Add time:09/03/2019         Source:sciencedirect.com

A comparison was made of the inhibition of deoxythymidine kinase derived froin Ehrlich ascites carcinoma cells by 5-iodo-2'-deoxyuridine 5'-triphosphate and deoxythymidine 5'-triphosphate. The inhibition produced by the halogenated triphosphate derivative was observed to be qualitatively similar to deoxythymidine 5'-triphosphate over a rangs of pH, temperature and substrate concentrations; however, 5-iodo-2'-deoxyuridine 5'-triphosphate was quantitatively more inhibitory. Although the apparent Km for thymidine was not significantly different at pH 5.0, 6.5 and 7.5, the inhibitory effect of deoxythymidine 5'-triphosphate and 5-iodo-2'-deoxyuridine 5'-triphosphate was 43- and 26-fold larger respectively at pH 5.0 compared to pH 6.5; whereas the inhibitory effect for both compounds at pH 6.5 was about 4-fold greater than that at pH 7.5. In the presence or absence of the inhibitors, analysis indicated “allosteric” kinetics prevailed at pH 5.0 and 6.5, whereas Michaelis competitive kinetics occurred at pH 7.5 for both. These triphosphate derivatives protected thymidine kinase froin thermal inactivation (65°, 30 min); however, the halogenated derivative was effective at lower concentrations. Thus the halogenated triphosphate not only exerted a greater inhibitory effect than deoxythymidine triphosphate, but also afforded greater protection to thermal inactivation.A Hill plot indicated cooperative interactions at pH 5.0 and 6.5 but not at pH 7.5, and the presence of either triphosphate derivative increased the Hill coefficient (n) at pH 5.0 and 6.5 but not at pH 7.5. The order of addition of the substrate (thymidine) or of the inhibitory effectors had no significant effect on the amount of inhibition observed.

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