Responses of normal and sickle cell hemoglobin to S-nitroscysteine: implications for therapeutic applications of NO in treatment of sickle cell disease.

Factors which govern transnitrosation reactions between hemoglobin (Hb) and low molecular weight thiols may define the extent to which S-nitrosated Hb (SNO-Hb) plays a role in NO in the control of blood pressure and other NO-dependent reactions. We show that exposure to S-nitrosylated cysteine (CysNO) produces equivalent levels of SNO-Hb for Hb A(0) and sickle cell Hb (Hb S), although these proteins differ significantly in the electron affinity of their heme groups as measured by their anaerobic redox potentials. Dolphin Hb, a cooperative Hb with a redox potential like that of Hb S, produces less SNO-Hb, indicating that steric considerations outweigh effects of altered electron affinity at the active-site heme groups in control of SNO-Hb formation. Examination of oxygen binding at 5-20 mM heme concentrations revealed increases due to S-nitrosation in the apparent oxygen affinity of both Hb A(0) and Hb S, similar to increases seen at lower heme concentrations. As observed at lower heme levels, deoxygenation is not sufficient to trigger release of NO from SNO-Hb. A sharp increase in apparent oxygen affinity occurs for unmodified Hb S at concentrations above 12.5 mM, its minimum gelling concentration. This affinity increase still occurs in 30 and 60% S-nitrosated samples, but at higher heme concentration. This oxygen binding behavior is accompanied by decreased gel formation of the deoxygenated protein. S-nitrosation is thus shown to have an effect similar to that reported for other SH-group modifications of Hb S, in which R-state stabilization opposes Hb S aggregation.