Alterations of human erythrocyte membrane fluidity by oxygen-derived free radicals and calcium.

Two possible reasons for the structural alterations of cell membranes caused by free radicals are lipid peroxidation and an increase in the intracellular calcium ion concentration. To characterize the alterations in membrane molecular dynamics caused by oxygen-derived free radicals and calcium, human erythrocytes were spin-labeled with 5-doxyl stearic acid, and alterations in membrane fluidity were quantified by electron spin resonance spectrometry. The in vitro generation of oxygen free radicals, using hypoxanthine (0.43 mM) plus xanthine oxidase (0.07 U/mL) decreased membrane fluidity, and the addition of superoxide dismutase and catalase inhibited the effect on membrane fluidity of the hypoxanthine-xanthine oxidase system. Hydrogen peroxide (0.1 and 1 mM) also decreased membrane fluidity and caused alterations to erythrocyte morphology. In addition, a decrease in membrane fluidity was observed in erythrocytes incubated with 2.8 mM CaCl2. On the other hand, incubation of erythrocytes with calcium-free solution decreased the changes in membrane fluidity caused by hydrogen peroxide. These results suggest that changes in membrane fluidity are directly due to lipid peroxidation and are indirectly the result of increased intracellular calcium concentration. We support the hypothesis that alterations of the biophysical properties of membranes caused by free radicals play an important role in cell injury, and that the accumulation of calcium amplifies the damage to membranes weakened by free radicals.