Dihydrofolate Reductase in Plasmodium lophurae and Duckling Erythrocytes*

SYNOPSIS Dihydrofolate reductase activity in duckling erythrocytes was found to be low, while activity in erythrocytes heavily infected with small uninucleate trophozoites was like that of uninfected erythrocytes. Activity of the enzyme in erythrocytes infected with large multinucleate parasites, however, was greatly increased. This activity was 5 times higher in erythrocyte‐free large trophozoites than in small ones. The dihydrofolate reductase of P. lophurae differed from the host enzyme in: greater molecular weight; higher sensitivity to pyrimethamine inhibition; pH optimum; substrate and cofactor specificity; and stimulation by salts. The parasite enzyme was partially purified by ammonium sulfate precipitation.