Primaquine-mediated oxidative metabolism in the human red cell. Lack of dependence on oxyhemoglobin, H2O2 formation, or glutathione turnover

Understanding the mechanisms for metabolism-linked hemolytic toxicity of primaquine against glucose 6-phosphate dehydrogenase deficient human erythrocytes: evaluation of eryptotic pathway

Therapeutic utility of primaquine, an 8-aminoquinoline antimalarial drug, has been limited due to its hemolytic toxicity in population with glucose 6-phosphate dehydrogenase deficiency. Recent investigations at our lab have shown that the metabolites generated through cytochrome P(450)-dependent metabolic reactions are responsible for hemotoxic effects of primaquine, which could be monitored with accumulation of methemoglobin and increased oxidative stress. The molecular markers for succeeding cascade of events associated with early clearance of the erythrocytes from the circulation were evaluated for understanding the mechanism for hemolytic toxicity of primaquine. Primaquine alone though did not induce noticeable methemoglobin accumulation, but produced significant oxidative stress, which was higher in G6PD-deficient than in normal erythrocytes. Primaquine, presumably through redox active hemotoxic metabolites generated in situ in human liver microsomal metabolism-linked assay, induced a dose-dependent methemoglobin accumulation and oxidative stress, which were almost similar in normal and G6PD-deficient erythrocytes. Primaquine alone or in presence of pooled human liver microsomes neither produced significant effect on intraerythrocytic calcium levels nor affected the phosphatidyl serine asymmetry of the normal and G6PD-deficient human erythrocytes as monitored flowcytometrically with Annexin V binding assay. The studies suggest that eryptosis mechanisms are not involved in accelerated removal of erythrocytes due to hemolytic toxicity of primaquine.

Effect of antimalarial drugs on rat enterocyte mitochondrial phospholipase D activity

We have earlier shown that enterocyte mitochondria contain a phospholipase D (PLD) activity which can be stimulated by oxygen free radicals, divalent cations and polyamines. The functional significance of this enzyme in mitochondria is not known but it can be investigated using selective inhibitors. In the present study, mitochondrial PLD was activated by exposure to oxidants (X+XO or menadione), calcium or polyamines and the effect of antimalarial drugs, chloroquine, amodiaquin and primaquine on PLD activity was studied. Chloroquine and amodiaquine inhibited Ca2+ stimulated PLD activity in dose dependent manner whereas these drugs had no significant effect on PLD activated by oxidants or polyamines. Increasing the calcium concentration relieved the PLD inhibition by these drugs. Primaquine did not have any effect on calcium stimulated PLD activity whereas it slightly activated the enzyme. These results indicate that chloroquine and amodiaquine may bind with calcium making it unavailable for PLD activation.

Glutathione peroxidase (EC 1.11.1.9) and superoxide dismutase (EC 1.15.1.1) activities in riboflavin-deficient rats infected with Plasmodium berghei malaria

Riboflavin deficiency interferes with the growth and multiplication of malaria parasites as well as the host response to malaria. The objective of the present work was to determine the effects of riboflavin deficiency on erythrocyte glutathione peroxidase (EC 1.11.1.9; GPx) and superoxide dismutase (EC 1.15.1.1; SOD) in rats infected with Plasmodium berghei malaria. Riboflavin in its co-enzyme form, FAD, is required by glutathione reductase (EC 1.6.4.1) to regenerate GSH and GSH is an important cellular antioxidant both in its own right and also as a substrate for the enzyme GPx. Weanling rats were deprived of riboflavin for 8 weeks before intraperitoneal injection of 1 x 10(6) P. berghei parasites. Control animals were weight-matched to the respective riboflavin-deficient group. At 10 d post-infection, parasite counts were higher in the weight-matched control group than the riboflavin-deficient group (P = 0.004). GPx activity was higher in erythrocytes of rats parasitized with P. berghei than comparable non-infected rats regardless of riboflavin status (P < 0.05). As mature erythrocytes do not synthesize new protein, the higher GPx activities were probably due to the presence of the parasite protein. In erythrocytes from riboflavin-deficient rats, GPx activity tended to be lower than in those rats fed on diets adequate in riboflavin (weight-matched controls) whether parasitized or not, but the difference was not significant. Neither riboflavin deficiency nor malaria had any effect on erythrocyte SOD activity. It was concluded that riboflavin deficiency has no marked effect on erythrocyte GPx or SOD activity in the rat.

