Transport of amino acid-related compounds mediated by L-type amino acid transporter 1 (LAT1): Insights into the mechanisms of substrate recognition

The L-type amino acid transporter 1 (LAT1) is an Na⁺-independent neutral amino acid transporter subserving the amino acid transport system L. Because of its broad substrate selectivity, system L has been proposed to be responsible for the permeation of amino acid-related drugs through the plasma membrane. To understand the mechanisms of substrate recognition, we have examined the LAT1-mediated transport using a Xenopus laevis oocyte expression system. LAT1-mediated [¹⁴C] phenylalanine uptake was strongly inhibited in a competitive manner by aromatic-amino acid derivatives including L-dopa, α-methyldopa, melphalan, triiodothyronine, and thyroxine, whereas phenylalanine methyl ester, N-methyl phenylalanine, dopamine, tyramine, carbidopa, and droxidopa did not inhibit [¹⁴C] phenylalanine uptake. Gabapentin, a γ-amino acid, also exerted a competitive inhibition on LAT1-mediated [¹⁴C] phenylalanine uptake. Although most of the compounds that inhibited LAT1-mediated uptake were able to induce the efflux of [¹⁴C]phenylalanine preloaded to the oocytes expressing LAT1 through the obligatory exchange mechanism, melphalan, triiodothyronine, and thyroxine did not induce the significant efflux. Based on the experimental and semiempirical computational analyses, it is proposed that, for an aromatic amino acid to be a LAT1 substrate, it must have a free carboxyl and an amino group. The carbonyl oxygen closer to the amino group needs a computed charge of -0.55∼-0.56 and must not participate in hydrogen bonding. In addition, the hydrophobic interaction between the substrate side chain and the substrate binding site of LAT1 seems to be crucial for the substrate binding. A substrate, however, becomes a blocker once Connolly accessible areas become large and/or the molecule has a high calculated IogP value, such as those for melphalan, triiodothyronine, and thyroxine.