Reactions of PTIO and Carboxy - PTIO with

Nitronyl nitroxides, such as derivatives of 2-phenyl4,4,5,5,-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) 10 M 1 s . 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 NO2 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.