Primaquine alters antioxidant enzyme profiles in rat liver and kidney

The effects of primaquine treatment on antioxidant enzyme activities were investigated in rat liver and kidney. Male Sprague-Dawley rats were treated with 0.21 mg/kg daily for two weeks (chronic treatment) or a single dose at 0.21 or 0.63 mg/kg. Antioxidant enzyme activities were determined in liver and kidney cytosolic fractions whereas glutathione (GSH) and malondialdehyde (MDA) levels were determined in tissue samples. Results for the liver showed increases in cytosolic superoxide dismutase (SOD) and glutathione peroxidase (GPX) enzymatic activities after chronic primaquine treatment. Levels of MDA, a marker for lipid peroxidation, were also increased by more than 50% indicating enhanced oxidative damage in the liver. In the single dose study, 0.63 mg/kg primaquine caused a more than 100% increase in liver SOD and a 36% increase in NAD (P) H: quinone oxidoreductase (NQOR) activities. Results for the kidney, however, showed fewer primaquine-induced changes in antioxidant enzyme activities when compared to the liver in both the chronic and single dose studies. Overall, our results indicate that primaquine treatment causes an oxidative stress in the two rat organs. These results are consistent with the known pro-oxidant effects of primaquine in vivo, and supplement current knowledge on the effects of antimalarial drugs on various enzyme systems.

Protective effects of rutin against hemoglobin oxidation

A prooxidant drug, primaquine (PQ) was used to produce oxidative stress in human red blood cells (RBC) in vitro. Rutin, a plant flavonoid, did not prevent PQ-induced cell lysis but protected against hemoglobin (Hb) oxidation inside RBC. After PQ removal, rutin failed to reduce preformed met-Hb indicating that the rutin protective effect manifests only in the presence of PQ. Since H2O2 was proved to mediate PQ-induced Hb oxidation, authentic Hb was studied for its reaction with H2O2 and rutin in solution. Rutin partially protected oxy-Hb against H2O2-induced oxidation and heme loss. Rutin was also shown to delay H2O2-induced met-Hb oxidation to ferryl-Hb. Rutin directly reduced ferryl-Hb to met-Hb in stoichiometric (1:1) reaction characterized by a rate constant of 100 to 130/M/sec. It is assumed that by reducing ferryl-Hb, rutin prevents oxy-Hb from reacting with ferryl-Hb (comproportionation reaction), thus preventing half of the oxy-Hb molecules from being converted to met-Hb. This mechanism is consistent with 50% inhibition by rutin (at the maximum of its activity) of PQ-induced oxy-Hb oxidation in RBC. The present results demonstrate new antioxidant properties of rutin that may be useful in diminishing oxidative damage to pathological red blood cells.

Oxidative activity of primaquine metabolites on rat erythrocytes in vitro and in vivo

The oxidative activities of primaquine [6-methoxy-8-(4-amino-1-methylbutylamino)quinoline] and its metabolites, the quinone-imine derivatives of 5-hydroxyprimaquine [5-hydroxy-6-methoxy-8-(4-amino-1-methylbutylamino)quinoline] and 5-hydroxydemethylprimaquine [5-hydroxy-6-demethyl-8-(4-amino-1-methylbutylamino)quinoline], 6-methoxy-8-amino quinoline and hydrogen peroxide, were studied on rat erythrocytes in vitro and in vivo. In both cases, the most effective metabolites in oxidizing hemoglobin and depleting non-protein sulfhydryl groups from erythrocytes were the quinone-imine derivatives of the ring-hydroxylated metabolites, 5-hydroxyprimaquine and 5-hydroxydemethyl-primaquine. The latter quinone-imines were shown by light absorption spectroscopy and oxygen consumption studies to be able to oxidize purified rat hemoglobin to methemoglobin but to be unable to react directly with reduced glutathione. In agreement with these results, no radical adduct was detected by electron paramagnetic resonance spectroscopy in incubations of rat erythrocytes with the quinone-imines and the spin-trap 5,5-dimethyl-1-pyrroline-N-oxide; metabolite-derived free radicals were detected instead. Taken together, the results suggest that 5-hydroxyprimaquine and 5-hydroxydemethylprimaquine are important metabolites in the expression of primaquine hemotoxicity, in contrast to 6-methoxy-8-aminoquinoline. Additionally, the results indicate that hydrogen peroxide is the ultimate oxidant formed from the ring-hydroxylated metabolites by redox-cycling of the corresponding quinone-imine derivatives both in vitro and in vivo.

