Heme biosynthesis in bacterium-protozoon symbioses: enzymic defects in host hemoflagellates and complemental role of their intracellular symbiotes

Phytomonas and other trypanosomatid parasites of plants and fruit

Trypanosomatid parasites are fairly common in the latex, phloem, fruit sap, seed albumen, and even in the nectar, of many plant families. They are transmitted to the plants in the saliva of phytophagous hemipterous bugs (Insecta). Morphologically, plant trypanosomatids have no special characteristic, except perhaps a very twisted cell body. Most occur in plants as promastigotes and a few as choanomastigotes. It is still controversial whether or not they are pathogenic in lactiferous plants or fruit, but it is certain that the phloem parasites are pathogenic in coconut palms and coffee bushes. In these plants, they cause lethal diseases responsible for the destruction of many plantations in Central and South America, but fortunately nowhere else in the world. Probably more than one genus of Trypanosomatidae is represented among the plant parasites. The most important is certainly Phytomonas, but Leptomonas, Crithidia and Herpetomonas may also be present. The distinction between them is difficult and only recently have molecular markers become available to help in their identification. At present, Phytomonas can be identified by DNA hybridization with a specific probe (SL3') complementary to a sequence of the mini-exon or spliced leader gene. The development of a polymerase chain reaction coupled to SL3' hybridization has facilitated the detection of Phytomonas in plants. The phylogeny of Phytomonas is still being worked out. For the moment it can only be said that the genus is very close to Herpetomonas.

Hemoglobin endocytosis in Leishmania is mediated through a 46-kDa protein located in the flagellar pocket

Four lines of evidence indicate that a specific high affinity binding site on the surface of Leishmania donovani promastigotes mediates rapid internalization and degradation of hemoglobin. 1) Binding and uptake of 125I-hemoglobin by Leishmania followed saturation kinetics and were competed by unlabeled hemoglobin but not by globin or hemin or other heme- or iron-containing proteins. 2) Immunogold labeling studies revealed that, at 4 degreesC, hemoglobin binding was localized in the flagellar pocket of the promastigotes. Indirect immunofluorescence assays showed that, at 37 degreesC, the bound hemoglobin in such cells entered an endocytic compartment within 2 min and dispersed throughout the cell body by 15 min. 3) After incubation with hemoglobin-gold conjugates at 25 degreesC or 37 degreesC, the particles accumulated in discrete intracellular vesicles. 4) A single biotinylated protein of 46 kDa was revealed when solubilized membranes from surface biotinylated intact Leishmania adsorbed by hemoglobin-agarose beads were subjected to SDS-polyacrylamide gel electrophoresis and Western blotting with avidin-horseradish peroxidase. Considered together, these data indicate that this 46-kDa protein on the cell surface of L. donovani promastigotes mediates the binding of hemoglobin and its rapid internalization through a vesicular pathway characteristic of receptor-mediated endocytosis.

Evidence for symbiont-induced alteration of a host's gene expression: irreversible loss of SAM synthetase from Amoeba proteus

Symbiont-bearing xD amoebae no longer produce a 45-kDa cytoplasmic protein that functions as S-adenosylmethionine synthetase in symbiont-free D amoebae. The absence of the protein in xD amoebae is attributable to xD amoeba's failure to transcribe the corresponding gene as a result of harboring bacterial symbionts. However, xD amoebae have about half the level of enzyme activity found in D amoebae, indicating that they use an alternative source for the enzyme. xD amoebae originated from D amoebae by bacterial infection and now depend on their symbionts for survival. xD amoebae exhibit irreversible nucleolar abnormalities when their symbionts are removed, suggesting that X-bacteria supply the needed enzyme. A monoclonal antibody against the 45-kDa protein was produced and used as a probe in cloning its corresponding cDNA. The product of the cDNA was found to have S-adenosylmethionine synthetase activity. These results show how symbiotic X-bacteria may become essential cellular components of amoeba by supplementing a genetic defect for an amoeba's house-keeping gene that is brought about by an action of X-bacteria themselves. This is the first reported example in which symbionts alter the host's gene expression to block the production of an essential protein.

