PHENYLALANINE TRANSPORT IN RABBIT SMALL-INTESTINE

1. The proposal that rabbit small intestine possesses a separate, sodium-dependent carrier of phenylalanine has been examined by measurements of the unidirectional influx of amino acids across the brush-border membrane of the intact epithelium of the rabbit small intestine. 2. We demonstrate that, like alanine, glycine and leucine, phenylalanine shares sodium-dependent as well as sodium-independent transport with lysine. 3. Using the distal ileum we applied the A (phenylalanine)-B (leucine)-C (alanine) test on the sodium-dependent, lysine-resistant transport of phenylalanine. For phenylalanine, K-1/2 (concentration required for half-maximal transport) was 3.1 +/- 0.2 mM (n=7) and K-i (inhibitor constant) against leucine transport was 3.1 +/- 0.2 mM (n = 4). For leucine K-1/2 was 1.1 +/- 0.1 mM (n = 4) and K-i against transport of phenylalanine was 1.1 +/- 0.1 mM (n = 4). For alanine, K-1/2 was 12.6 +/- 1.1 mM (n = 3), K-i against phenylalanine was 13.1 +/- 1.8 mM (n = 4) and K-i against leucine was 11.0 +/- 0.4 mM ( n = 4). 4. Using the jejunum we applied the A (phenylalanien)-B (alanine)-C (methionine) test on the lysine-resistant, sodium-dependent transport of pehnylalanine. For phenylalanine, K-1/2 was 4.7 +/- 0.2 mM (n = 7) and K-i against alanine was 4.8 +/- 0.2 mM (n = 4). For alanine, K-1/2 was 15.6 +/- 0.08 mM ( n = 7) and K-i against phenylalanine was 18.1 +/- 0.9 mM (n = 5). For methionine, K-i against phenylalanine was 1.1 +/- 0.2 mM (n = 3) and against alanine was 0.8 +/- 0.2 mM (n = 3). 5. These data demonstrate that one, and only one, lysine-resistant, sodium-dependent carrier is involved in transport of phenylalanine across the brush-border membrane of rabbit small intestine.