Magnetic and spectral properties of carp carbonmonoxyhemoglobin. Competitive effects of chloride ions and inositol hexakisphosphate

Investigating the magnetic susceptibility properties of fresh human blood for noninvasive oxygen saturation quantification

Quantification of blood oxygen saturation on the basis of a measurement of its magnetic susceptibility demands knowledge of the difference in volume susceptibility between fully oxygenated and fully deoxygenated blood (Δχ(do) ). However, two very different values of Δχ(do) are currently in use. In this work we measured Δχ(do) as well as the susceptibility of oxygenated blood relative to water, Δχ(oxy) , by MR susceptometry in samples of freshly drawn human blood oxygenated to various levels, from 6 to 98% as determined by blood gas analysis. Regression analysis yielded 0.273 ± 0.006 and -0.008 ± 0.003 ppm (cgs) respectively, for Δχ(do) and Δχ(oxy) , in excellent agreement with previous work by Spees et al. (Magn Reson Med 2001;45:533-542).

Rapid Exchange of Fluoroethylamine via the Rhesus Complex in Human Erythrocytes: 19F NMR Magnetization Transfer Analysis Showing Competition by Ammonia and Ammonia Analogues

A remarkable recent discovery in red blood cell function is that the Rhesus antigen complex that for so long was considered to be simply a means of cell recognition is also the ammonia transporter. It catalyzes transmembrane exchange of ammonia on the subsecond time scale, and yet because of a lack of rapid-exchange methodology its kinetics had not been characterized. The flux of ammonia varies appreciably in diverse clinical states, and a convenient method for its characterization would be of basic and of clinical diagnostic value. Fluoroethylamine is water-soluble and when added to a suspension of human red blood cells (RBCs) displays the experimentally useful property of giving separate 19F NMR spectral peaks for the populations inside and outside the cells. By using two-site, one-dimensional magnetization exchange spectroscopy (1D-EXSY), the transmembrane exchange of fluoroethylamine was measured; it was found to occur on the subsecond time scale with an apparent first-order rate constant for efflux, under the equilibrium exchange conditions, of 3.4 s(-1). The method was used to characterize the concentration, temperature, and pH dependence of the exchange rate constant. We determined the extent of competitive inhibition exhibited by ammonia and two molecules that contain an amine group (ethylamine and methylamine). Inhibition of the exchange by incubating the suspension with anti-RhAG antibody, and no inhibition by anti-RhD antibody, suggested specificity of exchange via the RhAG protein of the Rh complex.

New insights into the proton-dependent oxygen affinity of Root effect haemoglobins

A long-standing puzzle with regard to protein structure/function relationships is the proton-dependent modification of haemoglobin (Hb) structure that causes oxygen to be unloaded from Root effect Hbs into the swim bladders and eyes of fish even against high oxygen pressure gradients. Although oxygen unloading in Root effect Hbs has generally been attributed to proton-dependent stabilization of the T-state, protonation of Root effect Hbs can alter their ligand affinities in both R- and T-state conformations and either stabilize the T-state or destabilize the R-state. The C-terminal residues that are so important in the Bohr effect of human Hb appear to be involved in the Root effects of some fish Hbs and not in others, indicating that several evolutionary pathways have resulted in expression of highly pH-dependent Hbs. New data are presented that show surprising similarities in the pH- and anion-dependence of sulfhydryl group reactivity and anaerobic oxidation of human and fish Hbs. The available evidence supports the concept that in both Bohr effect and Root effect Hbs a large steric component acts in addition to quaternary shifts between R and T conformations to regulate ligand affinity. Allosteric effectors moderate these steric effects within both R- and T-state conformations and allow for an elegant match between Hb function and the wide-ranging physiological needs of diverse organisms.

Isolation and stability of partially oxidized intermediates of carp hemoglobin: kinetics of CO binding to the mono- and triferric species

The monoliganded and triliganded forms of the asymmetric valency hybrids of carp hemoglobin were isolated using high-performance liquid chromatography. These partially oxidized hybrids were shown to be sufficiently stable to permit the measurement of the kinetics of CO binding. The effects of protons and inositol hexaphosphate on the rates of these reactions were examined. The kinetics of CO recombination with these partially oxidized derivatives were compared to the kinetics of CO binding to the fully ferrous molecule. To a first approximation, the kinetic behavior of the monoferric derivative was consistent with a small shift in the T<==>R equilibrium in favor of the R state. The presence of three ferric ligands resulted in a still greater shift in the conformational equilibrium in favor of the R state. The kinetic behavior of the triferric molecule was similar, but not identical, to that of a fully ferrous molecule which is triliganded with CO. The properties of both asymmetric valency hybrids were responsive to the nature of the ligand; i.e., the rate of CO binding was increased more by the presence of cyanide on the ferric hemes than by water. Not all of the data could be accommodated within the two-state model. For example, there was evidence of an altered T state in the case of the tricyanomet derivative at low pH in the presence of inositol hexaphosphate.

