Polyamine metabolism in filarial worms

Developmental regulation of Dirofilaria immitis microfilariae and evaluation of ecdysone signaling pathway transcript level using droplet digital PCR

BACKGROUND

Current measures for the prevention of dirofilariasis, caused by the dog heartworm, Dirofilaria immitis, rely on macrocyclic lactones, but evidence of drug-resistant isolates has called for alternative approaches to disease intervention. As microfilariae are known to be in a state of developmental arrest in their mammalian host and then undergo two molts once inside the arthropod, the aim of this study was to look at the developmental regulation of D. immitis microfilariae that occurs in their arthropod host using in vitro approaches and to investigate the role of the ecdysone signaling system in this development regulation.

METHODS

Dirofilaria immitis microfilariae extracted from dog blood were incubated under various culture conditions to identify those most suitable for in vitro culture and development of the microfilariae, and to determine the effects of fetal bovine serum (FBS), mosquito cells, and ecdysteroid on the development of the microfilariae. Transcript levels of the ecdysone signaling pathway components were measured with droplet digital PCR (ddPCR).

RESULTS

In vitro conditions that best promote early development of D. immitis microfilariae to the "late sausage stage" have been identified, although shedding of the cuticle was not observed. FBS had inhibitory effects on the development and motility of the microfilariae, but media conditioned with Anopheles gambiae cells were favorable to microfilarial growth. The transcript level study using ddPCR also showed that ecdysone signaling system components were upregulated in developing microfilariae and that 20-hydroxyecdysone increased the proportion of larvae developing to the sausage and late sausage stages in vitro.

CONCLUSIONS

The arthropod host environment provides cues required for the rapid development of D. immitis microfilariae, and the ecdysone signaling system may play an important role in filarial nematode developmental transitions. This study contributes to a better understanding of the developmental process of D. immitis microfilariae.

Two decades of antifilarial drug discovery: a review

Filariasis is one of the oldest, most debilitating, disabling, and disfiguring neglected tropical diseases with various clinical manifestations and a low rate of mortality, but has a high morbidity rate, which results in social stigma.

Molecular and biochemical characterisation of ornithine decarboxylases in the sheep abomasal nematode parasites Teladorsagia circumcincta and Haemonchus contortus

Full length cDNA encoding ornithine decarboxylases (ODC; EC 4.1.1.17) were cloned from the sheep abomasal nematode parasites Teladorsagia circumcincta (TcODC) and Haemonchus contortus (HcODC). The TcODC (1272 bp) and HcODC cDNA (1266 bp) encoded 424 and 422 amino acid proteins respectively. The predicted TcODC amino acid sequence showed 87% identity with HcODC and 65% and 64% with Caenorhabditis elegans and Caenorhabditis briggsae ODC respectively. All binding sites and active regions were completely conserved in both proteins. Soluble N-terminal His-tagged ODC proteins were expressed in Escherichia coli strain BL21, purified and characterised. The recombinant TcODC and HcODC had very similar kinetic properties: K(m) ornithine was 0.2-0.25 mM, optimum [PLP] was 0.3 mM and the pH optima were pH 8. No enzyme activity was detected when arginine was used as substrate. One millimolar difluoromethylornithine (DFMO) completely inhibited TcODC and HcODC activity, whereas 2 mM agmatine did not inhibit activity. The present study showed that ODC is a separate enzyme from arginine decarboxylase and strictly uses ornithine as substrate.

Biochemical composition and metabolic pathways of filarial worms Setaria cervi: search for new antifilarial agents

The main problem regarding the chemotherapy of filariasis is that no safe and effective drug is available yet to combat the adult human filarial worms. Setaria cervi, the causal organism of setariasis and lumbar paralysis in cattle, is routinely employed as a model organism for conducting biochemical and enzymatic studies on filarial parasites. In view of the practical difficulties in procuring human strains of Wuchereria bancrofti and Brugia malayi for drug screening, the bovine filarial parasite S. cervi, resembling the human species in having microfilarial periodicity and chemotherapeutic response to known antifilarial agents, is widely used as a model in such studies. For a rational approach to antifilarial chemotherapy, knowledge of the biochemical composition and metabolic pathways of this helminth parasite may be of paramount importance, so that more potent antifilarial agents based on specific drug targets can be identified in drug discovery programmes. The present review provides an update on the biochemistry of the important metabolic pathways functioning within this potentially important bovine parasite, that have so far been studied, and on those that need to be investigated further so as to identify novel drug targets that can be exploited for designing new antifilarial drugs.

