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 the3H-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: Kt= 2.71 mM and Jmax= 1390 nmol mg-1s-1, Kt= 1.03 mM and Jmax= 176 nmol mg-1s-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 (Ki= 2.81 mM) and l-isoleucine (Ki= 2.62 mM). Similar results were obtained from self-inhibition experiments: the Kiof 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. © 1992.
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