Routes to S-nitroso-hemoglobin formation with heme redox and preferential reactivity in the β subunits

Previous studies of the interactions of NO with human hemoglobin have implied the predominance of reaction channels that alternatively eliminate NO by converting it to nitrate, or tightly complex it on the α subunit ferrous hemes. Both channels could effectively quench NO bioactivity. More recent work has raised the idea that NO groups can efficiently transfer from the hemes to cysteine thiols within the β subunit (cysβ-93) to form bioactive nitrosothiols. The regulation of NO function, through its chemical position in the hemoglobin, is supported by response to oxygen and to redox agents that modulate the molecular and electronic structure of the protein. In this article, we focus on reactions in which Fe(III) hemes could provide the oxidative requirements of this NO-group transfer chemistry. We report a detailed investigation of the reductive nitrosylation of human met-Hb, in which we demonstrate the production of S-nitroso (SNO)-Hb through a heme-Fe(III)NO intermediate. The production of SNO-Hb is strongly favored (over nitrite) when NO is gradually introduced in limited total quantities; in this situation, moreover, heme nitrosylation occurs primarily within the β subunits of the hemoglobin tetramer. SNO-Hb can similarly be produced when Fe(II)NO hemes are subjected to mild oxidation. The reaction of deoxygenated hemoglobin with limited quantities of nitrite leads to the production of β subunit Fe(II)NO hemes, with SNO-Hb produced on subsequent oxygenation. The common theme of these reactions is the effective coupling of heme-iron and NO redox chemistries. Collectively, they establish a connectivity between hemes and thiols in Hb, through which NO is readily dislodged from storage on the heme to form bioactive SNO-Hb.