[Adaptation of organisms to extreme conditions of deep-sea hydrothermal vents]

The deep-sea hydrothermal vents are located along the volcanic ridges and are characterized by extreme conditions such as unique physical properties (temperature, pression), chemical toxicity, and absence of photosynthesis. However, life exists in these particular environments. The primary producers of energy and organic molecules in these biotopes are chimiolithoautotrophic bacteria. Many animals species live in intimate and complex symbiosis with these sulfo-oxidizing and methanogene bacteria. These symbioses imply a strategy of nutrition and a specific metabolic organization involving numerous interactions and metabolic exchanges, between partners. The organisms of these ecosystems have developed different adaptive strategies. In these environments many microorganisms are adapted to high temperatures. Moreover to survive in these environments, living organisms have developed various strategies to protect themselves against toxic molecules such as H2S and heavy metals.

A novel member of the GCN5-related N-acetyltransferase superfamily from Caenorhabditis elegans preferentially catalyses the N-acetylation of thialysine [S-(2-aminoethyl)-L-cysteine]

The putative diamine N-acetyltransferase D2023.4 has been cloned from the model nematode Caenorhabditis elegans. The 483 bp open reading frame of the cDNA encodes a deduced polypeptide of 18.6 kDa. Accordingly, the recombinantly expressed His6-tagged protein forms an enzymically active homodimer with a molecular mass of approx. 44000 Da. The protein belongs to the GNAT (GCN5-related N-acetyltransferase) superfamily, and its amino acid sequence exhibits considerable similarity to mammalian spermidine/spermine-N1-acetyltransferases. However, neither the polyamines spermidine and spermine nor the diamines putrescine and cadaverine were efficiently acetylated by the protein. The smaller diamines diaminopropane and ethylenediamine, as well as L-lysine, represent better substrates, but, surprisingly, the enzyme most efficiently catalyses the N-acetylation of amino acids analogous with L-lysine. As determined by the k(cat)/K(m) values, the C. elegans N-acetyltransferase prefers thialysine [S-(2-aminoethyl)-L-cysteine], followed by O-(2-aminoethyl)-L-serine and S-(2-aminoethyl)-D,L-homocysteine. Reversed-phase HPLC and mass spectrometric analyses revealed that N-acetylation of L-lysine and L-thialysine occurs exclusively at the amino moiety of the side chain. Remarkably, heterologous expression of C. elegans N-acetyltransferase D2023.4 in Escherichia coli, which does not possess a homologous gene, results in a pronounced resistance against the anti-metabolite thialysine. Furthermore, C. elegans N-acetyltransferase D2023.4 exhibits the highest homology with a number of GNATs found in numerous genomes from bacteria to mammals that have not been biochemically characterized so far, suggesting a novel group of GNAT enzymes closely related to spermidine/spermine-N1-acetyltransferase, but with a distinct substrate specificity. Taken together, we propose to name the enzyme 'thialysine N(epsilon)-acetyltransferase'.

Polyamine metabolism of filaria and allied parasites

Putrescine and the polyamines spermidine and spermine occur both in prokaroytes and in eukaryotes where they seem intimately involved in regulatory processes of cellular growth and differentiation. They seem to play an important role related to the biosynthesis of nucleic acids and proteins, although at the molecular level their precise function remains unclear. In general, prokaryotes utilize putrescine and spermidine while eukaryotes tend to have higher concentrations of spermidine and spermine compared to putrescine(1-3.) Differences in polyamine metabolism between parasites and their hosts suggest several potential targets for chemotherapeutic attack As Rolf Walter discusses here, such approaches have already been exploited for African trypanosomes and also offer some leads for the chemotherapy of helminth infections.

Biochemical and enzymological aspects of the symbiosis between the deep-sea tubeworm Riftia pachyptila and its bacterial endosymbiont

