130,824 research outputs found
Prooxidant activity of beta-hematin (synthetic malaria pigment) in arachidonic acid micelles and phospholipid large unilamellar vesicles.
No full textIntraerythrocytic malaria parasite has evolved a unique pathway to detoxify hemoglobin-derived heme by forming a crystal of Ferri-protoporphyrin IX dimers, known as hemozoin or "malaria pigment." The prooxidant activity of beta-hematin (BH), the synthetic malaria pigment obtained from hematin at acidic pH, was studied in arachidonic acid micelles and phospholipid Large Unilamellar Vesicles (LUVs) and compared to that of alpha-hematin (AH, Ferri-protoporphyrin IX-hydroxide) and hemin (HE, Ferri-protoporphyrin-chloride). Lipid peroxidation was measured as production of thiobarbituric acid reactive substances (TBARS). The extent of peroxidation induced by either AH or BH was strongly dependent upon the content of pre-existing hydroperoxides and efficiently inhibited by triphenylphosphine, a deoxygenating agent able to reduce hydroperoxides to hydroxides and by lipophilic scavengers. BH prooxidant activity was linearly related to the material, whereas that of AH seemed dependent on the aggregation state of the porphyrin. Maximal activity was observed when AH was present in concentration lower than 2 microM. In this case a shift of spectra in the Soret region, leading to the increase of the O.D. 400/385 nm ratio, suggested a transition toward a less aggregated state. BH prooxidant activity was significantly lower than that of monomeric AH, yet higher than that of AH aggregates. Differently from AH aggregates, BH-induced peroxidation was unaffected by GSH and inhibited rather than enhanced by acidic pH (5.7) and chloroquine. UV/Vis spectroscopy of AH aggregates at acidic pH, low GSH concentrations and chloroquine suggests a shift of AH aggregates toward the less aggregated state, more active as peroxidation catalyst
A product-form network for systems with job stealing policies
No full textIn this paper, we introduce a new product-form queueing network model where servers are always busy. This is obtained by defining a job movement policy that admits instantaneous migrations of jobs from non-empty waiting buffers to empty ones. Thus, the resulting routing scheme is statedependent. This class of networks tends to maximize the system throughput and can be used to model situations where successive refinements on jobs improve the processes quality but are not strictly required to obtain a result
A microtitre-based method for measuring the haem polymerization inhibitory activity (HPIA) of antimalarial drugs
No full textThe malaria parasite metabolizes haemoglobin and detoxifies the resulting haem by polymerizing it to form haemozoin (malaria pigment). A polymer identical to haemozoin, beta-haematin, can be obtained in vitro from haematin at acidic pH. Quinoline-containing anti-malarials (e.g. chloroquine) inhibit the formation of either polymer. Haem polymerization is an essential and unique pharmacological target. To identify molecules with haem polymerization inhibitory activity (HPIA) and quantify their potency, we developed a simple, inexpensive, quantitative in-vitro spectrophotometric microassay of haem polymerization. The assay uses 96-well U-bottomed polystyrene microplates and requires 24 h and a microplate reader. The relative amounts of polymerized and unpolymerized haematin are determined, based on solubility in DMSO, by measuring absorbance at 405 nm in the presence of test compounds as compared with untreated controls. The final product (a solid precipitate of polymerized haematin) was validated using infrared spectroscopy and the assay proved reproducible; in this assay, activity could be partly predicted based on the compound's chemical structure. Both water-soluble and water-insoluble compounds can be quantified by this method. Although the throughput of this assay is lower than that of radiometric methods, the assay is easier to set up and cheaper, and avoids the problems related to radioactive waste disposal
Non-iron porphyrins inhibit β-haematin (malaria pigment) polymerisation
No full textAbstractInfrared spectroscopy was used to evaluate the effect of non-iron porphyrins (protoporphyrin IX and haematoporphyrin) on haematin polymerisation to β-haematin at acidic pH. Both molecules effectively inhibited the reaction, with haematoporphyrin 6 times as active as protoporphyrin IX. We postulated that the interaction between the π electron system of porphyrin rings leads to the formation of π–π adducts, which inhibit polymer elongation in the same way as antimalarial drugs (e.g., chloroquine); the presence of hydroxyl groups able to bind haem iron enhances activity
