Spectrophotometric determination of de novo hemozoin/β-hematin formation in an in vitro assay

Formation of hemozoin in the malaria parasite, due to its unique nature, is an attractive molecular target. Several laboratories have been trying to unravel the molecular mechanism of hemozoin biosynthesis within the parasite digestive vacuoles. Use of different assay protocols for in vitro β-hematin (synthetic identical to hemozoin) formation by these laboratories has led to inconsistent and often contradictory findings. Much of the difficulty may be attributed to oligomeric heme aggregates, which may be indistinguishable in some detection approaches if adequate separation of β-hemtin is not achieved. Therefore, there is an urgent need for a widely accepted protocol for in vitro β-hematin formation. We describe here a spectrophotometric assay for in vitro β-hematin formation. The assay has been validated with the Plasmodium falciparum lysate, the parasite lipid extracts, and some commercially available fatty acids, which are known to initiate/catalyze β-hematin formation in vitro. The necessity for multiple wash steps for accurate quantification of de novo hemozoin/β-hematin formation was verified experimentally. It was necessary to wash the pellet, which contains β-hematin and heme aggregates, sequentially with Tris/SDS buffer and alkaline bicarbonate solution for complete removal of monomeric heme and heme aggregates and accurate quantification of β-hematin formed during the assay. The pellets and side products in the supernatant were characterized by infrared spectroscopy. No β-hematin formation occurred in the absence of a catalytic/initiating factor. Based on these findings, a filtration-based assay that uses 96-well microplates, and which has important application in in vitro screening and identification of novel inhibitors of hemozoin formation as potential blood schizontocidal antimalarials, has been developed.