Transport of L-amino acids by brush border membrane vesicles from bovine small intestine.

Amino acid transport was studied using brush border membrane vesicles from bovine small intestine. Initial influxes of L-forms of alanine, lysine, methionine, phenylalanine, and proline were stimulated by the presence of a Na+ gradient (outside greater than inside). A relatively large proportion of total influx was by diffusion (e.g., as much as 50% of methionine and 62% of phenylalanine). Kinetic analyses indicated a single mediated transport system for alanine influx in the presence of Na+. This system had an apparent affinity for alanine of .67 mM with maximum flux of 129 pmol/mg protein/s. Mutual inhibition experiments indicated proline mediated transport is probably separate from the neutral amino acids. Alanine, methionine, and phenylalanine probably share common transport system(s), but in addition, methionine is also transported by a system(s) independent of alanine and phenylalanine. In general, methionine was a potent inhibitor of influx of other amino acids.