Riftia pachyptila (Vestimentifera) is a giant tubeworm living around the volcanic deep-sea vents of the East Pacific Rise. This animal is devoid of a digestive tract and lives in an intimate symbiosis with a sulfur-oxidizing chemoautotrophic bacterium. This bacterial endosymbiont is localized in the cells of a richly vascularized organ of the worm: the trophosome. These organisms are adapted to their extreme environment and take advantage of the particular composition of the mixed volcanic and sea waters to extract and assimilate inorganic metabolites, especially carbon, nitrogen, oxygen and sulfur. The high molecular mass hemoglobin of the worm is the transporter for both oxygen and sulfide. This last compound is delivered to the bacterium which possesses the sulfur oxidizing respiratory system, which produces the metabolic energy for the two partners. CO2 is also delivered to the bacterium where it enters the Calvin-Benson cycle. Some of the resulting small carbonated organic molecules are thus provided to the worm for its own metabolism. As far as nitrogen assimilation is concerned, NH3 can be used by the two partners but nitrate can be used only by the bacterium. This very intimate symbiosis applies also to the organization of metabolic pathways such as those of pyrimidine nucleotides and arginine. In particular, the worm lacks the first three enzymes of the de novo pyrimidine biosynthetic pathways as well as some enzymes involved in the biosynthesis of polyamines. The bacterium lacks the enzymes of the pyrimidine salvage pathway. This symbiotic organization constitutes a very interesting system to study the molecular and metabolic basis of biological adaptation.

Arginine metabolism in the deep sea tube worm Riftia pachyptila and its bacterial endosymbiont

The present study describes the distribution and properties of enzymes involved in arginine metabolism in Riftia pachyptila, a tubeworm living around deep sea hydrothermal vents and known to be engaged in a highly specific symbiotic association with a bacterium. The results obtained show that the arginine biosynthetic enzymes, carbamyl phosphate synthetase, ornithine transcarbamylase, and argininosuccinate synthetase are present in all of the tissues of the worm and in the bacteria. Thus, Riftia and its bacterial endosymbiont can assimilate nitrogen and carbon via this arginine biosynthetic pathway. The kinetic properties of ornithine transcarbamylase strongly suggest that neither Riftia nor the bacteria possess the catabolic form of this enzyme belonging to the arginine deiminase pathway, the absence of this pathway being confirmed by the lack of arginine deiminase activity. Arginine decarboxylase and ornithine decarboxylase are involved in the biosynthesis of polyamines such as putrescine and agmatine. These activities are present in the trophosome, the symbiont-harboring tissue, and are higher in the isolated bacteria than in the trophosome, indicating that these enzymes are of bacterial origin. This finding indicates that Riftia is dependent on its bacterial endosymbiont for the biosynthesis of polyamines that are important for its metabolism and physiology. These results emphasize a particular organization of the arginine metabolism and the exchanges of metabolites between the two partners of this symbiosis.

The activator-binding site of Onchocerca volvulus S-adenosylmethionine decarboxylase, a potential drug target

S-Adenosylmethionine decarboxylase (AdoMetDC) is a key enzyme in polyamine biosynthesis. In many eukaryotes its activity is stimulated specifically by putrescine. The AdoMetDC of the filarial parasite Onchocerca volvulus, however, is not only stimulated by putrescine but also by the naturally occuring polyamines spermidine and spermine. Several diamines, acetylated polyamines and polyamine analogues were used to analyse what molecular prerequisites are needed to stimulate nematode AdoMetDC activity. In the absence of an activator, the O. volvulus enzyme exhibits an extremely low specific activity. This fact, together with the unspecificity of activator binding, was thought to be useful for a new strategy to inhibit nematode AdoMetDC activity. Therefore, different polyamine analogues were tested as competitive inhibitors towards the stimulatory effect putrescine has on the O. volvulus and, in comparison, on the Caenorhabditis elegans and human AdoMetDC. Bis(aralkyl)- and bis(alkyl)-substituted polyamine analogues with a 3-7-3 backbone were found to inhibit AdoMetDC activities, however, probably without interfering with the putrescine stimulation. The best inhibitor, BW-1, was about 10-fold more effective against O. volvulus AdoMetDC than against the human enzyme. Unexpectedly, BW-1 was determined to be a competitive inhibitor with respect to AdoMet, having a Ki value of 310 microM for the putrescine-stimulated human AdoMetDC. Furthermore, we show for the O. volvulus and the human enzyme that the degree of inhibition by BW-1 depends on the actual putrescine concentration.

