The role of the C-terminus for catalysis of the large thioredoxin reductase from Plasmodium falciparum

The thioredoxin system is one of the major thiol reducing systems of the cell. Recent studies have revealed that Plasmodium falciparum and human thioredoxin reductase represent a novel class of enzymes, which are substantially different from the isofunctional prokaryotic Escherichia coli enzyme. We identified the cysteines Cys88 and Cys93 as the redox active disulfide and His509 as the active site base [Gilberger, T.-W., Walter, R.D. and Müller, S., J. Biol. Chem. 272 (1997) 29584-29589]. In addition to the active site thiols Cys88 and Cys93 the P. falciparum enzyme has another pair of cysteines at the C-terminus: Cys535 and Cys540. To assess the possible role of these peripheral cysteines in the catalytic process the single mutants PfTrxRC535A and PfTrxRC540A, the double mutant P/TrxRC535AC540A and the deletion mutant PfTrxRdelta9 (C-terminal deletion of the last nine amino acids) were constructed. All mutants are defective in their thioredoxin reduction activity, although they still show reactivity with 5,5'-dithiobis (2-nitrobenzoate). These data imply that the C-terminal cysteines are crucially involved in substrate coordination and/or electron transfer during reduction of the peptide substrate.

Identification of stress-responsive genes in Caenorhabditis elegans using RT-PCR differential display

In order to identify genes that are differentially expressed as a consequence of oxidative stress due to paraquat we used the differential display technique to compare mRNA expression patterns in Caenorhabditis elegans . A C.elegans mixed stage worm population and a homogeneous larval population were treated with 100 mM paraquat, in parallel with controls. Induction of four cDNA fragments, designated L-1, M-47, M-96 and M-132, was confirmed by Northern blot analysis with RNA from stressed and unstressed worm populations. A 40-fold increase in the steady-state mRNA level in the larval population was observed for the L-1/M-47 gene, which encodes the detoxification enzyme glutathione S-transferase. A potential stress-responsive transcription factor (M-132) with C2H2-type zinc finger motifs and an N-terminal leucine zipper domain was identified. The M-96 gene encodes a novel stress-responsive protein. Since paraquat is known to generate superoxide radicals in vivo , the response of the C.elegans superoxide dismutase (SOD) genes to paraquat was also investigated in this study. The steady-state mRNA levels of the manganese-type and the copper/zinc-type SODs increased 2-fold in the larval population in response to paraquat, whereas mixed stage populations did not show any apparent increase in the levels of these SOD mRNAs.

Identification and characterization of the functional amino acids at the active site of the large thioredoxin reductase from Plasmodium falciparum

The thioredoxin system, composed of the pyridine nucleotide-disulfide oxidoreductase thioredoxin reductase, the small peptide thioredoxin, and NADPH as a reducing cofactor, is one of the major thiol-reducing systems of the cell. Recent studies revealed that Plasmodium falciparum and human thioredoxin reductase represent a novel class of enzymes, called large thioredoxin reductases. The large thioredoxin reductases are substantially different from the isofunctional prokaryotic Escherichia coli enzyme. The putative essential amino acids at the catalytic center of large thioredoxin reductase from P. falciparum were determined by using site-directed mutagenesis techniques. To analyze the putative active site cysteines (Cys88 and Cys93) three mutant proteins were constructed substituting alanine or serine residues for cysteine residues. Further, to evaluate the function of His509 as a putative proton donor/acceptor of large thioredoxin reductase this residue was replaced by either glutamine or alanine. All mutants were expressed in the E. coli system and characterized. Steady state kinetic analysis revealed that the replacement of Cys88 by either alanine or serine and Cys93 by alanine resulted in a total loss of enzymatic activity. These results clearly identify Cys88 and Cys93 as the active site thiols of large thioredoxin reductase. The replacement of His509 by glutamine yielded in a 95% loss of thioredoxin reductase activity; replacement by alanine provoked a loss of 97% of enzymatic activity. These results identify His509 as active site base, but imply that its function can be substituted, although inefficiently, by an alternative proton donor, similar to glutathione reductase. Spectral analysis of wild-type P. falciparum thioredoxin reductase revealed a 550-nm absorption band upon reduction which resembles the EH2 form of glutathione reductase and lipoamide dehydrogenase. This spectral feature, recently also reported for the human placenta protein (Arscott, L. D., Gromer, S., Schirmer, R. H., Becker K., and Williams, C. H., Jr. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 3621-3626), further illustrates the similarity between large thioredoxin reductases and glutathione reductases and stresses the profound differences to small E. coli thioredoxin reductase.