Decreased catalase activity is the underlying mechanism of oxidant susceptibility in glucose-6-phosphate dehydrogenase-deficient erythrocytes

Historically, it has been theorized that the enhanced oxidant sensitivity of glucose-6-phosphate dehydrogenase (G6PD)-deficient erythrocytes arises as a direct consequence of an inability to maintain cellular glutathione (GSH) levels. This study alternatively hypothesizes that decreased NADPH concentration leads to impaired catalase activity which, in turn, underlies the observed oxidant susceptibility. To investigate this hypothesis, normal and G6PD-deficient erythrocytes and hemolysates were challenged with a H2O2-generating agent. The results of this study demonstrated that catalase activity was severely impaired upon H2O2 challenge in the G6PD-deficient cell while only a transient decrease was observed in normal cells. Supplementation of either normal or G6PD-deficient hemolysates with purified NADPH was found to significantly (P < 0.001) inhibit catalase inactivation upon oxidant challenge while addition of NADP+ had no effect. Analysis of these results demonstrated direct correlation between NADPH concentration and catalase activity (r = 0.881) and an inverse correlation between catalase activity and erythrocyte oxidant sensitivity (r = 0.906). In contrast, no correlation was found to exist between glutathione concentration (r = 0.170) and oxidant sensitivity. Analysis of NADPH/NADPt ratio in acatalasemic mouse erythrocytes demonstrated that NADPH maintenance alone was not sufficient to explain oxidant resistance, and that catalase activity was required. This study supports the hypothesis that impaired catalase activity underlies the enhanced oxidant sensitivity of G6PD-deficient erythrocytes and elucidates the importance of NADPH in the maintenance of normal catalase activity.

Towards more efficacious chemotherapy of trypanosomiasis: combination of alpha-difluoromethylornithine (DFMO) with reactive oxygen generating drugs

Trypanosomiasis (whether African sleeping sickness, or American Chaga's disease) is caused by an infection with a protozoan parasite, i.e. the trypanosome. This carries fatal sequences in the untreated host. Currently available chemotherapeutic drugs (some of which cure by involving reactive oxygen species (ROS] are not optimally adequate. They are toxic as well, and may also be carcinogenic. It is therefore desirable to devise better chemotherapeutic regimens. ROS destroy the parasite, but excess ROS damage host tissue and are potentially carcinogenic. Alpha-difluoromethylornithine (DFMO) inhibits ornithine decarboxylase and so lowers the levels of spermine and spermidine. This singular effect in the parasite inhibits its multiplication, whereas in the host tissue it prevents carcinogenesis by preventing cell proliferation. Thus, combination of ROS-generating drugs with DFMO would be very effective against trypanosomiasis, and would be without cancer risk too. The combination is therefore advocated for chemotherapy of trypanosoma infections. This necessities experimental investigations specifically directed towards establishing the optimally efficacious combination of DFMO with the drugs.

5,5-dimethyl-1-pyrroline-N-oxide alone enhances the spontaneous superoxide generation by primaquine

Primaquine (PQ), a well-known antimalarial drug, has been reported to generate superoxide (O2-) in the presence of reducing agents such as NADPH. In the present study, chemiluminescence was detected by adding only PQ to aqueous 2-methyl-6-[p-methoxyphenyl]-3,7-dihydroimidazo-[1,2-alpha] pyrazin-3-one (MCLA), which is a specific chemiluminescent probe for O2-, and was quenched by superoxide dismutase (SOD), indicating that PQ alone can generate O2- in aerobic conditions. Furthermore, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) enhanced the O2- generation by PQ. Superoxide spin adduct, DMPO-OOH, was also detected by ESR both in aqueous solutions and in dimethyl sulfoxide with DMPO. The level of O2- generation showed a linear correlation with the DMPO concentration, and SOD competitively inhibited the DMPO-OOH formation. The results suggested that in aerobic conditions PQ is autoxidized to 5-hydroxy-PQ, which generates O2-, and DMPO accelerates the autoxidation process by trapping O2-. DMPO or M4PO alone enhances the spontaneous O2- generation by PQ, therefore cautious evaluation is necessary in all studies using the ESR/spin trapping technique to elucidate the mechanism of PQ-related radical generation.

