Blood-brain barrier permeability to dipeptides and their constituent amino acids.

The penetration of two [14C]-labelled dipeptides, glycyl-L-phenylalanine and glycyl-L-leucine, and of their constituent amino acids into the brain of the rat was measured employing an intracarotid injection technique. The brain-uptakes of the dipeptides were about equal to that of sucrose suggesting a negligible extraction from the blood during the 15-s period of exposure to the peptides. Brain uptakes for L-phenylalanine and L-leucine were large and in agreement with earlier work on these amino acids; self-inhibition by unlabelled amino acids was marked as also inhibition by the typical L-transport system substrate, 2-aminobicyclo (2, 2, 1) heptane-2 carboxylic acid (BCH), whilst the substrate for the A-system, N-methyl-L-aminoisobutyric acid (MeAIB) was without effect. Uptake of L-phenylalanine and L-leucine was not inhibited by dipeptides in 10 mM concentration. The uptakes of [14C]-labelled MeAIB and glycine were not significantly different from that of sucrose. It is concluded that peptide formation effectively excludes the rapidly penetrating L-system amino acids, L-leucine and L-phenylalanine, from access to the L-system channel.