A third dipeptide carrier system typified by l-prolyl-lhydroxyproline and independent of l-leucyl and beta-alanyl dipeptides in rat gut loops.

1. The intestinal transport of L-prolyl-L-hydroxyproline (10 mmol/l) was investigated in rat small gut loops in vivo under pentobarbitone anaesthesia. Osmolality of test solutions was adjusted to eliminate any positive effect of solvent drag on disappearance of solutes from the lumen. 2. L-Leucylglycine and beta-alanyl-L-histidine (carnosine), representative members of two distinctly different dipeptide transport groups previously delineated, were tested for competitive action on L-prolyl-L-hydroxyproline uptake at ten times equimolar concentration (100 mmol/l), but were found to have no effect on the carrier system. 3. L-Prolyl-L-hydroxyproline uptake was markedly blocked by other L-prolyl dipeptides, indicating that they shared a common carrier system. Disappearance of L-prolyl-L-hydroxyproline from the gut lumen was reduced from 48% 15 min-1 10 cm-1 (control, containing 70 mmol/l mannitol) to 11% or 20% in the presence of L-prolylglycine (100 mmol/l) or L-prolyl-L-leucine (25 mmol/l), respectively. 4. It was concluded that at least three separate dipeptide carrier protein systems exist in the rat small gut, the disappearance of L-prolyl-L-hydroxyproline from the gut lumen being inhibited by two other L-prolyl dipeptides but not by L-leucyl or beta-alanyl dipeptides.