In vitro and in vivo inhibition of thymidylate synthase of human colon cancer by 5-fluorouracil.

Abstract

Thymidylate synthase (TS) is a key enzyme in de novo synthesis of TMP (Fig. 1). TS catalyzes the conversion of dUMP to TMP for which 5, 10-methylenetetrahydrofolate (CH2THF) acts as the methyl-donor; the Km for dUMP is about 1–5 μM (1–5). Inhibition of TS by FdUMP is one of the main mechanisms for the action of 5-fluorouracil (5FU). Without preincubation FdUMP acts as a potent competitive inhibitor of TS with a Ki of about 1 nM. The Km/Ki ratio is about 1000. The inhibition of TS by FdUMP is mediated by the formation of a tight-binding covalent ternary complex of FdUMP with TS and CH THF. Retention of the inhibition of TS is mainly related to the FdUMP/dUMP ratio (6), the FdUMP binding to TS and the stabilization of the ternary complex by CH2 THF or one of its polyglutama-tes (6, 7, 9). In vitro, resistance against 5FU or its analog 5-fluoro-2′-deoxyuridine (FUdR) has been related to altered kinetics of TS with respect to Km values for dUMP and FdUMP binding (2, 5, 8), disturbed folate pools (9), and the level of enzyme before treatment (6, 8). Gene amplification of TS has been demonstrated for FUdR-resistant sub-cell lines (10, 11). Recently evidence for gene amplication has also been obtained in a patient with colon cancer who developed resistance against 5FU (12), while in breast cancer patients binding of FdUMP and the effect of CH2THF decreased during development of resistance (13).KeywordsPyrimidine NucleosidePrimary Colorectal TumorPotent Competitive InhibitorTritium Release AssayFdUMP BindingThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.