Seasonal changes in enzymes of lipogenesis and triacylglycerol synthesis in the golden-mantled ground squirrel (Spermophilus lateralis)

In order to determine whether critical enzyme activities of glycerolipid synthesis change seasonally in the golden-mantled ground squirrel (Spermophilus lateralis), we collected summer and winter samples of liver, brown adipose tissue (BAT), and white adipose tissue (WAT). Compared with fatty acid synthase activity during hibernation, summer activities were 2.5- to 8-fold higher in adipose tissue and liver. Diacylglycerol acyltransferase (DGAT) activity was 2.6-fold higher in WAT during the summer, consistent with increased seasonal triacylglycerol storage, but the activity did not change in liver or BAT, suggesting that in these tissues, triacylglycerol synthesis is equally active in summer and winter. Lack of change in acyl-CoA synthetase in liver and BAT may reflect high synthetic rates for acyl-CoAs that are destined in the summer for glycerolipid synthesis and in the winter for beta-oxidation. Monoacylglycerol acyltransferase (MGAT) activity increased significantly in both liver and WAT during the summer but decreased in BAT. Although the changes were consistent with active year-round triacylglycerol synthesis, the higher summer MGAT activity observed in the squirrel liver and WAT suggest that MGATs function may not be limited to conserving essential fatty acids during physiological states of lipolysis. Seasonal changes observed in the ground squirrel were similar to those previously reported in the yellow-bellied marmot (Marmota flaviventris), confirming that important adjustments occur in energy metabolism necessitated by long seasonal hibernation.

Localization and functional analysis of the cytosolic and extracellular CuZn superoxide dismutases in the human parasitic nematode Onchocerca volvulus

This study describes the histological localization of two CuZn superoxide dismutases (SOD1 and SOD2) in the parasitic nematode Onchocerca volvulus, and a functional characterization of the 'extracellular' form of this enzyme (SOD2) which provides evidence that it is involved in the defense against environmental superoxide anion radicals. These essential enzymes are detected in larval and adult stages of the parasite, determined at the mRNA and protein levels by in situ hybridization and immunolocalization studies. These proteins are distributed throughout the worm, at various concentrations with particularly high levels produced in the hypodermis. In vitro maintenance of parasites indicated that SOD2 was secreted outside the parasite into the medium. Baculovirus constructs designed to test the ability of the SOD2 hydrophobic N-terminal region to function in processing and secretion confirmed the ability of this polypeptide sequence to direct the secretion of a marker protein, as well as of the mature SOD2 enzyme. Analyses of the native, mature SOD2 enzyme molecular mass, and the primary and quaternary structure, indicate that unlike other extracellular SODs, the SOD2 is active as a non-glycosylated dimer, rather than as a tetrameric glycoprotein. The detection of SOD2 outside of the parasite maintained in vitro, and the confirmation that the SOD2 is a secreted enzyme, indicate that this enzyme plays a role in the interactive biology of parasitic nematodes with their hosts.

Molecular characterization and expression of Onchocerca volvulus glutathione reductase

Glutathione metabolism represents a potential target for anti-parasite drug design. The central role of glutathione reductase (GR) in maintenance of the thiol redox state and in anti-oxidative defence has to be evaluated in more detail in order to establish the essential function of this enzyme for the survival of the filarial parasite Onchocerca volvulus. The O. volvulus GR (OvGR) gene was cloned and sequenced. The gene is composed of 13 exons and 12 introns and spans 4065 bp. The first intron is located within the 5'-untranslated region of the gene, 16 nucleotides upstream of the translation initiation codon. Southern-blot analysis and structural characterization of the genomic sequence indicate that OvGR is encoded by a single-copy gene. Isolation of various cDNA clones revealed a polymorphism of polyadenylation initiation with no consensus polyadenylation sites in any of the cDNAs analysed. The entire cDNA is 1977 bp long and carries the nematode-specific spliced leader sequence SL1 at its 5' end, 236 nucleotides upstream of the first in-frame methionine. The cDNA codes for a polypeptide of 462 amino acids with 53.5% sequence identity with human GR (HsGR). A total of 18 out of 19 residues contributing to glutathione binding are identical in OvGR and HsGR. However, one of the arginine residues (Arg-224 in HsGR) involved in discrimination between NADPH and NADH in all known GRs is substituted by tryptophan (Trp-207 in OvGR). The coding region of OvGR was expressed in Escherichia coli as a histidine-fusion protein, and it was established that the parasite protein still favours the binding of NADPH (Km 10.9 microM) over NADH (Km 108 microM). The histidine-fusion protein has a subunit size of 54 kDa and is active as a homodimer of 110 kDa.