Haemonchus contortus: cloning and functional expression of a cDNA encoding ornithine decarboxylase and development of a screen for inhibitors

Polyamines (PA) are essential for viability and replication of all cells; organisms either synthesize PA or acquire them from the environment. How nematodes that parasitize the gut satisfy their PA requirement has not been resolved. The primary regulatory enzyme in PA biosynthesis in most animals is ornithine decarboxylase (ODC). This enzyme has recently been characterized in free-living nematodes and in the parasitic species. Haemonchus contortus. Nematode and mammalian ODC are reported to differ in subcellular localization, kinetics, and sensitivity to inhibitors. We cloned an H. contortus cDNA that encodes a full-length ODC (sequence data from this article have been deposited with the GenBank Data Library under Accession Nos. AF016538 and AF016891). This cDNA was functionally expressed in strains of Escherichia coli and Saccharomyces cerevisiae that lack ODC and are dependent upon exogenous PA for survival. Expression of nematode ODC reversed the PA-dependence phenotype of both microorganisms. The complemented yeast strain was used to develop a nutrient-dependent viability screen for selective inhibitors of nematode ODC. The antiprotozoal drug stilbamidine isethionate was identified as active in this screen, but biochemical characterization revealed that this compound did not inhibit ODC. Instead, like other cationic diamidines, stilbamidine probably inhibits yeast S-adenosylmethionine decarboxylase. Nonetheless, the activity in the screen of the known ODC inhibitor difluoromethylornithine (DFMO) validates the concept that specific recombinant microorganisms can serve as the basis for extremely selective and facile screens.

Parasite sulphur amino acid metabolism

This paper reviews current knowledge regarding the metabolism of the sulphur-containing amino acids methionine and cysteine in parasitic protozoa and helminths. Particular emphasis is placed on the unusual aspects of parasite biochemistry which may present targets for rational design of antiparasite drugs. In general, the basic pathways of sulphur amino acid metabolism in most parasites resemble those of their mammalian hosts, since the enzymes involved in (a) the methionine cycle and S-adenosylmethionine metabolism, (b) the trans-sulphuration sequence, (c) the transminative catabolism of methionine, (d) the oxidative catabolism of cysteine and (e) glutathione synthesis have been demonstrated variously in several helminth and protozoan species. Despite these common pathways, there also exist numerous differences between parasite and mammalian metabolism. Some of these differences are relatively subtle. For example, the biochemical properties (and primary amino acid structures) of certain parasite methionine cycle enzymes and S-adenosylmethionine decarboxylases differ from those of the corresponding mammalian enzymes, and nematodes and trichomonads possess a novel, non-mammalian form of the trans-sulphuration enzyme cystathionine beta-synthase. The most profound differences between parasite and mammalian biochemistry relate to a number of unusual enzymes and thiol metabolites found in parasitic protozoa. In certain protozoa the pathway for methionine recycling from 5'-methylthioadenosine differs markedly from the mammalian route, and involves 2 exclusively microbial enzymes. Trypanosomatid protozoa contain the non-mammalian antioxidant thiol compounds ovothiol A and trypanothione, together with unique trypanothione-linked enzymes. Specific anaerobic protozoa possess another exclusively microbial enzyme, methionine gamma-lyase, which catabolises methionine (and homocysteine); the physiological significance of these non-mammalian activities is not fully understood. These unusual features offer opportunities for chemotherapeutic exploitation, and in some cases represent metabolic similarities with bacteria. Additionally, some anaerobic protozoa contain unidentified thiols and this implies the presence of further unusual enzymes/pathways in these organisms. So far, no truly unique targets for chemotherapy have been found in helminth sulphur amino acid metabolism, and to some degree this reflects the relative lack of detailed study in the area.

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.

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.

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.

Biogenic-amine acetylation: an additional function of the N-acetyltransferase from Fasciola hepatica

The previously described polyamine N-acetyltransferase from Fasciola hepatica has been observed to have an additional function, the acetylation of biogenic amines. The activities for biogenic amines, diamines and polyamines were in a constant ratio throughout the purification process. Biogenic amines found to be substrates for the enzyme included tyramine, tryptamine, beta-phenylethylamine and histamine, with Km values of 0.12 mM, 0.26 mM, 0.30 mM and 0.76 mM respectively. Octopamine, 5-hydroxytryptamine and alpha-phenylethylamine were also acceptable as substrates, though to a lesser degree. The optimum pH for biogenic-amine acetylation was 7.5, and CoA was inhibitory to the process, with a Ki of 5.5 microM. N-Acetylation appears to play a major role in the amine metabolism of this trematode. We presume that acetylation represents the process by which the parasite inactivates excess amines.

