Sulfate self-exchange and amino acid transport in calcium-loaded human erythrocytes

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Abstract
To analyze the effects of Ca2+-mediated membrane protein changes on the membrane function, we have studied the SO 4 2− self-exchange and amino acid transport in human erythrocytes after loading them with Ca2+ with the help of ionophore A23187. The SO 4 2− self-exchange is inhibited by 20–30% by loading the erythrocytes with 25 μm to 0.5mm Ca2+. The extent of this inhibition is almost doubled (50–60%) by increasing the Ca2+ loading concentration to 1.5mm. This additional effect of 1.5mm Ca2+ is not correlated with the Ca2+-induced ATP depletion or membrane protein degradation, but is caused by the transglutaminase-catalyzed membrane protein crosslinking. Like the SO 4 2− self-exchange,l-alanine andl-cysteine uptakes are also inhibited in Ca2+-loaded cells. However, no effect is observed on thel-lysine uptake under identical conditions. These results have been interpreted to suggest that the Ca2+-mediated effects on the SO 4 2− self-exchange and amino acid transport are caused perhaps by the Ca2+-induced structural rearrangement of the band 3 protein.