient data make it impossible at this time to determine the relative nutritional roles of these surfaces in any single parasitic helminth.

Purification of cysteine proteinases from adult Schistosoma mansoni

Proteolytic activity against hemoglobin and low molecular weight synthetic substrates has been previously found in homogenates and excretion/secretion products of adult Schistosoma mansoni worms. This activity is stimulated in the presence of thiol compounds and is maximally active at acidic pH. To characterize further this proteolytic activity, lyophilized adult worms were extracted, and proteinases were isolated and purified. From extracts prepared in 0.2 M citrate buffer, pH 4.9, two proteinase species were purified to homogeneity by centrifugation, gel filtration, dialysis, and chromatofocusing chromatography. The proteinases, designated SMw32 and SMw28, have apparent molecular weights (SDS-PAGE) of 31,700 +/- 1400 and 27,800 +/- 1700, respectively. Both are thiol-dependent, acidic endopeptidases that cleave hemoglobin and a synthetic substrate, CBZ-arg-arg-AFC. A statistical comparison of amino acid compositions reveals that the proteinases are highly related.

Schistosoma japonicum: ultrastructural localization of a hemoglobinase using mercury labeled pepstatin

Mercury labeled pepstatin was used to demonstrate the site of a pepstatin sensitive hemoglobinase in paraformaldehyde fixed adult Schistosoma japonicum. Pepstatin was covalently attached to glutathione using dicyclohexylcarbodiimide followed by addition to methyl mercury chloride. Deposition of mercury was observed in lipid-like globules and autophagic vacuoles in the gastrodermis. Control studies were negative in all instances. These results complement previous cytochemical studies on the distribution of other acid hydrolases in the gastrodermis of schistosomes. It is hypothesized that this pepstatin sensitive enzyme probably belongs to the carboxyl class of proteinases.

Schistosoma mansoni: purification and characterization of the major acidic proteinase from adult worms

We report purification of the major digestive proteinase from adult worms of Schistosoma mansoni. This enzyme is a thiol proteinase with a pH optimum of 5 and is activated by thiol reagents. It was purified 300-fold using a combination of gel chromatography and chromatofocusing. It readily hydrolyzed hemoglobin with an apparent Km of 0.29 microM and a specific activity of 27 micrograms degraded/min/mg enzyme at 37 C. Peptides with positively charged amino acids were preferentially cleaved. The enzyme degraded Boc-Arg-Arg-7-amino-4-methyl coumarin with a kcat/Km of 9083 M-1 sec-1. Lengthening the peptide chain to 3 amino acids or substituting glycine for the amino terminal arginine resulted in decreased activity. The enzyme was inhibited by chloromethylketone-derivatized peptides of similar sequence and by leupeptin. The purified proteinase exhibits microheterogeneity in different preparations with forms ranging in molecular weight from 30,000 to 35,000, and pI 5.7-6.0.

Schistosoma mansoni: proteinase activity of "hemoglobinase" from the digestive tract of adult worms

A method of collecting samples from the Schistosoma mansoni digestive tract was used to study proteinase activity. Activity against hemoglobin and a low molecular weight synthetic substrate, carbobenzoxy-arginyl-arginyl-7-amino-4-trifluoromethylcoumarin, was demonstrated in the soluble fraction of material regurgitated by S. mansoni adults and was dependent on the addition of a thiol compound, cysteine, to the assays. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration chromatography (AcA54), two proteins with estimated mol wt of 32,500 and 28,500 were found in the regurgitant and were associated with proteinase activity against both hemoglobin and the synthetic substrate. Homogenates of intact worms showed greater specific activity (synthetic substrate) in the females. Further, in bisected worms proteinase activity paralleled protein content, suggesting that, once secreted into the lumen, proteinase activity was distributed throughout the worm digestive tract.

Schistosoma mansoni: characterization of hemolytic activity from adult worms

Homogenates of adult Schistosoma mansoni worms contain a hemolytically active component(s). Centrifugation at 10,000 g shows the major activity is present in the pellet fraction. Red blood cell lysis with the schistosome hemolytic agent is optimal at acid pH (5.0) and highly temperature dependent. The hemolytic component is resistant to boiling (5 min) and stable for extended periods of time at 38 C (22 hr). The length of the lag phase prior to hemolysis and the rate of hemolysis are both concentration and temperature dependent. Following hemolysis, red blood cell ghosts remain.