Purification and characterization of polyamine oxidase from Ascaris suum

The interconversion of polyamines in the parasite nematode Ascaris suum by a novel type of polyamine oxidase was demonstrated. The nematode enzyme was clearly distinguishable from monoamine and diamine oxidases as well as from the mammalian polyamine oxidase, as shown by the use of the specific inhibitors pargyline, aminoguanidine and MDL 72527 respectively. All three inhibitors had no effect on the parasite polyamine oxidase, and the enzyme did not accept diamines such as putrescine, cadaverine or histamine as substrates. The parasite polyamine oxidase selectively oxidizes spermine and spermidine but not N-acetylated polyamines, whereas the mammalian tissue-type polyamine oxidase shows preference for the N-acetylated polyamines. These results suggest a regulatory function of the nematode polyamine oxidase in the degradation and interconversion of polyamines in parasite nematodes. The enzyme was purified to homogeneity by gel filtration, preparative isoelectric focusing and subsequent affinity chromatography on spermine- and berenil-Sepharose 4B. With respect to reaction type, the prosthetic group FAD, the molecular mass (66 kDa) and the contents of thiol and carbonyl groups, the polyamine oxidase from A. suum is similar to the isofunctional enzyme of mammalian tissue.

Polyamine N-acetyltransferase from Fasciola hepatica

A cytosolic polyamine N-acetyltransferase which catalyses polyamine and diamine acetylation has been partially purified from the liver fluke Fasciola hepatica. The enzyme has an apparent Mr of 50,000 and unlike the corresponding mammalian liver counterpart is capable of putrescine acetylation. Among the substrates tested, spermidine had the highest reaction rate but putrescine had a lower Km value. The Km values for spermidine, spermine, norspermidine, putrescine, cadaverine and 1,3-diaminopropane were 20 microM, 1.30 mM, 20 microM, 7 microM, 10 microM and 50 microM, respectively. Acetylated polyamines were also substrates for the trematode acetylase, but histones were inactive. The partially purified enzyme had no deacetylase activity. The Km for acetyl-CoA was 4.4 microM. Coenzyme A was strongly inhibitory with a Ki value of 5.3 microM. Bis(benzyl)polyamine analogue MDL 27695 was a potent competitive inhibitor of the enzyme with a Ki of 22 microM. Inhibition by 1,4-dimethyl-putrescine was non-competitive and had a Ki value of 15 microM. The trematode acetylase is highly dependent on sulfhydryl groups for its activity. As had been reported in nematodes, polyamine acetylation could represent a process by which trematodes convert excess polyamines to forms suitable for transport and excretion. On the other hand, this could be the regulatory step of a functional interconversion pathway in these parasites.

Polyamine oxidase from Acanthamoeba culbertsoni specific for N8-acetylspermidine

Polyamine oxidase plays a key role in the catabolism of polyamines and regeneration of spermidine and putrescine. The mammalian enzyme utilises N1-acetylspermidine, and N8-acetylspermidine, although formed in the mammals, is not catabolised further. We have characterised an enzyme from Acanthamoeba culbertsoni which acts preferentially on N8-acetylspermidine. The highly unstable enzyme was stabilised in the presence of glycerol or dimethylsulphoxide together with spermine and purified 400-fold by a combination of DEAE-cellulose, CM-cellulose, spermine-Sepharose and Sephacryl S-300 chromatography. The enzyme has a pH optimum of 8 and a temperature optimum of 45 degrees C. The relative activities on different substrates are: N8-acetylspermidine 100%, N1-acetylspermine 40%, N1-acetylspermidine 1%, N1,8-diacetylspermidine 1% and N1,12-diacetylspermine 15%. Free polyamines and substrates of monoamine oxidase were not attacked. The enzyme yielded diaminopropane as an end product of catabolism and could be involved in the biosynthesis of this unusual polyamine present in large amounts in this organism.

Pyridoxal 5'-phosphate dependent enzymes in the nematode Nippostrongylus brasiliensis

Eight classes of pyridoxal 5'-phosphate dependent enzymes have been investigated in Nippostrongylus brasiliensis in parallel with rat tissues. The range of decarboxylases detected in N. brasiliensis was limited in comparison with rat tissues. N. brasiliensis possessed a highly active L-serine hydroxymethyltransferase, but in contrast with rat liver, 5-aminolevulinic acid synthetase was absent. Similar levels of L-serine and L-threonine dehydratase activities were detected in N. brasiliensis and rat liver, and both organisms lacked L-alanine racemase, L-tryptophan synthetase and L-methionine gamma-lyase. The demonstration of cystathionine beta-synthase and gamma-cystathionase in N. brasiliensis suggests the presence of a functional trans-sulphuration sequence. The substrate specificities of the nematode cystathionine beta-synthase and gamma-cystathionase varied significantly from those of the corresponding mammalian enzymes. Particularly striking was the ability of N. brasiliensis cystathionine beta-synthase to catalyse the non-mammalian 'activated L-serine sulphydrase' reaction (L-cysteine + R-SH----cysteine thioether + H2S). N. brasiliensis and rat liver exhibited comparable abilities to transaminate amino acids via the 2-oxoglutarate: glutamate system.

