Does L‐leucine use a dipeptide transport protein for absorption by crustacean intestine?

Abstract

Previous work with mammals and crustaceans has shown that L‐histidine forms a bis‐complex with zinc ([His]‐Zn‐[His]) in solution. A PEPT‐1‐like dipeptide carrier was suggested as a transport mechanism by which the bis‐complex would pass across the intestinal luminal membrane of the lobster, Homarus americanus. In addition, this early work showed that L‐leucine inhibited transport of the histidine bis‐complex. In the present study, lobster intestines were isolated and mounted in a perfusion chamber in order to determine how 3H‐L‐leucine is transported across lobster intestine in the presence of zinc and to deduce whether 3H‐L‐leucine and L‐histidine are using the same transport system. It was found that 3H‐L‐leucine transport was a hyperbolic function of luminal zinc concentration. In addition, transmural M to S (mucosa to serosa) 3H‐L‐leucine transport in physiological saline with 25 μM Zn was stimulated by decreasing the pH from 7.1 to 6.0. Transmural M to S 3H‐L‐leucine fluxes (0.5 to 10 mM) in physiological saline at pH 5.5 and 25 uM Zn displayed sigmoidicity (Hill Coefficient ≈ 2.0). If leucine was transported by a PEPT‐1‐like transporter, the amino acid may form a bis‐complex with zinc in solution (two amino acids coupled with a zinc ion) and its transport would be sigmoidal, displaying a Hill coefficient of 2. Finally, it was found that luminal zinc increased the net M to S flux of 3H‐L‐leucine across the intestine. These experiments support the hypothesis that L‐leucine forms bis‐complexes with zinc ([Leu]‐Zn‐[Leu]) and uses a PEPT‐1‐like protein system for transport across lobster intestine. Supported by NSF grant IBN04‐21986.