Oxidative and reductive reactions of sulphhaemoglobin with various reagents correlated with changes in quaternary structure of the protein

The absorption maxima in the Soret region and near 620nm of sulphhaemoglobin shifted from 419.5nm to 423nm and from 623nm to 619nm respectively with a decrease in oxygen concentrations of the sulphhaemoglobin solution [101.3, 20.3 and 0 kPa (760, 152 and 0 mmHg) partial pressures]. The major changes in the positions of the absorption maxima occurred drastically at oxygen concentrations between 20.3 kPa and 0 kPa, suggesting that the quaternary structure of sulphhaemoglobin has changed from the R to the T state. Inositol hexaphosphate, a known allosteric effector of haemoglobin, enhanced the shift in peak location. This result supports the view that the shift of the absorption maxima reflects changes in the conformation of sulphhaemoglobin. To investigate the relationship between the conformation of sulphhaemoglobin and the reactivity of the protein with various reagents, we studied the oxidation and reduction reactions of the protein with ascorbic acid, ferricyanide and nitrite under aerobic and anaerobic conditions. The results showed that the rates of oxidation and reduction of ferrous and ferric sulphhaemoglobins with these compounds are associated with the conformation of the protein.

Simplified determination of carboxyhemoglobin

This spectrophotometric method for estimating carboxyhemoglobin is based on the production of a two-pigment mixture by reducing oxyhemoglobin and methemoglobin with sodium hydrosulfite . Absorbances of the pigment are measured at 420 and 432 nm. Protecting the solutions against exposure to air is unnecessary. Values are stable and reproducible when the solutions are buffered at pH 6.85.

Photolytic studies on the carbon monoxide complex of sulphaemoglobin

The CO complex of sulphaemoglobin was found to be photolytically dissociable at pH 6.0 and pH 8.5. The recombination kinetics of the CO complex after flash photolysis show two phases, which differ in rate by approximately 20-fold. At both pH 6.0 and pH 8.5 the two phases show an identical linear dependence on CO concentration, with associated second-order rate constants of 6.0 x 10(3) M-1 . s-1 and 1.2 x 10(5) M-1 . s-1 respectively. The percentage contribution of each of the two phases to the total absorbance change is independent of the CO concentration employed, but is a function of the protein concentration used. The percentage of the faster phase increases with protein dilution. These results taken together with computer simulation suggest the existence of appreciable amounts of monomeric species in solution at both pH 6.0 and pH 8.5 at low concentration of liganded protein. Estimates of the dissociation constants for the dimer in equilibrium monomer equilibrium yield values of 8 microM and 3.5 microM at pH 6.0 and pH 8.5 respectively. Stopped-flow delayed-flash-photolysis studies show that the liganded dimer decays to the liganded monomer with a half-life of approx. 0.5 s. Constant-illumination experiments allow an estimation of the half-life for the combination of deoxy monomers to deoxy dimers of 22 s. A model explaining the range of CO recombination kinetics seen for sulphaemoglobin does not require the existence of a photolytically produced high-activity form, even at high pH, in contrast with the case for normal haemoglobin.

Microvessel mean transit time and blood flow velocity of sulfhemoglobin-RBC

An indicator dilution technique is described for obtaining time-concentration curves subsequent to bolus injections of sulfhemoglobin red blood cells (SH-RBC), which have a deep greenish-brown color (absorption peak 620 nm vs. 542 and 564 nm for normal red cells). The series- and parallel-coupled microvessels of cat mesentery were studied. This is accomplished by means of video microscopy with a two-window intensity-sensitive video sampler system. The relationship between SH-RBC concentration in blood and optical measurement is linear. Blood flow velocities were calculated from the difference in mean transit times between two points along a vessel. When this technique is used in association with the previously reported method for determining time-concentration curves for the plasma indicator FITC-dextran the mean transit time (t) for red blood cells was less than for plasma in arterioles. The reproducibility of t and flow velocity for both SH-RBC and FITC-dextran from successive injections were reported. The mean transit time ratio of arteriolar SH-RBC to FITC-dextran averages 0.89. Blood flow velocity calculated from SH-RBC is greater than that calculated from FITC-dextran in these same arterioles. The ratio of the velocities averages 1.29.

The reversible binding of oxygen to sulfhemoglobin

The O2 binding properties of sulfhemoglobin were studied. The oxygen tension required for half-saturation of sulfhemoglobin is more than 2 orders of magnitude higher than that for hemoglobin A. The binding of O2 exhibits an alkaline Bohr effect larger than that observed for hemoglobin, yet the Hill number is unity. From the Bohr titration curve, 0.68 proton is released during O2 binding at 0 degrees C. Sulfhemoglobin prepared from carboxypeptidase A-treated hemoglobin has an affinity for O2 which is about the same as that of sulfhemoglobin at the theoretical limit of the Bohr titration curve. Like its carboxypeptidase A-treated hemoglobin precursor, this sulfhemoglobin does not bind O2 cooperatively. Thus, sulfhemoglobin appears to be in a high affinity form at alkaline pH and a low affinity form at acid pH, similar to hemoglobin A. These results demonstrate that the magnitude of the Hill number is not always an indicator of the interaction between oxygen binding and other functions in a hemoglobin.

Sulfhaemoglobin. Absorption spectrum, millimolar extinction coefficient at lambda = 620 nm, and interference with the determination of haemoglobin and of haemiglobincy anide

The spectrophotometry properties of sulfhaemoglobin (SHb) and some derivatives were investigated using an improved technique for measuring the SHb fraction induced in human blood samples. The millimolar extinction coefficient of SHb at lambda = 620 nm was found to be 20.8 (S.D. 1.48; S.E. = 0.44; n = 11). In addition it was demonstrated that the spectral changes occurring in SHb containing haemoglobin solutions upon the addition of KCN, K3Fe(CN)6 and K3Fe(CN)6 +KCN invalidate the KCN addition method for the determination of haemiglobin. The influence of clinically occurring SHb fractions on the internationally standarized total haemoglobin determination were shown to be insignificant.

Oxidative injury to erythrocytes, cell rigidity, and splenic hemolysis in hemodialyzed uremic patients

In 23 hemodialyzed patients the metabolic defect of erythrocytes related to uremia and potentiated by dialysis was studied to establish a possible link with the shortened erythrocyte survival. A highly significant correlation was found between sulfhemoglobin provoked by oxidative stress and erythrocyte rigidity measured by filterability. Impaired filterability also correlated with the degree of splenic sequestration. Both sulfhemoglobin and impaired filterability correlated with the degree of hemolysis. Finally, erythrocyte survival showed a highly significant correlation with the degree of anemia. From osmotic fragility, plasma trapping, erythrocyte ATP and 2,3-diphosphoglycerate (DPG), and serum phosphorus, calcium, and magnesium measurements it seems unlikely that either spheric transformation or sol-gel transformation of the membrane is causing the increased erythrocyte rigidity. In view of the impaired erythrocyte defense capacity against oxidative injury, cell content rigidity as mediated by reduced hemoglobin solubility is a more likely explanation. Our results give a rational basis for a trial of splenectomy in severely anemia hemodialyzed patients.