Reactive oxygen production against malaria--a potential cancer risk factor

In response to malaria infection, phagocytes, such as macro-phages and neutrophils, produce superoxide and thence the other reactive oxygen species (ROS) with which to kill the parasites. Excess ROS is normally eliminated by the body's natural scavenger molecules; however, in the event of a vast excess of ROS, as may be the case in acute as well as chronic malaria patients, the natural scavengers may be overwhelmed. We hypothesize that unscavenged ROS in malaria patients causes DNA damage in normal host cells which, if unrepaired or incorrectly repaired, could result in oncogene activation and eventually lead to cancer. An epidemiologic study may be warranted in malaria-endemic regions to investigate the possible relationship between malaria infection and cancer risk.

Plasmodium falciparum: inhibitor sensitivity of the endogenous superoxide dismutase

Plasmodium falciparum, unlike P. berghei, contains two superoxide dismutases (SOD). We have previously found that the major isozyme is cyanide sensitive and appears, like the P. berghei SOD, to be adopted from its host, whereas the minor isozyme was found to be cyanide insensitive. We now report that the minor parasite-associated enzyme is peroxide insensitive, suggesting that it is manganese containing.

Studies on the mechanisms of oxidation in the erythrocyte by metabolites of primaquine

The interaction of certain metabolites of the 8-aminoquinoline antimalarial primaquine with both normal and glucose-6-phosphate dehydrogenase (G6PD)-deficient erythrocytes and with haemoglobin preparations was studied in an attempt to elucidate the mechanisms of methaemoglobin formation and haemolytic anaemia associated with the use of primaquine. Studies using erythrocytes revealed that oxidation of haemoglobin and reduced glutathione (GSH) was due to the metabolites rather than the parent drug. Incubation of free haemoglobin with 5-hydroxylated metabolites of primaquine also led to oxidation of oxyhaemoglobin and GSH. Oxidation of GSH also occurred in the absence of oxyhaemoglobin. The results suggest a dual mechanism for these oxidative effects, involving autoxidation of the 5-hydroxy-8-aminoquinolines and their coupled oxidation with oxyhaemoglobin. The initial products of these processes would be drug metabolite free radicals, superoxide radical anions, hydrogen peroxide and methaemoglobin. Further free radical reactions would lead to oxidation of GSH, more haemoglobin and probably other cellular constituents. NADPH had no effect on the oxidative effects of the primaquine metabolites in these experiments. In the G6PD-deficient erythrocyte, the oxidation of haemoglobin and GSH leads to Heinz body formation and eventually to haemolysis, the mechanisms of which are as yet unclear. The possible role of oxygen free radicals in the mode of action of 8-aminoquinolines against the malaria parasite is also briefly discussed.

Oxidant defense enzymes of Plasmodium falciparum

We have measured and characterized three oxidant defense enzymes in early and late intraerythrocytic stages of the human malarial parasite, Plasmodium falciparum. Isolated early intraerythrocytic stages contain catalase (24.1 mumol min-1 (mg protein)-1) and superoxide dismutase (SOD; 6.3 units (mg protein)-1) but little or no glutathione peroxidase (GPX; less than 2 mumol min-1 (mg protein)-1). Isolated late intraerythrocytic stages of P. falciparum contain slightly less catalase (17.0 mumol min-1 (mg protein)-1) but significantly more GPX (7.7 mumol min-1 (mg protein)-1) and SOD (25.1 units (mg protein)-1). P. falciparum, like P. berghei, probably acquires most of its SOD from its host, since parasite-associated SOD is predominantly cyanide-sensitive, and has the same pI as host SOD. Unlike P. berghei, however, late stages of P. falciparum contain an additional SOD isozyme which is not cyanide-sensitive and may represent an endogenous enzyme. Parasites grown in red cells that have been partially depleted of SOD are more sensitive to exogenously generated superoxide, suggesting some dependence of the parasite on host SOD.

