Pigment biocrystallization in Plasmodium falciparum.

Abstract

Malaria parasites catabolize hemoglobin as an important source of amino acids. One consequence of this dependence on hemoglobin degradation is the need for an effective detoxification mechanism for hematin (Fig. 1Fig. 1a,b). To date, the only definitively characterized product of hematin in the parasite is the malaria pigment hemozoin. This pigment is chemically and structurally identical to synthetic β-hematin [1xCharacterization of the products of the heme detoxification pathway in malarial late trophozoites by X-ray diffraction. Bohle, D.S. et al. J. Biol. Chem. 1997; 272: 713–716Crossref | PubMed | Scopus (131)See all References, 2xCharacterisation of synthetic β-haematin and effects of the antimalarial drugs quinidine, halofantrine, desbutylhalofantrine and mefloquine on its formation. Egan, T.J. et al. J. Inorg. Biochem. 1999; 73: 101–107Crossref | PubMed | Scopus (70)See all References]. Until recently, this material was believed to be a polymer of hematin [1xCharacterization of the products of the heme detoxification pathway in malarial late trophozoites by X-ray diffraction. Bohle, D.S. et al. J. Biol. Chem. 1997; 272: 713–716Crossref | PubMed | Scopus (131)See all References, 3xAn iron-carboxylate bond links the heme units of malaria pigment. Slater, A.F.G. et al. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 325–329Crossref | PubMed | Scopus (317)See all References], but the structure determination by Pagola et al. has revealed that it is a dimer (Fig. 1Fig. 1c) with hydrogen bonding between the dimer units in the crystal [4xThe structure of malaria pigment (β-haematin). Pagola, S. et al. Nature. 2000; 404: 307–310Crossref | PubMed | Scopus (561)See all References[4]. Malaria pigment is thus no more polymeric than ice or sugars in which the individual molecules interact through intermolecular hydrogen bonding in the crystalline state. In the light of this, the continued use of the word polymer to describe malaria pigment or β-hematin, or the word polymerization to describe its formation is inappropriate and inaccurate.Fig. 1Chemical structures are shown for (a) hematin (aqua or hydroxyferriprotoporphyrin IX) formed by autoxidation of the heme (b) released from hemoglobin. The dimeric structure for β-hematin is also indicated (c). The OH−, H2O group is represented by X in (a) and the histidine is represented by Y in (b).View Large Image | Download PowerPoint SlidePigment formation is also not a result of a simple precipitation or aggregation because it is chemically distinct from the species present in solution (hematin), and such a simple process would not remove the toxic soluble portion from the solution in the cell. Interestingly, the crystals are remarkably uniform in shape and size when viewed by electron microscopy [2xCharacterisation of synthetic β-haematin and effects of the antimalarial drugs quinidine, halofantrine, desbutylhalofantrine and mefloquine on its formation. Egan, T.J. et al. J. Inorg. Biochem. 1999; 73: 101–107Crossref | PubMed | Scopus (70)See all References[2]. Pigment crystals from eight different strains of Plasmodium falciparum have very similar external shapes and sizes, all exhibiting a triclinic habit (E. Hempelmann, unpublished data). Three recent papers have referred to malaria pigment formation as a biomineralization-type process [5xZiegler, J. et al. J. Am. Chem. Soc. 1999; 121: 2395–2400Crossref | Scopus (49)See all References, 6xThe mechanism of β-hematin formation in acetate solution. Parallels between hemozoin formation and biomineralization processes. Egan, T.J. et al. Biochemistry. 2001; 40: 204–213Crossref | PubMed | Scopus (104)See all References, 7xHeme aggregation inhibitors: antimalarial drugs targeting an essential biomineralization process. Ziegler, J. et al. Curr. Med. Chem. 2001; 8: 171–189Crossref | PubMedSee all References]. Such processes are known detoxification mechanisms in other microorganisms (e.g. mineralization of copper sulfide confers copper tolerance in Saccharomyces cerevisiae [8xIdentification of SLF1 as a new copper homeostasis gene involved in copper sulfide mineralization in Saccharomyces cerevisiae. Yu, W. et al. Mol. Cell. Biol. 1996; 16: 2464–2472PubMedSee all References[8]). Strictly speaking, biomineralization refers to the formation of insoluble inorganic salts, and the term biocrystallization is probably the more appropriate term to describe malaria pigment formation.Currently, the mechanism of malaria pigment formation in vivo is unknown, although proteins (in the form of histidine rich proteins) [9xPlasmodium hemozoin formation mediated by histidine-rich proteins. Sullivan, D.J. et al. Science. 1996; 271: 219–222Crossref | PubMedSee all References, 10xHeme binding and polymerization by Plasmodium falciparum histidine rich protein II: influence of pH on activity and conformation. Lynn, A. et al. FEBS Lett. 1999; 459: 267–271Abstract | Full Text | Full Text PDF | PubMed | Scopus (45)See all References, 11xSpectroscopic characterization of the heme-binding sites in Plasmodium falciparum histidine-rich protein 2. Choi, C.Y.H. et al. Biochemistry. 1999; 38: 16916–16924Crossref | PubMed | Scopus (56)See all References] and lipids [12xHaem polymerization in malaria. Bendrat, K. et al. Nature. 1995; 378: 138–139Crossref | PubMedSee all References, 13xHaem polymerization in malaria. Ridley, R.G. et al. Nature. 1995; 378: 138–139Crossref | PubMedSee all References, 14xA comparison and analysis of several ways to promote haematin (haem) polymerisation and an assessment of its initiation in vitro. Dorn, A. et al. Biochem. Pharmacol. 1998; 55: 737–747Crossref | PubMed | Scopus (87)See all References] have been implicated. Both proteins and lipid membranes are typically involved in biomineralization processes [15xCrystallochemical strategies in biomineralization. Mann, S. : 35–62See all References[15]. The detailed role of such constituents in malaria pigment formation and the precise effect of chloroquine and other antimalarials, which appear to act by disturbing its formation [16xInhibition by chloroquine of a novel haem polymerase enzyme activity in malaria trophozoites. Slater, A.F.G. and Cerami, A. Nature. 1992; 355: 167–169Crossref | PubMed | Scopus (411)See all References, 17xQuinoline anti-malarial drugs inhibit spontaneous formation of β-haematin (malaria pigment). Egan, T.J. et al. FEBS Lett. 1994; 352: 54–57Abstract | Full Text PDF | PubMed | Scopus (277)See all References, 18xMalarial haemozoin/β-haematin supports haem polymerization in the absence of protein. Dorn, A. et al. Nature. 1995; 374: 269–271Crossref | PubMedSee all References, 19xOn the molecular mechanism of chloroquine's antimalarial action. Sullivan, D.J. et al. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 11865–11870Crossref | PubMed | Scopus (324)See all References, 20xAn assessment of drug-haematin binding as a mechanism for inhibition of haematin polymerisation by quinoline antimalarials. Dorn, A. et al. Biochem. Pharmacol. 1998; 55: 727–736Crossref | PubMed | Scopus (257)See all References, 21xStructural specificity of chloroquine-hematin binding related to inhibition of hematin polymerization and parasite growth. Vippagunta, S.R. et al. J. Med. Chem. 1999; 42: 4630–4639Crossref | PubMed | Scopus (118)See all References, 22xStructure–function relationships in aminoquinolines: effect of amino and chloro groups on quinoline–hematin complex formation, inhibition of (-hematin formation, and antiplasmodial activity. Egan, T.J. et al. J. Med. Chem. 2000; 43: 283–291Crossref | PubMed | Scopus (225)See all References], remains to be determined.