Inhibition of a protein tyrosine kinase activity in Plasmodium falciparum by chloroquine.

The aim of the present study was to establish the importance of phosphorylation events for parasite growth and maturation. Investigations into the cytosolic Plasmodium falciparum protein tyrosine kinase (PTK) activity revealed that there is a stage specific increase in the activity, in the order ring < trophozoite < schizont in both chloroquine sensitive (CQ-S) and chloroquine resistant (CQ-R) strains (p < 0.05). Our data also show that in vivo conversion of the schizont stage to ring stage via release of merozoites is associated with a decrease in PTK activity. Piceatannol, a specific inhibitor of PTK inhibited the activity in both the CQ-S and CQ-R strains of the parasites. The presence of low levels of chloroquine (CQ) inhibited the cytosolic PTK activity in a dose dependent manner (IC50 = 45 mumoles or 23 micrograms/ml) in CQ-S strains. The effect of varying concentration of CQ on the kinetics of peptide phosphorylation reveal that CQ was a competitive inhibitor of PTK with respect to peptide substrate and non-competitive with respect to ATP indicating that CQ inhibits PTK activity by binding with protein substrate binding site. These data thus suggests that maturation of malaria parasite may be due to this cellular PTK and its inhibition by CQ could provide a hypothesis to explain its antimalarial activity and efficacy.

Inhibitory effect of piceatannol, a protein tyrosine kinase inhibitor, on asexual maturation of Plasmodium falciparum.

This report describes the effect of piceatannol (3,4,3',5'-tetrahydroxy trans stilbene), a plant secondary natural product, on protein tyrosine kinase (PTK) activity in different stages of P. falciparum grown in vitro. Piceatannol inhibited PTK activity in trophozoites and schizonts suggesting that PTK may be important in the initial asexual maturation of the parasite. Inhibition of PTK activity by piceatannol may thus provide new insights into more specific tools for chemotherapeutic interventions for P. falciparum.

Receptor heterogeneity and invasion on erythrocytes by Plasmodium falciparum merozoites in Indian isolates.

Malarial parasites, P. falciparum with different capabilities of invasion on sialic acid deficient erythrocytes have been identified. All three parasite lines (FDL-RI; FSJ-B5; FJB-D2) required sialic acid for invasion. However, parasite FSJ-B5 cultured in neuraminidase treated erythrocytes invaded at 20% efficiency, whereas in the cells treated with neuraminidase and trypsin together, the parasites FDL-R1 and FJB-D2 invaded at less than 10% efficiency. It is therefore suggested that different parasites isolated from different geographical regions of India possess two receptors--one that binds at sialic acid dependent ligand and other that binds in sialic acid independent ligand as demonstrated by ELISA using monoclonals against glycophorin A and glycophorin B. The sialic acid independent ligand may be having different affinities of their receptors for the malarial parasites.