L-alanine: 4,5-dioxovalerate transaminase in Leishmania donovani that differs from mammalian enzyme

Leishmania protozoans are the causative agents of leishmaniasis, a major parasitic disease in humans. The parasites manifest a nutritional requirement for heme compounds since they are deficient in heme biosynthesis. In this study we have demonstrated for the first time the presence of the enzyme L-alanine: 4,5-dioxovalerate transaminase in Leishmania donovani. This enzyme catalyzes the synthesis of 5-aminolevulinic acid (ALA), the first committed step in heme synthesis. Thus the defect in heme biosynthesis pathway in Leishmania must lie at some enzymatic level subsequent to synthesis of ALA. The enzyme was found to be present in both virulent and avirulent strains of L. donovani. The virulent promastigotes showed a 41% higher specific activity as compared to the avirulent strain. The enzyme activity was found to be inhibited in the presence of heme and methylglyoxal. Immunoblot analysis revealed that L-alanine: 4,5-dioxovalerate transaminase in L. donovani was immunologically different from that in mammals.

Developmental regulation of mitochondrial biogenesis in Trypanosoma brucei

The metabolism of Trypanosoma brucei undergoes a significant change as the parasite differentiates from the mammalian bloodstream form to the form found in the tse-tse fly vector. Because the mitochondria of bloodstream form cells lack cytochromes and several key citric acid cycle enzymes, the metabolism of these cells is mostly limited to glycolysis. The reducing equivalents generated by this process are passed to oxygen by a plant-like alternative oxidase. As cells differentiate to the insect form, they begin to oxidatively metabolize proline. The mitochondria of insect form cells contain functional, cytochrome-mediated electron transport chains and have complete complements of citric acid cycle enzymes. Although the characterization is far from complete, the nuclear and mitochondrial genes involved in the expression of these mitochondrial functions appear to be developmentally regulated at posttranscriptional and posttranslational levels. This review outlines some of the molecular processes that are associated with the developmental regulation of mitochondrial biogenesis and suggests some possible mechanisms of regulation.

Membrane-associated polysaccharides composition, nutritional requirements and cell differentiation in Herpetomonas roitmani: influence of the endosymbiont

Herpetomonas roitmani, a trypanosomatid containing a bacterial endosymbiont, was cured by high doses of chloramphenicol. Wild-type and cured flagellates were compared as to polysaccharide composition, nutritional requirements and cellular differentiation. Fucose (18.0%), xylose (15.7%), mannose (38.9%), galactose (10.8%), glucose (16.4%) and inositol (< 1.0%) were identified as polysaccharide components of cured H. roitmani as assessed by gas-liquid chromatography. However, the wild-type strain displayed a markedly different sugar profile, in that xylose was absent and inositol preferentially synthesized, whereas the other monosaccharide components remained unchanged. Variations in nutritional pattern also occurred between both strains. The bacterial endosymbiont seems to provide the flagellates with nutritional factors, including usual amino acids, vitamins, purine (as adenine) and hemin. The process of cell differentiation was also significantly influenced by the endosymbiont. Opisthomastigote forms predominate (72.0%) in cured as compared with wild-type H. roitmani (37.0%).

Endocytosis of gold-labeled proteins and LDL by Trypanosoma cruzi

Endocytosis was studied at the ultrastructural level in different developmental forms of Trypanosoma cruzi after incubation of the parasites in the presence of gold-labeled proteins (albumin-Au, peroxidase-Au and transferrin-Au) and low-density lipoprotein (LDL-Au). Epimastigote (culture) forms actively ingested LDL and proteins. Initially, gold particles were seen adhering only to the cytostome and inside the flagellar pocket. In parasites incubated at 4 degrees C with transferrin-Au or peroxidase-Au, labeling was found only at these two sites, showing that receptor-mediated endocytosis occurs in both regions. In the cytoplasm, gold particles were seen only inside two different compartments: membrane-bound vesicles and reservosomes. Incubation of epimastigotes with acridine orange followed by fluorescence microscopy revealed intense orange staining, indicating that the reservosomes have an acidic pH. This staining was abolished after incubation of the parasites in the presence of ammonium chloride. These data confirm that this compartment is the site of accumulation of ingested lipids and proteins. Little intracellular labeling with transferrin-Au was found in in vitro-derived amastigotes and trypomastigotes (both lack reservosomes). However, although in amastigotes very few gold particles were seen bound to the cells, in trypomastigotes they were observed bound to the membrane that encloses the cell body, the flagellar pocket, and the flagellum, suggesting that the receptors are more abundant in this form.

