Mechanism of HRP-catalyzed nitrite oxidation by HO revisited: Effect of nitroxides on enzyme inactivation and its catalytic activity.

The peroxidative activity of horseradish peroxidase (HRP) undergoes progressive inactivation while catalyzing the oxidation of nitrite by HO. The extent of inactivation increases as the pH increases, [nitrite] decreases or [HO] increases, and is accompanied by a loss of the Soret peak of HRP along with yellow-greenish coloration of the solution. HRP-catalyzed nitrite oxidation by HO involves not only the formation of compounds I and II as transient heme species, but also compound III, all of which in turn, oxidize nitrite yielding NO. The rate constant of nitrite oxidation by compound III is at least 10-fold higher than that by compound II, which is also reducible by NO where its reduction by nitrite is the rate-determining step of the catalytic cycle. The extent of the loss of the Soret peak of HRP is lower than the loss of its peroxidative activity implying that deterioration of the heme moiety leading to iron release only partially contributes toward heme inactivation. Cyclic stable nitroxide radicals, such as 2,2,6,6-tetramethyl-piperidine-N-oxyl (TPO), 4-OH-TPO and 4-NH-TPO at µM concentrations detoxify NO thus protecting HRP against inactivation mediated by this radical. Hence, HRP inactivation proceeds via nitration of the porphyrin ring most probably through compound I reaction with NO, which partially leads to deterioration of the heme moiety. The nitroxide acts catalytically since its oxidation by NO yields the respective oxoammonium cation, which is readily reduced back to the nitroxide by HO, superoxide ion radical, and nitrite. In addition, the nitroxide catalytically inhibits tyrosine nitration mediated by HRP/HO/nitrite reactions system as it efficiently competes with tyrosyl radical for NO. The inhibition by nitroxides of tyrosine nitration is demonstrated also in the case of microperoxidase (MP-11) and cytochrome c revealing an additional role played by nitroxide antioxidants.