Lysine transport by brush-border membrane vesicles of eel intestine: interaction with neutral amino acids.

L-[H-3] lysine uptake was measured in brush-border membrane vesicles prepared from intestinal mucosa of the European eel Anguilla anguilla. Lysine uptake occurred via 1) a nonsaturable component with an apparent diffusional permeability (P) of 0.58-mu-l.mg protein-1.min-1, 2) a Na-dependent transport system [half-saturation constant (K(app)) 0.16 mM, maximal transport rate (J(max)) 3.57 nmol.mg protein-1.min-1]; 3) a Na-independent transport system (K(app) 0.17 mM, J(max) 2.77 nmol.mg protein-1.min-1). Both carrier-mediated processes were accelerated by the presence of an intravesicular negative membrane potential. Hill analysis of L-lysine influx, over a wide range of external Na concentrations, resulted in a Hill coefficient (n) of approximately 2, suggesting that two or more Na ions may be associated with amino acid transport. The inhibition of lysine uptake by other amino acids was studied. Na-dependent lysine uptake was competitively inhibited by proline [inhibitory constant (K(i)) approximately 2 mM] and may occur by a system specific for cationic amino acids. Na-independent lysine uptake was competitively inhibited by alanine (K(i) approximately 1 mM) and may occur by a classic L system.