Polyamine metabolism in Acanthamoeba: polyamine content and synthesis of ornithine, putrescine, and diaminopropane

Synthetic Dihydropyridines as Novel Antiacanthamoebic Agents

BACKGROUND

Acanthamoeba is an opportunistic pathogen widely spread in the environment. Acanthamoeba causes excruciating keratitis which can lead to blindness. The lack of effective drugs and its ability to form highly resistant cyst are one of the foremost limitations against successful prognosis. Current treatment involves mixture of drugs at high doses but still recurrence of infection can occur due to ineffectiveness of drugs against the cyst form. Pyridine and its natural and synthetic derivatives are potential chemotherapeutic agents due to their diverse biological activities.

OBJECTIVE

To study the antiamoebic effects of four novel synthetic dihydropyridine (DHP) compounds against Acanthamoeba castellanii belonging to the T4 genotype. Furthermore, to evaluate their activity against amoeba-mediated host cells cytopathogenicity as well as their cytotoxicity against human cells.

METHODS

Dihydropyridines were synthesized by cyclic dimerization of alkylidene malononitrile derivatives. Four analogues of functionally diverse DHPs were tested against Acanthamoeba castellanii by using amoebicidal, encystation and excystation assays. Moreover, Lactate dehydrogenase assays were carried out to study cytopathogenicity and cytotoxicity against human cells.

RESULTS

These compounds showed significant amoebicidal and cysticidal effects at 50 μM concentration, whereas, two of the DHP derivatives also significantly reduced Acanthamoebamediated host cell cytotoxicity. Moreover, these DHPs were found to have low cytotoxicity against human cells suggesting a good safety profile.

CONCLUSION

The results suggest that DHPs have potential against Acanthamoeba especially against the more resistant cyst stage and can be assessed further for drug development.

Structures and Mechanism of the Monoamine Oxidase Family

Members of the monoamine oxidase family of flavoproteins catalyze the oxidation of primary and secondary amines, polyamines, amino acids, and methylated lysine side chains in proteins. The enzymes have similar overall structures, with conserved FAD-binding domains and varied substrate-binding sites. Multiple mechanisms have been proposed for the catalytic reactions of these enzymes. The present review compares the structures of different members of the family and the various mechanistic proposals.

Drug target identification, validation, characterisation and exploitation for treatment of Acanthamoeba (species) infections

New more efficacious antimicrobials as required for the treatment of Acanthamoeba infections as those currently available require arduous treatment regimes, are not always effective and are poorly active against the cystic stages. Herein, we review potential drug targets including tubulin, alternative oxidase, amino acid biosynthesis and myosin. In addition, we review the literature for current missing tools and resources for the identification, validation and development of new antimicrobials for this organism. Additional targets should come to light through a concerted genome sequencing effort.

Characterization of the Entamoeba histolytica ornithine decarboxylase-like enzyme

BACKGROUND

The polyamines putrescine, spermidine, and spermine are organic cations that are required for cell growth and differentiation. Ornithine decarboxylase (ODC), the first and rate-limiting enzyme in the polyamine biosynthetic pathway, is a highly regulated enzyme.

METHODOLOGY AND RESULTS

To use this enzyme as a potential drug target, the gene encoding putative ornithine decarboxylase (ODC)-like sequence was cloned from Entamoeba histolytica, a protozoan parasite causing amoebiasis. DNA sequence analysis revealed an open reading frame (ORF) of approximately 1,242 bp encoding a putative protein of 413 amino acids with a calculated molecular mass of 46 kDa and a predicted isoelectric point of 5.61. The E. histolytica putative ODC-like sequence has 33% sequence identity with human ODC and 36% identity with the Datura stramonium ODC. The ORF is a single-copy gene located on a 1.9-Mb chromosome. The recombinant putative ODC protein (48 kDa) from E. histolytica was heterologously expressed in Escherichia coli. Antiserum against recombinant putative ODC protein detected a band of anticipated size approximately 46 kDa in E. histolytica whole-cell lysate. Difluoromethylornithine (DFMO), an enzyme-activated irreversible inhibitor of ODC, had no effect on the recombinant putative ODC from E. histolytica. Comparative modeling of the three-dimensional structure of E. histolytica putative ODC shows that the putative binding site for DFMO is disrupted by the substitution of three amino acids-aspartate-332, aspartate-361, and tyrosine-323-by histidine-296, phenylalanine-305, and asparagine-334, through which this inhibitor interacts with the protein. Amino acid changes in the pocket of the E. histolytica enzyme resulted in low substrate specificity for ornithine. It is possible that the enzyme has evolved a novel substrate specificity.

