Inhibitory effect of chloroquine on the peroxidase activity of ferriprotoporphyrin IX

Abstract

The effect of the antimalarial chloroquine {7-chloro-4-[4-(diethylamino)-1-methylbutylamino]quinoline} on the oxidation of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)(abts) promoted by ferriprotoporphyrin IX [(3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropanoato)iron(III)] and H₂O₂ was studied. Under the conditions [abts][H₂O₂][catalyst], the kinetic results showed a first-order dependence on both the peroxide (up to 8 × 10^–3 mol dm^–3) and the catalyst concentration (up to 4 × 10^–6 mol dm^–3), although a saturation effect was observed at higher concentrations. The values of the apparent second-order rate constants in the absence and presence of chlorcquine were (1.71 ± 0.05)× 10⁵ and (1.23 ± 0.04)× 10⁵ dm⁶ mol^–2 s^–1 at pH 6.86, 303.1 ± 0.2 K, and I= 0.10 mol dm^–3 NaCl. Therefore, the antimalarial inhibits the peroxidase activity by about 28% due the formation of a catalytically inactive complex with the haem. A first-order dependence on the concentration of abts was also verified, followed by a zero-order dependence at concentrations higher than 5 × 10^–3 mol dm^–3. Some EPR spin-trapping experiments, and comparative studies in the presence of bromide ions, provided evidence for the participation of analogues of the horseradish peroxidase compounds I and II rather than hydroxyl radicals, in the peroxidase activity of the haem. The results are discussed in the context of the antimalarial activity of chloroquine.