Heme synthesis in Crithidia deanei: influence of the endosymbiote

The activity of the following enzymes involved in the biosynthesis of porphyrins was determined in endosymbiote-free and endosymbiote-containing Crithidia deanei grown in a chemically defined medium: succinyl Coenzyme A synthetase (Suc.CoA-S), 5-aminolevulinate synthetase (ALA-S), 4,5-dioxovaleric acid transaminase (DOVA-T), 5-aminolevulinate dehydratase (ALA-D), porphobilinogenase (PBGase), deaminase and heme synthetase (Heme-S). The amount of 5-aminolevulinic acid (ALA) and porphobilinogen, porphyrins and heme was also determined. ALA and PBG were detected in C. deanei. The levels of free porphyrins was low. Heme concentration was nil. The activity of ALA-D, deaminase and PBGase was not detected in C. deanei. The activity of Suc.CoA-S and ALA-S were twice higher in symbiote-containing than in aposymbiotic C. deanei. Aposymbiotic cells had a higher activity of DOVA-T than symbiote-containing cells. The level of Heme-S, measured using protoporphyrin as substrate, was twice as high in symbiote-containing than in symbiote-free cells.

The in vitro trypanocidal activity of N-substituted p-benzoquinone imines: assessment of biochemical structure-activity relationships using the Hansch approach

It has previously been found that naphthoquinones can potentiate the rate of hydrogen peroxide production by mitochondrial preparations of Trypanosoma brucei brucei and that organisms treated with naphthoquinones are more susceptible to lysis, especially in the presence of compounds such as heme, which promote the homolytic cleavage of hydrogen peroxide. We have evaluated the lytic effect of various N-substituted p-benzoquinone imines both in vitro and in vivo and have attempted to correlate their structure with trypanocidal activity using the Hansch approach. While none of the compounds tested proved to be active in vivo, all caused the lysis of trypanosomes in vitro. The parameters that correlated best with trypanocidal activity were the conditional redox potential, the lipophilicity of the substituent attached to the nitrogen atom and the number of active hydrogens on the quinonoid ring. These findings suggest two possible modes of action, which may in fact be related. Conjugate nucleophilic addition and/or oxidative damage could be responsible for lysis of the parasites. These same compounds were previously found to be active against the ascitic sarcoma 180 in mice. The strong correlation between antineoplastic activity in vivo and trypanocidal activity in vitro suggests a similar mode of action in both cases. Further studies aimed at developing a quinonelike compound that will be active against trypanosomes in vivo are now in progress.

Growth cycle-dependent overproduction and accumulation of protoporphyrin IX in Tetrahymena: effect of heavy metals

Cells of the ciliate Tetrahymena pyriformis GL overproduce and accumulate massive quantities of the heme intermediate, protoporphyrin IX. Protoporphyrin is localized intracellularly in discrete membranous compartments. The amount of porphyrin stored in the cell changes dramatically as cells progress through the growth cycle. Porphyrin overproduction is stimulated by delta-aminolevulinic acid, but only during the mid-stationary phase. Overproduction of protoporphyrin IX apparently results from an increase, late in the growth cycle, of activities subsequent to delta-aminolevulinic acid synthetase. Feedback inhibition in the pathway by accumulated protoporphyrin IX does not occur. The presence of Co2+ completely inhibits accumulation of protoporphyrin IX in a manner reversed by delta-aminolevulinic acid. Sn4+ stimulates protoporphyrin IX accumulation in the culture.

Ultrastructural differences between species of trypanosomatids with and without endosymbionts

Species of trypanosomatids without endosymbionts (Leptomonas seymouri, L. collosoma, L. samueli, crithidia fasciculata, C. luciliae, C. acanthocephali, Herpetomonas megaseliae, H. mariadeanei, H. samuelpessoai, H. muscarum muscarum, Trypanosoma cruzi) and species of trypanosomatids with endosymbionts (Crithidia deanei, C. oncopelti, Blastocrithidia culicis) were comparatively studied by means of electron microscopy. Artificially aposymbiotic strains derived from species with symbiont were also included in the survey. Species with symbiont were found to differ in some ultrastructural aspects from the group of species without symbiont. Paraxial rods of flagella or intraflagellar structure were found exclusively in species without symbiont. Peripheral branching of mitochondria, accompanied by absence of subpellicular microtubules in sites where the mitochondrial branches are appressed to the cell membrane, were found exclusively in species with symbiont. Networks of kinetoplast DNA fibrils were found to be larger and looser in species with symbiont. Symbiont-free strains of species with symbiont retained the same morphological characteristics of their parental species.

