Inhibition of xanthine oxidase by various aldehydes

The inactivation of bovine milk xanthine oxidase by various aldehydes has been investigated. For each aldehyde, the inactivation reaction gives rise to a unique molybdenum(V) electron paramagnetic resonance signal from xanthine oxidase (the Inhibited signal). Of the aldehydes tested, only a few (mainly aromatic) failed to undergo this reaction. The g values of the Inhibited signals vary systematically from one aldehyde to another. As the substituents of the alpha-carbon atom become more electron withdrawing, so the gav increases. The inactivation rate depends on both enzyme and aldehyde concentration. Oxygen or another oxidizing substrate is also required for inhibition by 3-pyridinecarboxaldehyde and butyraldehyde but not formaldehyde. Reactivation of xanthine oxidase inhibited by an aldehyde occurs spontaneously after removal of excess aldehyde. For butyraldehyde or 3-pyridinecarboxaldehyde, greater than 95% recovery of activity was observed. The rate of reactivation is dependent both on the nature of the molecule bearing the aldehyde group and on a pK (6.6) of the complex with the enzyme. Evidence is presented that the modifying aldehyde in the Inhibited signal-giving species has (contrary to earlier assumptions) not been oxidized. These results are discussed in relation to the structure of the molybdenum center, and a mechanism for the inhibiting reaction is suggested.