Uptake of lysine and proline via separate α-neutral amino acid transport pathways in Mytilus gill brush border membranes

Brush border membrane vesicles (BBMV) were prepared from the gills of the marine mussel, Mytilus edulis. These membranes contained two distinct pathways for cotransport of Na⁺ and α-neutral amino acids. The major pathway in mussel gill BBMV was the alanine-lysine (AK) pathway, which had a high affinity for alanine and for the cationic amino acid, lysine. The AK pathway was inhibited by nonpolar α-neutral amino acids and cationic amino acids, but was not affected by β-neutral amino acids or imino acids. The kinetics of lysine transport were consistent with a single saturable process, with a J_max of 550 pmol/mg-min and a K_t of 5 μm. The AK pathway did not have a strict requirement for Na⁺, and concentrative transport of lysine was seen in the presence of inwardly directed gradients of Li⁺ and K⁺, as well as Na⁺. Harmaline inhibited the transport of lysine in solutions containing either Na⁺ or K⁺. The alanine-proline (AP) pathway transported both alanine and proline in mussel gill BBMV. The AP pathway was strongly inhibited by nonpolar α-neutral amino acids, proline, and α-(methylamino)isobutyric acid (Me-AIB). The kinetics of proline transport were described by a single saturable process, with a J_max of 180 pmol/mg-min and K_t of 4 μm. In contrast to the AK pathway, the AP pathway appeared to have a strict requirement for Na⁺. Na⁺-activation experiments with lysine and proline revealed sigmoid kinetics, indicating that multiple Na⁺ ions are involved in the transport of these substrates. The transport of both lysine and proline was affected by membrane potential in a manner consistent with electrogenic transport.