PRESENCE AND PROPERTIES OF DIHYDROFOLATE REDUCTASES WITHIN GENUS CRITHIDIA

1. 1. The properties of dihydrofolate reductases in the non-pathogenic easily cultured trypanosomatid flagellates Crithidia oncopelli and Crithidia fasciculata have been investigated.Similar amounts of reductase activity/mg protein were found in high speed centrifugal supernatant fractions of these organisms after disruption in the Mickle disintegrator. The enzyme in C. oncopelti was highly active between pH values of 5.6 and 7.0 and that in C. fasciculata between pH values of 5.6 and 8.0. Michaelis' constants for the two enzymes were about 3·10-6 M for dihydrofolate and 1·10-5 M for NADPH. Folate, biopterin and dihydrobiopterin (all at concentrations of 10-4 M) were ineffective as substrates for either enzyme; however, folate inhibited both enzymes at this concentration by about 50% while dihydrobiopterin and biopterin were without effect. The enzymes were also inhibited about 50% by methotrexate (6·10-9-8·10-9 M), pyrimethamine (5·10-6 M) and trimethoprin (2·10-5-2·10-4 M). 2. 2. The dihydrofolate reductases in these two flagellates are thus similar in general properties to all such enzymes that have so far been described. 3. 3. Some evidence is presented which suggests that C. oncopelti preparations may contain more than one dihyrofolate reductase. © 1969.