Reactions of PTIO and Carboxy-PTIO with ^.NO, ^.NO ₂, and O₂^.-

Nitronyl nitroxides, such as derivatives of 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl 3-oxide (PTIOs), react with ^.NO to form the corresponding imino nitroxides (PTIs) and ^.NO₂. PTIOs are considered as monitors of ^.NO, stoichiometric sources of ^.NO₂, biochemical and physiological effectors, specific tools for the elimination of ^.NO, and potential therapeutic agents. However, a better understanding of the chemical properties of PTIOs, especially following their reaction with ^.NO, is necessary to resolve many of the reported discrepancies surrounding the effects of PTIOs and to better characterize their potential therapeutic activity. We have generated electrochemically the oxidized and reduced forms of PTIO and carboxy-PTIO (C-PTIO), characterized their absorption spectra, and determined the reduction potentials for the oxoammonium/nitroxide and nitroxide/hydroxylamine couples. The rate constants for the reaction of ^.NO₂ with PTIO and C-PTIO to form the corresponding oxoammonium cations (PTIO⁺s) and nitrite were determined to be (1.5 - 2) × 10⁷ M^-1 S^-1. We have also shown that the reactions of PTIO⁺s with ^.NO form PTIOs and NO₂^-. The rate constants for these reactions are approximately 30-fold higher than those for the reactions of PTIOs with ^.NO or O₂^.-. The present results show that (i) the reaction of PTIOs with ^.NO forms solely PTIs and NO ₂^- where [NO₂^-]/[PTI] varies between 1 and 2 depending on the steady-state concentrations of ^.NO. Consequently, quantitation of 'NO is valid only at sufficiently low fluxes of ^.NO; (ii) the reaction of PTIOs with ^.NO can be used as a valid source of ^.NO₂ only when the latter is effectively scavenged by an appropriate reductant; and (iii) the formation of peroxynitrite cannot be efficiently inhibited by PTIOs even under relatively low fluxes of ^.NO and O₂^.- and millimolar levels of PTIOs.