Mechanism of malarial haem detoxification inhibition by chloroquine

The haem detoxification pathway of the malaria parasite Plasmodiumfalciparum is a potential biochemical target for drug development. Free haem, released after haemoglobin degradation, is polymerized by the parasite to form haemozoin pigment. Plasmodiumfalciparum histidine-rich protein-2 (Pfhrp-2) has been implicated as the catalytic scaffold for detoxification of haem in the malaria parasite. Previously we have shown that a hexapeptide repeat sequence (Ala-His-His-Ala-Ala-Asp), which appears 33 times in Pfhrp-2, may be the major haem binding site in this protein. The haem binding studies carried out by ourselves indicate that up to 18 equivalents of haem could be bound by this protein with an observed K_d of 0.94µM. Absorbance spectroscopy provides evidence that chloroquine is capable of extracting haem bound to Pfhrp-2. This was supported by the K_d value, of 37nM, observed for the haem-chloroquine complex. The native PAGE studies reveal that the formation of the haem-Pfhrp-2 complex is disrupted by chloroquine. These results indicate that chloroquine may be acting by inhibiting haem detoxification/binding to Pfhrp-2. Moreover, the higher affinity of chloroquine for haem than Pfhrp-2 suggests a possible mechanism of action for chloroquine; it may remove the haem bound to Pfhrp-2 and form a complex that is toxic to the parasite.