Enzymatic function of hemoglobin as a nitrite reductase that produces NO under allosteric control

Abstract

Hypoxic vasodilation is a fundamental, highly conserved physiological response that requires oxygen and/or pH sensing coupled to vasodilation. While this process was first characterized more than 80 years ago, the precise identity and mechanism of the oxygen sensor and mediators of vasodilation remain uncertain. In support of a possible role for hemoglobin (Hb) as a sensor and effector of hypoxic vasodilation, here we show biochemical evidence that Hb exhibits enzymatic behavior as a nitrite reductase, with maximal NO generation rates occurring near the oxy-to-deoxy (R-to-T) allosteric structural transition of the protein. The observed rate of nitrite reduction by Hb deviates from second-order kinetics, and sigmoidal reaction progress is determined by a balance between 2 opposing chemistries of the heme in the R (oxygenated conformation) and T (deoxygenated conformation) allosteric quaternary structures of the Hb tetramer -the greater reductive potential of deoxyheme in the R state tetramer and the number of unligated deoxyheme sites necessary for nitrite binding, which are more plentiful in the T state tetramer. These opposing chemistries result in a maximal nitrite reduction rate when Hb is 40-60% saturated with oxygen (near the Hb P 50 ), an apparent ideal set point for hypoxiaresponsive NO generation. These data suggest that the oxygen sensor for hypoxic vasodilation is determined by Hb oxygen saturation and quaternary structure and that the nitrite reductase activity of Hb generates NO gas under allosteric and pH control.

Plasma methemoglobin as a potential biomarker of anemic stress in humans

“…The proposed mechanism for increased MetHb may include NO-mediated Hb oxidation or reduction of nitrite (NO 2 -) by DeoxyHb, as suggested by experimental and clinical studies. 3,[18][19][20][21][22] It will be important to assess these changes in subsequent studies to determine if the increase in MetHb is reflective of activated adaptive mechanisms (nNOS) 7 and/ or increased OxyHb desaturation to DeoxyHb and tissue hypoxia [18][19][20][21][22] during acute hemodilutional anemia.…”

“…2). [18][19][20][21][22] In both cases, the increase in MetHb may signify activation of adaptive mechanisms, likely triggered by inadequate oxygen delivery and tissue hypoxia.…”

“…HO -. [18][19][20][21][22] In either case, MetHb may be a biomarker of increased NO production and/or decreased oxygen saturation in local vascular beds. These proposed mechanisms may be particularly favoured during anemia since local production of NO would be supported by an increase in tissue oxyHb extraction which is known to occur in anemic vascular beds.…”

Cardioprotection and mitochondrial S-nitrosation: Effects of S-nitroso-2-mercaptopropionyl glycine (SNO-MPG) in cardiac ischemia–reperfusion injury

“…Such a mechanism may underlie the cardioprotective effects of diverse agents such as SNO-MPG, NO • donors [16], and nitrite [21,50,[85][86][87]. Mitochondria were treated with GSNO or SNO-MPG at the concentrations given on the x-axis, as detailed in the methods.…”

The role of β93 Cys in the inhibition of Hb S fiber formation

“…These and other findings have led to an alternate model, in which Hb acts as a nitrite reductase and nitrite is a storage pool for NO. In this model deoxy R-state hemes exhibit greater reductase activity than deoxy T-state hemes, leading to maximal rates of NO production when Hb is partially saturated with O 2 to a level of 40-60% [22][23][24][25]. Release of NO under these conditions leads to vasodilation; thus, NO release and vasodilation are intrinsically coupled to the allosteric state of Hb.…”