Bound NO in human red blood cells: fact or artifact?

There has been considerable debate over the nature and chemistry of the interaction between nitric oxide (NO) and red blood cells (RBCs), in particular whether hemoglobin consumes or conserves NO bioactivity. Given the vast range of nitrosation levels reported for human RBCs in the literature, we sought to investigate whether there was a common denominator that could account for such discrepancies across different methodologies and reaction conditions and if such a pathway may exist in physiology. Here, we show that there are marked differences in reactivity toward NO between human and rat hemoglobin, which offers a mechanistic explanation for why basal levels of NO-adducts in primate RBCs are considerably lower than those in rodents. We further demonstrate that the inadvertent introduction of trace amounts of nitrite and incomplete thiol alkylation lead to rapid heme and thiol nitros(yl)ation, with generation of nitrosylhemoglobin (NOHb) and S-nitrosohemoglobin (SNOHb), while neither species is detectable in human RBCs at physiological nitrite concentrations. Thus, caution should be exercised in interpreting experimental results on SNOHb/NOHb levels that were obtained in the absence of knowledge about the degree of nitrite contamination, in particular when a physiological role for such species is implicated.