Participation of chlorobiumquinone in the transplasma membrane electron transport system of Leishmania donovani promastigote: Effect of near-ultraviolet light on the redox reaction of plasma membrane

Presence of Phylloquinone in the Intraerythrocytic Stages of Plasmodium falciparum

Malaria is one of the most widespread parasitic diseases, especially in Africa, Southeast Asia and South America. One of the greatest problems for control of the disease is the emergence of drug resistance, which leads to a need for the development of new antimalarial compounds. The biosynthesis of isoprenoids has been investigated as part of a strategy to identify new targets to obtain new antimalarial drugs. Several isoprenoid quinones, including menaquinone-4 (MK-4/vitamin K2), α- and γ-tocopherol and ubiquinone (UQ) homologs UQ-8 and UQ-9, were previously detected in in vitro cultures of Plasmodium falciparum in asexual stages. Herein, we described for the first time the presence of phylloquinone (PK/vitamin K1) in P. falciparum and discuss the possible origins of this prenylquinone. While our results in metabolic labeling experiments suggest a biosynthesis of PK prenylation via phytyl pyrophosphate (phytyl-PP) with phytol being phosphorylated, on the other hand, exogenous PK attenuated atovaquone effects on parasitic growth and respiration, showing that this metabolite can be transported from extracellular environment and that the mitochondrial electron transport system (ETS) of P. falciparum is capable to interact with PK. Although the natural role and origin of PK remains elusive, this work highlights the PK importance in plasmodial metabolism and future studies will be important to elucidate in seeking new targets for antimalarial drugs.

Exploring Ubiquinone Biosynthesis Inhibition as a Strategy for Improving Atovaquone Efficacy in Malaria

Atovaquone (AV) acts on the malaria parasite by competing with ubiquinol (UQH₂) for its union to the mitochondrial bc₁ complex, preventing the ubiquinone-8 and ubiquinone-9 (UQ-8 and UQ-9) redox recycling, which is a necessary step in pyrimidine biosynthesis. This study focused on UQ biosynthesis in Plasmodium falciparum and adopted proof-of-concept research to better elucidate the mechanism of action of AV and improve its efficacy. Initially, UQ biosynthesis was evaluated using several radioactive precursors and chromatographic techniques. This methodology was suitable for studying the biosynthesis of both UQ homologs and its redox state. Additionally, the composition of UQ was investigated in parasites cultivated at different oxygen saturations or in the presence of AV. AV affected the redox states of both UQ-8 and UQ-9 homologs by increasing the levels of the respective reduced forms. Conversely, low-oxygen environments specifically inhibited UQ-9 biosynthesis and increased the antimalarial efficacy of AV. These findings encouraged us to investigate the biological importance and the potential of UQ biosynthesis as a drug target based on its inhibition by 4-nitrobenzoate (4-NB), a 4-hydroxybenzoate (4-HB) analog. 4-NB effectively inhibits UQ biosynthesis and enhances the effects of AV on parasitic growth and respiration rate. Although 4-NB itself exhibits poor antimalarial activity, its 50% inhibitory concentration (IC₅₀) value increased significantly in the presence of a soluble UQ analog, p-aminobenzoic acid (pABA), or 4-HB. These results indicate the potential of AV combined with 4-NB as a novel therapy for malaria and other diseases caused by AV-sensitive pathogens.

Characterization of Transplasma Membrane Electron Transport Chain in Wild and Drug-Resistant Leishmania donovani Promastigote and Amastigote

BACKGROUND

Leishmania donovani (L. donovani) is one of the parasites that cause leishmaniasis. The mechanisms by which L. donovani fights against adverse environment and becomes resistant to drugs are not well understood yet.

OBJECTIVE

The present study was designed to evaluate the effects of different regulators on the modulation of Transplasma Membrane Electron Transport (transPMET) systems of susceptible and resistant L. donovani cells.

MATERIALS AND METHODS

Effects of UV, different buffers, and electron transport inhibitors and stimulators on the reduction of α-lipoic acid (ALA), 1,2-naphthoquinone-4-sulphonic acid (NQSA) and ferricyanide were determined.

