Abstract
Malaria is an infectious disease caused by protozoan parasites from the genus Plasmodium falciparum. Parasites infect red blood cells (RBCs) which thus adhere to the microvascular endothelium. The accumulation of infected RBCs causes a reduction of oxygenation and impaired perfusion resulting in, eventually, organ damage. The characteristics of severe malaria are decreased nitric oxide (NO) production, low extracellular concentration of L-arginine (the substrate of NO production) and increased cell free haemoglobin (CFHb) concentration compared to healthy control subjects. Arginine catabolism is part of the urea cycle.The aim of this study is to develop a model of sufficient complexity to describe arginine catabolism and NO production and quenching in malaria and can be used to interpret empirical findings from our previous studies in Timika. This model also aims to identify the optimal dosing schedule of arginine infusion in malaria.
Models from the literature, a steady state acyclic intracellular model of arginine catabolism and a model for NO quenching were used as the initial building blocks for a model for arginine catabolism and NO production and quenching. Further model development was conducted to include intra- and extracellular transport pathways, cyclic catabolism and also arginine supplementation in non-steady state conditions. Model development was undertaken in MATLAB. No formal estimation procedures were included. The optimal dosing schedule was determined by calculating the total number of NO molecules produced by the endothelial cell (or that reach the adjacent muscle cells) for any given dosing regimen of arginine.
A full arginine catabolic model was developed that was able to describe available in vivo data. Modification of the maximum enzyme velocities was required for some enzymes, such as inducible nitric oxide syntheses (iNOS), suggesting (but not proving) induction in malaria. In addition, some affinity constants for transporters were also required to be increased, suggesting that a pool of arginine may exist intracellularly, which is inaccessible to the transporter. Modelling also indicated that the presence of induced iNOS would increase the intracellular and extracellular citrulline concentration by approximately 1000-fold, therefore an increased in (argininosuccinate synthase) activity was required. The model predicted that the drop in extracellular arginine concentration seen empirically in moderate severe malaria (MSM) is greater than the sum of the individual effects obtained from an increase in iNOS or arginase alone. Hence, an increase in NOS activity is unlikely to be a significant cause of reduced extracellular arginine concentration and, at most, would be a minor contributor to the hypoargininemia of MSM and by extrapolation also severe malaria (SM). The cumulative NO molecules produced by endothelial cells is significantly increased with the supplementation of extracellular arginine. An increase in dose of arginine infusion leads only to a minimal increase in cumulative NO molecules. However, an increase in duration of the arginine infusion from 0.5 to 12 hours resulted in a significant increase in the cumulative NO molecules and, therefore, the administration of arginine is shown to be highly schedule-dependent. Additionally, an increase in CFHb decreases the cumulative number of NO molecules that reach muscle cells via a nonlinear inverse relationship.
The model represented adequately arginine catabolism in malaria and the findings are in agreement with the current in vivo data. The model hypothesizes that the most important determinant of the benefits seen with exogenously administered arginine is the duration of the infusion. Further work is required to test this hypothesis.
| Date of Award | Sep 2010 |
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| Original language | English |
| Supervisor | Tsin Yeo (Supervisor) & Nicholas Anstey (Supervisor) |