2024 AIChE Annual Meeting
(288c) Inhibition of Hematin Crystal Nucleation By Suppressing the Mesoscopic Precursors: Implications for Heme Detoxification in Malaria Parasites
Authors
Sayeed, A., Johns Hopkins University
Rimer, J., University of Houston
Sullivan, D. J., Johns Hopkins University
Vekilov, P. G., University of Houston
We report the discovery of a novel mechanism of suppression of heme detoxification of malaria parasites by stalling of hematin crystal nucleation. In the their intraerythrocytic stage, malaria parasites metabolize hemoglobin and release its prosthetic group, the heme, which immediately transforms to toxic hematin. To survive, the intracellular parasite sequesters hematin into innocuous crystals, called hemozoin. Numerous antimalarials drugs operate by inhibiting hemozoin formation. Laboratory grown b-hematin crystals are isostructural with hemozoin and a suitable model to explore how hematin detoxification may be suppressed. The most recent antimalarial, approved by the FDA, is tafenoquine (TQ) and 8-aminoquinoline which as a class target malaria liver and gametocyte stages. TQ also kills the malaria-causing Plasmodium parasites, falciparum and vivax, in their blood stage, suggesting that it interferes with hematin crystallization in addition to active the metabolites from the hepatocyte cytochrome P450s, which target the Plasmodium mitochondria like widely used 8-aminoquinoliine, primaquine. We reveal a pathway of b-hematin suppression, mobilized by TQ, which is distinct from all previously known mechanisms. In contrast to several quinoline analogues and metabolites of three artemisinin-class drugs, TQ does not inhibit the growth of hematin crystals. The combination of mild delay of step propagation with a boost in new layer generation invokes, instead, faster b-hematin crystal growth. Uniquely, TQ strongly suppresses the rate of b-hematin crystal nucleation. Crystal nucleation suppression falls outside of the realm of classical nucleation theory, which only envisions how foreign compounds and surfaces may enhance the nucleation rate. We demonstrate that the nucleation of hematin crystals is nonclassical and is hosted by mesoscopic hematin-rich clusters, a novel phase with unusual properties. We show that TQ suppresses the population of the mesoscopic clusters in hematin solutions, in parallel with the drug effects on crystal nucleation. To test whether nucleation suppression is the mechanism employed by TQ to kill blood stage Plasmodium parasites, we are exposing intraerythrocytic parasites at distinct ages, from early to late rings, trophozoites with visible hemozoin and schizonts where nuclear division commences (the parasites’ intraerythrocytic life cycle encompasses 48 hours). In freshly divided parasites (also called “young rings”) heme release starts and hemozoin crystals nucleate. As the parasite ages, crystals nucleated at the young ring stage grow in parallel with hemoglobin digestion. While the 4 amino-quinolines mainly target trophozoite stages with rapid hemoglobin degradation, pyronaridine has greater effect on young rings. These new mechanisms of action enable much slower acquisition of resistance.