RESULTS AND DISCUSSION

ALA reductions were inhibited in susceptible, sodium antimony gluconate (SAG)-resistant and paromomycin (PMM)-resistant AG83 amastigote cells, and stimulated in susceptible and SAG-resistant AG83 promastigote cells upon UV exposure. The results indicate that UV irradiation almost oppositely affect ALA reductions in amastigotes and promastigotes. ALA reductions were stimulated in sensitive and inhibited in resistant GE1 amastigotes upon UV exposure. Susceptible amastigotes and promastigotes inhibited, and resistant amastigotes and promastigotes stimulated NQSA reduction under UV irradiation. Thus, susceptible and drug-resistant amastigotes and promastigotes are different in the reduction of ALA. Susceptible and resistant AG83 amastigotes and promastigotes inhibited the ferricyanide reductions upon UV exposure, which indicates, there is no such difference in ferricyanide reductions among susceptible as well as resistant AG83 amastigotes and promastigotes. The reductions of extracellular electron excerptors in susceptible promastigotes requires the availability of Na⁺ and Cl^- ions for maximal activity but susceptible amastigotes are mostly not dependent on the availability of Na⁺ and Cl^- ions. Both in promastigotes and amastigotes, reductions of electron acceptors were strongly inhibited by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone. Furthermore, antimycin A, rotenone and capsaicin markedly inhibited the reductions of electron acceptors in promastigotes, but not in amastigotes.

CONCLUSION

Results of this study suggest that the transPMET system is functionally different in wild and resistant strains of L. donovani.

Prenylquinones in Human Parasitic Protozoa: Biosynthesis, Physiological Functions, and Potential as Chemotherapeutic Targets

Human parasitic protozoa cause a large number of diseases worldwide and, for some of these diseases, there are no effective treatments to date, and drug resistance has been observed. For these reasons, the discovery of new etiological treatments is necessary. In this sense, parasitic metabolic pathways that are absent in vertebrate hosts would be interesting research candidates for the identification of new drug targets. Most likely due to the protozoa variability, uncertain phylogenetic origin, endosymbiotic events, and evolutionary pressure for adaptation to adverse environments, a surprising variety of prenylquinones can be found within these organisms. These compounds are involved in essential metabolic reactions in organisms, for example, prevention of lipoperoxidation, participation in the mitochondrial respiratory chain or as enzymatic cofactors. This review will describe several prenylquinones that have been previously characterized in human pathogenic protozoa. Among all existing prenylquinones, this review is focused on ubiquinone, menaquinone, tocopherols, chlorobiumquinone, and thermoplasmaquinone. This review will also discuss the biosynthesis of prenylquinones, starting from the isoprenic side chains to the aromatic head group precursors. The isoprenic side chain biosynthesis maybe come from mevalonate or non-mevalonate pathways as well as leucine dependent pathways for isoprenoid biosynthesis. Finally, the isoprenic chains elongation and prenylquinone aromatic precursors origins from amino acid degradation or the shikimate pathway is reviewed. The phylogenetic distribution and what is known about the biological functions of these compounds among species will be described, as will the therapeutic strategies associated with prenylquinone metabolism in protozoan parasites.

Tocopherol biosynthesis in Leishmania (L.) amazonensis promastigotes

Leishmaniasis is a neglected disease caused by a trypanosomatid protozoan of the genus Leishmania. Most drugs used to treat leishmaniasis are highly toxic, and the emergence of drug‐resistant strains has been observed. Therefore, new therapeutic targets against leishmaniasis are required. Several isoprenoid compounds, including dolichols or ubiquinones, have been shown to be important for cell viability and proliferation in various trypanosomatid species. Here, we detected the biosynthesis of tocopherol in Leishmania (L.) amazonensis promastigotes in vitro through metabolic labelling with [1‐(n)‐3H]‐phytol. Subsequently, we confirmed the presence of vitamin E in the parasite by gas chromatography–mass spectrometry. Treatment with usnic acid or nitisinone, inhibitors of precursors of vitamin E synthesis, inhibited growth of the parasite in a concentration‐dependent manner. This study provides the first evidence of tocopherol biosynthesis in a trypanosomatid and suggests that inhibitors of the enzyme 4‐hydroxyphenylpyruvate dioxygenase may be suitable for use as antileishmanial compounds.

