Inactivation of dihydropyrimidine dehydrogenase by 5-iodouracil.

摘要: 5-Iodouracil was a substrate for bovine liver dihydropyrimidine dehydrogenase (DHPDHase) and potent inactivator of the enzyme. NADPH increased rate inactivation thymine protected against inactivation. These findings suggest that 5-iodouracil mechanism-based inactivator. However, dithiothreitol excess enzyme Thus, reactive product, presumably 5-iodo-5,6-dihydrouracil generated through enzymatic reduction 5-iodouracil, released from DHPDHase during processing 5-iodouracil. Since only 18% [6-3H]5-iodouracil reduced by covalently bound to radiolabel not lost solvent as tritium, partition coefficient 4.5. activity completely titrated with 1.7 mol per enzyme-bound flavin. results indicate there 0.31 This suggests were 3.2 flavins active site, which is consistent report multiple enzymic subunit (Podschun, B., Wahler, G., Schnackerz, K. D. (1989) Eur. J. Biochem. 185, 219-224). inactivated 2.1 racemic sites. The stoichiometry nonenzymatically enantiomer calculated be 1. Two radiolabeled fragments isolated tryptic digest amino acid sequences these peptides Asn-Leu-Ser-X-Pro-His Asn-Leu-Ser-X-Pro-His-Gly-Met-Gly-Glu-Arg where X modified containing [6-3H]5-iodouracil. Fast atom bombardment mass spectral analysis smaller peptide yielded protonated parent ion 782 daltons being S-(hexahydro-2,4-dioxo-5-pyrimidinyl)cysteinyl residue.