The case of the missing NO-hemoglobin: Spectral changes suggestive of heme redox reactions reflect changes in NO-heme geometry

When low levels of gaseous nitric oxide (NO) are equilibrated with deoxygenated Hb, all NO added can be accounted for in terms of hexacoordinate and pentacoordinate forms of NO-Hb, despite recent reports on NO disappearance from heme groups to form nitroxyl anions or S-nitrosated Hb at low ratios of NO to Hb. We demonstrate that a fraction of the spectral signature of fully nitrosylated (largely hexacoordinate) Hb disappears as the pentacoordinate state forms and reappears when pentacoordinate NO-Hb is reconverted to the hexacoordinate condition. We show that the spectral changes associated with these reversible shifts in NO-heme geometry can be remarkably well approximated as variations in the contributions from fully nitrosylated Hb and oxidized Hb (MetHb). As a result, increases in the level of pentacoordinate NO-Hb that occur at low NO to Hb ratios can be misinterpreted as increases in MetHb levels associated with NO-dependent heme oxidation. Conversely, any decrease in levels of pentacoordinate NO-Hb can be misinterpreted as a disappearance of MetHb associated with NO-dependent heme reduction. Transitions between pentacoordinate and hexacoordinate forms of NO-Hb with spectral changes suggestive of changes in levels of heme-bound NO are sensitive to the protein's quaternary conformation and can be brought about by alterations in anion levels or the degree of heme saturation with either O-2 or NO.