Inhibition of glutathione synthesis as a chemotherapeutic strategy for trypanosomiasis

With the expectation that trypanosomal glutathione (GSH) plays a major protective role against the endogenous oxidant stress that results form high intracellular levels of H2O2, we sought to deplete Trypanosoma brucei brucei of their GSH through inhibition of its biosynthesis. Administration of buthionine sulfoximine (BSO), a reversible inhibitor of gamma-glutamylcysteine synthetase, to parasitemic mice resulted in a progressive decrease in trypanosome GSH content, such that parasites isolated after 5 h or BSO treatment contained 50% of normal values. When BSO administration was continued for 18 h (intraperitoneal injection of 4 mmol/kg every 1.5 h), parasitemias temporarily cleared. When inhibitory plasma levels of BSO were maintained for about 27 h, two out of six infected mice were cured and the rest had significantly prolonged survival. These findings demonstrate the potential value of GSH depletion for the treatment of trypanosomiasis.

Dense Crithidia growth and heme sparing: relation to Fe, Cu, Mo chelation

SYNOPSIS. Heme, intrinsically required by Trypanosomatidae, is unstable, especially in conventional alkaline (pH 7.2–8.0) media. Low solubility of heme in a pH 6.5 basal medium (developed to assay biopterin with Crithidia fasciculata) posed a problem: in media acidified during growth because of glycolysis, heme precipitated, perhaps contributed to acid‐limited growth and interfered with densitometric estimation of growth. The remedy was to: replace glucose with less rapidly metabolized mannitol; distribute media in thin layers to promote oxidation of acetate, fumarate, and malate (presumably leaving an alkaline residue); and buffer heavily with histidine + Good zwitterionic buffers, and superimpcse physiological buffering by arginine + asparagine whose catabolism appeared to yield an excess of NH+4 over acid. Thereupon, Fe and Cu deficiencies sharply limited growth in the medium whose main chelators were: (a) 2,3–dihydroxybenzoic + 5‐sulfosalicylic acids (which preferentially bind transitional elements at their higher valences; (b) malic and gluconic acids; and (c) histidine. With unconventionally heightened concentrations of Fe, Cu, and Mo (the latter serving as Cu buffer as well as nutrient per se), the hemin concentration could be lowered, widening the margin of safety for heme solubility. Growth then reached 1.4 × 108 cell/ml. This medium may serve to screen for ligands promoting uptake or release of Fe and Cu. The increased growth is a step towards improving the assay medium for biopterin and practical use of Crithidia to assay several B vitamins and essential amino acids for metazoa.

An approach to the development of new drugs for African trypanosomiasis

The inability of the bloodstream form of Trypanosoma brucei brucei to decompose hydrogen peroxide forms the basis of our attempt to develop new pharmacological agents to kill these organisms. Approximately 1-3% of the oxygen consumed by these parasites appears in the form of hydrogen peroxide. Our previous observation that free radical initiators such as heme and hematoporphyrin D proved to be trypanocidal in vitro and in vivo, respectively, prompted this investigation into the mechanism of action of this class of compounds to enhance their therapeutic efficacy. The locus of H2O2 production within the trypanosome was examined using cell-free homogenates. Experiments described herein suggest that H2O2 is formed by the alpha-glycerol phosphate dehydrogenase in an adventitious manner, and that no enzymatic means of disposing of this potentially toxic compound are present with the organisms. Naphthoquinones were found to substantially increase the rate of both oxygen consumption and H2O2 production by trypanosomal mitochondrial preparations. Presumably, the naphthoquinones are acting as coenzyme Q analogues. The addition of sublytic concentrations of both naphthoquinones and heme leads to a synergistic lysis of the organisms in vitro. Another approach to increasing the susceptibility of T. b. brucei to free radical damage involved reduction of the intracellular concentration of glutathione. This was accomplished through the use of trypanocidal arsenicals. Melarsenoxide and heme acted synergistically in vitro, an effect which was further enhanced via addition of a naphthoquinone. Moreover, hematoporphyrin D and tryparsamide were shown to have a synergistic effect in T. b. brucei-infected mice.