Database

The amino acid sequence of a conserved hypothetical protein [Leishmania mexicana MHOM/GT/2001/U1103] has been deposited in GenBank (CBZ28005.1)

Biosynthesis and applications of prenylquinones

Prenylquinones are isoprenoid compounds with a characteristic quinone structure and isoprenyl tail that are ubiquitous in almost all living organisms. There are four major prenylquinone classes: ubiquinone (UQ), menaquinone (MK), plastoquinone (PQ), and rhodoquinone (RQ). The quinone structure and isoprenyl tail length differ among organisms. UQ, PQ, and RQ contain benzoquinone, while MK contains naphthoquinone. UQ, MK, and RQ are involved in oxidative phosphorylation, while PQ functions in photosynthetic electron transfer. Some organisms possess two types of prenylquinones; Escherichia coli has UQ₈ and MK₈, and Caenorhabditis elegans has UQ₉ and RQ₉. Crystal structures of most of the enzymes involved in MK synthesis have been solved. Studies on the biosynthesis and functions of quinones have advanced recently, including for phylloquinone (PhQ), which has a phytyl moiety instead of an isoprenyl tail. Herein, the synthesis and applications of prenylquinones are reviewed.

Involvement of thermoplasmaquinone-7 in transplasma membrane electron transport of Entamoeba histolytica trophozoites: a key molecule for future rational chemotherapeutic drug designing

The quinone composition of the transplasma membrane electron transport chain of parasitic protozoa Entamoeba histolytica was investigated. Purification of quinone from the plasma membrane of E. histolytica and its subsequent structural elucidation revealed the structure of the quinone as a methylmenaquinone-7 (thermoplasmaquinone-7), a napthoquinone. Membrane bound thermoplasmaquinone-7 can be destroyed by UV irradiation with a concomitant loss of plasma membrane electron transport activity. The abilities of different quinones to restore transplasma membrane electron transport activity in UV irradiated trophozoites were compared. The lost activity was recovered completely by the addition of thermoplasmaquinone-7, but ubiquinones are unable to restore the same. These findings clearly indicate that thermoplasmaquinone-7 acts as a lipid shuttle in the plasma membrane of the parasite to mediate electron transfer between cytosolic reductant and non permeable electron acceptors. This thermoplasmaquinone-7 differs from that of the mammalian host and can provide a novel target for future rational chemotherapeutic drug designing.

Occurrence, biosynthesis and function of isoprenoid quinones

Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K(1), a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.

Membrane bound pyrophosphatase and P-type adenosine triphosphatase of Leishmania donovani as possible chemotherapeutic targets: similarities and differences in inhibitor sensitivities

The activities of inorganic pyrophosphatase (PPase) and adenosine triphosphatase (ATPase) were studied in the plasma membrane of Leishmania donovani promastigotes and amastigotes. It was shown that the specific activity of PPase was greater than that of ATPase in the promastigote plasma membrane. We characterized H+-PPase present in the plasma membrane of L. donovani and investigated its possible role in the survival of promastigote and amastigote. PPase activity was stimulated by K+ and sodium orthovanadate and inhibited by pyrophosphate analogs (imidodiphosphate and alendronate), KF, N,N'-dicyclohexylcarbodiimide (DCCD), thiol reagents (p-chloromercuribenzenesulfonate (PCMBS), N-ethylmaleimide (NEM), and phenylarsine oxide (PAO)), the ABC superfamily transport modulator verapamil, and also by the F(1)F(o)-ATPase inhibitor quercetin. ATPase activity was stimulated by K+ and verapamil, inhibited by DCCD, PCMBS, NEM, sodium azide, sodium orthovanadate, and quercetin, and was unaffected by PAO. We conclude that there are significant differences within promastigote, amastigote, and mammalian host in cytosolic pH homeostasis to merit the inclusion of PPase transporter as a putative target for rational drug design.