One-electron reduction of the antimalarial drug primaquine, studied by pulse radiolysis

One-electron reduction of the antiparasitic drug primaquine has been studied by pulse radiolysis. Primaquine is reduced by the hydrated electron at neutral pH with a rate constant of (2.47 +/- 0.1) x 10(10) dm3mol-1s-1. Reduction by formate and isopropanol radicals is relatively slow (less than or equal to 10(7) dm3mol-1s-1) at neutral pH, but increases in rate with decreasing pH on protonation of the quinoline moiety. The one-electron reduction product form reaction of the hydrated electron with primaquine at neutral pH reacts with O2, benzyl viologen and NAD+ with rates of (1-2.3) x 10(9) dm3mol-1s-1. The relevance of these observations to the mechanisms proposed by Thornalley et al. (Biochem. Pharmacol. 32, 357, (1983] for oxygen free radical generation in solutions of NADPH and primaquine and the antiparasitic action of the drug is discussed.

Human red cells scavenge extracellular hydrogen peroxide and inhibit formation of hypochlorous acid and hydroxyl radical

The ability of intact human red cells to scavenge extracellularly generated H2O2 and O2-, and to prevent formation of hydroxyl radicals and hypochlorous acid has been examined. Red cells inhibited oxidation of ferrocytochrome c by H2O2. Cells treated with aminotriazole no longer inhibited, indicating that protection was almost entirely due to intracellular catalase. Contribution by the GSH system was slight, and apparent only with low H2O2 concentrations when catalase was inhibited by aminotriazole. The cells were about a quarter as efficient at inhibiting cytochrome c oxidation as an equivalent concentration of purified catalase. No inhibition of O2(-)-dependent reduction of ferricytochrome c or nitroblue tetrazolium was observed, although extracted red cell superoxide dismutase inhibited nitroblue tetrazolium reduction at one fortieth the concentration of that in the cells. Red cells efficiently inhibited deoxyribose oxidation by hydroxyl radicals generated from H2O2, O2- and Fe(EDTA), and myeloperoxidase-dependent oxidation of methionine to methionine sulfoxide by stimulated neutrophils. Most of the red cell inhibition of hydroxyl radical production, and all the inhibition of methionine oxidation, was prevented by blocking intracellular catalase with aminotriazole. Thus red cells are able to efficiently scavenge H2O2, but not O2-, produced in their environment, and to inhibit formation of hydroxyl radicals and hypochlorous acid. They may therefore have an important role in extracellular antioxidant defense.

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.

Effects of nine synthetic putative metabolites of primaquine on activity of the hexose monophosphate shunt in intact human red blood cells in vitro

Suspensions of washed human red blood cells were treated with nine synthetic putative metabolic derivatives of primaquine (PQ'), and their individual effects on activity of the hexose monophosphate shunt (HMS) were quantitated by radiometric analysis of 14CO2 from [14C] glucose. The most potent HMS stimulant was 5-hydroxy-6-methoxy-8-aminoquinoline (5H6MQ), which caused 10-fold elevation of HMS activity at an estimated concentration of 0.004 mM. Ten millimolar primaquine (PQ) was required to achieve the same effect. Thus, 5H6MQ was approximately 2500-fold more reactive with the HMS than PQ. Other analogs achieved less than 0.4- to 154-fold increases in HMS reactivity. Patterns of effects on HMS activity indicated that 5-hydroxylation and/or N-dealkylation of PQ strongly enhanced HMS reactivity. In contrast, none of the putative metabolites of PQ activated the proteolytic system known to degrade oxidized protein in red cells, indicating that stimulation of the HMS by the PQ analogs was not related to an injurious oxidative stress. Red cells pretreated with 1.0 mM N-ethylmaleimide (NEM) or with 1.0% (w/v) sodium nitrite to cause glutathione sulfhydryl blockage and conversion of red cell hemoglobin to methemoglobin (metHb), respectively, also showed elevation of HMS activity when exposed to 5H6MQ. These observations suggested that 5H6MQ-induced elevation of HMS activity was at least partially independent of glutathione redox reactions, hydrogen peroxide accumulation and reaction with oxyhemoglobin. The relevance of these observations to proposed mechanisms of hemolytic toxicity of PQ is discussed.

