Effects of S-nitrosation on oxygen binding by normal and sickle cell hemoglobin

S-Nitrosated hemoglobin (SNO-Hb) is of interest because of the allosteric c ontrol of NO delivery from SNO-Hb made possible by the conformational diffe rences between the R- and T-states of Rb, To better understand SNO-Hb, the oxygen binding properties of S-nitrosated forms of normal and sickle cell H b were investigated. Spectral assays and electrospray ionization mass spect rometry were used to quantify the degree of S-nitrosation. Hb A(0) and unpo lymerized Hb S exhibit similar shifts toward their R-state conformations in response to S-nitrosation, with increased oxygen affinity and decreased co operativity, Responses to 2,3-diphosphoglycerate were unaltered, indicating regional changes in the deoxy structure of SNO-Hb that accommodate NO addu ction, A cycle of deoxygenation/reoxygenation does not cause loss of NO or appreciable heme oxidation, There is, however, appreciable loss of NO and h eme oxidation when oxygen-binding experiments are carried out in the presen ce of glutathione, These results indicate that the in vivo stability of SNO -Hb and its associated vasoactivity depend on the abundance of thiols and o ther factors that influence transnitrosation reactions. The increased oxyge n affinity and R-state character that result from S-nitrosation of Hb S wou ld be expected to decrease its polymerization and thereby lessen the associ ated symptoms of sickle cell disease.