Plasmodium falciparum: Association with Erythrocytic Superoxide Dismutase1

Levels of superoxide dismutase (SOD) activity and its properties in Plasmodium falciparum-infected erythrocytes, isolated parasites, and noninfected erythrocytes were studied. A higher specific activity was found in P. falciparum-infected erythrocytes compared to noninfected erythrocytes, resulting from the lower protein content of infected cells and not enzyme synthesis by the parasite, as the superoxide dismutase activity expressed per number of cells was decreased. Superoxide dismutase from noninfected erythrocytes and isolated P. falciparum parasites showed similar sensitivities to various inhibitors and had identical molecular weights and electrophoretic mobilities. These results support the hypothesis of uptake and use of the erythrocytic SOD enzyme by the parasite as a possible mechanism of defense against oxidative stress.     

Plasmodium falciparum regulatory subunit of cAMP-dependent PKA and anion channel conductance

Malaria symptoms occur during Plasmodium falciparum development into red blood cells. During this process, the parasites make substantial modifications to the host cell in order to facilitate nutrient uptake and aid in parasite metabolism. One significant alteration that is required for parasite development is the establishment of an anion channel, as part of the establishment of New Permeation Pathways (NPPs) in the red blood cell plasma membrane, and we have shown previously that one channel can be activated in uninfected cells by exogenous protein kinase A. Here, we present evidence that in P. falciparum-infected red blood cells, a cAMP pathway modulates anion conductance of the erythrocyte membrane. In patch-clamp experiments on infected erythrocytes, addition of recombinant PfPKA-R to the pipette in vitro, or overexpression of PfPKA-R in transgenic parasites lead to down-regulation of anion conductance. Moreover, this overexpressing PfPKA-R strain has a growth defect that can be restored by increasing the levels of intracellular cAMP. Our data demonstrate that the anion channel is indeed regulated by a cAMP-dependent pathway in P. falciparum-infected red blood cells. The discovery of a parasite regulatory pathway responsible for modulating anion channel activity in the membranes of P. falciparum-infected red blood cells represents an important insight into how parasites modify host cell permeation pathways. These findings may also provide an avenue for the development of new intervention strategies targeting this important anion channel and its regulation.     

Malarial parasite hexokinase and hexokinase-dependent glutathione reduction in the Plasmodium falciparum-infected human erythrocyte

The metabolism of glucose in Plasmodium falciparum-infected human erythrocytes is increased 50- to 100-fold. This is accomplished in part by parasite-directed synthesis of a protozoan hexokinase with unique kinetic, electrophoretic, and heat stability properties. The total hexokinase activity is increased approximately 25-fold over that of control uninfected erythrocytes of the same age from the same donor. The parasite hexokinase has a lower affinity for glucose than the mammalian enzyme (Km = 431 microM +/- 21 S.D. for the parasite enzyme versus 98 microM +/- 10 for the erythrocyte enzyme), but the Km for ATP and the Vmax for both glucose and ATP are similar. The NADPH-dependent reduction of oxidized glutathione (GSSG) requires the formation of glucose 6-phosphate which in turn is metabolized by the pentose shunt pathway in which NADPH is generated. Using glucose as the substrate, lysates of P. falciparum-infected normal erythrocytes demonstrated enhanced ability to reduce GSSG. The rate of GSSG reduction was proportional both to the parasitemia and the hexokinase activity of the lysates. However, infected glucose-6-phosphate dehydrogenase-deficient red cell lysates displayed a severely restricted ability to reduce GSSG under the same conditions. In conclusion, P. falciparum-infected red cells contain a parasite-encoded hexokinase with unique properties which initiates the large increase in glucose consumption. In normal infected red cells, reduction of GSSG is also dependent upon hexokinase activity, but in infected glucose-6-phosphate dehydrogenase-deficient red cells, the absence of this pentose shunt enzyme remains the rate-limiting step in GSSG reduction.     

Regulation of superoxide dismutase synthesis in Candida albicans

The synthesis of superoxide dismutase [SOD: EC 1.15.1.1] in response to various cultural conditions was examined in Candida albicans, an opportunistic yeast which causes candidiasis in immunosuppressed patients. SOD plays an important role in protecting cells from the oxidative damage of superoxide radicals. Maximum SOD activity was found after 72 hrs of yeast growth. The optimum pH and temperature for the SOD activity were 7 and 40 °, respectively. The major SOD activity was found in the cytosol fraction and the level of extracellular SOD was very low. The enzyme was stimulated to varying degrees by cholic acid, procaine and tocopherol. On the basis of inhibitor studies and other enzyme properties, the isolated enzyme from C. albicans is identified as copper and zinc superoxide dismutase. This revised version was published online in June 2006 with corrections to the Cover Date.     

