Purification and characterization of recombinant mouse thymidylate synthase

Abstract

Recombinant mouse thymidylate synthase (TS) expressed at high levels in Escherichia coli was purified to homogeneity in greater than 70% yield by a rapid three-step procedure. Both 0.1% Triton X-100 and 10% glycerol were required to stabilize the enzyme whose activity remained unchanged after 1 month when stored at −20°C. Thermal inactivation of the enzyme was a first-order process at 37°C, with t 1 2 values of 6.9, 15.6 and 3.0 min at pH 5.5, 7.0 and 8.5, respectively. The presence of saturating levels of dUMP at pH 8.5 increased the t 1 2 of inactivation to 38 min. The pH profile for enzyme activity showed a narrow optimum region centered at pH 7.0, which was mirrored by the shape of the K m, dUMP/ V max plot. The pH dependence of K d for the covalent inhibitory ternary complex of enzyme, 5-fluoro-2′-deoxyuridylate and 5,10-methylenetetrahydrofolate exhibited a broad minimum between pH 5.5 and 8.5, and ranged between 3.1, 0.8 and 1.1 nM at pH 5.5, 7.0 and 8.5, respectively. The UV/VIS spectrum of the native enzyme exhibited a maximum at 280 nm ( ϵ = 98 200 M −1 cm −1), while that of the inhibitory ternary complex showed an additional maximum at 320 nm. The 19F-NMR spectrum of the mouse enzyme: FdUMP binary complex revealed two new resonances at −2.8 and −34.8 ppm. The most deshielded resonance represented the noncovalent binary complex while the other resonance was assigned to the nucleotide covalently bound to the enzyme. The alteration of nucleotide binding equilibria produced by addition of H 4 folate was exemplified by both an increase in intensity and a 5 ppm deshielding of the resonance attributed to the covalent FdUMP-enzyme complex. Addition of formaldehyde to the latter mixture produced the covalent ternary complex which resulted in the collapse of the resonances at −2.8 and −39.5 ppm and the appearance of a new resonance at −12.4 ppm.