Intestinal absorption of L-phenylalanine in vitro

¹ Cancer Research Institute, New England Deaconess Hospital; Peter Bent Brigham Hospital; and Department of Biological Chemistry, Harvard Medical School, Boston, Massachusetts

When initially present at the same concentration on both sides, net transport of L-phenylalanine by everted hamster intestinal sacs reached a steady state at 1 hour. The net amount transported, as a function of the final mucosal concentration, was described by a Langmuir adsorption isotherm. An equation was derived showing that the concentration gradient also obeyed such a formulation. Sacs from the midintestine transported more L-phenylalanine than proximal and distal end sacs. Lineweaver-Burk plots of the concentration gradient against final mucosal concentrations showed the process in end sacs to have the same K_M (1.8 x 10^–3 M) as that in midintestinal sacs; those from midgut, however, had a greater capacity. L-Phenylalanine absorption likely occurs by different amounts of the same mechanism throughout the small intestine. High concentrations of L-phenylalanine resulted in a depression of net transport. The importance of the concentration gradient in revealing the fall in net transport in this and other systems was stressed. l-Phenylalanine transport was also depressed by L-tryptophan and L-methionine.