Malarial haemozoin/β-haematin supports haem polymerization in the absence of protein

Abstract

Malarial parasites growing inside erythrocytes digest up to 80% of the host cell's haemoglobin within a lysosomal organelle, the digestive vacuole. They sequester the potentially toxic haem (Fe (II) protohaematoporphyrin) that is released during this process into an insoluble pigment called haemozoin, which consists of polymerized Fe (III) protohaematoporphyrin subunits. We have studied this process of haem polymerization, which was previously reported to be enzyme-mediated and the target of the quinoline antimalarial drugs chloroquine and quinine. Here we show that, rather than being enzyme-mediated, haem polymerization is actually a chemical process, dependent only on the presence of haem-derived material associated with haemozoin and not on protein. This discovery does not invalidate haem polymerization as a target for drug intervention and the mechanism by which haemozoin formation is initiated is still not understood, but our view of this process and of the action of choroquine must be reconsidered.

Formation of haemozoin/β‐haematin under physiological conditions is not spontaneous

“…Recently reaction of haem polymerization was studied using a HPLC method and no polymerization was found to occur without malarial parasite extract [4]. This is in accordance with the previous studies [10,20,21]. On the basis of all these evidences we propose that the product reported by Egan et al [14] was not ]3-haematin but haem-acetate adduct and the polymerization of haem to form haemozoin/[3-haematin is not spontaneous but requires the presence of some parasitic or biological material for the initiation and/or continuous formation of haemozoin, whose identity is as yet unestablished.…”

“…Initially the presence of a parasite-specific enzyme termed as 'haem polymerase' was proposed [8,9]. However, the involvement of the enzyme component was questioned later as the reaction was reported to occur even in the presence of preformed J3-haematin or malarial haemozoin [10]. A phospholipid component isolated from the non-infected as well as malaria-infected erythrocytes or malarial histidine-rich protein were also found to initiate the formation of haemozoin [11,12], Haemozoin is a polymer of haem units linked through an iron~zarboxylate bond between central iron of one haem and propionate side chain of the other [13].…”

Inhibition of the peroxidative degradation of haem as the basis of action of chloroquine and other quinoline antimalarials

“…The interaction of chloroquine with haem has previously been shown to inhibit haem polymerization [4][5][6]. We have therefore examined the ability of chloroquine to inhibit the peroxidative decomposition of haem under conditions resembling those of the food vacuole.…”

“…The parasite lacks haem oxygenase [3] and it has been proposed that it disposes of the haem, at least in part, by polymerizing it into long insoluble polymers of haemozoin [4]. The polymerization process is autocatalytic [5] and has been shown to be relatively inefficient in itro [6]. This suggests that the parasite has an alternative means of disposing of toxic haem moieties.…”

Estudo de modelagem molecular de complexos ferriprotoporfirina-IX e quinolinocarbinolaminas antimaláricas: proposta de um farmacóforo

“…Esta conclusão foi baseada nas condições extremas que, aparentemente, seriam requeridas para a formação por síntese da β-hematina e na observação que um extrato de membrana plasmodial aparentemente catalisa esta formação. Entretanto, Dorn e colaboradores 13 encontraram que a formação de β-hematina é independente de proteínas e sugeriram que ela é autocatalítica, sendo que o agente catalítico no extrato do parasita é, de fato, a própria hemozoína.…”