A physiochemical mechanism of hemozoin (β-hematin) synthesis by malaria parasite

Malaria parasite homogenate, the lipid extracts, and an unsaturated fatty acid, linoleic acid, which have been shown to promote β-hematin formation in vitro, were used to investigate the mechanism of hemozoin biosynthesis, a distinct metabolic function of the malaria parasite. In vitro β-hematin formation promoted by Plasmodium yoelii homogenate, the lipid extracts, and linoleic acid were blocked by ascorbic acid, reduced glutathione, sodium dithionite, β-mercaptoethanol, dithiothreitol, and superoxide dismutase. Oxidized glutathione did not show any effect. Preoxidized preparations of the lipids extracts or the P. yoelii homogenate failed to catalyze β-hematin formation. Depletion of oxygen in the reaction mixtures also inhibited the lipid-catalyzed β-hematin formation. Under the reaction conditions similar to those used for the in vitro β-hematin formation assay, the antioxidants and reducing agents mentioned above, except the DTT and β-mercaptoethanol, did not cause degradation of heme. β-Hematin formation was also inhibited by p-aminophenol, a free radical chain reaction breaker. Hemozoin biosynthesis within the digestive vacuoles of the malaria parasite may be a lipid-catalyzed physiochemical reaction. An oxidative mechanism may be proposed for lipid-mediated β-hematin formation, which may be mediated by generation of some free radical intermediates of heme.