Impact of exogenous factor on 5-FU metabolism. Leucovorin (LV) treatment and uridine metabolism are presented in two mechanisms of 5-FU action. The FdUMP, TS, and 5,10, methylene-THF ternary complex is stabilised by the administration of LV (A). Administration of uridine lowers UMP catabolisation; higher amounts of UMP, UDP, and UTP facilitate UTP on FUTP RNA incorporation (B). FdUMP, 5-fluoro-deoxyuridine monophosphate; FUTP, fluorouridine triphosphate; TS, thymidylate synthase; UMP, uridine monophosphate; UDP, uridine diphosphate; UTP, uridine triphosphate.