What is pure hemozoin? A close look at the surface of the malaria pigment.

Abstract The malaria parasite, Plasmodium spp., produces hemozoin (Hz) crystals as a by-product of hemoglobin digestion. Purification methods used to remove host or parasite products adsorbed on Hz surface lead to variable and undetermined residues. This compositional variation likely accounts for the assortment of contradictory results in studies of Hz's biomineralization, immunomodulating properties, and the mechanism of action of some antimalarials. In this work, we study the surface of Hz cleaned with two methods, both reported in the literature, one stricter than the other. We find that biomolecules are adsorbed on Hz treated with either method, they bind through carboxylate groups, and may be present within Hz structure. Their composition and amount depend on the washing protocol, which also introduces contaminants. This finding led us to question the concept of "pure" Hz, and to propose x-ray photoelectron spectroscopy (XPS) and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) as characterization tools to assess surface contamination prior to further work on Hz crystals. Graphical abstract Organic contaminants are strongly adsorbed onto hemozoin surface despite extensive purification. The contaminants adsorb on hemozoin mainly through surface carboxylate groups, and could be present within its crystalline structure. These results explain discrepancies related to the immunomodulatory properties of hemozoin and will help designing new antimalarials and understanding hemozoin formation. Unlabelled Image Highlights • Organic contaminants remain adsorbed onto hemozoin even after purification. • The purification protocol introduces contamination to hemozoin surface. • The residual biomolecules could also be part of the crystalline structure of hemozoin. • Surface carboxylate groups in hemozoin are the main adsorption sites for biomolecules. [ABSTRACT FROM AUTHOR]