CONCLUSION

To our knowledge this is the first report on the molecular characterization of putative ODC-like sequence from E. histolytica. Computer modeling revealed that three of the critical residues required for binding of DFMO to the ODC enzyme are substituted in E. histolytica, resulting in the likely loss of interactions between the enzyme and DFMO.

Heterologous expression and characterization of mouse spermine oxidase

Polyamine oxidases are key enzymes responsible of the polyamine interconversion metabolism in animal cells. Recently, a novel enzyme belonging to this class of enzymes has been characterized for its capability to oxidize preferentially spermine and designated as spermine oxidase. This is a flavin adenine dinucleotide-containing enzyme, and it has been expressed both in vitro and in vivo systems. The primary structure of mouse spermine oxidase (mSMO) was deduced from a cDNA clone (Image Clone 264769) recovered by a data base search utilizing the human counterpart of polyamine oxidases, PAOh1. The open reading frame predicts a 555-amino acid protein with a calculated M(r) of 61,852.30, which shows a 95.1% identity with PAOh1. To understand the biochemical properties of mSMO and its structure/function relationship, the mSMO cDNA has been subcloned and expressed in secreted and secreted-tagged forms into Escherichia coli BL21 DE3 cells. The recombinant enzyme shows an optimal pH value of 8.0 and is able to oxidize rapidly spermine to spermidine and 3-aminopropanal and fails to act upon spermidine and N(1)-acetylpolyamines. The purified recombinant-tagged form enzyme (M(r) approximately 68,000) has K(m) and k(cat) values of 90 microm and 4.5 s(-1), respectively, using spermine as substrate at pH 8.0. Molecular modeling of mSMO protein based on maize polyamine oxidase three-dimensional structure suggests that the general features of maize polyamine oxidase active site are conserved in mSMO.

Entamoeba histolytica: purification and characterization of ornithine decarboxylase

Ornithine decarboxylase, a rate-limiting enzyme in polyamine biosynthesis in eukaryotes, was stabilized and purified from trophozoites of the parasite protozoan E. histolytica. Analytical electrophoresis revealed the presence in the purified preparations of a major polypeptide of 45 kDa and barely detectable amounts of two other proteins of 70 and 120 kDa. Both the 45 and 70 kDa polypeptides were recognized by a mouse anti-ODC monoclonal antibody. The major polypeptide exhibited amino terminal sequence homology in the range of 40-73% with ODCs from other organisms. The immunoreactive polypeptide of 70 kDa was not identified. The molecular masses of 216 and 45 kDa determined for the native enzyme by gel filtration and for the major polypeptide by SDS-PAGE, respectively, suggest that the amoeba ODC is a homopentamer. Dialysis against hydroxylamine rendered the enzyme activity fully dependent on pyridoxal 5'-phosphate (PLP). As expected for an oligomeric enzyme, ODC activity exhibited sigmoidal kinetics when it was measured as a function of increasing concentrations of L-ornithine and PLP yielding S(0.5) values of 0.45 and 0.18 mM, respectively. Purified ODC was inhibited by 1,3-diaminopropane and 2,4-diamino-2-butanone but was largely insensitive to inhibition by alpha-difluoromethylornithine (DFMO), indicating that the enzyme may not be a suitable target for this anti-parasitic drug. Other features of the amoeba ODC were common with the enzyme from prokaryotes and eucaryotes.

