beta-carboxylic acid esterified D-Asp-Ala retains a high affinity for the oligopeptide transporter in Caco-2 monolayers

D-Asp-Ala, a metabolically stable dipeptide, possesses a relatively high affinity for the Caco-2 oligopeptide transporter (IC50 = 5.75 +/- 0.09 mM) as demonstrated by its ability to compete with [C-14]Gly-Sar in cellular uptake experiments. When the beta-carboxylic acid of D-Asp-Ala is modified by esterification with a cyclohexyl group (D-Asp(OcHx)-Ala) or a benzyl group (D-Asp(OBzl)-Ala), the resulting compounds are still able to inhibit [C-14]Gly-Sar binding to the oligopeptide transporter, i.e., IC50 values for D-Asp(OcHx)-Ala and D-Asp(OBzl)-Ala were 2.80 +/- 0.1 and 2.62 +/- 0.35 mM, respectively. HPLC analysis shows that both D-Asp(OcHx)-Ala and D-Asp(OBzl)-Ala are fully resistant to degradation for up to 5 h when incubated in the apical media of confluent Caco-2 monolayers. These results demonstrate that it is possible to covalently modify the side chain of one amino acid in an enzymatically stabilized dipeptide with small, aromatic molecules while enabling them to retain their affinity for the oligopeptide transporter. (C) 1997 Elsevier Science B.V.