Antigenicity of filarial superoxide dismutase in human bancroftian filariasis

Superoxide dismutase activity was measured in different stages of growth of filarial parasites (human and cattle). The activity was almost undetected or very low in microfilarial stage but in adult worms, the enzyme activity was high. The enzyme was characterized to be a Cu/Zn superoxide dismutase. Most of the enzyme activity was associated with a detergent extractable fraction of adult (Setaria) parasite. The enzyme was also detected in thein vitro released products of adult worms. The superoxide dismutase activity was completely inhibited with IgG antibody from chronic filarial patients in contrast to IgG from normal people. Filarial patients particularly have high IgG and IgM antibody levels to purified enzyme. However, individuals from non-filarial regions of Orissa are sero-negative for superoxide dismutase antibodies. Antibody response to superoxide dismutase could thus be used for filarial diagnosis.     

The H89 cAMP-dependent protein kinase inhibitor blocks Plasmodium falciparum development in infected erythrocytes.

In Plasmodium falciparum, the causative agent of human malaria, the catalytic subunit gene of cAMP-dependent protein kinase (Pfpka-c) exists as a single copy. Interestingly, its expression appears developmentally regulated, being at higher levels in the pathogenic asexual stages than in the sexual forms of parasite that are responsible for transmission to the mosquito vector. Within asexual parasites, PfPKA activity can be readily detected in schizonts. Similar to endogenous PKA activity of noninfected red blood cells, the parasite enzyme can be stimulated by cAMP and inhibited by protein kinase inhibitor.Importantly, ex vivo treatment of infected erythrocytes with the classical PKA-C inhibitor H89 leads to a block in parasite growth. This suggests that the PKA activities of infected red blood cells are essential for parasite multiplication. Finally, structural considerations suggest that drugs targeting the parasite, rather than the erythrocyte enzyme, might be developed that could help in the fight against malaria.     

Plasmodium falciparum signal sequences: simply sequences or special signals?

The malaria parasite, Plasmodium falciparum, synthesises and exports several proteins inducing morphological and biochemical modifications of erythrocytes during the erythrocytic cycle. The protein trafficking machinery of the parasite is similar to that of other eukaryotic cells in several ways. However, some unusual features are also observed. The secretion of various polypeptides was inhibited when P. falciparum-infected erythrocytes were incubated with Brefeldin A. Immunoelectron microscopy studies revealed substantial morphological changes in the endoplasmic reticulum following exposure of parasitised erythrocytes to the drug. Immunofluorescence studies of Brefeldin A-treated parasites suggest that polypeptide sorting to different intracellular destinations begins at the endoplasmic reticulum. The parasite also secretes polypeptides by a Brefeldin A-insensitive route that bypasses the classical endoplasmic reticulum-Golgi complex pathway.     

Changes in superoxide dismutase activity in the presence of electron donors and acceptors

The activity of superoxide dismutase (SOD) from bovine erythrocytes was measured by the inhibition of nitrotetrazolium blue reduction rate in superoxide anion radical generation systems--xanthine/xanthine oxidase of NADH/phenazine methasulfate. The enzyme activity increases in the presence of compounds acting as electron donors in radical-involving reactions and decreased in the presence of compounds possessing the properties of electron acceptors. Activation of SOD by electron donors and its inhibition by electron acceptors was dependent on the concentration of the above compounds. In the absence of SOD electron donors and acceptors did not change the rate of tetrazolium blue reduction by superoxide anion radicals. The role of the new type of SOD regulation for the enzyme functioning in the cell is discussed.     

Superoxide dismutase and catalase in the murine malaria, Plasmodium berghei: content and subcellular distribution

Plasmodium berghei, a murine malaria, lacks endogenous superoxide dismutase (SOD). Instead it appears to take up and concentrate SOD from its host cell, the erythrocyte. We now demonstrate that the adopted host enzyme is localized in granules which are probably lysosomes. In addition, isolated P. berghei parasites contain only low levels of catalase, probably as a result of contamination of the preparation with host cell material. Thus, the cytosol of this organism appears to be deficient in enzymes which protect against damage by activated oxygen.     

A case of 21q-syndrome with half normal SOD-1 activity

A male Japanese infant was found to have a chromosomal aberration of del(21)(qter leads to q22.1-2) and decreased superoxide dismutase (SOD) activity in erythrocytes and polymorphonuclear and mononuclear leukocytes. The cuprozinc enzyme (SOD-1) level was 40-50% of normal, while the cyanide-insensitive manganese enzyme (SOD-2) activity was within the normal range. Determination of SOD activity in blood cells is a valuable method of classification of the syndrome.     