Studies on the linkage between spin equilibria and protein structure in carp ferric hemoglobin

The effects of protein conformation on the spin-state equilibria of several derivatives of carp hemoglobin have been examined. This has been done by measuring the pH dependence of the paramagnetic susceptibilities of these derivatives in the presence and absence of inositol hexakisphosphate, P6-inositol. In all cases the addition of P6-inositol at low pH and the lowering of the pH in the presence of P6-inositol shift the spin-state equilibrium in favor of the high-spin electronic configuration. The P6-inositol and pH dependence of these magnetic properties parallels the pH and P6-inositol dependence of the conformational state of the hemoglobin as determined in earlier studies and further supports a thermodynamic linkage between the electronic state of the iron atoms and the quaternary structure of the hemoglobin molecule.

Reexamination of the evidence for paramagnetism in oxy- and carbonmonoxyhemoglobins

Experiments have been carried out in an attempt to confirm previous reports of paramagnetism in the oxy- and carbonmonoxy derivatives of human and carp hemoglobin. When care is taken to ensure complete saturation of the hemoglobins with ligand and the diamagnetic contributions of all of the buffers are carefully evaluated, these hemoglobin derivatives are found to have the same gram susceptibilities as exhibited by a set of metal free proteins, suggesting that they are fully diamagnetic.

Magnetic susceptibility of oxy- and carbonmonoxyhemoglobins

The room temperature magnetic susceptibilities of human and carp oxy- and carbonmonoxyhemoglobin solutions were measured in a 30-gauss (1 G = 10(-4) T) field with a superconducting magnetometer. To within experimental uncertainty, the susceptibility was the same for both the oxy and carbonmonoxy forms, and salt concentration did not effect it. A variety of sample preparations were used; the iron chemical state was verified by Mössbauer spectroscopy. A value of -0.580 +/- 0.010 X 10(-6) centimeter-gram-second (cgs) system was obtained for the mass susceptibility of the protein. We attribute the paramagnetism sometimes observed in oxyhemoglobin solutions to the presence of a small amount of the deoxy form.

A high-resolution proton nuclear-magnetic-resonance investigation of carp hemoglobin. Conformational differences between carp and human normal adult hemoglobins in solution

The high-resolution proton nuclear magnetic resonance spectra of carp hemoglobin have been compared to those of human normal adult hemoglobin. Carp deoxy and carbonmonoxy hemoglobins in the deoxy-type quaternary state exhibit two downfield exchangeable proton resonances as compared to four seen in human normal adult deoxyhemoglobin. This suggests that two of the hydrogen bonds present in human normal adult deoxyhemoglobin are absent or occur in very different environments in carp hemoglobin. One of the exchangeable proton resonances of carp hemoglobin, while present in the deoxy-type quaternary state of the carbonmonoxy and deoxy derivatives, is absent in the oxy-type quaternary state of both, in agreement with the assignments of these quaternary structures by other methods. The ring-current-shifted proton resonances (sensitive tertiary structural markers) of carp carbonmonoxyhemoglobin are substantially different from those of human normal adult hemoglobin. The aromatic proton resonance region of carp hemoglobin has fewer resonances than that of human normal adult hemoglobin, consistent with its much reduced histidine content. The hyperfine-shifted proximal histidyl NH-exchangeable proton resonances of carp hemoglobin suggest that during the transition from the oxy to the deoxy quaternary structure, there is a greater alteration in the heme pocket of one type of subunits (presumably the beta chain) than that in the other subunit. The present results suggest that there are differences in both tertiary and quaternary structures between carp and human normal adult hemoglobins which could contribute to the great differences in the functional properties between these two proteins.

Quaternary structure and spin equilibria in ferric hemoglobins. A room temperature study

The effective magnetic moments for a number of human and carp methemoglobin derivatives were determined in solution at room temperature. The data permit us to confirm the dependence of the spin-state equilibrium of azide methemoglobin on the quarternary state of the hemoglobin and to demonstrate a similar dependence for both human and carp aquomethemoglobin. In addition, the pH dependence of the effective magnetic moment and the Soret spectrum of carp azidemethemoglobin are compared.