Mechanism-based inactivation of thymine hydroxylase, an .alpha.-ketoglutarate-dependent dioxygenase, by 5-ethynyluracil

5-Ethynyluracil was shown to be a mechanism-based inactivator of thymine 7-hydroxylase, with Ki = 22 micromolar and a k2 = 2.6 min-1. Inactivation resulted in covalent modification of the enzyme with a stoichiometry of approximately 1 adduct/enzyme molecule. The reaction of thymine 7-hydroxylase with 5-ethynyluracil also generated two products: 5-carboxyuracil and uracil-5-acetylglycine. The enzyme adduct was stable at pH 2, 8, and 10 and stable to treatment with hydroxylamine. Following trypsin digestion of labeled enzyme, two labeled peptides corresponding to 45% of the adduct were isolated and sequenced. The results demonstrated the presence of a single modified amino acid. Tandem mass spectrometry suggested that the modified amino acid is tyrosine, which is linked to the inhibitor in an unprecedented fashion