Thymidylate synthase, mechanism-based inactivation

Trifluridine (5-trifluoromethyl-2 -deoxyuridine) (Viroptic ) is marketed for the topical treatment of herpes simplex virus infection in the eyes. This antiviral drug is a mechanism-based inactivator of thymidylate synthase. The mechanism of inhibition and synthesis of trifluoridine are reported in Chapter 7. 

An interesting dinically useful prodrug is 5-fluorouracil, which is converted in vivo to 5-fluoro-2 -deoxyuridine 5 -monophosphate, a potent irreversible inactivator of thymidylate synthase It is sometimes charaderized as a dead end inactivator rather than a suicide substrate since no electrophile is unmasked during attempted catalytic turnover. Rathei since a fluorine atom replaces the proton found on the normal substrate enzyme-catalyzed deprotonation at the 5 -position of uracil cannot occur. The enzyme-inactivator covalent addud (analogous to the normal enzyme-substrate covalent intermediate) therefore cannot break down and has reached a dead end (R. R. Rando, Mechanism-Based Enzyme Inadivators , Pharm. Rev. 1984,36,111-142). 

Other useful targets for pharmaceutical agents are thymidylate synthase and dihydrofolate reductase, enzymes that provide the only cellular pathway for thymine synthesis (Fig. 22-49). One inhibitor that acts on thymidylate synthase, fluorouracil, is an important chemotherapeutic agent. Fluorouracil itself is not the enzyme inhibitor. In the cell, salvage pathways convert it to the deoxynucleoside monophosphate FdUMP, which binds to and inactivates the enzyme. Inhibition by FdUMP (Fig. 22-50) is a classic example of mechanism-based enzyme inactivation. Another prominent chemotherapeutic agent, methotrexate, is an inhibitor of dihydrofolate reductase. This folate analog acts as a competitive inhibitor the enzyme binds methotrexate with about 100 times higher affinity than dihydrofolate. Aminopterin is a related compound that acts similarly. 

Fluoropropynyl-dUMP (23), a novel mechanism based inhibitor of the A A -methylenetetrahydrofolate-dependent enzyme thymidylate synthase, has been synthesised. This 5-fluoropropynyl-dUMP caused rapid, irreversible inactivation of thymidylate synthase both in the presence and in the absence of the cofactor. 

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