A barley polyamine oxidase isoform with distinct structural features and subcellular localization

Two cDNAs encoding polyamine oxidase (PAO) isoforms (BPAO1 and BPAO2) and the corresponding gene copies were isolated from barley cultivar Aura. Gene organization is not conserved between these two nonallelic coding sequences. Both precursor proteins include a cleavable N-terminal leader of 25 amino acids. N-terminal sequencing of PAO purified from barley seedlings reveals a unique amino-acid sequence corresponding to the BPAO2 N-terminus as predicted from the corresponding cDNA. BPAO2 has been purified, characterized and compared to maize PAO (MPAO), the best characterized member of this enzyme class. The two proteins show different pH optima for catalytic activity, Km and Vmax values with spermidine and spermine as substrates. Molecular modelling of BPAO2 reveals the same global fold as in MPAO. However, substitution of the active site residue Phe403 by a tyrosine, provides a rationale for the different catalytic properties of the two enzymes. In barley leaves PAO-specific activity is higher in isolated mesophyll protoplasts than in the extracellular fluids, whereas in maize the reverse is true. The C-terminus of BPAO2 shows homology with the endoplasmic reticulum retention signal that might be responsible for the subcellular localization observed. We conclude that BPAO2 is a symplastic PAO in barley mesophyll cells. Production of BPAO2 mRNA and the corresponding protein is induced by light, and has a different pattern of accumulation in leaves and coleoptiles.

Encystation in Acanthamoeba castellanii: development of biocide resistance

Since the early 1960s, axenic culture and the development of procedures for the induction of encystation have made Acanthamoeba spp. superb experimental systems for studies of cell biology and differentiation. More recently, since their roles as human pathogens causing keratitis and encephalitis have become widely recognized, it has become urgent to understand the parameters that determine differentiation, as cysts are much more resistant to biocides than are the trophozoites. Viability of trophozoites of the soil amoeba Acanthamoeba castellanii (Neff), is conveniently measured by its ability to form plaques on a lawn of Escherichia coli. Use of confocal laser scanning microscopy with Calcofluor white, Congo Red or the anionic oxonol dye, DiBAC4(3) or flow cytometry with propidium iodide diacetate and fluorescein or oxonol provides more rapid assessment. For cysts, the plaque method is still the best, because dye exclusion does not necessarily indicate viability and therefore the plate count method has been used to study the sequence of development of biocide resistance during the differentiation process. After two hours, resistance to HCl was apparent. Polyhexamethylene biguanide, benzalkonium chloride, propamidine isethionate, pentamidine isethionate, dibromopropamine isethionate, and H2O2 and moist heat, all lost effectiveness at between 14 and 24 h after trophozoites were inoculated into encystation media. Chlorhexidine diacetate resistance was observed at between 24 and 36 h. The molecular biology and biochemistry of the modifications that underlie these changes are now being investigated.

Dependence of Trichomonas vaginalis upon polyamine backconversion

Trichomonas vaginalis grown for 16 h in the presence of [(14)C]spermine formed a high intracellular pool of [(14)C]spermidine and a small but detectable pool of [(14)C]putrescine. When [(3)H]putrescine was added to the growth medium, a large intracellular pool of [(3)H]putrescine was found, but it was not further metabolized, confirming previous studies suggesting the absence of a forward-directed polyamine synthetic pathway in T. vaginalis. Spermidine:spermineN:(1)-acetyltransferase (SSAT) and polyamine oxidase enzyme activities were detected which collectively converted spermine to spermidine. Polyamine oxidase was localized in the hydrogenosome-enriched fraction, whereas SSAT was found predominantly in the cytosolic fraction. In the presence of saturating substrate, the trichomonad SSAT had an activity of 0. 39+/-0.09 nmol min(-1) (mg protein)(-1) (the mean of five analyses) and an apparent K:(m) for spermine of 1.7 microM. The enzyme was competitively inhibited by di(ethyl)norspermine with a K:(i) of 28 microM. Growth studies indicated that 50 microM di(ethyl)norspermine caused a 68% and 84% reduction in the intracellular concentrations of spermidine and spermine, respectively. The trichomonad polyamine oxidase required FAD as a cofactor and had an apparent K:(m) of 6.0 microM for N(1)-acetylspermine. The potential of bis(alkyl) polyamine analogues as antitrichomonad agents is discussed.

