Inhibition of Xanthine-Oxidase by 2,4-Dihydroxy- Benzophenone and 2,3,4-Trihydroxy-Benzophenone BioChem Press Inhibition of Xanthine-Oxidase by 2,4-Dihydroxy- Benzophenone and 2,3,4-Trihydroxy-Benzophenone

Abstract Motivation. Xanthine oxidase is a very important enzyme that catalyzes the oxidation of xanthine to uric acid, which plays a crucial role in gout. The search for new compounds that are capable of inhibiting the activity of XO therefore constitutes a field of investigation that attracts great interest. As a continuation of studies performed on benzophenones, in this work we determine the inhibition of XO by 2,4-dihydroxybenzophenone and 2,3,4-trihydroxybenzophenone. We propose a model for the representation of the active site of XO, which was used to describe the possible enzyme-substrate and enzyme-inhibitor interactions. Method. The IC 50 of the compounds was determined using a UV-visible spectroscopic kinetic method. Basis sets at B3LYP/6-31+G(d) and HF/3-21G were used for performing the calculations that permitted to describe the interactions involving the enzyme, the substrate and the inhibitors. Results. The IC 50 data obtained indicate that 2,4-dihydroxybenzophenone and 2,3,4-trihydroxybenzophenone exert a moderate inhibiting activity on XO, which is comparable to that exhibited by other polyphenolic BPs, but is lower to that of potent inhibitors of XO like allopurinol or quercetin. On the other hand, a model for the active site of XO was proposed, which includes a pentaheteroatomic ring and two extracyclic heteroatoms (S and O). The structures of the enzyme-substrate and enzyme-inhibitor-complex were calculated using this model. Conclusions. The inhibition of XO by 2,4-dihydroxybenzophenone and 2,3,4-trihydroxybenzophenone is produced by a Michaelis-Menten mechanism of the competitive reversible type. It was also established that the inhibiting activity of the compounds is determined by the stability of the enzyme-inhibitor complex and is independent of their acidity constants. Keywords. Xanthine oxidase; inhibiting activity of benzophenones; inhibitor-enzyme interactions; mechanism; ab initio and DFT calculations. Abbreviations and notation