Polyamine metabolism in some helminth parasites

Polyamine levels of some helminth parasites were analyzed by reverse phase HPLC of benzoyl derivatives. Setaria cervi, Acanthocheilonema viteae, Hymenolepis nana, H. diminuta, and Ascaridia galli contained higher levels of spermine than spermidine while in Ancylostoma ceylanicum and Nippostrongylus brasiliensis the spermidine levels were higher than spermine; putrescine was either absent or present in minor quantities. The enzymes of polyamine biosynthesis viz., ornithine decarboxylase, S-adenosyl methionine (SAM)-decarboxylase, and arginine decarboxylase were present in very low to negligible amounts in all the parasites examined. A. ceylanicum exhibited high activity of ornithine amino transferase (OAT) and catalyzed appreciable decarboxylation of ornithine. The ornithine decarboxylating activity of A. ceylanicum was localized in the particulate fraction containing mitochondria, not inhibited by alpha-difluoromethyl ornithine, the specific inhibitor of ornithine decarboxylase (ODC), but inhibited in the presence of glutamate, suggesting the involvement of mitochondrial OAT rather than a true ODC in ornithine decarboxylation in this parasite. Significant activity of polyamine oxidase was also detected in helminth parasites. The absence of polyamine biosynthesizing enzymes in helminth parasites suggests their dependence on hosts for uptake and interconversion of polyamines, providing a potential target for chemotherapy.

Amino acid metabolism in helminths

Amino acids are major constituents of biological material. Chemically they are extremely stable and combine a relatively simple molecular structure with a wide range of properties and functions. In general, amino acid metabolism in helminths has been relatively neglected and the information available is often uneven and of uncertain quality. However, the search for new target sites for anthelmintic development has led to a renewed interest in this area. The amino acid composition of helminths is similar to that of other invertebrates and no unique amino acids have been reported. With the possible addition of tyrosine, helminths seem to require the same 10 essential amino acids as mammals and, where studied in detail, the pathways of amino acid synthesis in helminths are similar to those of mammals. Although amino acids are not a significant energy source in parasites, helminths are able to catabolize amino acids by pathways which, again, appear identical to those found in mammals. Helminths have also been shown to carry out a number of oxidative reactions associated with amino acid metabolism, including cysteine dioxygenase, proline hydroxylase and tryptophan hydroxylase. There are, however, differences in detail between the pathways of amino acid metabolism in helminths and mammals, particularly in the metabolism of the sulphur amino acids and arginine and proline. These differences may be exploitable in anthelmintic design and proline analogues and proline biosynthesis inhibitors show some potential as fasciolicides (Sheers et al., 1982). Differences in metabolism between parasites and their hosts may be the result of parasitic adaptation or they may merely reflect general features of the invertebrate phyla as a whole. Thus a comparison of amino acid metabolism in parasitic helminths with that of their free-living relatives may give some insight into the biochemical basis of parasitism.

Characterization of a high-affinity membrane-associated ornithine decarboxylase from the free-living nematode Caenorhabditis elegans

Ornithine decarboxylase has been identified and characterized in the free-living nematode Caenorhabditis elegans. Unlike previously described ornithine decarboxylases, the enzyme activity is membrane-associated and remains in the membrane fraction after treatment with high salt, detergents or phosphatidylinositol-specific phospholipase C. Ornithine has an apparent Km value of 2.7 microM for ornithine decarboxylase. The enzyme is competitively inhibited by arginine and lysine with Ki values of 4.0 and 24.4 microM respectively. None of the other naturally occurring amino acids inhibited more than 10% of the enzyme activity at concentrations up to 1 mM. Agmatine, putrescine, spermidine and spermine inhibit ornithine decarboxylase in a non-competitive manner with Ki values of 10, 53.5, 59 and 855 microM respectively. A similar ornithine decarboxylase activity was also identified in membrane preparations from the parasitic nematode Haemonchus contortus.