Structural and functional analysis of a glutathione S-transferase from Ascaris suum

A recombinant glutathione S-transferase (GST) (EC 2.5.1.18) from the parasitic nematode Ascaris suum (AsGST1) displays specific activity with a variety of model substrates and secondary products of lipid peroxidation. The AsGST1 interacts with a range of model inhibitors, haematin-related compounds, bile acids and anthelminthics. The reported variations in biochemical activity correlate with structural differences observed by homology modelling. Here, differences in the topography of the proposed substrate binding site between the AsGST1 and the host GSTs were identified. A rabbit polyclonal antiserum was raised against the glutathione-binding proteins of A. suum and specific antibodies against AsGST1 were affinity-purified using the recombinant protein. These antibodies were used to localize the AsGST1 in adult worms by immunohistochemical staining. The strongest immunostaining for AsGST1 was localized in the intestine in all worms examined. This suggests that the enzyme may be responsible for the metabolism of materials that are incorporated from the environment, as well as for molecules that are excreted or secreted from the parasite to the environment. It also demonstrates the accessibility of the enzyme to an inhibitor or blocking antibody. In addition, the structure and sequence of the gene encoding AsGST1 have been determined. Southern-blot analyses of the AsGST1 gene suggests that it is a single-copy gene. The nucleotide sequence analysis revealed that the gene is composed of four exons and three introns, and potential regulatory elements were identified in the 5' flanking sequence.

Isolation and partial characterization of a fatty-acid-binding protein from Ascaris suum reproductive tissue

A 12-kDa fatty-acid-binding protein was purified to homogeneity from Ascaris suum reproductive tissue as confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. N-terminal amino-acid-sequence analysis of the protein revealed its identity with the ABA-1 allergen protein isolated from A. suum pseudocoelomic fluid. Fatty-acid binding by the protein from A. suum reproductive tissue was investigated using the Lipidex 1000 assay, which revealed the presence of a single class of fatty-acid-binding sites with an apparent dissociation constant for palmitate of about 0.8 microM.

A novel trans-spliced mRNA from Onchocerca volvulus encodes a functional S-adenosylmethionine decarboxylase

Complete cDNA and genomic sequences encoding the Onchocerca volvulus S-adenosylmethionine decarboxylase (SAMDC), a key enzyme in polyamine biosynthesis, have been isolated and characterized. The deduced amino acid sequence encodes a 42 kDa proenzyme with a moderate level of sequence homology to eukaryotic SAMDCs. Enzymically active O. volvulus SAMDC was expressed at a high level in an Escherichia coli mutant strain lacking endogenous SAMDC. The recombinant enzyme was purified to homogeneity using DEAE-cellulose, methylglyoxal bis(guanylhydrazone)-Sepharose and Superdex S-200 chromatography. It was determined that the recombinant proenzyme is cleaved to produce 32 and 10 kDa subunits. The sequence of the N-terminal portion of the large subunit was determined and comparison with the sequence of the proenzyme revealed that the precise cleavage site lies between Glu86 and Ser87. Gel-filtration experiments demonstrated that these two subunits combine to form an active heterotetramer. Comparison of the cDNA and genomic sequences revealed that the SAMDC mRNA undergoes both cis- and trans-splicing in its 5'-untranslated region (UTR). Anchored PCR on O. volvulus mRNA confirmed the cDNA sequence and identified two distinct trans-spliced products, a 22-nucleotide spliced-leader sequence and a 138 bp sequence containing the 22 nucleotide spliced-leader sequence. Genomic Southern-blot analysis suggests that the O. volvulus SAMDC is encoded by a single-copy gene. This gene spans 5.3 kb and is comprised of nine exons and eight introns. The first intron is located in the 5'-UTR and processing of this intron has a potential regulatory function. The 5'-flanking region of the gene contains potential transcriptional regulatory elements such as a TATA box, two CAAT boxes and AP-1-, C/EBP-, ELP-, H-APF-1-, HNF-5- and PEA3-binding sites.

