A study of the substrate specificity of Na+-dependent and Na+-independent neutral amino acid transport systems in dog intestinal brush-border membrane vesicles using L-alanine analogues.

Neutral amino acids are mainly transported across the intestinal brush-border membrane by two Na+-dependent systems (system B0 and system B0+) and one Na+-independent system (system b0,+). To investigate potential differences in substrate specificity between these systems, we screened ten different alanine analogues for their ability to inhibit the transport of L-alanine in dog intestinal brush-border membrane vesicles. The results suggested that a phenyl group directly attached to the alpha-carbon has different effects on the Na+-dependent and Na+-independent transport systems, with an increased affinity for the former and a decreased affinity for the latter. Based on these inhibition studies, we investigated [14C]L-phenylglycine transport kinetics in comparison with L-alanine. Similar to L-alanine, L-phenylglycine transport followed at least three routes, however, the Km of the Na+-dependent transport system was lower and the K'm of the Na+-independent system was higher than the corresponding values for L-alanine. These results corroborated the conclusions drawn from the inhibition studies. Based on these data, we conclude that different sizes of immediate parts to the alpha-carbon in functional groups of amino acid analogues have differential effects on the interaction of these amino acid analogues with the Na+-dependent and Na+-independent transport systems for neutral amino acids.