Dihydrofolate Reductase from Pyrimethamine-resistant Plasmodium berghei

An investigation of the H2-folate reductases from pyrimethamine-sensitive (Pb/WLTM) and -resistant (Pb/WLTM/50–63) strains of Plasmodium berghei has revealed that the specific activity of the enzyme from the resistant strain is 10-fold higher than that of the sensitive strain. Since turnover numbers (based on amethopterin binding) are equivalent, the increase in specific activity must be due to an increased number of catalytic sites. Marked differences were found between the two enzymes in several of the properties studied. The Km value for H2-folate is 12.4-fold higher for the enzyme from the resistant strain. The Pb/WLTM/50–63 H2-folate reductase is only slightly stimulated by KCl, in contrast to the 3-fold stimulation observed with the enzyme from the sensitive strain. The Ki values for pyrimethamine and two other antifolates are higher for the enzyme from the resistant strain. In addition, the inhibition is competitive with H2-folate for the enzyme from Pb/WLTM and noncompetitive for the enzyme from Pb/WLTM/50–63. It is proposed that Pb/WLTM/50–63 is resistant in vivo to the action of pyrimethamine because of the combined effects of the increase in enzyme content and the decrease in inhibitor binding. It is suggested that these alterations in enzymatic properties and enzyme levels reflect mutations in the gene or genes responsible for the coding of this enzyme.