5-Fluoro-2'-deoxyuridylate: covalent complex with thymidylate synthetase.

5-Fluoro-2'-deoxyuridylate causes a rapid inactivation of thymidylate synthetase that is dependent upon prior complexation of the cofactor 5,10-methylenetetrahydrofolate. The enzyme-5-fluoro-2'-deoxyuridylate complex may be isolated on nitrocellulose membranes and is not disrupted by 6 M urea. Upon reaction of 5-fluoro-2'-deoxyuridylate with the enzyme in the presence of 5,10-methylenetetrahydrofolate a rapid loss of absorbance is observed at 269 nm, the absorption maximum for the pyrimidine chromophore. It is concluded that a covalent bond is formed between the 6-position of 5-fluoro-2'-deoxyuridylate and a nucleophilic group of the enzyme that is involved in catalysis.