Accumulation of L-phenylalanine by segments of small intestine

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

Segments of hamster small intestine accumulate l-phenylalanine from an in vitro solution without degradation of the amino acid. Uptake by the segments equilibrates in about 20 minutes, while transport of the amino acid across intestinal sacs requires nearly 1 hour. This suggests that the initial event preceding transintestinal transport is cellular uptake of l-phenylalanine. Concentration of the amino acid in the wall of intestinal sacs during l-phenylalanine transport was equal to or greater than that in the serosal fluid. Hence it is possible that intramural accumulation is the active process and outward diffusion from wall to serosa is the passive event. Dependence of segmental uptake of l-phenylalanine on external concentration, and its increased magnitude in midintestinal segments, resembles the process occurring in intestinal sacs. On the basis of the time course of events, concentration within the wall, and concentration and site dependence, it is proposed that intestinal uptake of l-phenylalanine is the initial and essential event in its transport. For l-phenylalanine uptake, the K_M calculated from initial rate data was identical to that found from equilibrium data. Usefulness of intestinal segment uptake in determining initial rates and other parameters is pointed out.