Lipid or aqueous medium for hematin crystallization?
Hematin crystallization is the primary heme detoxification mechanism of malaria parasites infecting human erythrocytes and the target of currently applied antimalarial medications. The composition of the crystallization medium within the parasite's digestive vacuole (DV), the aqueous or lipid sub-phases, has been the subject of intense debate. Here we show that a blend of lipids, designed to mimic the lipid sub-phase in the parasite DV, contains significant amounts of soluble water that facilitates hematin crystal formation. We show that the hematin solubility in citric buffer-saturated n-octanol (CBSO), a model for the DV lipid sub-phase, is 100 000-fold greater than in a biomimetic aqueous solution, indicating that organic-based crystallization provides an environment for faster hematin crystallization and more efficient heme detoxification. We demonstrate that hematin crystals grow with physiologically-relevant rates from CBSO and do not grow from our biomimetic aqueous solvents. Our findings suggest that hematin crystallization most likely requires the participation of lipid structures. We propose a mechanism of hematin crystallization in the parasite DV that reconciles this conclusion with data on hemoglobin transport and hematin generation and crystallization in vivo. The proposed mechanism suggests that hematin becomes incorporated into crystals via a layer of neutral lipids or possibly after penetrating the phospholipid bilayer of the DV. We specify guidelines for tests of the proposed mechanism and highlight its clinical and pharmacological implications.