Distinguishing transport systems having overlapping specificities for neutral and basic amino acids in the rabbit ileum.

1. The kinetic parameters of lysine and alanine uptake by rabbit ileal mucosa have been measured in the presence and absence of Na. 2. Using lysine as an inhibitor of part of alanine uptake in the absence of Na it has been possible to define two mediated systems for alanine entry. One of these systems (Km 14.2 mM; Jmax 41.3 n-mole cm-2 min-1) is inhibited by lysine (Ki 0.96 mM) while the other (Km 115 mM; Jmax 323 n-mole cm-2 min-1) is not. 3. Methionine is an effective inhibitor of both Na-independent uptake mechanisms for alanine. 4. Na-independent lysine uptake also takes place through two mediated systems. One of these systems (Km 1.0 mM; Jmax 12.8 n-mole cm-2 min-1) is inhibited by alanine (Ki 4.3 mM) while the other (Km 108 mM; Jmax 194 n-mole cm-2 min-1) is not. 5. The use of a naturally occurring basic amino acid to inhibit a portion of neutral amino acid uptake extends considerably the ability to distinguish multiple acid transport systems present in the same cell membrane. The physiological implications of these findings are discussed.