The Role of Extracellular Ca 2+ in Regulating the Distribution and Conformation of Hemoglobin in Erythrocytes

It was found that a decrease in the concentration of extracellular Ca 2+ ([Ca 2+ ] out ) caused an increase in the surface charge of the erythrocyte membrane, which was accompanied by a change in the conformation of heme and globin, as well as in a redistribution of hemoglobin in the cell (a decrease in the optical path difference (OPD) of the cytoplasm and the content of the membrane-associated hemoglobin). It was also found that when [Ca 2+ ] out was decreased from 10 –3 to 10 –6 M, the morphology of the erythrocyte practically did not change, the ζ-potential increased from –15.4 ± 0.2 to –14.5 ± 0.3 mV (at [Ca 2+ ] out decrease from 10 –3 to 5 × 10 –4 M), and the OPD of the cell cytoplasm decreased. A decrease in [Ca 2+ ] out to 10 –6 M changed the conformation of heme, as well as the distribution of hemoglobin in the erythrocyte, as assessed by a decrease in the symmetric vibrations of the side CH groups of pyrrole semirings and an increase in the vibrations of the vinyl groups of hemoporphyrin as well as the affinity of heme to ligands. At the same time, the probability of finding heme in a flat conformation did not change, while the rigidity of its protein surrounding decreased. When [Ca 2+ ] out was decreased from 10 –3 to 10 –4 M, a change in the globin conformation was observed as a decrease in the ordering of the CH groups of globin amino acids, an increase in the globin packing density, and the polarity of the globin surrounding. When erythrocytes were incubated in a medium containing [Ca 2+ ] out = 10 –6 M, the maximum intensity of the Raman scattering of erythrocyte hemoglobin (Raman spectrum bands at 1375 and 1580 cm –1 ) was detected in the center of the cell; it was 2.6 times less than that of erythrocytes incubated in a medium containing [Ca 2+ ] out = 10 –3 M.