A novel endogenous antimalarial : Fe(II)-protoporphyrin IXα (heme) inhibits hematin polymerization to β-hematin (malaria pigment) and kills malaria parasites
No full textThe polymerization of hemoglobin-derived ferric-protoporphyrin IX [Fe(III)PPIX] to inert hemozoin (malaria pigment) is a crucial and unique process for intraerythrocytic plasmodia to prevent heme toxicity and thus a good target for new antimalarials. Quinoline drugs, i.e., chloroquine, and non-iron porphyrins have been shown to block polymerization by forming electronic π-π interactions with heme monomers. Here, we report the identification of ferrous-protoporphyrin IX [Fe(II)PPIX] as a novel endogenous anti-malarial. Fe(II)PPIX molecules, released from the proteolysis of hemoglobin, are first oxidized and then polymerized to hemozoin. We obtained Fe(II)PPIX on preparative scale by electrochemical reduction of Fe(III)PPIX, and the reaction was monitored by cyclic voltammetry. Polymerization assays at acidic pH were conducted with the resulting Fe(II)PPIX using a spectrophotometric microassay of heme polymerization adapted to anaerobic conditions and the products characterized by infrared spectroscopy. Fe(II)PPIX (a) did not polymerize and (b) produced a dose- dependent inhibition of Fe(III)PPIX polymerization (IC50 = 0.4 molar equiv). Moreover, Fe(II)PPIX produced by chemical reduction with thiol- containing compounds gave similar results: a dose-dependent inhibition of heme polymerization was observed using either L-cysteine, N-acetylcysteine, or DL-homocysteine, but not with L-cystine. Cyclic voltammetry confirmed that the inhibition of heme polymerization was due to the Fe(II)PPIX molecules generated by the thiol-mediated reduction of Fe(III)PPIX. These results point to Fe(II)PPIX as a potential endogenous antimalarial and to Fe(III)PPIX reduction as a potential new pharmacological target
Macrophage preconditioning with synthetic malaria pigment reduces cytokine production via heme iron-dependent oxidative stress
No full textHemozoin (malaria pigment), a polymer of hematin (ferri-protoporphyrin IX) derived from hemoglobin ingested by intraerythrocytic plasmodia, modulates cytokine production by phagocytes. Mouse peritoneal macrophages (PM) fed with synthetic beta-hematin (BH), structurally identical to native hemozoin, no longer produce tumor necrosis factor alpha (TNFalpha) and nitric oxide (NO) in response to lipopolysaccharide (LPS). Impairment of NO synthesis is due to inhibition of inducible nitric oxide synthase (iNOS) production. BH-mediated inhibition of PM functions cannot be ascribed to iron release from BH because neither prevention by iron chelators nor down-regulation of iron-regulatory protein activity was detected. Inhibition appears to be related to pigment-induced oxidative stress because (a) thiol compounds partially restored PM functions, (b) heme oxygenase (HO-1) and catalase mRNA levels were up-regulated, and (c) free radicals production increased in BH-treated cells. The antioxidant defenses of the cells determine the response to BH: microglia cells, which show a lower extent of induction of HO-1 and catalase mRNAs and lower accumulation of oxygen radicals, are less sensitive to the inhibitory effect of BH on cytokine production. Results indicate that BH is resistant to degradation by HO-1 and that heme-iron mediated oxidative stress may contribute to malaria-induced immunosuppression. This study may help correlate the different clinical manifestations of malaria, ranging from uncomplicated to severe disease, with dysregulation of phagocyte functions and promote better therapeutic strategies to counteract the effects of hemozoin accumulation
Stability of the antimalarial drug dihydroartemisinin in under physiologically-relevant conditions : implications for clinical treatment, pharmacokinetic and in vitro assays