Biochemical analysis, gene structure and localization of the 24 kDa glutathione S-transferase from Onchocerca volvulus

Survival of Onchocerca volvulus, a pathogenic human filarial parasite, is likely to depend upon the detoxification activities of the glutathione S-transferases (GSTs). The 24 kDa O. volvulus GST, OvGST2, was expressed in a bacterial system and the recombinant protein was purified to homogeneity by affinity chromatography. Specific activities of the recombinant OvGST2 (rOvGST2) with a variety of substrates, and in the presence of inhibitors, were determined. With the universal substrate 1-chloro-2,4-dinitrobenzene, the specific activity of rOvGST2 was 2130 nmol min-1 mg-1. The rOvGST2 showed relatively limited selenium-independent glutathione peroxidase activity, but secondary products of lipid peroxidation, namely members of the trans,trans-alka-2,4-dienal,trans-alk-2-enal and 4-hydroxyalk-2-enal series, were conjugated to glutathione via OvGST2 dependent activity. The gene encoding the OvGST2 was isolated and the nucleotide sequence determined. The ovgst2 gene was found to possess seven exons with six intervening sequences, with all except one having consensus splice-site junctions. This intron/exon organisation of the ovgst2 gene is almost identical with those described for the mammalian Pi class GST genes, consistent with the protein structural evidence that the OvGST2 is related to the Pi class GSTs. Southern blot analysis with total parasite genomic DNA indicated a single copy gene, with a restriction pattern consistent with that of the isolated gene. The tissue distribution of the OvGST2 was examined in O. volvulus by immunohistochemistry and was shown to be distinct from that of the OvGST1. The OvGST2 was located throughout the syncytial hypodermis of male and female adult worms, as well as in the uterine epithelium. Microfilariae, and infective third stage larvae of O. volvulus, isolated from Simulium neavei, were immunopositive for OvGST2.

Panagrellus redivivus ornithine decarboxylase: structure of the gene, expression in Escherichia coli and characterization of the recombinant protein

A southern blot analysis of the Panagrellus redivivus ornithine decarboxylase (ODC) gene suggests that it is a single-copy gene that resides on a genomic 3.2 kb EcoRI fragment. Phage clones possessing ODC gene sequences were isolated from a genomic EMBL-4 library and purified. The phage DNA inserts were analysed and a 3.2 kb EcoRI fragment containing the entire ODC gene was isolated. The nucleotide sequence analysis of this fragment reveals that the gene is interrupted by two introns of 47 and 49 bp. In the 5' non-translated region of the gene, putative AP1, VPE2 and c-Myc binding sites were identified. The ODC cDNA was expressed in a bacterial system as a His-fusion protein and the enzyme was purified by Ni(2+)-chelating affinity chromatography. The subunit molecular mass, as deduced from the cDNA and shown by SDS/PAGE, is 47.1 kDa. On the basis of gel filtration analyses it is shown that the active enzyme is a dimer. The specific enzyme activity was determined to be 4.2 mumol CO2/min/mg protein. The enzyme is dependent on pyridoxal 5-phosphate as a cofactor, and the presence of dithioerythritol or other thiol-reducing agents is essential for maximal activity. The Km value for L-ornithine was determined as 44 microM. The Ki values for putrescine, alpha-diffluoromethylornithine, alpha-hydrazino-ornithine and alpha-methylornithine were calculated as 51, 34, 0.34 and 42 microM respectively.