Oxidative activity of hydroxylated primaquine analogs. Non-toxicity to glucose-6-phosphate dehydrogenase-deficient human red blood cells in vitro

The individual effects of two putative metabolites of primaquine (5,6-dihydroxyprimaquine and 5,6-dihydroxy-8-aminoquinoline) on the hexose monophosphate shunt (HMS) and on the ATP-dependent proteolytic system which rapidly degrades oxidized erythrocyte protein were measured in intact red blood cells in vitro from two blood donors. In red cells treated with nitrite (1-40 mM) or phenylhydrazine (0.01-10 mM), proteolytic activity was detected only with concentrations (7.5 mM NaNO2 and 0.25 mM phenylhydrazine) causing greater than 15-fold elevation of HMS activity, and glucose-6-phosphate dehydrogenase (G6PD)-deficient (25% of normal activity) red cell suspensions thus treated showed approximately 30% greater proteolysis. G6PD-normal and deficient red cells treated with the primaquine analogs, however, did not experience proteolysis with concentrations (0.25 mM) in excess of those causing 17-fold elevation of HMS activity. Stimulation of the HMS by the primaquine analogs thus appears unrelated to an erythrotoxic oxidative stress. Methylene blue is known to cause an elevation of HMS activity through direct and diaphorase II-dependent oxidation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) which is independent of injurious oxidative stress. It was found that the putative primaquine metabolites also caused direct and diaphorase II-dependent oxidation of NADPH in dilute hemolysate, thus suggesting that the putative primaquine metabolites have a methylene blue-like redox disposition in red blood cells. Results obtained in this study suggest that the hemolytic toxicity of primaquine may be unrelated to processes which lead to oxidative deterioration of red cell protein.

Hydroxyl radical formation as a result of the interaction between primaquine and reduced pyridine nucleotides. Catalysis by hemoglobin and microsomes

Kinetic, circular dichroism, and NADH and NADPH fluorescence quenching studies indicate that these compounds interact with the antimalarial drug primaquine (PQ). The affinity of both pyridine nucleotides for PQ is similar. The data are in contrast with a previous report (Thornalley et al. (1983) Biochem. Pharmacol. 32, 3571-3575) suggesting specificity for the interaction with NADPH. The complex was seen to facilitate electron transfer from NAD(P)H to oxygen, generating oxygen-free radicals which were detected by the spin-trapping technique and to flavin nucleotides, giving rise to flavin semiquinone radicals which were demonstrated by direct ESR spectroscopy under anaerobic conditions. A twofold increase in oxygen uptake and hydroxyl radical generation by the NAD(P)H-PQ complex was observed in the presence of hemoglobin. This effect was independent of heme concentration (in the range 1 X 10(-5)-1 X 10(-4) M) and oxidation state of the iron. Under anaerobic conditions, the NAD(P)H-PQ complex reduces Fe-III to Fe-II hemoglobin, and under aerobic conditions about 65% of the heme chromophore is irreversibly destroyed. Superoxide dismutase inhibits hydroxyl radical generation by the NAD(P)H-PQ pair; this effect is not observed in the presence of hemoglobin. In the presence of microsomes there is a 10-fold increase in both oxygen consumption and hydroxyl radical generation by the NAD(P)H-PQ pair. The fact that both pyridine nucleotides are active, and the inability of SKF 525A in decreasing hydroxyl radical generation, suggests that microsomal reductases are involved in the catalysis.

The modes of action of some anti-protozoal drugs

In spite of the continuing need for new and improved anti-protozoal drugs for use in man, a considerable contraction of industrially based research on anti-protozoal drugs has occurred in recent years. Newton (1983) reviewed the reasons for this decline and presented a compelling argument that fundamental research on the biology of the parasites is essential for the discovery of leads for the development of a new generation of drugs – a rational chemotherapy. The rapid advance in knowledge of the biochemistry of parasitic protozoa which has occurred in recent years has provided a number of potential leads to new drug development and has permitted a greater understanding of the mode of action of many current drugs. The account of these advances which follows is necessarily selective and relates to protozoan parasites of man.

Methylene blue-mediated hexose monophosphate shunt stimulation in human red blood cells in vitro: independence from intracellular oxidative injury

The red blood cell hexose monophosphate shunt (HMS) and proteolytic responses to several concentrations of Methylene Blue or sodium nitrite were measured. The results suggested two distinct mechanisms for activation of the HMS: (1) nitrite treatment increased HMS activity in response to oxidative challenge to red cell protein; (2) Methylene Blue treatment activated HMS without injurious oxidative challenge. Nitrite-treated cells actively degraded protein, whereas Methylene Blue-treated red cells did not activate proteolytic systems that degrade oxidized red cell protein. These observations are relevant to proposed in vitro systems for evaluation of drug hemolytic toxicity potential on the basis of HMS stimulation capacity.