Putrescine N-acetyltransferase in Onchocerca volvulus and Ascaris suum, an enzyme which is involved in polyamine degradation and release of N-acetylputrescine

A novel type of N-acetyltransferase, clearly different from the nuclear and cytosolic polyamine N-acetyltransferases of mammals, was recently found in the intestinal nematode Ascaris suum. The occurrence of this putrescine N-acetylating enzyme has also been noted in the filarial parasite Onchocerca volvulus. The enzyme was partially purified from adults of O. volvulus and A. suum by chromatography on DEAE-cellulose and cadaverine-Sepharose. Substrate specificities of the filarial enzyme resemble those of the N-acetyltransferase from A. suum, with respect to its preference for putrescine and other diamines above polyamines and histones. Additionally, both nematode enzymes acetylated histamine, whereas dopamine and serotonin were not accepted as substrates. The activities of the N-acetyltransferase from O. volvulus and A. suum were potently inhibited by the drug berenil; the type of inhibition was competitive with respect to putrescine. The inhibition constants for berenil were determined as 4.2 and 1.2 microM for the enzymes of O. volvulus and A. suum, the Km values for putrescine were found to be 330 microM and 250 microM, respectively. Putrescine N-acetyltransferase is discussed as a regulatory step in the degradation of excessive polyamines via polyamine oxidase to putrescine. At this branching point, putrescine is retained in the cell for de novo synthesis of spermidine and spermine, catabolized via diamine oxidase or acetylated to a suitable transport product for excretion.

A novel type of putrescine (diamine)-acetylating enzyme from the nematode Ascaris suum

A cytosolic enzyme catalysing the acetylation of the diamines putrescine, cadaverine, 1,3-diaminopropane and 1,6-diaminohexane has been partially purified from reproductive tissue of the intestinal parasitic nematode Ascaris suum. The enzyme formed N-acetylated derivatives of the above diamines when incubated in the presence of acetyl-CoA. The Michaelis constants (Km) for the above diamines were 0.25 nM, 0.1 mM, 1.25 mM and 0.4 mM respectively, and the apparent Km for acetyl-CoA was 7.7 microM. sym-Norspermidine was also acetylated by this enzyme preparation, and, at a much lower rate, the enzyme acted on sym-norspermine. The common polyamines, spermidine and spermine, and histones were not substrates. Purification steps involved a freezing-and-thawing procedure to release enzyme activity from unknown inhibitors, DEAE-cellulose chromatography and affinity chromatography on cadaverine-Sepharose, from which the enzyme was eluted by increasing ionic strength. The enzyme exhibited an apparent Mr of about 38,000-40,000, and it consisted of at least two subunits, of which the catalytic one had an Mr of about 13,000. The partially purified enzyme showed no deacetylase activity, and its activity was competitively inhibited by the product N-acetylputrescine, but not by CoA. The name putrescine N-acetyltransferase is suggested for this enzyme, which may have an important function in the degradation of diamines of lower eukaryotes.

Polyamine metabolism in Ancylostoma ceylanicum and Nippostrongylus brasiliensis

Spermidine was detected as the major polyamine of Ancylostoma ceylanicum as well as Nippostrongylus brasiliensis. Spermine was present in lower amounts whereas the level of putrescine was even less. S-Adenosylmethionine decarboxylase, a rate-limiting enzyme in the biosynthetic pathway of polyamines, was demonstrated at low levels in both parasites. Decarboxylation of lysine and arginine was absent or negligible and that of ornithine questionable, as the enzyme activity was not inhibited by alpha-difluoromethylornithine while RMI 71,645, an irreversible inhibitor of ornithine aminotransferase, strongly inhibited the liberation of CO2 from ornithine. High activity of ornithine aminotransferase was observed in both the parasites and may interfere with the assay for ornithine decarboxylase. Adults of A. ceylanicum were found to rapidly take up spermidine and spermine from incubation medium while uptake of putrescine was very low. These results indicate that hookworms depend on uptake and interconversion rather than de novo synthesis for their polyamine requirement.

Polyamine metabolism in Setaria cervi, the bovine filarial worm

Spermine and spermidine were found to be the principal polyamines in the bovine filarial parasite Setaria cervi, whereas putrescine was observed in very low amounts. Studies conducted on the enzymes of polyamine biosynthesis revealed low activity for S-adenosyl-methionine decarboxylase, questionable and negligible activities for the decarboxylation of ornithine and arginine, and appreciable activity for ornithine aminotransferase. Uptake studies with radiolabeled putrescine, spermidine and spermine showed that these amines are rapidly taken up from the medium by an active uptake process. The uptake was temperature-sensitive and abolished at 0-4 degrees C. The questionable presence of biosynthetic enzymes such as ornithine and arginine decarboxylase and, on the other hand, an effective uptake mechanism indicate that the parasite may depend on the host for its polyamine requirement, thereby indicating a possible target for chemotherapy.

