Investigations into the Peroxidase Activity of Ferriprotoporphyrin IX and its Complexes with Clinically Relevant Antimalarial Drugs

Sammy, Chandre Jade (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: A major class of antimalarial drugs act by increasing levels of free haem, Fe(III)PPIX, within the malaria parasite. While Fe(III)PPIX is known to be toxic, the manner in which it brings about parasite death is not entirely clear. Furthermore, it is unknown as to whether the antimalarial drugs which interact with Fe(III)PPIX, modulate its toxicity. The work presented here aims to address these uncertainties. The first part of this work focusses on the interactions between clinically relevant antimalarial drugs and Fe(III)PPIX. Spectrophotometric titrations were used to investigate drug-Fe(III)PPIX complex formation under three biologically relevant environments, namely an aqueous environment (pH 7.4) representative of the malaria parasite cytosol; acetonitrile which mimics the non-aqueous interior of a lipid and the detergent SDS was used to model a lipid-water interface system, that is present at membrane surfaces. Three antimalarial drugs chloroquine (CQ), quinidine (QD) and artemisinin (Art), belonging to three distinct classes, were investigated to determine binding strength between the antimalarial drug and Fe(III)PPIX. Strong association constants for the interaction of Fe(III)PPIX with QD and CQ were determined in the aqueous (log K = 5.8 ± 0.1 and 6.5 ± 0.1), aqueous SDS system (log K = 6.2 ± 0.1 and 6.5 ± 0.1) and acetonitrile (log K = 5.92 ± 0.1 and 5.3 ± 0.2). An association constant for the interaction of Fe(III)PPIX with Art could, however, only be determined in acetonitrile (log K = 4.48 ± 0.04) owing to a weakened interaction in aqueous media. Furthermore, the first crystal structure of the complex formed between Fe(III)PPIX and the QD analogue, cinchonine, was obtained following slow evaporation from a solution of acetonitrile. To probe the possible mechanism of toxicity of Fe(III)PPIX, its peroxidase activity was investigated using the chromogenic substrate, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). The oxidation of ABTS to ABTS˙+ by Fe(III)PPIX and hydrogen peroxide (H2O2) was investigated in an aqueous and aqueous SDS solvent system. Radical production was found to be more stable in the aqueous SDS system, and furthermore, the rate of radical production was also found to be lower in this system compared to aqueous solution. From the available experimental data, attempts were made to determine a preliminary kinetic model for the oxidation of ABTS by Fe(III)PPIX and H2O2 in aqueous solution. The modulatory effect of CQ, QD and Ar on the oxidation of ABTS by Fe(III)PPIX was monitored in both aqueous solution and aqueous SDS. Both CQ and QD were found to significantly inhibit the rate of radical production in aqueous solution by 30% and 80%, respectively. While the rate of ABTS oxidation is lower, CQ was found to extend the life of Fe(III)PPIX by reducing the rate of its degradation by H2O2 by 70%, while QD reduced the rate of degradative attack by 90%. Consequently, Fe(III)PPIX has reduced peroxidative activity but is present for a longer period. The same was true for QD in the aqueous SDS system, however, CQ was found to have minimal effect on the yield of radical production compared to Fe(III)PPIX in aqueous SDS, however, the rate was still reduced. Art was found to have the opposite effect of CQ and QD and enhanced radical production by three times that of Fe(III)PPIX. This is thought to be related to the presence of the endoperoxide bridge in the structure of Art and it is proposed that the induction of oxidative stress is one of its possible mechanisms of action. The results obtained in this study provide insight into the mechanism of drug-Fe(III)PPIX interaction. Furthermore, valuable information regarding the ABTS oxidation catalytic cycle was determined through careful consideration of the kinetic model and the assay could be used to provide insight into the mechanism of toxicity of CQ, QD and Ar.

