Resistance of Pediococcus cerevisiae to amethopterin as a consequence of changes in enzymatic activity and cell permeability. I. Dihydrofolate reductase, thymidylate synthetase and formyltetrahydrofolate synthetase in amethopterin-resistant and -sensitive strains of Pediococcus cerevisiae.

Pediococcus cerevisiae/AMr, resistant to amethopterin, possesses a higher dihydrofolate reductase (5, 6, 7, 8-tetrahydrofolate: NADP+ oxidoreductase, EC 1.5.1.3) activity than the parent, a folate-permeable and thus amethopterin-susceptible strain and than the wild-type. The properties of dihydrofolate reductase from the three strains have been compared. Temperature, pH optima, heat stability, as well amethopterin binding did not reveal significant differences between the enzymes from the susceptible and resistant strains. The enzyme from the wild-type was 10 times more sensitive to inhibition by amethopterin and more susceptible to heat denaturation. The apparent Km values for dihydrofolate in enzymes from the three strains were in the range of 4.8--7.2 muM and for NADPH 6.5--8.0 muM. The amethopterin-resistant strain exhibited cross-resistance to trimethoprim and was about 40-fold more resistant to the latter than the sensitive parent and the wild-type. The resistance to trimethoprim appears to be a direct result of the increased dihydrofolate reductase activity. Inhibition of dihydrofolate reductase activity by this drug was similar in the three strains. 10--20 nmol caused 50% inhibition of 0.02 enzyme unit. Trimethoprim was about 10 000 times less effective inhibitor of dihydrofolate reductase than amethopterin. The cell extract of the AMr strain possessed a folate reductase activity three times higher than that of the sensitive strain. The activities of other folate-related enzymes like thymidylate synthetase and 10-formyltetrahydrofolate synthetase (formate: tetrahydrofolate ligase (ADP-forming), EC 6.3.4.3) were similar in the three strains studied.