14C-desferrioxamine B: uptake into erythrocytes infected with Plasmodium falciparum

Plasmodium falciparum-infected human erythrocytes (early trophozoite stages) and non-infected erythrocytes were incubated in 1.7 mM 14C-desferrioxamine B (specific activity 1 microCi/2.6 mg desferrioxamine B). After 270 min the cells were washed and the radioactivity was measured in the cell pellet and, after lysis, in cytoplasm and membranes. The results indicate that Desferrioxamine B can the red blood cell and pass through the parasite membrane and that the parasites are killed by the intracellular action of the chelator.     

Effects of some drugs on purified human erythrocyte CuZnSOD and in vitro inhibitiory effect of 5-fluorouracil on leukocyte total SOD activity

The inhibition and activation effects of some drugs on the activities of superoxide dismutase enzymes (SOD) in human erythrocyte and leukocyte cells was investigated. Firstly, CuZnSOD enzyme was purified 837–fold and 12% efficiency from human erythrocytes by ethanol-chloroform treatment to remove hemoglobin and then ion exchange chromatography (DEAE-Sepharose) and copper chelate affinity chromatography techniques. Inhibition or activation effects of fourteen drugs on CuZnSOD was investigated. None of the studied drugs except for 5-fluorouracil showed any effects on the enzyme. 5-fluorouracil showed activation effects on CuZnSOD at 3.33 mg/ml and 4 mg/ml concentrations with 33% and 32% activation, respectively. Leukocytes were isolated from healthy human blood, lysed in liquid nitrogen and the effect of 5-fluorouracil on the lysate SOD activity investigated. 5-Fluorouracil showed inhibition effects on total SOD activity of human leukocytes at 2 mg/ml and 4 mg/ml concentrations with 42% and 62% inhibition, respectively.     

Effects of some drugs on purified human erythrocyte CuZnSOD and in vitro inhibitiory effect of 5-fluorouracil on leukocyte total SOD activity

The inhibition and activation effects of some drugs on the activities of superoxide dismutase enzymes (SOD) in human erythrocyte and leukocyte cells was investigated. Firstly, CuZnSOD enzyme was purified 837-fold and 12% efficiency from human erythrocytes by ethanol-chloroform treatment to remove hemoglobin and then ion exchange chromatography (DEAE-Sepharose) and copper chelate affinity chromatography techniques. Inhibition or activation effects of fourteen drugs on CuZnSOD was investigated. None of the studied drugs except for 5-fluorouracil showed any effects on the enzyme. 5-fluorouracil showed activation effects on CuZnSOD at 3.33mg/ml and 4mg/ml concentrations with 33% and 32% activation, respectively. Leukocytes were isolated from healthy human blood, lysed in liquid nitrogen and the effect of 5-fluorouracil on the lysate SOD activity investigated. 5-Fluorouracil showed inhibition effects on total SOD activity of human leukocytes at 2 mg/ml and 4 mg/ml concentrations with 42% and 62% inhibition, respectively.     

Plasmodium falciparum: an invasion inhibitory human monoclonal antibody is directed against a malarial glycolipid antigen

A Plasmodium falciparum malaria blood stage antigen was detected using a human monoclonal antibody (MAb A52A6) obtained from a clinically immune donor. Immunofluorescence analysis showed that the MAb reacted with the intracellular parasite throughout the asexual blood stage cycle as well as with gametocytes. The MAb also reacted with the surface of erythrocytes containing late stage P. falciparum parasites. The antigen seen by the MAb was species- but not strain- or isolate-specific. At rupture of the infected erythrocytes, antigenic material was deposited on the membrane of uninfected cells surrounding the parasite. At merozoite invasion MAb reactive material was present on the invaginating erythrocyte membrane, indicating an involvement of the antigen in the invasion process. This was also indicated by the high capacity of the MAb to inhibit merozoite invasion in vitro. The antigen appears to be a phosphoglycolipid, sensitive to phospholipase and present in lipid extracts of P. falciparum-infected erythrocytes.     

Room temperature electron spin resonance of superoxide dismutase-loaded liposomes and erythrocytes. A direct approach to the interaction of O2- with cells

Human erythrocytes were enriched with bovine superoxide dismutase by fusion with liposomes containing the entrapped enzyme. Liquid solution ESR of intact cells at room temperature was used to measure directly the increase in the superoxide dismutase content. From the spectral characteristics (g-value and hyperfine splitting tensor), the structural integrity of the Cu site of the enzyme was found to be unaffected by the liposome preparation procedure or the incubation with cells. Changes in the ESR signal size were used to test directly the interaction of superoxide with the enzyme entrapped in liposomes or delivered to erythrocytes. It was found that the liposome-entrapped enzyme does not react with externally generated O2-, but once delivered to red blood cells this reaction can take place. This is the first demonstration of O2- -scavenging activity by superoxide dismutase delivered into an intact cell structure and is therefore to be considered as strong evidence for activity of this enzyme under in vivo conditions.     

