Effect of some physical features and of amino acid substitutions on the mechanical precipitation of hemoglobin

Role of Nitric Oxide Carried by Hemoglobin in Cardiovascular Physiology: Developments on a Three-Gas Respiratory Cycle

A continuous supply of oxygen is essential for the survival of multicellular organisms. The understanding of how this supply is regulated in the microvasculature has evolved from viewing erythrocytes (red blood cells [RBCs]) as passive carriers of oxygen to recognizing the complex interplay between Hb (hemoglobin) and oxygen, carbon dioxide, and nitric oxide-the three-gas respiratory cycle-that insures adequate oxygen and nutrient delivery to meet local metabolic demand. In this context, it is blood flow and not blood oxygen content that is the main driver of tissue oxygenation by RBCs. Herein, we review the lines of experimentation that led to this understanding of RBC function; from the foundational understanding of allosteric regulation of oxygen binding in Hb in the stereochemical model of Perutz, to blood flow autoregulation (hypoxic vasodilation governing oxygen delivery) observed by Guyton, to current understanding that centers on S-nitrosylation of Hb (ie, S-nitrosohemoglobin; SNO-Hb) as a purveyor of oxygen-dependent vasodilatory activity. Notably, hypoxic vasodilation is recapitulated by native S-nitrosothiol (SNO)-replete RBCs and by SNO-Hb itself, whereby SNO is released from Hb and RBCs during deoxygenation, in proportion to the degree of Hb deoxygenation, to regulate vessels directly. In addition, we discuss how dysregulation of this system through genetic mutation in Hb or through disease is a common factor in oxygenation pathologies resulting from microcirculatory impairment, including sickle cell disease, ischemic heart disease, and heart failure. We then conclude by identifying potential therapeutic interventions to correct deficits in RBC-mediated vasodilation to improve oxygen delivery-steps toward effective microvasculature-targeted therapies. To the extent that diseases of the heart, lungs, and blood are associated with impaired tissue oxygenation, the development of new therapies based on the three-gas respiratory system have the potential to improve the well-being of millions of patients.

Hb Shelby [beta 131(H9)Gln-->Lys] in association with Hb S [beta 6(A3)Glu-->Val]: characterization, stability, and effects on Hb S polymerization

When first tested for abnormal hemoglobins, a 2-year-old boy, appeared to have Hb F, Hb S and Hb A2. Confirmatory testing revealed a beta chain variant inherited from his father and beta S from his mother. Analysis of tryptic peptides in conjunction with automated DNA sequence analysis showed that the variant hemoglobin was Hb Shelby [beta 131(H9)Gln-->Lys (CAG-->AAG)]. Heat and mechanical stabilities of various liganded Hb Shelby tetramers were compared to those of Hb A and Hb S. Oxy-Hb Shelby precipitated more readily than oxy-Hb A, but was much more stable than oxy-Hb S during mechanical agitation. In contrast, oxy-Hb Shelby was much less stable than oxy-Hb A and oxy-Hb S following heat treatment. Met-Hb Shelby was most unstable compared to other liganded forms of Hb Shelby, while deoxy- and carbonmonoxy-forms of Hb Shelby showed similar heat-induced precipitation rates. These data indicate that heat instability of Hb Shelby is accompanied by heme oxidation, and that denaturation by mechanical agitation occurs in the absence of heme oxidation. Hb Shelby, like Hb A, can form hybrids with Hb S which participate in polymer formation in vitro. However, Hb S/Hb Shelby hybrids copolymerized with Hb S less than A/S hybrids. Since the patient's MCHC value is normal, this finding coupled with the elevated Hb A2 and Hb F levels, both of which are known to inhibit polymerization of Hb S, may contribute to the patient's mild clinical presentation.

Effects of beta 6 amino acid hydrophobicity on stability and solubility of hemoglobin tetramers

The relationship between different amino acids at the beta 6 position of hemoglobin and tetramer stability was addressed by a site-directed mutagenesis approach. Precipitation rates during mechanical agitation of oxyhemoglobins with Gln, Ala, Val, Leu and Trp at the beta 6 position increased 2, 5, 13, 21 and 53 times, respectively, compared with that for Hb A. There was a linear relationship between the log of the precipitation rate constant and amino acid hydrophobicity at the beta 6 position, suggesting that enhanced precipitation of oxy Hb S during mechanical agitation results in part from increased hydrophobicity of beta 6 Val. Deoxyhemoglobin solubility increased in the order of beta 6 Ile, Leu, Val, Trp, Gln, Ala and Glu suggesting that hydrophobic interactions between beta 6 Val and the acceptor site of another hemoglobin molecule during deoxy-Hb S polymerization not only depend on hydrophobicity but also on stereospecificity of the amino acid side chain at the beta 6 position. Furthermore, our results indicate that hydrophobic amino acids at the beta 6 position which promote tetramer instability in the oxy form do not necessarily promote polymerization in the deoxy form.