title: Double mode of inhibition-inducing interactions of 1,4-naphthoquinone with urease: arylation versus oxidation of enzyme thiols.
abstract: In their inhibition-inducing interactions with enzymes, quinones primarily utilize two mechanisms, arylation and oxidation of enzyme thiol groups. In this work, we investigated the interactions of 1,4-naphthoquinone with urease in an effort to estimate the contribution of the two mechanisms in the enzyme inhibition. Jack bean urease, a homohexamer, contains 15 thiols per enzyme subunit, six accessible under non-denaturing conditions, of which Cys592 proximal to the active site indirectly participates in the enzyme catalysis. Unlike by 1,4-benzoquinone, a thiol arylator, the inactivation of urease by 1,4-naphthoquinone under aerobic conditions was found to be biphasic, time- and concentration-dependent with a non-linear residual activity-modified thiols dependence. DTT protection studies and thiol titration with DTNB suggest that thiols are the sites of enzyme interactions with the quinone. The inactivated enzyme had approximately 40% of its activity restored by excess DTT supporting the presence of sulfenic acid resulting from the oxidation of enzyme thiols by ROS. Furthermore, the aerobic inactivation was prevented in approximately 30% by catalase, proving the involvement of hydrogen peroxide in the process. When H2O2 was directly applied to urease, the enzyme showed susceptibility to this inactivation in a time- and concentration-dependent manner with the inhibition constant of H2O2 Ki = 3.24 mM. Additionally, anaerobic inactivation of urease was performed and was found to be weaker than aerobic. The results obtained are consistent with a double mode of 1,4-naphthoquinone inhibitory action on urease, namely through the arylation of the enzyme thiol groups and ROS generation, notably H2O2, resulting in the oxidation of the groups.
authors.affiliation: Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland. krajewsk@chemia.uj.edu
authors.forename: Barbara
authors.initials: B
authors.lastname: Krajewska
authors.forename: Wiesława
authors.initials: W
authors.lastname: Zaborska
mesh_terms: D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D009285:Naphthoquinones / Q000494:pharmacology*; D010084:Oxidation-Reduction; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D014510:Urease / Q000037:antagonists & inhibitors*
chemical_list: D004791:Enzyme Inhibitors; D009285:Naphthoquinones; D013438:Sulfhydryl Compounds; D014510:Urease; C035342:1,4-naphthoquinone