Browsing by Author "Bergh, Wikus"

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    Exploring the peroxidase activity of ferriprotoporphyrin IX: towards understanding its cytotoxicity in the malaria parasite
    (Stellenbosch : Stellenbosch University, 2019-03) Bergh, Wikus; De Villiers, Katherine A.; Gerber, Wilhelmus J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The peroxidase reaction, catalysed by ferriprotoporphyrin IX (Fe(III)PPIX), was selected as a model reaction by which to explore Fe(III)PPIX toxicity via the formation of reactive oxygen species. This has direct relevant in, for example, antimalarial chemotherapy. To analyse potential reaction steps, model-based analysis is used. A program was coded incorporating a simplex algorithm for the multivariate global analysis of experimental data using non-linear least squares (NLLS) regression analysis. The reaction between Fe(III)PPIX and hydrogen peroxide was studied, with tests conducted regarding reversibility of proposed reaction steps, as well as the addition of potential side reactions. NLLS regression analysis of proposed models yielded fitted data that could be used to determine the goodness-of-fit of the model, as well as compare fits of models as a function of their complexity. Model comparison analysis showed that reversibility of reactions were irrelevant, and that the best depiction of the reaction proceeds via three steps. The first is complexation of Fe(III)PPIX and H2O2. Secondly, the complex forms an active intermediate š¼. The last step is denoted by the conversion of the intermediate to Feā€¢+(IV)PPIX=O. Degradation of Feā€¢+(IV)PPIX=O was incorporated in model extensions as side reaction but did not form part of the model determined to be best. ABTS is used as chromogen to follow the peroxidase-like reaction of Fe(III)PPIX spectrophotometrically. Studies conducted on plausible side reactions of the chromogen ABTS showed that the compound does not react with hydrogen peroxide at pH levels of 4.8 and 7.5. The disproportionation reaction is shown to occur under highly acidic conditions but is concluded not to transpire at pH levels approaching physiologically relevant conditions. Degradation of ABTS is not observed for physiologically relevant conditions. ABTSā€¢+ degradation is considered a viable side reaction under the premise that it can be reduced to a null parameter following non-linear least squares regression in the case that it does not prove relevant. Results obtained for the reaction between Fe(III)PPIX and H2O2 and ABTS studies are incorporated in the proposition of plausible reaction models for the peroxidase-like reaction of Fe(III)PPIX. NLLS analysis of models presented based on literary review indicate that the peroxidase-like activity of Fe(III)PPIX involves two parallel catalytic cycles. One involves the reaction steps presented for the reaction of Fe(III)PPIX with H2O2. From the formation of Feā€¢+(IV)PPIX=O, the radical porphyrin compound can react with ABTS to form Fe(IV)PPIX=O and ABTSā€¢+. Fe(IV)PPIX=O can react with another molecule of ABTS to form ABTSā€¢+ and regenerate Fe(III)PPIX, thus concluding the cycle. The second catalytic cycle is initiated by the complexation of Fe(III)PPIX with ABTS. The complex formed can react with H2O2 to produce a Feā€¢+(IV)PPIX=O.ABTS complex. The porphyrin radical compound can react with the ABTS molecule associated to it to produce ABTSā€¢+ and Fe(IV)PPIX=O, thus joining into the first cycle presented. The model proposed can be used to compare the kinetics of this reaction in the presence of antimalarial compounds to elicit the effect these compounds have on the toxicity of Fe(III)PPIX; such insights may advance rational design efforts towards new antimalarial chemotherapeutics.