Polyamine N-acetyltransferase in Leishmania amazonensis

N-Acetyltransferase, which is suggested to be responsible for the production of N1-acetylspermidine in Leishmania amazonensis and to be involved in the process of inactivation and degradation of excessive polyamines, was partially purified and characterized. Among the substrates tested, sym-norspermidine, sym-norspermine, and 1,3-diaminopropane had the highest reaction rates, but the naturally occurring polyamines spermine and spermidine were also acetylated at considerable rates, whereas putrescine was a poor substrate. The Michaelis constants (Km values) for spermine and spermidine were 0.66 and 3.3 mM, respectively. The Km value for acetylcoenzyme A (acetyl-CoA) was determined to be 34 microM. CoA inhibited the reaction in a competitive manner; the inhibition constant was 5 microM. The enzyme showed an apparent relative molecular mass of 35,000.

Characterization of the arylalkylamine N-acetyltransferase in Onchocerca volvulus

The characteristics and kinetic properties of an arylalkylamine N-acetyltransferase were studied in partially purified preparations of the human filarial parasite Onchocerca volvulus. The enzyme, which had a relative molecular mass (M(r)) of 37-38 kDa, catalyzed the acetylation of arylalkylamines but did not accept arylamines or polyamines as substrates. The optimal pH for enzyme activity was found to be 8.5 in TRIS-HCI. The apparent Michaelis constant (K(m)) and maximum velocity (Vmax) determined from Lineweaver-Burk plots for tryptamine were 1.8 microM and 29 nmol min-1 mg protein-1, respectively. Except for the catecholamines, the other arylalkylamines such as 5-hydroxytryptamine (5-HT), tyramine, and octopamine similarly exhibited high affinities and reaction rates. Whereas the enzyme is inhibited by metals and p-chloro-mercuribenzoate, it is inactivated neither by amethopterin nor by cystamine and is thereby distinguished from the mammalian arylamine N-acetyltransferase. Like other N-acetyltransferases whose function is the regulation of intracellular amine levels, the enzyme may have a role in the inactivation of excess biogenic amine in this parasite.

Inhibition of glutathione synthesis of Ascaris suum by buthionine sulfoximine

We investigated the effect of DL-buthionine-S,R-sulfoximine (BSO), a selective glutathione (GSH)-depleting agent, on the GSH synthesis of Ascaris suum. The GSH concentrations of the reproductive and muscle tissues of A. suum were determined to be 8.5 +/- 0.3 and 14.3 +/- 1.3 (n = 3) nmol/mg protein, respectively. After treatment of the parasites with 10 microM BSO for 24 h, the GSH content of the reproductive tissue of A. suum was totally depleted as compared with that of untreated controls. However, the GSH levels of the muscle tissue were reduced to only 50% after treatment of the worms for 24 h with 10 microM BSO. Exogenous GSH had no significant effect on the GSH level of the parasites when the worms were incubated for 4 h in RPMI 1640 medium supplemented with 1 mM GSH. In the presence of exogenous GSH, BSO was less effective in depleting the GSH levels of the parasites, which may indicate that the parasites can replenish their GSH levels. GSH depletion, which has been discussed as being therapeutically effective when normal and tumor cells or parasites have markedly different requirements for GSH, may have applications in the development of drugs against nematode infections.

Purification and characterization of gamma-glutamyl transpeptidase from Ascaris suum

gamma-Glutamyl transpeptidase, which is of central importance in the degradation of glutathione, was purified from Ascaris suum to apparent homogeneity. The enzyme was found to be a predominantly membrane-bound protein and was solubilized by Triton X-100. The purified enzyme, which exhibits a specific activity of 1009 U (mg protein)-1, showed a molecular mass of 70 kDa and was found to be composed of two non-identical subunits of molecular mass 43 and 30 kDa. Concerning the kinetic properties of the enzyme, the data presented in this study showed that various amino acids and dipeptides with L-configuration served as acceptors for the gamma-glutamyl moieties of the enzyme reaction products and showed Km-values in the mM range. The apparent Km-value for the gamma-glutamyl donor L-glutamyl-gamma-7-amido-4-methylcoumarin of the enzyme was found to be 0.03 mM. L- and D-serine in combination with borate ions were competitive inhibitors of the enzyme activity with Ki-values of 0.30 and 0.61 mM, respectively. Acivicin was an irreversible inhibitor of the enzyme with a Ki-value of 0.42 mM and with a pseudo-first-order kinetics (kinact) of 0.18 min-1. In vitro treatment of the adult A. suum with acivicin resulted in a dose-dependent inhibition of the enzyme activity and an increase of the glutathione levels. These findings indicate the physiological role of the gamma-glutamyl transpeptidase of this parasitic nematode in the catabolism of glutathione.

