Inhibition by chloroquine of a novel haem polymerase enzyme activity in malaria trophozoites

Abstract

THE incidence of human malaria has increased during the past 20 years; 270 million people are now estimated to be infected with the parasite¹. An important contribution to this increase has been the appearance of malaria organisms resistant to quinoline-containing antimalarials such as chloroquine and quinine². These drugs accumulate in the acid food vacuoles of the intraerythrocytic-stage malaria parasite^3–5, although the mechanism of their specific toxicity in this organelle is uncertain. The primary function of the food vacuole is the proteolysis of ingested red cell haemoglobin^6,7 to provide the growing parasite with essential amino acids. Haemoglobin breakdown in the food vacuole releases haem, which if soluble can damage biological membranes⁸ and inhibit a variety of enzymes^9,10. Rather than degrading or excreting the haem, the parasite has evolved a novel pathway for its detoxification by incorporating it into an insoluble crystalline material called haemozoin or malaria pigment¹¹. These crystals form in the food vacuole of the parasite concomitant with haemoglobin degradation, where they remain until the infected red cell bursts. The structure of haemozoin comprises a polymer of haems linked between the central ferric ion of one haem and a carboxylate side-group oxygen of another¹². This tructure does not form spontaneously from either free haem or haemoglobin under physiological conditions^12–14, and the biochemistry of its formation is unclear. Here we report the identification and characterization of a haem polymerase enzyme activity from extracts of Plasmodium fal-ciparum trophozoites, and show that this enzyme is inhibited by quinoline-containing drugs such as chloroquine and quinine. This provides a possible explanation for the highly stage-specific anti-malarial properties of these drugs.