Purification and characterization of dihydrofolate reductase from wild-type and trimethoprim-resistant Mycobacterium smegmatis

Abstract

Dihydrofolate reductase (DHFR) from extracts of Mycobacterium smegmatis strain mc 26 and trimethoprim-resistant mutant mc 226 was purified to homogeneity. In crude extracts, the specific activity of the enzyme from the trimethoprim resistant strain was comparable to that from the sensitive strain. The DHFR from both sources was purified using affinity chromatography on MTX-Sepharose followed by Mono Q FPLC. The enzyme has an apparent molecular mass of 23 kDa from gel filtration on Sephadex G-100 and from SDS-PAGE. Amino terminal sequence analysis showed homology with DHFRs from a subset of other gram-positive organisms. The purified enzyme from the trimethoprim-sensitive organism exhibited K m values for H 2folate and NADPH of 0.68 ± 0.2 μ M and 21 ± 4 μ M, respectively. The K m values for H 2folate and NADPH for the enzyme from the drug-resistant organism were 1.8 ± 0.4 μ M and 5.3 ± 1.5 μ M, respectively. A K cat of 4.5 sec −1 was determined for the DHFR from both sources. The enzyme from both sources was competitively inhibited by pyrimethamine and trimethoprim. The K i value of trimethoprim, for the enzyme from the drug-resistant organism was about six-fold higher than for the enzyme from drug-sensitive strain. Our data suggest that mutation of DHFR contributes to trimethoprim resistance in the mc 226 strain of M. smegmatis.