Biochemical factors affecting the tightness of 5-fluorodeoxyuridylate binding to human thymidylate synthetase

Metabolic Resistance: The Protection of Enzymes against Drugs which are Tight‐Binding Inhibitors by the Accumulation of Substrate

“…The solid line represents the best fit to the experimental data as determined by nonlinear regression using the CRICF computer program [9] and it was from this analysis that the Ki value of 6.2 nM was derived Table 1. The arrows indicate the enzyme concentration (0.14 pM) and the dotted lines are the asymptotes calculated from Eqn (6) inhibition is shown by zUMP with no curvature visible in the plot of Ai versus IT (Fig. 2a) due to its relatively high Ki value of 0.51 pM.…”

“…Under these conditions, Ai is a linear function of IT and the calculated slopes and intercepts on the abscissa of the asymptote for each drug-target-enzyme interaction are included in Table 1. These slopes and intercepts have predictive value: if the cellular concentration of the inhibitor is known, the concentration of substrate (Ai) necessary to overcome the blockade can be calculated by substituting the values for the slope (A,/Ai) and intercept (E;--Ki), from Table1 into Eqn (6). Naturally, a more accurate simulation of metabolic resistance will be obtained if values for ET, Ki, X, A , are For the trifunctional protein hOro synthetase, the local concentration of Cbm-P at the aspartate transcarbamoylase site is 2.2-fold higher than that prevailing in the bulk solvent [4].…”

“…Allowance must therefore be made for the reduction of free inhibitor concentration which occurs as a result of its binding to the enzyme [2]. In general, it is tight-binding inhibitors which are of therapeutic value, but no quantitative relationship equivalent to Eqn (2) has been developed for this more complex case although several authors [5,6] have acknowledged the importance of substrate accumulation in response to drug administration in vivo. In this paper, we present the equation which describes metabolic resistance to tight-binding inhibitors and demonstrate the use of this equation in describing published experimental data [4] for the interaction of PAcAsp with aspartate transcarbamoylase.…”

Oral fluoropyrimidine S-1 combined with leucovorin is a promising therapy for colorectal cancer: Evidence from a xenograft model of folate-depleted mice

“…Of note, we found that the co-administration of LV continued to enhance TS inhibition at 24 h after treatment. This finding is supported by the observation that a much greater amount of ternary complex was formed at 24 h after the single administration of S-1/LV than after the administration of S-1 alone, and can be explained by the increased levels of intratumoral reduced folate, which possibly stabilized the ternary complex (14,(24)(25)(26)(27). On the other hand, the accumulation of ternary complex in tumor cells after fluoropyrimidine treatment is reportedly capable of inducing TS translation (28).…”

Escherichia coli thymidylate synthase expression protects human cells from the cytotoxic effects of 5-fluorodeoxyuridine more effectively than human thymidylate synthase overexpression

“…26 -29 In addition, studies using purified forms of either hTS or bacterial TS reported that the presence of high levels of reduced folate cofactor enhanced FdUMP binding to TS by slowing the rate of dissociation of the ternary complex. 30,31 Therefore, the potentiation of FdUrd toxicity in VAHGC cells by LV can be explained by the enhanced binding of FdUMP to TS in those cells. This hypothesis is supported by the enhanced inhibition of extractable TS catalytic activity in VAHGC cell lysates by LV (Table 2).…”