Molecular cloning, expression and characterization of a recombinant glutathione S-transferase from Echinococcus multilocularis

We report the identification and characterization of the first cestode glutathione S-transferase (GST) cDNA sequence. A fragment of an Echinococcus multilocularis glutathione S-transferase cDNA was isolated by the polymerase chain reaction. Subsequently, a Lambda zap cDNA library prepared from mRNA from protoscolices of E. multilocularis was screened with this PCR fragment. A complete cDNA clone was isolated and the nucleotide sequence determined. Analysis of the E. multilocularis GST-deduced amino acid sequence indicates that it is clearly related to the mammalian mu-class GSTs. The E. multilocularis GST cDNA was expressed in Escherichia coli, using a protocol designed to produce the native enzyme rather than a fusion protein. The 25.5-kDa enzyme subunit was purified to homogeneity using glutathione-sepharose chromatography. Gel filtration demonstrated that this GST is enzymatically active as a homodimer. The recombinant enzyme had conjugating activity with organic hydroperoxides and with members of the trans,trans-2,4 alkadienal and trans-2-alkenal series, which are secondary products of lipid peroxidation.

Identification of the polyamine N8-acetyltransferase involved in the pathway of 1,3-diaminopropane production in Acanthamoeba culbertsoni

A cytosolic polyamine N-acetyltransferase that preferentially catalyzes the acetylation of spermidine in the N8-position was identified in the free-living pathogenic amoeba Acanthamoeba culbertsoni. In addition to spermidine, the enzyme also catalyzed the acetylation of spermine and putrescine with Michaelis constants (Km values) of 97, 12, and 10 microM, respectively. The Km value for acetylcoenzyme A (acetyl-CoA) was estimated to be 11 microM, whereas CoA had an inhibitory constant of 6 microM. The N-acetylase has a molecular mass of approximately 45 kDa. That the enzyme preferentially catalyzed the acetylation of spermidine at the N8-position, resulting in N8-acetylspermidine, the preferred substrate of the polyamine oxidase found in A. culbertsoni, indicates a role for the enzyme in the production of 1,3-diaminopropane, the major polyamine found in the Acanthamoeba.

Effect of haloallylamines on polyamine oxidase activity and spermine levels in Ascaris suum

In parasitic nematodes the rate-limiting step in the polyamine interconversion pathway is catalysed by polyamine oxidase. MDL 72527, the specific inhibitor of mammalian polyamine oxidase, had no effect on the Ascaris suum enzyme, whereas its activity was inhibited in a time-dependent manner by the haloallylamine MDL 72145, originally designed as a specific inhibitor of monoamine oxidase A and B. The dissociation constant (Ki) was found to be 0.9 microM and the enzyme half-life under saturation conditions (t50) was determined to be 0.8 min. Incubation of A. suum in vitro in the presence of 50 microM MDL 72145 for 6 h resulted in a decrease in polyamine oxidase activity to about 20% of the control value, and spermine concentrations simultaneously increased about 200%. Both results suggest that MDL 72145 might be a chemical lead compound for the design of new chemotherapeutic agents against nematode infections.

Plasmodium falciparum glutathione reductase exhibits sequence similarities with the human host enzyme in the core structure but differs at the ligand-binding sites

The homodimeric flavoenzyme glutathione reductase (GR) which catalyzes the reduction of glutathione disulfide is a cornerstone of the malaria parasite antioxidant defense and repair mechanisms. Here we report on the identification of the GR gene from Plasmodium falciparum. A 1.4-kb fragment of the gene was amplified by polymerase chain reaction (PCR). Using this PCR fragment as a probe a full length cDNA clone (2085 bp) was isolated from a P. falciparum gametocyte library. The deduced amino acid sequence of 541 residues shows an overall identity of 35% when compared to the human enzyme. Most amino acids of known function are identical. However, notable differences between human and parasite protein occur in the glutathione-binding pocket (for instance, Glu374 instead of the expected basic residue) and at the intersubunit contact area. These regions are of particular interest since they represent binding sites of known GR inhibitors. Consequently, parasite GR can serve as a target structure for the design of antimalarial drugs.

