HNO trapping and assisted decomposition of nitroxyl donors by ferric hemes

The role of nitric oxide (NO) as a signalling molecule in biological systems has been thoroughly studied in the last decades. More recently, there has been an increasing interest in the one-electron reduction product of NO, namely nitroxyl (HNO/NO−). Some studies suggest that nitroxyl can be produced by nitric oxide synthases under certain conditions, and that distinct pharmacological effects are observed for NO and nitroxyl donors. HNO is capable of react with heme proteins, thiols, molecular oxygen, NO and HNO itself. However, only recently the different reactivity patterns are being thoroughly understood. Heme model compounds offer the opportunity to study the reaction kinetics without the complexity arising from ligand interactions with the protein matrix. In this study we analyzed the reaction between the commonly used nitroxyl donors sodium trioxodinitrate and toluene sulfohydroxamic acid, with the ferric model compounds microperoxidase-11 (MP11) and the cationic metalloporphyrin [FeIIITEPyP]5+ (Tetrakis N-ethylpyridinium-2yl porphyne). Our results show that there are two alternative modes of reactivity for nitroxyl donors towards heme in aqueous solutions. The first one comprises the heme assisted decomposition of the donor, enhancing its decomposition rate more than 100-fold. In the second, the donor produces HNO which subsequently reacts with the porphyrin. The observed rate constants (of about 105 M−1 s−1) are consistent with the estimated data for the HNO reaction with heme proteins, and may be controlled by the leaving water ligand. This rate constant probably represents an upper limit for the bimolecular rate constant of HNO towards these proteins.

The reactions between two nitroxyl donors with MP11 and FeIIITEPyP show two alternative modes of reactivity. One comprises the heme assisted decomposition of the donor enhancing its decomposition rate, while in the other the donor produces HNO which subsequently reacts with the porphyrin.Figure optionsDownload as PowerPoint slide