Superoxide dismutase activity in nematodes

The activities of the enzymes superoxide dismutase (E.C.:1.15.1.1) and catalase (E.C.:1.11.1.6) were studied in purified extracts of four nematodes: Ascaris suum, Toxascaris leonina, Toxocara canis and T. cati adult males and females. No catalase activity was found in any of the extracts. The results reveal that the SOD activities of the four parasites presented species differences and also sexual differences within each species. Polyacrylamide gel electrophoresis pattern analysis confirmed that the mobilities, widths and band intensities varied according to the species and sex of the parasite from which the enzyme was obtained.     

Molecular cloning and characterization of copper/zinc-superoxide dismutase of Paragonimus westermani

Superoxide dismutases (SODs; EC 1.15.1.1) play important roles in the protection of the parasites against cellular oxygen-mediated killing of the hosts. A copper/zinc-containing SOD (Cu/Zn-SOD) was identified previously from lung fluke, Paragonimus westermani. To expand our understanding of P. westermani SOD, we isolated a complementary DNA encoding a Cu/Zn-SOD, expressed the active enzyme in Escherichia coli, and characterized its biochemical properties. The deduced amino acid (aa) sequence of the gene shared up to 73.7% identities with Cu/Zn-SODs of other helminths and shared well-conserved characteristic motifs and essential aa residues involved in coordinating copper and zinc enzymatic functions. Recombinant Cu/ Zn-SOD exhibited comparable biochemical properties with that of the native enzyme, including pH optima and potassium cyanide-and hydrogen peroxide-sensitive inhibition profiles. The active enzyme consisted of 2 identical subunits covalently linked by disulfide bonds. The enzyme was constitutively expressed throughout various developmental stages of the parasite. The levels increased as P. westermani matured and plateaued in adult stage. Our result suggests the enzyme might play an important role for parasites to survive in the hosts through its superoxide anion-detoxifying function.     

Dismutation of superoxide radicals by ceruloplasmin--details of the mechanism

Like superoxide dismutase (SOD), human ceruloplasmin (Cp) scavenges superoxide anion radicals injected into the solution with the aid a high-voltage generator, hydrogen peroxide being the product of reaction. The O2-/H2O2 ratio is close to 2:1. The dismutase activity of Cp is about 1500 times lower than that of Cu, Zn-SOD isolated from human erythrocytes. The dismutation of O2- accomplished by SOD, "free" copper ions, native Cp or partly copper-depleted Cp, is inhibited with equal efficiency by cyanide. All the copper ions of the multicopper catalytic center of Cp are not essentially required for the dismutation of O2-, since the enzyme depleted of all type 2 Cu2+ and partly of type 1 Cu2+ lost none of its dismutase activity. Type 1 copper ions of Cp seem to play the leading role in the one-electron transfer occurring upon dismutation of O2-.     

Choline kinase activity in Plasmodium-infected erythrocytes: characterization and utilization as a parasite-specific marker in malarial fractionation studies

Choline kinase (EC 2.7.1.32) was investigated in plasmodium falciparum-infected erythrocytes. Disrupted infected erythrocytes had a choline kinase activity of 1.9 +/- 0.2 nmol phosphorylcholine/10(7) infected cells per h, whereas the activity in normal uninfected erythrocytes was less than 6 pmol/10(7) cells per h. A broad alkaline optimal pH (7.9-9.2) was observed. The Km values for choline and ATP were 79 +/- 20 microM, and 1.3 +/- 0.3 mM, respectively. ATP concentrations higher than 12 mM inhibited choline kinase. Maximal activity was registered with a Mg2+ concentration of 10 mM, whereas its replacement by Mn2+, or other divalent cations, involved a decrease in choline kinase activity of at least 75%. Inhibition by products of the reaction, such as phosphorylcholine and ADP was investigated. In plasmodium knowlesi-infected erythrocytes, choline kinase had similar properties, but with a much higher specific activity of 16.4 +/- 2.1 nmol/10(7) infected cells per h. Subcellular fractionation of P. knowlesi-infected erythrocyte suspensions revealed that choline kinase was located exclusively in the cytosol of the parasite. We show that this enzyme is a useful index of parasite cytosolic content leakage, when infected erythrocytes are fractionated by saponin lysis or nitrogen decompression.     

A novel,thermostable manganese-containing superoxide dismutase from <Emphasis Type="Italic">Bacillus licheniformis</Emphasis>

A new, thermostable superoxide dismutase (SOD) from Bacillus licheniformis M20, isolated from Bulgarian mineral springs, was purified 11-fold with 11% recovery of activity. From native PAGE and SDS-PAGE, the enzyme was composed of two subunits of 21.5 kDa each. The SOD was inhibited only by NaN₃, which suggested that this SOD is of the manganese superoxide dismutase type. The purified enzyme had maximum activity at pH 8 and 55°C. The half-life of the SOD was 10 min at 95°C.