No full textArtemisinins are peroxidic antimalarial drugs known to be very potent but chemically highly unstable; they degrade in the presence of ferrous iron, Fe(II)-heme or biological reductants. Less documented is how this translates into chemical stability and antimalarial activity across a range of conditions applying to in vitro testing and clinical situations. Dihydroartemisinin (DHA) is studied here because it is both an antimalarial drug on its own and the main metabolite of other artemisinins. The behavior of DHA in PBS, plasma or erythrocytes lysate at different temperatures and pH ranges was examined. The antimalarial activity of the residual drug was evaluated using the chemosensitivity assay on P. falciparum, and the extent of decomposition of DHA was established through use of HPLC-ECD analysis. The role of the Fe(II)-heme was investigated by blocking its reactivity using carbon monoxide. A significant reduction in the antimalarial activity of DHA was seen after incubation in plasma and to a lesser extent in erythrocytes lysate: activity was reduced by half after 3 hours and almost completely abolished after 24 hours. Serum-enriched media also affected DHA activity. Effects were temperature and pH-dependent and paralleled the increased rate of decomposition of DHA from pH 7 upwards and in plasma. These results suggest that particular care should be taken in conducting and interpreting in vitro studies, prone as they are to experimental and drug storage conditions. Disorders such as fever, hemolysis or acidosis associated with malaria severity may contribute to artemisinins instability and reduce their clinical efficacy
The Non-Saturated Multiserver Job Queuing Model with Two Job Classes: A Matrix Geometric Analysis
No full textDatacenters comprise large quantities of processors, memory, and input/output modules. These resources are shared among requests (jobs) submitted by datacenter users. Jobs differ in their frequency of arrivals, demand for resources, and execution times. Resource sharing generates contention, especially in heavily loaded systems, that must therefore implement effective scheduling policies for incoming jobs. The First-In First-Out (FIFO) policy is often used for batch jobs, but may produce under-utilization of resources, in terms of wasted servers. This is due to the fact that a job that requires many resources can block jobs arriving later that could be served because they require fewer resources. The mathematical construct often used to study this problem is the Multiserver Job Queuing Model (MJQM), where servers represent resources which are requested and used by jobs in different quantities. Unfortunately, very few explicit results are known for the MJQM, especially at realistic system loads (i.e., before saturation). In this paper, we propose the first exact analytical model of the non-saturated MJQM in case of two classes of customers with exponentially distributed service times and an arbitrary number of identical servers. Our analysis is based on the matrix geometric method. Our results provide insight into datacenter dynamics, thus supporting the design of more complex schedulers, capable of improving performance and energy consumption within large datacenters
Stability Condition for the Multi-server Job Queuing Model: Sensitivity Analysis
No full textA Multiserver Job Queuing Model (MJQM) is a queuing system that can be instrumental in the study of the dynamics of resource allocation in datacenters. The queue comprises a waiting line with infinite capacity and a large number of servers. In this paper, we look at the case of an infinite number of servers. Jobs are termed “multiserver” because each one is characterized by a resource demand in terms of number of simultaneously used servers and by a service duration. In a MJQM, jobs are clustered into classes, and a number of used servers is deterministically associated with each class. Instead, holding times are independent and identically distributed random variables whose distributions depend on the class of the job. We consider the case of just two job classes: “small” jobs use just one server, while “big” jobs use all servers in the system. The service discipline is First-Come-First-Served (FCFS). This means that if the job at the head-of-line (HOL) cannot enter service because the number of free servers is not sufficient to meet the job requirement, it blocks all subsequent jobs, even if there are sufficient free servers for them. Despite its importance, only few results exist for the MJQM, whose analysis is challenging, especially because the MJQM is not work-conserving. This implies that even the stability region of the MJQM is known only in special cases. In a previous work, we obtained a closed-form stability condition for MJQM with big and small jobs under the assumption of exponentially distributed service times for small jobs. In this paper, we compute the stability condition of MJQM with big and small jobs, with an infinite number of servers, considering different distributions of the service times of small jobs. Simulations are used to support the analytical results and to investigate the impact of service time distributions on the expected job waiting time before saturation
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