A mechanism for primaquine mediated oxidation of NADPH in red blood cells

The incubation of NADPH with primaquine results in the formation of free radicals which were demonstrated by the electron spin resonance (ESR) technique of spin trapping using 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) as the spin trap. The free radicals formed were identified as the superoxide (DMPO-OOH) and hydroxyl (DMPO-OH) spin adducts of DMPO. Copper/zinc superoxide dismutase inhibited the formation of DMPO-OOH while it only partly inhibited the formation of DMPO-OH which could be totally inhibited by catalase. This indicates that the formation of hydroxyl radicals is not totally arising from the Haber-Weiss reaction. However since the formation of hydroxyl radicals is dependent on hydrogen peroxide, a non-metal catalysed reduction of hydrogen peroxide is postulated for their formation. Oxygen consumption during the reaction between primaquine and NADPH was found to be consistent with the spin trapping experiments and the rate of production of DMPO-OH indicates the formation of 1:1 catalytic complex between the two reactants. Quenching of the fluorescence of NADPH at 460 nm in the presence of primaquine indicates the formation of a charge transfer complex. When red blood cells are incubated with primaquine a hydroxyl spin adduct (DMPO-OH) is observed. The formation of this radical is probably the main cause of primaquine mediated toxicity.

Effects of physiologic concentrations of lactate, pyruvate and ascorbate on glucose metabolism in unstressed and oxidatively stressed human red blood cells

Glucose metabolism was studied in human red blood cells incubated in the presence of physiologic concentrations of ascorbate (0.1 mM) and/or lactate (2 mM) plus pyruvate (0.1 mM). The total flux through glycolysis, as measured by 14C-labeling of glycolytic intermediates, was increased about 15% by ascorbate, 30% by lactate plus pyruvate, and 40% by ascorbate plus lactate plus pyruvate. We found, however, that physiologic concentrations of ascorbate and/or lactate plus pyruvate had no effect on flux of glucose or recycling of pentoses through the hexose monophosphate shunt. Increased formation of lactate accounted for most of the observed increase in glycolysis with little change in pyruvate formation, indicating that the increased flux of reducing equivalents from glucose was stored as lactate rather than being consumed by red cell metabolism. In all experiments, there was a net increase with time in the absolute amount of both lactate and pyruvate in red cell suspensions, indicating that lactate or pyruvate present at zero time did not function as a stoichiometric source or sink for reducing equivalents. There was little effect on steady-state levels of ATP or 2,3-diphosphoglycerate. Equilibration of ascorbate between red cells and the medium was complete before the addition of 14C-labeled glucose to the medium. Glucose metabolism prevented net oxidation of ascorbate in the incubation medium. Physiologic concentrations of ascorbate, lactate and pyruvate appear to increase flux through glycolysis by increasing the turnover of ATP and/or 2,3-diphosphoglycerate. Red cells were exposed to mild oxidative stress by incubation with 0.27 mM 6-hydroxydopamine, 0.27 mM 6-aminodopamine, 0.13 mM 1,4-naphthoquinone-2-sulfonic acid or 0.27 mM phenylhydrazine. The metabolic response to oxidative stress was determined by measuring the formation of methemoglobin, pyruvate, lactate and CO2 in the presence and absence of physiologic concentrations of lactate, pyruvate and ascorbate. Lactate, pyruvate and ascorbate had no effect on the net methemoglobin accumulation but rather on the distribution of the metabolic sources of reducing equivalents and on the flux of reducing equivalents to oxygen. Physiologic lactate and pyruvate allowed increased flow of reducing equivalents from glycolysis to methemoglobin and ultimately oxygen without the necessity of increased flux through glycolysis. This was accomplished by a decrease in the ratio of newly formed lactate to newly formed pyruvate with no increase in total lactate plus pyruvate.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of ATP hydrolysis by chloroquine and primaquine in human red blood cells

Primaquine, an 8-aminoquinoline, and chloroquine, a 4-aminoquinoline, both stimulate ATP hydrolysis in human red blood cells incubated in the absence of glucose. In the presence of glucose, ATP levels are partially maintained by increased flux of glucose through glycolysis. Glucose dependence of chloroquine uptake and the activity of primaquine as a redox reagent explain quantitative differences in ATP hydrolysis and accumulation of specific glycolytic products.