Malaria-infected red cells and free parasites have limited capabilities for the biosynthesis of amino acids. Therefore, the principal amino acid sources for parasite protein synthesis are the plasma free amino acids and host cell haemoglobin. Infected cells and plasmodia incorporate exogenously supplied amino acids into protein. However, the hypothesis that amino acid utilization (from an external source) is related to availability of that amino acid in haemoglobin is without universal support: it is true for isoleucine and for Plasmodium knowlesi and P. falciparum, but not for methionine, cysteine, and other amino acids, and it does not apply to P. lophurae. More by default than by direct evidence, haemoglobin is believed to be the main amino acid reservoir available to the intraerythrocytic plasmodium. Haemoglobin, ingested via the cytostome, is held in food vacuoles where auto-oxidation takes place. As a consequence, haem is released and accumulates in the vacuole as particulate haemozoin (= malaria pigment). Current evidence favours the view that haemozoin is mainly haematin. Acid and alkaline proteases (identified in crude extracts from mammalian and avian malarias) are presumably secreted directly into the food vacuole. They then digest the denatured globin and the resulting amino acids are incorporated into parasite protein. Cell-free protein synthesizing systems have been developed using P. knowlesi and P. lophurae ribosomes. In the main these systems are typically eukaryotic.Studies of amino acid metabolism are exceedingly limited. Arginine, lysine, methionine, and proline are incorporated into protein, whereas glutamic acid is metabolized via an NADP-specific glutamic dehydrogenase. Glutamate oxidation generates NADPH and auxiliary energy (in the form of alpha-ketoglutarate). The role of red cell glutathione in the economy of the parasite remains obscure. Important goals for future research should be: quantitative assessment of the relative importance of amino acid sources for parasite protein synthesis; purification and characterization of plasmodial proteinases; and in vitro translation of parasite messenger RNA.

Amino acid metabolism and protein synthesis in malarial parasites.

Inorganic pyrophosphate promoted the acidification of an intracellular compartment in permeabilized promastigotes of Leishmania donovani, as measured by Acridine Orange uptake. The proton gradient generated by pyrophosphate was collapsed by addition of nigericin or NH4Cl. Pyrophosphate-driven proton translocation was stimulated by potassium ions, and inhibited by NaF, the pyrophosphate analogues imidodiphosphate and aminomethylenediphosphonate (AMDP), dicyclohexylcarbodiimide, and the thiol reagents p-hydroxymercuribenzoate and N-ethylmaleimide, all at concentrations similar to those that inhibit the plant vacuolar proton-pumping pyrophosphatase (H+-PPase). The proton translocation activity had a pH optimum in the range 7.0-7.5, and was unaffected by bafilomycin A1 (40 nM), concanamycin A (5 nM), sodium o-vanadate (500 microM) and KNO3 (200 mM). AMDP-sensitive pyrophosphate hydrolysis was also detected in promastigotes, and potassium ions also stimulated this activity. Sodium ions disrupted pH gradients established in the presence of ATP but not in the presence of pyrophosphate, and sequential addition of ATP and pyrophosphate resulted in partially additive Acridine Orange accumulation, suggesting that the vacuolar H+-PPase is in a different intracellular compartment from the vacuolar H+-ATPase and Na+/H+ exchanger of L. donovani promastigotes. Separation of promastigote extracts on Percoll gradients yielded a dense fraction that contained H+-PPase activity but lacked ATPase activity and markers for mitochondria, glycosomes and lysosomes. The organelles in this fraction appeared by electron microscopy to consist of electron-dense vacuoles. In summary, these results indicate that, in contrast to plant vacuoles, vacuolar H+-PPase and vacuolar ATPase activities are present in different compartments in L. donovani promastigotes.

/pdf/presence-of-a-vacuolar-h-pyrophosphatase-in-promastigotes-of-30e10tvucg.pdf

Presence of a vacuolar H+-pyrophosphatase in promastigotes of Leishmania donovani and its localization to a different compartment from the vacuolar H+-ATPase

Publisher Summary This chapter focuses on the ribosomal RNA (rRNA) genes of plasmodium. The study of rRNA genes offers the biologist at least three possible research areas: (1) molecular biology, (2) molecular inference of phylogeny based upon molecular sequence data, and (3) rRNA-based specific detection of organisms, which can be used diagnostically. The members of the unicellular apicomplexan parasitic genus Plasmodium employ a dichotomous life style existing in association with an invertebrate (usually Anopheline) mosquito vector and a vertebrate host. The demonstration of the ability of the parasite to express different distinct rRNA genes at different points in its life cycle has a number of implications, which must be considered in the light of the circumstances of the life cycle. Ribosomes consist of a complex of the rRNA species already described and proteins. Two distinct subunits, the large and the small, associate around messenger RNA (mRNA) to form the active ribosome. The genus Plasmodium is contained within the phylum Apicomplexa, which is defined by the possession of all or part of a characteristic set of structures that together form what is termed “the apical complex.” Classical detection of microorganisms for medical diagnosis has relied upon culture of those organisms on defining media. The study of rRNA genes in Plasmodium has provided valuable information about the molecular biology, biology, and evolutionary history of members of the genus.

The ribosomal RNA genes of Plasmodium

Parasitic protozoa comprise a large number of species, some of which are agents of important diseases. They are also of interest from the point of view of cell biology since they contain special organelles and structures. This review analyses our present knowledge of (1) the glycosomes, found in members of the Kinetoplastida order, (2) the hydrogenosomes found in some anaerobic protozoa, especially in trichomonads, (3) the acidocalcisomes, recently described in several protozoa, and (4) structures and organelles participating in the endocytic pathway in trypanosomatids.

Special organelles of some pathogenic protozoa.

3 – Cell Biology

. Phosphatase activity in Trypanosoma rhodesiense has been examined histochemically by light and electron microscopy and by enzymatic assay in homogenate fractions. Using a method with lead as capture ion, acid phosphatase was found in lysosome-like vesicles and in the flagellar pocket. No alkaline adenosine triphosphatase (ATPase) was detectable by this method. Direct assay of p-nitrophenylphosphatase activity in homogenate fractions showed that acid phosphatase activity was strongly membrane-bound, but that activity at pH 9 was minimal in both soluble and particulate fractions. “Endogenous” ATPase activity was localized specifically and reproducibly in the mitochondrial membranes and under the plasma membrane of the flagellum. This nonenzymic reaction product could not be eradicated by glycerol extraction or glucose depletion. Unlike the membrane staining, which was manifest only after lead treatment, heat-resistant electron-dense material was found in the matrix of lysosomal vesicles in trypanosomes fixed in glutaraldehyde only and not subjected to further treatment with heavy metal reagents. X-ray emission analysis showed the presence of calcium and phosphorus, indicating that the matrix might have a phosphate storage function.

Localization of phosphatases in Trypanosoma rhodesiense.

Monomeric ribosomes, isolated from the monkey malaria Plasmodium knowlesi by the combined use of low concentrations of Triton X-100 and differential centrifugation, sedimented in the 80S range and could be dissociated into 60 and 40S subparticles. The base composition of the rRNA was typically protozoan; that is, low in guanine and cytosine residues.

The ribosomes of the simian malaria, Plasmodium knowlesi. I. Isolation and characterization.

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Localization of phosphatases in Trypanosoma rhodesiense.

The ribosomes of the simian malaria, Plasmodium knowlesi. I. Isolation and characterization.