Allosteric effects on oxidative and nitrosative reactions of cell‐free hemoglobins

Abstract

SummaryA review of the oxidative and nitrosative reactions of cell-free hemoglobin-based oxygen carriers (HBOCs) shows that these reactions are intimately linked and are subject to allosteric control. Cross-linking reactions used to produce HBOCs introduce conformational constraints and result in Hbs with reduced responses to heterotropic and homotropic allosteric effectors. The Nernst plots of heme oxidation of cross-linked HBOCs are shifted to higher potentials relative to unmodified Hb in the absence of allosteric effectors, in accord with their T-state stabilization and right-shifted Hill plots of O 2 binding. They exhibit enhanced rates of autoxidation and nitrite-induced oxidation, features that appear due to their having more solvent-accessible heme pockets. The stability of their NO-Hb derivatives varies as a result of allosteric effects on the extent of formation of pentacoordinate NO-heme geometry by a chains and subsequent oxidation of partner b chains. The physiological implications of these findings on the safety, efficacy and design of second generation HBOCs are discussed in the framework of a reaction scheme showing linkages between Hbmediated redox reactions. These redox reactions can drive formation of SNO-Hb and other reactive species and are of significance for the use of cell-free Hbs in vivo.

Nitrite Reductase Activity of Hemoglobin S (Sickle) Provides Insight into Contributions of Heme Redox Potential Versus Ligand Affinity

“…In both Hb and Mb these two properties are intrinsically coupled: as ligand affinity (P 50 ) increases due to conversion to R-state, the heme redox potential (E1 ⁄ 2 ) decreases. Recently, the half-life of the reaction of nitrite with HbA and modified hemoglobins developed as potential blood substitutes was shown to decrease (so the reaction gets faster) as the redox potential increased (31). Based on these data the authors concluded that ligand affinity, rather than redox potential, is the major determinant of the nitrite/Hb rate in these samples.…”

Molecular Controls of the Oxygenation and Redox Reactions of Hemoglobin

“…The results in Table 1 also show that inorganic anions and inositol hexaphosphate (IHP), effectors that lower Hb's O 2 affinity, exert opposite effects on the rate of nitrite-induced oxidation. These results show that the conformation and reactivity of Hb in its physiologically important redox reactions can be variably affected by the allosteric effects of anions, and by constraints induced by cross-linking reactions (18).…”

“…The reduction occurs at different rates for varied forms of HBOCs. Notably, when the oxidized forms of Bis(3, 5-dibromosalicyl)fumarate (DBBF)-Hb and a bovine polymerized Hb (OxyglobinÔ) were rapidly mixed with ascorbate, reduction of ferryl Hb back to ferric by ascorbate was found to be significantly faster for Oxyglobin (t 1/2 = 0.018 s) than for DBBF (t 1/2 = 0.13 s) (18). These findings offer important implications for clinical therapies for conditions where cell-free Hbs are present at abnormally high levels, or for improved conditions for injection of HBOCs.…”

“…For example, as shown in Table 1, the rates of NO production via the reaction of nitrite with deoxy cross-linked Hbs are faster than those of unmodified deoxy Hb, in spite of the fact that they have lowered O 2 affinity and have redox potentials which are indicative of reduced ease of oxidation. The faster reactions of the crosslinked Hbs with nitrite appear to arise as a result of their having more open heme pockets, which facilitates the access of nitrite to the heme sites (18).…”

The functional nitrite reductase activity of the heme-globins

“…Recently, the half-life of the reaction of nitrite with deoxyhemoglobin and modified hemoglobins developed as potential blood substitutes was shown to decrease (reaction getting faster) as the redox potential increased. 27 Based on these data, the authors concluded that ligand affinity, rather than redox potential, is the major determinant of the nitrite-deoxyhemoglobin reaction rate in these samples. In another study, the rate that deoxygenated normal adult hemoglobin, HbA 0 , reacts with nitrite was compared with the rate that sickle cell hemoglobin, HbS, reacts with nitrite.…”