Antimalarial drugs influence the pH dependent solubility of heme via apparent nucleation phenomena

Recently, we measured a more acid digestive vacuolar pH for drug resistant Plasmodium falciparum [Dzekunov S, Ursos LMB, Roepe PD. Mol Biochem Parasit ol 2000;in press; Ursos LMB, Dzekunov S, Roepe PD. Mol Biochem Parasitol 20 00;in press]. We suggested this acidification contributes to drug resistanc e via the profound effects that pH has on the solubility of unpolymerized h eme found in the vacuole (ferriprotoporphyrin IX mu oxo dimers). In this re port, we measure how FPIX concentration, time, NaCl concentration, and seve ral antimalarial drugs affect FPIX pH dependent solubility. Aggregation is essentially instantaneous below pH 5.3, but at vacuolar pH previously measu red for HB3 parasites [Dzekunov S, Ursos LMB, Roepe PD. Mol Biochem Parasit ol 2000;in press] can increase to several minutes as NaCl is lowered. As FP IX is decreased, the midpoint of the pH dependent solubility curve shifts t o higher values. Addition of antimalarial drugs also increases the midpoint of the pH dependent FPIX solubility curve, with the net shift proportional to the relative affinity of the drug for FPIX. Surprisingly, however, for all drugs tested shifts of essentially identical magnitude are found at all drug: FPIX molar ratios inspected, spanning eight orders of magnitude (to as low as 0.0000001:1). This suggests that changes in pH dependent FPIX sol ubility by addition of antimalarial drugs is via previously unrecognized dr ug/FPIX nucleation phenomena. These data could have important implications for understanding the role of previously observed changes in pH(vac) [Dzeku nov S, Ursos LMB, Roepe PD. Mol Biochem Parasitol 2000;in press; Ursos LMB, Dzekunov S, Roepe PD. Mol Biochem Parasitol 2000;in press] upon developmen t of antimalarial drug resistance. (C) 2001 Elsevier Science B.V. All right s reserved.