Cytotoxicity of polyamines to Amoeba proteus: role of polyamine oxidase

It has been shown that oxidation of polyamines by polyamine oxidases can produce toxic compounds (H2O2, aldehydes, ammonia) and that the polyamine oxidase-polyamine system is implicated, in vitro, in the death of several parasites. Using Amoeba proteus as an in vitro model, we studied the cytotoxicity to these cells of spermine, spermidine, their acetyl derivatives, and their hypothetical precursors. Spermine and N1-acetylspermine were more toxic than emetine, an amoebicidal reference drug. Spermine presented a short-term toxicity, but a 48-h contact time was necessary for the high toxicity of spermidine. The uptake by Amoeba cells of the different polyamines tested was demonstrated. On the other hand, a high polyamine oxidase activity was identified in Amoeba proteus crude extract. Spermine (theoretical 100%) and N1-acetylspermine (64%) were the best substrates at pH 9.5, while spermidine, its acetyl derivatives, and putrescine were very poorly oxidized by this enzyme (3-20%). Spermine oxidase activity was inhibited by phenylhydrazine (nil) and isoniazid (approximately 50%). Mepacrine did not inhibit the enzyme activity at pH 8. Neither monoamine nor diamine oxidase activity (approximately 10%) was found. It must be emphasized that spermine, the best enzyme substrate, is the most toxic polyamine. This finding suggests that knowledge of polyamine oxidase specificity can be used to modulate the cytotoxicity of polyamine derivatives. Amoeba proteus was revealed as a simple model for investigation of the connection between cytotoxicity and enzyme activity.

S-adenosyl-L-methionine decarboxylase of Acanthamoeba castellanii (Neff): purification and properties

S-Adenosyl-L-methionine decarboxylase (AdoMetDC) has been purified to near homogeneity from the Neff strain of Acanthamoeba castellanii. The holoenzyme molecular mass is 88.8 kDa, including two copies each of a 32.8 kDa alpha-subunit and a 10-15 kDa beta-subunit. The alpha-subunit contains the active site. It has an N-terminal pyruvoyl group, and the first 19 amino acids are 63 and 74% identical with comparable sequences from yeast and mammals, respectively. The apparent Km for S-adenosylmethionine (AdoMet) in the presence of 2 mM putrescine was 30.0 microM. The enzyme was stimulated 2-fold by putrescine, but was unaffected by spermidine. It was inhibited by the following anti-metabolites, listed with their Ki values: Berenil (0.17 microM), pentamidine (19.4 microM), propamidine (334 microM), hydroxystilbamidine (357 microM), methylglyoxal bis(guanylhydrazone) (604 microM) and ethidium bromide (1.3 mM). Activity of the enzyme fell to undetectable levels during cell differentiation (encystment).

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 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.

Norspermidine inhibits LPS-induced immunoglobulin production in an FCS-independent mechanism different from spermidine and spermine

The immunosuppressive effects of several polyamines were compared. The triamines (norspermidine (NSPD) and spermidine (SPD] and the tetramines (norspermine (NSPM) and spermine (SPM] but not the diamines (1,3-diaminopropane and putrescine) inhibited IgM production from murine splenocytes stimulated with LPS. The estimated IC50 of NSPD was 2.29 x 10(-7) M. The inhibitory effect of NSPD on IgM production was associated with the inhibition of cell growth, because DNA and RNA syntheses measured by 3H-TdR and 3H-UR incorporation were also similarly reduced. Interestingly, the inhibitory effects of NSPD were over ten times greater than those of SPD in spite of the fact that their difference in chemical structure is only one carbon chain. In a FCS-free medium NSPD retained its suppressive activities on LPS-induced IgM production, but the other polyamines were remarkably weakened in their activities. These immunosuppressive effects of NSPD were prevented by adding substances of the intracellular polyamine metabolism, putrescine, SPD or SPM. These findings suggest that NSPD inhibits B cell growth and differentiation by interfering with the polyamine metabolism pathway.