AFRIKAANSE OPSOMMING: ‘n Hoof klas van antimalariese dwelms tree op deur bykomende vlakte van vrye heem, Fe(III)PPIX, binne die malaria parasiet. Terwyl Fe(III)PPIX bekend is as giftig, is die manier hoe dit die dood in die parasiet bewerkstellig hoegenaamd nie duidelik nie. Die werk wat hier aangebied word, beoog om die onsekerhede aan te spreek. In die eerste deel van hierdie werkstuk word daar gefokus op die interaksie tussen kliniese relevante teen malaria dwels en Fe(III)PPIX. Spektrofotometriese titrasies was gebruik om die Fe(III)PPIX-dwelm komplekse vormasie te ondersoek onder drie biologiese relevante omgewings, naamlik ‘n waterige omgewing (pH 7.4) verteenwoordigend na die malaria parasiet sitisol; acetonitrile wat die nie-waterige interne lipied naboots en die reinigingsmiddel, SDS, was gebruik om ‘n lipied-water koppelvlak sisteem te modeliseer, wat by die membraan oppervlakte teenwoordig is. Drie teen-malariese dwelms, chlorokien (CQ), kinidien (QD) en artemisinin (Art), wat aan drie unieke klasse behoort, was ondersoek om die bindende sterkte tussen die teen-malariese dwelms en Fe(III)PPIX te bepaal. Sterk assosiasiese konstantes vir die interaksie van Fe(III)PPIX met QD en CQ was bepaal in die waterige (log K = 5.8 ± 0.1 and 6.5 ± 0.1), waterige SDS sisteem (log K = 6.2 ± 0.1 and 6.5 ± 0.1) en acetonitril (log K = 5.92 ± 0.1 and 5.3 ± 0.2). ‘n Assosiasie konstante vir die interaksie van Fe(III)PPIX met Art kon egter slegs in acetonitrile (log K = 4.48 ± 0.04) bepaal word te wyte aan ‘n verswakte interaksie in die waterige media. Verder was die eerste kristal struktuur met die komplekse vorm tussen Fe(III)PPIX en die QD analoog, cinchonine verkry deur ‘n stadige evaporasie van die acetonitrile oplossing. Om die moontlike meganisme van toksisiteit van Fe(III)PPIX te ondersoek , was die peroksidase aktiwiteit ondersoek deur die chromogeniese substraat 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) te gebruik. Die oksidasie van ABTS na ABTS˙+ deur Fe(III)PPIX en waterstof peroksied (H2O2) was ondersoek in ‘n waterige oplossing en waterige SDS oplossing. Radikale produksie was gevind wat meer stabile was in die waterige SDS sisteem, die koers van radikale produksie was egter betekenisvol stadiger wanneer vergelyk word met die van die waterige SDS sisteem. Verder was ‘n poging tot die bepaaling van ‘n voorlopige kinetiese model vir die oksidasie van ABTS deur Fe(III)PPIX en H2O2 in ‘n waterige oplossing uitgevoer. Die modulatoriese effek van CQ, QD en Ar op die oksidase van ABTS deur Fe(III)PPIX was gemonitor in beide die waterige oplossing en waterige SDS. Beide CQ en QD was gevind om radikale produksie beduidende te inhibeer by 30% en 80%, onderskeidelik. Terwyl die koers van ABTS oksidasie verlaag is, verleng CQ die lewe van Fe(III)PPIX deur die koers van sy agteruitgang deur H2O2 by 70% te inhibeer, terwyl QD dit by 90% geinhibeer het. Gevolglik, het Fe(III)PPIX peroxidatiewe aktiwiteite verminder, maar bly teenwoordig vir ‘n langer tydperk. Dieselfde is gevind vir QD in die waterige SDS stelsel, CQ was egter gevind dat dit ‘n minimale effek het op die opbrengs van radikale produksie vergelykend met Fe(III)PPIX in waterige SDS terwyl die koers steeds verminder het. Dit was gevind dat Art die teenoorgestelde effek van CQ en QD en gevorderde radikale produksie drie keer meer as Fe(III)PPIX. Die gedagte is dat dit verwant is aan die teenwoordigheid van ‘n endoperoksied brug in die struktuur van Art en dit word aanbeveel dat die induksie van oksidatiewe spanning een van die moontlike meganismes van aksie is. Die resultate wat in hierdie studie gevind is versksaf insig in die meganisme van die dwelm-Fe(III)PPIX se interaksie. Verder, was waardevolle inligting ten opsigte van die ABTS oksidasie katalitiese siklus ontdek deur sorgvuldige oorweging van die kinetiese model en die toets kon gebruik word om insig te beverkstellig in di meganisme van toksisiteit van CQ, QD en Ar.

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