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

To analyze the effects of Ca2(+)-mediated membrane protein changes on the membrane function, we have studied the SO4(2-) self-exchange and amino acid transport in human erythrocytes after loading them with Ca2+ with the help of ionophore A23187. The SO4(2-) self-exchange is inhibited by 20-30% by loading the erythrocytes with 25 microM to 0.5 mM Ca2+. The extent of this inhibition is almost doubled (50-60%) by increasing the Ca2+ loading concentration to 1.5 mM. This additional effect of 1.5 mM 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 SO4(2-) self-exchange, L-alanine and L-cysteine uptakes are also inhibited in Ca2(+)-loaded cells. However, no effect is observed on the L-lysine uptake under identical conditions. These results have been interpreted to suggest that the Ca2(+)-mediated effects on the SO4(2-) self-exchange and amino acid transport are caused perhaps by the Ca2(+)-induced structural rearrangement of the band 3 protein.