DIPEPTIDE TRANSPORT AND HYDROLYSIS IN ISOLATED LOOPS OF RAT SMALL-INTESTINE - EFFECTS OF STEREOSPECIFICITY

1. Isolated jejunal loops of rat small intestine were perfused by a si ngle pass of bicarbonate Krebs-Ringer solution containing either D- or L-phenylalanine or one of eight dipeptides formed from D- or L-alanin e plus D- or L-phenylalanine. 2. At 0.5 mM L-phenylalanyl-L-alanine in creased serosal phenylalanine appearance to forty times the control ra te giving a value similar to that found with 0.5 mM free L-phenylalani ne. No serosal dipeptide could be detected. 3. Perfusions with the two mixed dipeptides with N-terminal D-amino acids (D-alanyl-L-phenylalan ine and D-phenylalanyl-L-alanine) gave rise to the appearance of intac t dipeptides in the serosal secretions although there were substantial differences in their rates of absorption and subsequent hydrolysis. 4 . L-Alanyl-D-phenylalanine was absorbed from the lumen three to five t imes as fast as L-phenylalanyl-D-alanine. At 1 mM L-alanyl-D-phenylala nine transferred D-phenylalanine across the epithelial layer at more t han seven times the rate found with the same concentration of the free D-amino acid. 5. Perfusions with D-alanyl-D-phenylalanine or D-phenyl alanyl-D-alanine showed that these two dipeptides are poor substrates for both transport and hydrolysis by the rat small intestine. 6. Analy sis of mucosal tissue extracts after perfusion with the two mixed dipe ptides with N-terminal D-amino acids revealed that both dipeptides wer e accumulated within the mucosa and suggested that exit across the bas olateral membrane was rate limiting for transepithelial dipeptide tran sport.