Characterization of chloroquine-hematin mu-oxo dimer binding by isothermal titration calorimetry

Numerous studies indicate that a key feature of chloroquine's (CQ) antimalarial activity is its interaction with hematin. We now characterize this CQ-hematin interaction in detail using isothermal titration calorimetry (ITC). Between pH 5.6 and 9.0, association constants (K(a) values) for enthalpy-driven CQ-hematin mu-oxo dimer binding fell in the narrow range of 2.3-4.4 x 10(5) M(-1). It is therefore probable that CQ-hematin mu-oxo dimer binding affinity does not diminish at the pH range (4.8-5.4) of the parasite food vacuole. The binding affinity was unaffected by high salt concentrations, suggesting that ionic interactions do not contribute significantly to this complexation. With increasing ionic strength, the entropic penalty of CQ-hematin mu-oxo dimer binding decreased accompanied by increased hematin mu-oxo dimer aggregation. A stoichiometry (n) of 1:4 in the pH range 6.5-9.0 indicates that CQ binds to two hematin mu-oxo dimers. At pH 5.6, a stoichiometry of 1:8 suggests that CQ binds to an aggregate of four hematin mu-oxo dimers. This work adds further evidence supporting the hypothesis that CQ impedes hematin monomer incorporation into hemozoin by producing a forward shift in the hematin monomer-hematin mu-oxo dimer equilibrium, contributing to a destructive accumulation of soluble forms of hematin in the parasite and leading to its death by hematin poisoning.