Polyamine biosynthesis and inhibition in Trichomonas vaginalis

Polyomines - particularly putrescine, spermidine and spermine - are ubiquitous components of eukaryote and most prokaryote cells, and are essential for optimal cell proliferation. But since routes of polyamine synthesis may differ, for example between parasites and their hosts, selective inhibition of polyamine metabolism offers an attractive target for chemotherapy - as already shown with the success of difluoromethylomithine (DFMO) as an inhibitor of polyamine synthesis in African trypanosomes. Parasitology Today has featured a series of articles reviewing research on polyamine metabolism of various parasites (eg. vol. 3, pp 190-192, pp 312-315; vol. 4, pp 18-20) and here, Nigel Yorlett discusses these metabolic aspects of Trichomonas vaginalis (Fig. 1)-a common parasite of the urogenital tract.

Ascaris suum and Onchocerca volvulus: S-adenosylmethionine decarboxylase

Putrescine-dependent S-adenosylmethionine decarboxylase (EC 4.1.1.50) was demonstrated in Ascaris suum and Onchocerca volvulus; activation was found to be about fourfold by putrescine. Mg2+ did not affect the enzyme activity. A. suum was taken as a model nematode and its S-adenosylmethionine decarboxylase was partially purified and characterized. The molecular weight was estimated to be 220,000. The apparent Km-value for adenosylmethionine was determined to be 17 microM. Methylglyoxal bis(guanylhydrazone) and berenil competitively inhibited the enzyme activity; the apparent Ki-values were found to be 0.24 microM and 0.11 microM, respectively. The dependence of filarial worms on uptake and interconversion of putrescine and polyamines as well as properties of the S-adenosylmethionine decarboxylase, different from the host enzyme, points to the polyamine metabolisms as a useful target for chemotherapy.

Characterization of an immunodominant Onchocerca volvulus antigen with patient sera and a monoclonal antibody

Adult Onchocerca voluvlus and infective larvae, but not microfilariae contain an immunodominant antigen (33,000 and 21,000 Mr in females, 39,000, 33,000, and 21,000 Mr in males, 133,000 Mr in infective larvae) which is recognized by an Onchocerca-specific mAb. The component is part of the reproductive organs and muscles. 96.2% of onchocerciasis sera contained antibodies detectable by immunoblotting against it. Antigen purified by immunoaffinity chromatography was specifically recognized in immunoblots by onchocerciasis sera, but not by sera from other filarial infections. The high immunogenicity, the specificity, and the occurrence in infective larvae of this antigen indicate an immunodiagnostic potential and a possible role in the immunobiology of the parasite.

The polyamine metabolism of filarial worms as chemotherapeutic target

Parasite-specific putrescine-N-acetyltransferase and polyamine oxidase, both involved in the reversed pathway of polyamine metabolism, were demonstrated for Ascaris suum and Onchocerca volvulus. Berenil-treatment was found to be correlated with accumulation of polyamines, especially spermine, obviously due to blockaded polyamine interconversion. Furthermore it was shown that added spermine to the culture medium led to the death of worms. These specificities might be exploited for chemotherapy of filarial infections. Polyamines are widely distributed in the nature. They are found in higher and lower eucaryotes and in procaryotes as well as in viruses (Tabor and Tabor, 1984). During the last years there have been many approaches to examine the role of polyamines in cell growth and differentiation in vertebrates (Tabor and Tabor, 1984; Pegg, 1986). The polyamine metabolism of parasites also has attracted increasing interest, e.g. in African trypanosomes the initial enzyme of polyamine synthesis - ornithine decarboxylase - has been exploited as a target for chemotherapy by using DFMO (DL alpha-difluoromethylornithine) (Bacchi et al., 1980; Bacchi et al., 1983; Fairlamb et al., 1985; Giffin et al., 1986). The polyamine metabolism of filaria and other helminths is still a neglected area of research, although there are reports about distribution pattern of polyamines and some peculiarities of polyamine metabolism in filarial worms (Srivastava et al., 1980; Wittich et al., 1987; Walter, 1988). DFMO and MGBG (methylglyoxal bis-(guanylhydrazone], both of which are potent inhibitors of polyamine synthesis in mammals, do not significantly effect the viability of filarial worms (Wittich et al., 1987).(ABSTRACT TRUNCATED AT 250 WORDS)