Sequential amino acid exchange across b(0,+)-like system in chicken brush border jejunum

In the small intestine, cationic amino acids are transported by y(+)-like and b(0,+)-like systems present in the luminal side of the epithelium. Here, we report the characterization of a b(0,+)-like system in the apical membrane of the chicken jejunum, and its properties as an amino acid exchanger. Analysis of the brush border membrane by Western blot points out the presence of rBAT (protein related to b0,+ amino acid transport system) in these membranes. A functional mechanism for amino acid exchange across this system was established by kinetic analysis measuring fluxes at varying substrate concentrations both in internal (in) and external (out) vesicle compartments. This intestinal b(0,+)-like system functions for L-arginine as an obligatory exchanger since its transport capacity increases 100-200 fold in exchange conditions, thus suggesting an important role in the intestinal absorption of cationic amino acids. The kinetic analysis of Argin efflux velocities is compatible with the formation of a ternary complex and excludes a model involving a ping-pong mechanism. The binding affinity of Argout is higher than that of Argin, suggesting a possible order of binding (Argout first) for the formation of the ternary complex during the exchange cycle. A model of double translocation pathways with alternating access is discussed.