Comparison of polyamine and S-adenosylmethionine contents of growing and encysted Acanthamoeba isolates

We have used High Performance Liquid Chromatography to determine metabolite characteristics of three recent isolates of Acanthamoeba which exhibit cultural characteristics consistent with those of established potential pathogens. Growing amoebae and dormant cysts of these isolates were explored in regard to their qualitative and quantitative intracellular levels of polyamine and S-adenosylmethionine metabolites. The polyamine found in the greatest concentration in the growing cells was 1,3-diaminopropane (DAP), followed by spermidine (SPD). A low level of putrescine was also found in the growing cells. These polyamines significantly decreased in concentration as the amoebae differentiated to cysts. N8-acetylspermidine and acetylspermine were found in both developmental stages while acetylcadaverine was found only in growing amoebae and N1-acetylspermidine only in cysts. Acetylputrescine was present in both stages of two isolates but only in the growing amoebae of the third isolate. Spermine was not detected in any of the isolates. S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) were present in growing amoebae but SAM was undetectable or barely detectable in cysts. SAH also decreased in concentration during encystation of two of the isolates to a level comparable to that of the other isolate. The developmental transition from growing amoebae to dormant cysts is characterized metabolically by a threshold adjustment in concentration of SAM, SAH and of the polyamines (esp., DAP and SPD).

Stable ornithine decarboxylase in promastigotes of Leishmania mexicana mexicana

Studies on the decarboxylation of ornithine in Leishmania mexicana have shown that this activity corresponds to a true ornithine decarboxylase rather than to an oxidative decarboxylation or aminotransferase reaction, both of which also give rise to the release of CO2. The stoichiometric relationship between substrate and products has indicated that extracts of L. mexicana were able to catalyse the formation of an unknown compound besides putrescine and CO2. The addition of cycloheximide to cultures of L. mexicana allowed us to demonstrate that ornithine decarboxylase degradation in vivo was extremely slow in this parasite. This remarkable stability of the enzyme is only comparable to that found in Trypanosoma brucei and contrasts with the high turnover rate of ornithine decarboxylases of different mammalian cells.

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.

Novel biochemical pathways in parasitic protozoa

Throughout evolution, enzymes and their metabolites have been highly conserved. Parasites are no exception to this and differ most markedly by the absence of metabolic pathways that are present in the mammalian host. In general, parasites are metabolically lazy and rely on the metabolism of the host both for a supply of prefabricated components such as purines, fatty acids, sterols and amino acids and for the removal of end-products. Nonetheless, parasites are metabolically highly sophisticated in that (1) they retain the genetic capacity to induce many pathways, when needed, and (2) they have developed complex mechanisms for their survival in the host. Certain unique features of the metabolism of trypanosomes, leishmania, malaria and anaerobic protozoa will be discussed. This will include (1) glycolysis and electron transport with reference to the unique organelles: the glycosome and the hydrogenosome, (2) purine salvage, pyrimidine biosynthesis and folic acid metabolism and (3) polyamine and thiol metabolism with special reference to the role of the unique metabolite of trypanosomes and leishmanias, trypanothione.

Inhibition of multiplication in Acanthamoeba castellanii by specific inhibitors of ornithine decarboxylase

Proliferation of Acanthamoeba castellanii (Neff strain) in either a broth medium or a defined medium was arrested by alpha-monofluoromethyldehydroornithine (delta-MFMOme), alpha-difluoromethylornithine (DFMO), and (R,R')-delta-methyl-alpha-acetylenic putrescine (MAP), three specific inhibitors of ornithine decarboxylase. Although all three inhibited the ameba enzyme, delta-MFMOme was the most effective inhibitor of multiplication. Growth inhibition was reversed by the addition of polyamines. The inhibitors did not induce differentiation by themselves although DFMO caused encystment when supplemented with CaCl2 or MgSO4.