The membrane changes which occur during cellular maturation of erythroid cells have been investigated. The transport of
alpha-aminoisobutyric acid,
alanine, and N-methylated-
alpha-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
alpha-aminoisobutyric acid and
alanine by Na-dependent systems, but only small amounts of N-methylated-
alpha-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 (greater than 90%) for
alpha-aminoisobutyric acid and
alanine transport, and greater than 80% for the Na-dependent transport of
N-methyl-alpha-aminoisobutyric acid. The steady state distribution ratio for the Na-dependent transport was greater than 4. In the erythroblastic leukemic cell, at least three Na-dependent systems are present: one includes
alanine and
alpha-aminoisobutyric acid, but excludes
N-methyl-alpha-aminoisobutyric acid; one is for
alpha-aminoisobutyric acid,
alanine and also
N-methyl-alpha-aminoisobutyric acid; and one is for
N-methyl-alpha-aminoisobutyric acid alone. In the reticulocyte, the number of Na-dependent systems are reduced to two: one for
alpha-aminoisobutyric acid and
alanine; one for
N-methyl-alpha-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 system for
amino acids.