Red-cell amino acid transport

The properties of Na+-dependent L-alanine transport in human erythrocytes were investigated using K+ as the Na+ substitute. Initial rates of Na+-dependent L-alanine uptake (0.2mM extracellular amino acid) for erythrocytes from 22 donors ranged from 40 to 180pmol/litre of cells per h at 370C. Amino acid uptake over the concentration range 0.1-8mm was consistent with a single saturable component of Na+-dependent L-alanine transport. Apparent Km and V... values at 37 and 50C measured in erythrocytes from the same donor were 0.27 and 0.085 mm respectively, and 270 and 8.5 pmol/litre of cells per h respectively. The transporter responsible for this uptake was identified as system ASC on the basis of cross-inhibition studies with a series of 42 amino acids and amino acid analogues. Apparent K, values for glycine, L-aamino-n-butyrate, L-serine and L-leucine as inhibitors of Na+-dependent L-alanine uptake at 370C were 4.2, 0.12, 0.16 and 0.70mM respectively. Reticulocytes from a patient with inherited pyruvate kinase deficiency were found to have a 10-fold elevated activity of Na+-dependent L-alanine uptake compared with erythrocytes from normal donors. Separation of erythrocytes according to cell density (cell age) established that even the oldest mature erythrocytes retained significant Na+-dependent L-alanine transport activity. Amino acid transport was, however, a more sensitive indicator of cell age than acetylcholinesterase activity. Erythrocytes were found to accumulate L-alanine against its concentration gradient (distribution ratio approx. 1.5 after 4 h incubation), an effect that was abolished in Na+-free media. Na+-dependent L-alanine uptake was shown to be associated with L-alanine-dependent being 1:1.