During intra-erythrocytic development, malaria trophozoites digest hemoglobin, which leads to parasite growth and asexual replication while accumulating toxic heme. To avoid death, the parasite synthesizes insoluble hemozoin crystals in the digestive vacuole through polymerization of β-hematin dimers. In the process, the heme is converted to a high-spin ferriheme whose magnetic properties were studied as early as 1936 by Pauling et al. Here, by magnetophoretic cell motion analysis, we provide evidence for a graduated increase of live cell magnetic susceptibility with developing blood-stage parasites, compatible with the increase in hemozoin content and the mechanism used by P. falciparum to avoid heme toxicity. The measured magnetophoretic mobility of the erythrocyte infected with a late-stage schizont form was m = 2.94 × 10⁻⁶ mm³ s/kg, corresponding to the net volume magnetic susceptibility (relative to water) of Δχ = 1.80 × 10⁻⁶, significantly higher than that of the oxygenated erythrocyte (–0.18×10⁻⁶) but lower than that of the fully deoxygenated erythrocyte (3.33×10⁻⁶). The corresponding fraction of hemoglobin converted to hemozoin, calculated based on the known magnetic susceptibilities of hemoglobin heme and hemozoin ferriheme, was 0.50, in agreement with the published biochemical and crystallography data. Magnetophoretic analysis of live erythrocytes could become significant for antimalarial drug susceptibility and resistance determination.

Key words: erythrocyte magnetic susceptibility • Plasmodium falciparum • hemozoin • ferriheme • heme • toxicity