Purines and pyrimidines in malarial parasites.

In order for the plasmodium malarial parasite to replicate in the human erythrocyte it requires metabolic pathways which are not operative in the host erythrocyte. Thus, the malarial parasite not only synthesizes enzymes for purine salvage and interconversion, for the pyrimidine biosynthetic pathway de novo, and for the folate cycle, but it also alters the host erythrocyte membrane in respect to the transport of purines. Several of the plasmodium enzymes from these pathways have been cloned and these appear to be highly homologous to the corresponding human enzymes. However, enzymes which have been purified from Plasmodium, have demonstrated physicochemical and kinetic differences and may be potential targets for chemotherapy. Inhibition of individual enzymes, such as the dihydroorotate dehydrogenase (DHO-DHase), and inhibition of the inserted pathway from IMP to AMP and IMP to GMP hold considerable promise as chemotherapeutic targets. An entirely new approach in inhibiting malarial growth involves the altered nucleoside transporter in the infected cell membrane through which cytotoxic compounds may be selectively targeted into only the infected cell.