Oxygen and carbon dioxide exchange by human hemoglobin and erythrocytes

[摘要] O$sb2$ transport was examined by measuring the fractional saturation of concentrated hemoglobin solutions flowing through an artificial capillary (diameter $approx$ 27 $mu$m). The measured effects of pH, hemoglobin concentration, O$sb2$ tension, temperature, and organic phosphate were analyzed by a mathematical model which included the geometry of the capillary, parabolic flow inside the lumen, and cooperative O$sb2$ binding. Oxygen exchange was limited by diffusion and therefore governed by the magnitude of the O$sb2$ gradient between the intracapillary fluid phase and the external gas space. In uptake experiments, O$sb2$ flux was determined primarily by the external O$sb2$ tension (160 mm Hg) because the internal O$sb2$ pressure was kept small due to chemical combination with hemoglobin. In release experiments, the external O$sb2$ tension was maintained at zero, and the transport rate was determined by the intracapillary oxygen partial pressure, which was proportional to the O$sb2$ half-saturation pressure of the hemoglobin sample. Thus, factors that change the affinity of hemoglobin for oxygen, such as pH, temperature, and organic phosphate concentration, influence strongly the rate of O$sb2$ release but have little effect on the rate of O$sb2$ uptake. The rate-limiting step for erythrocyte CO$sb2$ transport is the transmembrane exchange of Cl$sp-$ and HCO$sb3sp-$ anions. This process was measured by following extracellular pH changes using a pH-sensitive fluorescent dye in a stopped-flow mixing device equipped with front-face optics. Initial pH and chloride gradients induced pH relaxations which were sensitive to the specific inhibitor 4,4$prime$-diisothiocyano-2,2$prime$-stilbenedisulfonic acid (DIDS). Special mixing experiments, designed to minimize anion competition for the transporter binding site(s), were simulated mathematically by models written for ping-pong and random ternary complex mechanisms. The experimentally observed rate dependence on initial extracellular bicarbonate concentration was approximated better by the ping-pong model, and the theoretically derived values for Cl$sp-$ and HCO$sb3sp-$ binding (both K$sb {m D}$;;s = 5 mM) as well as the translocation rate (1.1 $imes$ 10$sp4$ s$sp{-1}$) agreed very well with literature values for inhibition constants and protein turnover number, respectively. Finally the simultaneous measurement of O$sb2$ uptake and subsequent HCO$sb3sp-$/Cl$sp-$ exchange was demonstrated. The hemoglobin color change upon oxygen binding caused a rapid change in fluorescence, followed by a slower, DIDS-sensitive pH equilibration.

[发布机构] Rice University

[效力级别] Physiology