The State of the Nitric Oxide Cycle in Respiratory Tract Diseases

Scheme 1

O₂ complex with hemoglobin (Hb-O₂) and other heme proteins oxidize it to nitrate anion (NO₃^-) (1); when interacting with superoxide (О₂^-), toxic peroxynitrite (OONO^-) is formed (2). In the absence of other reactive molecules, they are combined to form NO₃^- and CO₂. But in the presence of reacting molecules, for example, tyrosine, NO₃^- interacts with the resulting tyrosyl radical () with the formation of nitrotyrosine (3 and 4). Small amounts of NO are reduced to N₂O. NO is oxidized to NO₂ dioxide, which gives higher oxides of nitrogen (N₂O₃ and N₂O₄) and many other reactions [8] (5). Thus, nitric oxide is able to interact with thiol groups (R-SH) and with iron-containing complexes, which play a leading role in the transportation and deposition of NO (6) [9].