title: Determination of intra- and intermolecular tritium isotope effects in the reaction of thymine 7-hydroxylase.
abstract: Different [7-3H]thymine preparations have been used to determine the inter- and intramolecular isotope effects of the 2-oxoglutarate-dependent thymine hydroxylation, catalyzed by thymine 7-hydroxylase (thymine, 2-oxoglutarate:oxygen oxidoreductase, EC 1.14.11.6). Specific activity ratios of products, viz., 3H2O and 5-hydroxymethyluracil, and remaining substrate to initial substrate have been determined. The influence on these ratios of intra- and intermolecular isotope effects at different degrees of tritium substitution has been analyzed. An intramolecular isotope effect with a kH/kT of about 6.5 has been found. No intermolecular isotope effect of TV/K could be detected when oxygen concentration was varied from 0.4 to 0.01 mM. This agrees with a mechanism in which 2-oxoglutarate is irreversibly changed before the bond-breaking in thymine takes place.
mesh_terms: D007700:Kinetics; D008433:Mathematics; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009492:Neurospora crassa / Q000201:enzymology; D011865:Radioisotope Dilution Technique; D014316:Tritium
chemical_list: D014316:Tritium; D006899:Mixed Function Oxygenases; C023164:thymine, 2-oxoglutarate dioxygenase