Molecular cloning and characterization of ornithine decarboxylase cDNA of the nematode Panagrellus redivivus

In a PCR with degenerate primers encoding highly conserved amino acids within ornithine decarboxylases (ODCs) of several organisms, a fragment of the ODC gene of the free-living nematode Panagrellus redivivus was isolated. Northern blot analysis revealed a single 1.7 kb transcript in a mixed-stage population of animals. From this RNA source, a cDNA library was constructed and screened with the PCR fragment. Several cDNA clones were isolated, one of which encodes the complete 435-amino-acid ODC enzyme with a calculated molecular mass of 47.1 kDa. The P. redivivus ODC possesses 126 of the 136 highly conserved amino acids in the enzymes from fungi, invertebrates and vertebrates. Functional amino acids are conserved, suggesting that the two active sites of the P. redivivus ODC are formed at the interface of a homodimer, as described for mammalian ODCs.

Characterization of the manganese superoxide dismutase cDNA and gene from the human parasite Onchocerca volvulus

The manganese-containing superoxide dismutase (MnSOD) is a major component of the cellular defence mechanisms against the toxic effects of the superoxide radical. Within the framework of studies on anti-oxidant enzymes and their protective role in the human parasitic nematode Onchocerca volvulus, sequences encoding the MnSOD were isolated and examined in this study. Degenerate primers were designed based upon conserved regions of MnSOD sequences from other organisms, and were used in PCR on reverse-transcribed O. volvulus total RNA and genomic DNA to identify partial cDNA and genomic DNA fragments encoding the O. volvulus MnSOD (OvMnSOD). The genomic DNA PCR product was used to screen an O. volvulus adult worm lambda unizap II cDNA library and the nucleotide sequence of the longest clone determined. The complete 5'-end of the OvMnSOD cDNA was obtained using the rapid amplification of cDNA ends (RACE) procedure with O. volvulus total RNA and was found to possess a spliced leader sequence at the 5'-terminus. The deduced primary sequence encodes a 25 kDa protein, which has the conserved residues required for enzyme activity and metal binding. The 24 N-terminal amino acids encoded by the OvMnSOD cDNA comprise a putative mitochondrial transit peptide. The OvMnSOD gene was also isolated from an O. volvulus adult worm lambda fix II genomic library, a restriction map was constructed and the nucleotide sequence determined. The OvMnSOD gene was found to possess five exons and four introns with consensus splice-site junctions. Potential regulatory elements were identified in the 5' genomic flanking sequence. Southern-blot analysis with total worm genomic DNA indicates a single-copy gene, with a restriction pattern consistent with that of the isolated gene.

Purification and characterization of gamma-glutamylcysteine synthetase from Ascaris suum

We have purified and characterized the Ascaris suum gamma-glutamylcysteine synthetase, the rate-limiting step in the glutathione biosynthesis. The purified enzyme exhibited a specific activity of 18 U (mg protein)-1. Estimation of the molecular mass of the native enzyme by FPLC on Superdex S-200 revealed the presence of two enzyme activity peaks corresponding to molecular masses of 100 and 70 kDa. The higher-molecular-mass component could be dissociated by repeated gel filtration into the 70-kDa protein which is the enzymatically active subunit. The apparent Km values of the A. suum enzyme for L-aminobutyrate, L-cysteine and L-glutamate were 0.31, 0.41 and 0.94 mM, respectively. D,L-Buthionine-S,R-sulfoximine and cystamine showed time-dependent irreversible inhibitory effects on the A. suum enzyme activity with Ki values of 0.05 and 1.11 microM, respectively. The Ki values for the corresponding enzyme from rat kidney with D,L-buthionine-S,R-sulfoximine and cystamine were 7.19 and 22.2 microM, respectively. The time of half-inactivation of the enzyme at infinite concentration of D,L-buthionine-S,R-sulfoximine, tau 50, was determined to be 3.1 and 1.34 min, for the parasite and mammalian enzymes respectively. For cystamine, a tau 50 value of 3.32 min for the A. suum gamma-glutamylcysteine synthetase was determined, while a value of 2 min in case of rat kidney enzyme was found. The A. suum enzyme activity was competitively inhibited by glutathione with a Ki value of 0.11 mM.(ABSTRACT TRUNCATED AT 250 WORDS)