In vitro β-hematin formation assays with plasma of mice infected with Plasmodium yoelii and other parasite preparations: Comparative inhibition with quinoline and endoperoxide antimalarials

Formation of β-hematin in vitro could be catalyzed in the presence of various preparations related to the malaria parasite viz., the cell free homogenate of Plasmodium yoelii, lipid extract of the parasite homogenate, purified malarial hemozoin and synthetic β-hematin. Plasma from mice infected with P. yoelii also catalyzed in vitro β-hematin formation with highly significant efficiency. The plasma based β-hematin formation assay was highly sensitive, as the background absorbance was almost negligible due to absence of any preformed hemozoin. The plasma β-hematin synthesizing activity was recovered in the lipid extract. The quinoline and endoperoxide antimalarials act by inhibiting hemozoin biosynthesis in the malaria parasite. Therefore, the in vitro β-hematin formation assay is useful for the screening and identification of blood schizontocidal antimalarials acting through interruption of heme detoxification in the parasite. Quinoline and endoperoxide antimalarials showed about three fold greater inhibition of β-hematin synthesizing activity in the plasma-based assays as compared to that of P. yoelii homogenate-based assays. The specificity of the inhibition was similar in both preparations. The plasma-based assay therefore provides a better alternative than the parasite homogenate-based assay for in vitro screening and identification of novel inhibitors of hemozoin biosynthesis as potential blood schizontocidal antimalarials.