Nitric oxide (NO) is known to possess antiparasitic activity towards Plasmodium species. Parasite proteases are currently considered to be promising targets for antima-larial chemotherapy. In the present study, we have studied the inhibitory effect of NO on the activity of plasmepsin in Plasmodium vivax, the pepsin-like aspartic protease which is believed to be involved in the cleavage during hemoglobin degradation in Plasmodium falciparum. NO donors (±) (E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR-3), S-nitrosoglutathione (GSNO), and sodium nitroprusside (SNP) were found to inhibit this plasmepsin activity in a dose-dependent manner in purified P. vivax aspartic protease enzyme extracts. This inhibitory effect may be attribut-able to the nitrosylation of the cysteine residue at the catalytic site. However, an inhibitor of aspartic protease activity, namely pepstatin, was also found to inhibit (IC₅₀3 μM) the enzyme activity, which we have used as a positive control. Our results therefore provide novel insights into the pathophysiological mechanisms, and will be useful for designing strategies for selectively upregulating NO production in P. vivax infections for antimalarial chemotherapy and also biochemical adaptations of the malaria parasite for survival in the host erythrocytes with a better understanding of the protease substrate interactions.

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