Maturation of membrane function: Transport of amino acid by rat erythroid cells

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Abstract
The membrane changes which occur during cellular maturation of erythroid cells have been investigated. The transport of α‐aminoisobutyric acid, alanine, and N‐methylated‐α‐aminoisobutyric acid have been studied in the erythroblastic leukemic cell, the reticulocyte, and the erythrocyte of the Long‐Evans rat. The dependence of amino acid transport on extracellular sodium concentration was investigated.Erythrocytes were found to transport these amino acids only by Na‐independent systems. The steady state distribution ratio was less than 1. Reticulocytes were found to transport α‐aminoisobutyric acid and alanine by Na‐dependent systems, but only small amounts of N‐methylated‐α‐aminoisobutyric acid. Small amounts of these amino acids were transported by Na‐independent systems. The steady state distribution ratio was greater than one for Na‐dependent transport. The erythroblastic leukemia cell, a model immature erythroid cell, showed marked Na‐dependence (>90%) for α‐aminoisobutyric acid and alanine transport, and >80% for the Na‐dependent transport of N‐methyl‐α‐aminoisobutyric acid. The steady state distribution ratio for the Na‐dependent transport was >4.In the erythroblastic leukemic cell, at least three Na‐dependent systems are present: one includes alanine and α‐aminoisobutyric acid, but excludes N‐methyl‐α‐aminoisobutyric acid; one is for α‐aminoisobutyric acid, alanine and also N‐methyl‐α‐aminoisobutyric acid; and one is for N‐methyl‐α‐aminoisobutyric acid alone. In the reticulocyte, the number of Na‐dependent systems are reduced to two: one for α‐aminoisobutyric acid and alanine; one for N‐methyl‐α‐aminoisobutyric acid. In the erythrocytes, no Na‐dependent transport was found. Therefore, maturation of the blast cell to the mature erythrocyte is characterized by a systematic loss in the specificity and number of transport systems for amino acids.