Cationic Amino Acid Transport into Cultured Animal Cells. I. Influx into Cultured Human Fibroblasts

Cationic amino acid transport into human skin fibroblasts occurs by a saturable mediation which we designate System y+, which may prove identical with the earlier Ly. This plasma membrane agency is kinetically distinguishable from systems transporting neutral and acidic amino acids and serves to catalyze the flows of omega-guanidino amino acids and alpha, omega-diamino acids. Uptake of cationic substrates by System y+ is Na+-independent, pH-insensitive, stereoselective, and inhibitable by neutral amino acids in the presence of Na+. Uptake and exodus of System y+ substrates are strongly stimulated by cationic amino acids inside and outside the cell, respectively. Arginine and homoarginine accumulate in human fibroblasts reaching distribution ratios of more than 20 at external amino acid concentrations in the physiological range.