Human solute carrier SLC6A14 is the β-alanine carrier

The β-alanine carrier was characterized functionally in the 1960s to 1980s at the luminal surface of the ileal mucosal wall and is a Na(+)- and Cl(−)-dependent transporter of a number of essential and non-essential cationic and dipolar amino acids including lysine, arginine and leucine. β-Alanine carrier-like function has not been demonstrated by any solute carrier transport system identified at the molecular level. A series of experiments were designed to determine whether solute carrier SLC6A14 is the molecular correlate of the intestinal β-alanine carrier, perhaps the last of the classical intestinal amino acid transport systems to be identified at the molecular level. Following expression of the human SLC6A14 transporter in Xenopus laevis oocytes, the key functional characteristics of the β-alanine carrier, identified previously in situ in ileum, were demonstrated for the first time. The transport system is both Na(+) and Cl(−) dependent, can transport non-α-amino acids such as β-alanine with low affinity, and has a higher affinity for dipolar and cationic amino acids such as leucine and lysine. N-methylation of its substrates reduces the affinity for transport. These observations confirm the hypothesis that the SLC6A14 gene encodes the transport protein known as the β-alanine carrier which, due to its broad substrate specificity, is likely to play an important role in absorption of essential nutrients and drugs in the distal regions of the human gastrointestinal tract.