Mechanism and Mechanism-Based Inactivation of 4-Hydroxyphenylpyruvate Dioxygenase

Article abstract of DOI:10.1006/bioo.1994.1028

Six substrate analogs of 4-hydroxyphenylpyruvate, specifically pentafluorophenylpyruvate, 4-hydroxytetrafluorophenylpyruvate, 2-thienylpyruvate, 3-thienylpyruvate, thiophenol oxalate, and p-thiocresol oxalate were synthesized and their interactions with porcine liver 4-hydroxyphenylpyruvate dioxygenase investigated.Both pentafluorophenylpyruvate and thiophenol oxalate are competitive inhibitors of the enzyme with K1 values of 14 and 150 μM, respectively, but p-thiocresol oxalate has no effect on the enzymic activity.The other three substrate analogs are both substrates and mechanism-based inactivators of the enzyme with the following kinetic characteristics (compound, K_m, V_max,k_inact, K', partition ratio) at pH 6.0, 37 deg C, and an air atmosphere: 4-hydroxytetrafluorophenylpyruvate, 50 μM, 1.9 mkat/kg, 1.5/min, 70 μM, 4.2; 2-thienylpyruvate, 500μM, 7.8 mkat/kg, 0.6/min, 400 μM, 41; 3-thienylpyruvate, 250 μM, 2.9 mkat/kg, 0.6/min, 300 μM, 22.When inactivated, the dioxygenase was found to contain per mole of active enzyme, 0.78 mol of label from 3-thienyl-3<3H>pyruvate and 0.85 mol of label from 4-hydroxytetrafluorophenyl-3<3H>pyruvate.The product formed from the enzyme-catalyzed oxidation of 3-thienylpyruvate was determined to be 3-carboxymethyl-3-thiolene-2-one.The implication of these results to the mechanism of the dioxygenase is considered.