10-Formyltetrahydrofolate Synthetase Activity in Normal and Brugia pahangi-Infected Aedes aegypti

Abstract

The activity of 10-formyltetrahydrofolate synthetase (f1oFHS) in crude extracts of 4- to 5-day-old adult male and uninfected female Aedes aegypti was higher than that of other folate-related enzymes studied so far. L-Cysteine, Mg2?, and NH4? were required for optimal activity of this enzyme in the forward direction (forming f1oFH4, ADP and P1). The apparent Km values of the substrates and cofactors for the forward reaction showed correspondence between ATP and Mg2' (both 0.07 mM) and formate and NH? (13 mM and 7 mM); the apparent Km value of FH, (0.26 mM) corresponded closely to the Ki value of the 4-amino analog of folate methotrexate (0.24 mM), the latter being a competitive inhibitor of the enzyme. The estimated molecular weight of A. aegypti PfoFHS was 260,000. Two compounds were relatively strong inhibitors of the enzyme, as indicated by their IDo50 value: metho- trexate (2 CM) and suramin (8 /tM). The activity of PoFHS was approximately 40% lower in unin- fected female mosquitoes more than a week old than in those only 5 days old. The activity of this enzyme in female mosquitoes 7 days after they fed upon Brugia pahangi-infected jirds was approximately the same as that in age-matched uninfected females. However, the activity of fpoFHS in female mos- quitoes 12 days after infection with B. pahangi was 50% lower than that in age-matched uninfected fe- males. The significance of this latter finding is discussed.