Uptake of l-leucine and l-phenylalanine across the basolateral cell surface in isolated oxyntic glands

Abstract

The time course kinetic specificity and sodium-dependence of l-leucine and l-phenylalanine uptake by rabbit isolated oxyntic glands were studied in order to identify the systems involved in the transport of branched-chain and aromatic neutral amino acids through the basolateral cell membrane. The uptake was measured directly in the disrupted cells after incubation of the glands with the 3H-labelled amino acid both in a sodium-containing and a sodium-free medium. The uptake of l-leucine was largely carrier-mediated whilst l-phenylalanine was taken up by either carrier-mediated and nonsaturable processes. Both amino acids were taken up by a Na +-independent process. The kinetic parameters of l-leucine and l-phenylalanine carrier-mediated influx were, respectively: K t = 2.71 mM and J max = 1390 nmol mg −1 s −1, K t = 1.03 mM and J max = 176 nmol mg −1 s −1. From cross-inhibition studies it can be inferred that l-leucine is primarily transported by a Na +-independent system which shows specificity for bulky side chains dipolar amino acids. The system displays similar affinities for l-phenylalanine ( K i = 2.81 mM) and l-isoleucine ( K i = 2.62 mM). Similar results were obtained from self-inhibition experiments: the K i of the carrier-mediated uptake of l-leucine and phenylalanine were 2.12 and 2.40 mM (from a Hanes plot) or 3.2 and 0.8 mM (from a Dixon plot), respectively. It is concluded that a sodium-independent transport system, like Christensen's L′ type, is shared by branched-chain and aromatic dipolar amino acids, which only shows slight differences in their affinities for the carrier.