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

Abstract

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 SO 4 ^2- self-exchange is inhibited by 20-30% by loading the erythrocytes with 25 μ m to 0.5 mm 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 SO₄^2- 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 SO₄^2- self-exchange and amino acid transport are caused perhaps by the Ca²⁺-induced structural rearrangement of the band 3 protein.