Figure 1 The effect of trifluorothymidine (TFT) on the uptake of [ 3 H]-dT (●), TK (■) and TS (▲) activity. Mpn wild type cells were cultured in the find more presence of [3H]-dT and different concentrations of TFT. The cells were incubated at 37°C for 70 hours and harvested. The total uptake and incorporation of [3H]-dT were analysed, and TK and TS activity were determined in total protein extracts. Expression, purification, and characterization of HPRT The purine analog 6-TG strongly inhibited Mpn growth, which promoted further investigation of potential targets of this compound. HPRT is the first enzyme in the salvage pathway of purine bases
for nucleotide biosynthesis, and is the enzyme responsible for metabolizing 6-TG in human patients treated with this
drug . Mpn HPRT (MPN672) consists of 175 amino acids and shares 29% sequence identity to human HPRT. Mpn HPRT cDNA was cloned and expressed in E. coli. Recombinant Mpn HPRT was expressed as an N-terminal fusion protein with a 6 × histidine tag and a tobacco etch virus (TEV) cleavage site at the N-terminus, and was purified to >98% purity by metal affinity chromatography, see more as assessed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis (data not shown). The purified Mpn HPRT used both hypoxanthine (Hx) and guanine (Gua) as substrates but not adenine or uracil. With Hx as substrate the Selleckchem ABT 263 reaction was linear with time for up to 25 min and the substrate saturation curve was hyperbolic, which indicated that the enzyme followed Michaelis–Menten kinetics with a Km value of 100.1 ± 6.5 μM and Vmax value of 15.8 ± 0.8 μmol min-1 mg-1 (Figure 2A). However, Dolutegravir nmr with Gua as a substrate, the reverse reaction rate was very high and the reaction reached equilibrium in less than 5 min under the same conditions used
for Hx. Therefore, the kinetic study with Gua was conducted differently as described in the experimental procedures. Substrate saturation for Gua exhibited a biphasic curve and therefore, data was fitted using the Hill equation. The Vmax value was 2.7 ± 0.1 μmol min-1 mg-1 and S0.5 was 107.6 ± 6.2 μM with a Hill coefficient of 3.5 (Figure 2B), indicating positive cooperativity with Gua binding. Figure 2 Substrate saturation curves of hypoxanthine (A) and guanine (B) with Mpn HPRT. Kinetic parameters for Hx and Gua were determined by using the DE81 filter paper assay with [3H]-Hx and [3H]-Gua as the labelled substrates as described in the experimental procedures. Data are from at least three independent measurements and are presented as mean ± standard deviation (SD).