Study of factors causing excess protoporphyrin accumulation in cultured skin fibroblasts from patients with protoporphyria

The activity of heme synthetase, which catalyzes the chelation of ferrous iron to protoporphyrin to form heme, is deficient in sonicates of skin fibroblasts cultured from patients with protoporphyria. During culture in Eagle's medium supplemented with fetal calf serum, these cells do not accumulate protoporphyrin, however. This may be due to a minimal requirement for heme synthesis, since glycine is incorporated into heme at a low rate which is similar to that in normal fibroblasts. In addition, the activity of delta-aminolevulinic acid (ALA) synthetase, the first and rate-limiting enzyme of heme biosynthesis which catalyzes the formation of ALA from glycine, is normal in lysates of the fibroblasts. Cultured fibroblasts were therefore incubated with ALA in order to bypass the rate-limiting step of heme biosynthesis. In the presence of 25 muM iron, protoporphyrin was detected in protoporphyria cell lines when the concentration of ALA in the medium reached 50 muM, but not in normal lines. As the concentration of ALA was increased above 50 muM, all lines accumulated protoporphyrin. However, the amount was 2-3 times more in cultured fibroblasts from patients with protoporphyria, reflecting their deficiency of heme synthetase activity. When iron was not added to the medium, protoporphyrin accumulated to a similar degree in normal and protoporphyria fibroblasts; this was significantly more than that in the presence of iron. These studies indicate that excessive protoporphyrin accumulation in protoporphyria, which is due principally to deficient heme synthetase activity, may be modified by the rate of ALA formation in heme-producing tissues, and by the availability of iron.

Catabolism of deoxythymidylate in some trypanosomatids

An initial observation concerning the failure of [3H]thymidine at high specific activity to be incorporated into the DNA of Crithidia fasciculata for more than a brief initial period has been correlated with the presence at high specific activity in the organism of a thymidine phosphorylase activity with an equilibrium in the direction of catabolism. This enzyme degrades thymidine to thymine which is not utilized by the organism. The enzyme has also been shown to be present in a number of other trypanosomatids, including the culture forms of Trypanosoma cruzi, where the specific activity was nearly as high as that in C. fasciculata. Evidence is presented that in C. fasciculata, the culture forms of T. cruzi and possibly other species of trypanosomatid, the thymidine phosphorylae, together with a thymidylate phosphatase, forms a catabolic pathway which degrades thymine nucleotides to thymine, which is then excreted. About 60% of the thymine nucleotides made by organisms appear to be metabolized through the pathway, suggesting that their synthesis is not subject to completely effective regulatory control.

Symbiote-free hemoflagellates, Blastocrithidia culicis and Crithidia oncopelti: their liver factor requirement and serologic identity

Several aposymbiotic strains of Blastocrithidia culicis and Crithidia oncopelti were cultivated in Trager's chemically defined medium as well as in a blood broth, both supplemented with 0.25% (v/v) liver extract concentrate. For all such strains, the liver extract was found to serve as an essential growth factor in the defined medium and as growth promoting additive in the blood broth. The active molecules were found to be water-soluble, heat stable, dialyzable, and probably nonlipid fractions. Antisera were developed in rabbit against all the available aposymbiotic strains. An almost total cross-reactivity at very high titers was observed in reciprocal agglutination test using strains with and without the bacterial symbiotes. These results indicate that the loss of the symbiotes does not affect the antigenic identity of B. culicis and C. oncopelti.

Surface membrane carbohydrate alterations of a flagellated protozoan mediated by bacterial endosymbiotes

Crithidia oncopelti, a parasitic trypanosomatid protozoan of insects, normally contains intracellular symbiotic bacteria. As shown earlier, the protozoa can be rid of their endosymbiotes by chloramphenicol, producing a symbiote-free cell line. Here surface-membrane carbohydrate ligands of the symbiote-containing and symbiote-free strains were compared by lectin-mediated agglutination, lectin-ultrastructure localization. [3H] lectin-binding, and fluorescent lectin staining. Symbiote-free organisms consistently had 3-fold higher agglutination titers than symbiote-containing cells with concanavalin A. Conversely, symbiote-containing flagellates had 2- to 3-fold greater agglutination titers with a fucose-binding lectin than symbiote-free organisms. Ultrastructure results showed that more of concanavalin A-horseradish peroxidase-diaminobenzidine reaction product was present at the surface of symbiote-free than on symbiote-containing cells. Treatment with [3H]concanavalin A revealed that surface membrane sites available per cell for [3H]lectin-binding ranged from 6.2 to 7.4 x 10(4) and from 24 to 27 x 10(4) for symbiote-containing and symbiote-free organisms, respectively, i.e., the mean binding level of the latter for the lectin was 3.5 times greater than that of the former. Moreover, symbiote-free cells fluoresced more than symbiote-containing organisms after staining with fluorescein-labeled concanavalin A. Apparently, the prokaryotic endosymbiotes somehow alter the quantity of saccharide ligands in the C. oncopelti surface membrane.