An endogenous inhibitor of thiol protease from adult lung fluke Paragonimus miyazakii

An endogenous inhibitor of thiol protease from adult Paragonimus miyazakii was found to occur during the gel filtration on Sephacryl S-300. The partially purified inhibitor was specific for thiol proteases such as ficin and papain. The inhibitor also suppressed tosyl-L-lysine alpha-naphthylester hydrolytic activity of Paragonimus thiol protease. The molecular weight of the inhibitor was found to be 430,000 by the gel filtration. This inhibitor was thermostable and resistant to trypsin and glycosidase digestions.

Purification and properties of a thiol protease from lung fluke adult Paragonimus ohirai

The thiol protease was purified from adult Paragonimus ohirai by alpha 1-antitrypsin-Sepharose, Sephadex G-75 and CM-cellulose, measuring its activities to hydrolyze hemoglobin and tosyl-L-lysine alpha-naphthyl-ester. The purified protease showed a single band on polyacrylamide disc gel isoelectrophoresis as zymogram with Tos-Lys-NE and also by protein staining, and its pI was found to be 6.4. The molecular weight was calculated to be 29,000 by gel filtration and 27,000 by SDS-polyacrylamide gel electrophoresis as a single polypeptide. The protease hydrolyzed hemoglobin and Tos-Lys-NE optimally at pH 4.0 and 5.0, respectively. The both hydrolyzing activities were inhibited by alpha 1-AT and soybean trypsin inhibitor as well as thiol protease inhibitors such as antipain, E-64 and p-hydroxymercuriphenylsulfonate. These results indicate that this enzyme is a new type thiol protease.

Strongyloides ransomi: proteolytic enzymes from larvae

The filariform larvae of Strongyloides ransomi can infect their hosts by penetration through skin. In this report, homogenates of these organisms were prepared and their proteolytic enzymes examined. Homogenates prepared in 0.2 M citrate, pH 4.0, contain two thiol-dependent proteinases with molecular weights of approximately 32,000 and 28,000. These proteinases have an acidic pH optimum and show substrate preferences and inhibitor susceptibilities similar to the vertebrate acidic cysteinyl proteinases. Homogenates prepared in 0.1 M Tris, pH 7.5, contain multiple proteolytic enzymes, active against both Azocoll and synthetic substrates. These enzymes do not require thiols for activity and they have an alkaline pH optimum. The enzymes are inhibited by both chelating agents and heavy metals, but not by serine-proteinase inhibitors. Extracts prepared in 0.1 M Tris-HCl, pH 7.5, contain endogenous proteinase inhibitors.

Isolation, partial purification and some properties of two acid proteases from adult Dirofilaria immitis

Two acid proteases were isolated from the soluble extracts of adult Dirofilaria immitis, the filarial heartworm of canines. Activity of these proteases was detected using 3H-labeled bovine alpha-casein as substrate, and they were designated Fp-I and Fp-II in order of their elution from a CM-cellulose column. The molecular weight of partially purified Fp-I was approximately 170000, and it was active between pH 4.6-5.8. The activity of Fp-I doubled in the presence of various sulfhydryl reagents at 5 mM, and it was inhibited 50-60% by the sulfhydryl inhibitors p-hydroxymercuribenzoate and iodoacetate at 1 mM, the heavy metal chelating agent o-phenanthroline at 1 mM and the peptide aldehyde protease inhibitors pepstatin (10 microM), leupeptin, antipain and chymostatin (50 microM). The molecular weight of the more extensively purified Fp-II is approximately 48000. This protease was active between pH 2.6-3.4 and was highly sensitive to inhibition by pepstatin (80% inhibition at 10 nM). Fp-II was not significantly affected by sulfhydryl reagents, sulfhydryl inhibitors, metal chelating agents or peptide aldehyde protease inhibitors other than pepstatin. These properties of dirofilarial Fp-II resemble those of mammalian cathepsin D.

Acidic thiol proteinase activity of Schistosoma mansoni cultured in vitro

Schistosoma mansoni cultured in vitro for periods from 12 days to 33 weeks were analyzed for proteinase activity using a fluorescent substrate and compared with adult S. mansoni worms from infected mice. The enzyme activity measured was that of an acidic thiol-dependent proteinase previously described. Proteinase activity appears early in development, and rises rapidly to an approximately constant proportion of the total extractable worm protein. Cultured worms have 5 to 20 times higher specific activity than in vivo-developed worms. However, cultured worms and in vivo worms have approximately equivalent activities per worm.