1. Preliminary observations concerned with the effect of the lipophilic properties of the amino acid side-chains of peptides on their apparent affinity for uptake by rings of everted hamster jejunum showed that of the series glycylglycine, l-alanyl-l-alanine, l-valyl-l-valine and l-leucyl-l-leucine, with increasingly lipophilic side-chains, l-valyl-l-valine, not l-leucyl-l-leucine, was the most powerful inhibitor of uptake of the hydrolysis-resistant dipeptide glycylsarcosine. This apparently anomalous observation indicated a need for further investigation, and this paper reports investigations of the kinetics of uptake of l-valyl-l-valine and of competition for uptake between l-valyl-l-valine and glycylsarcosine.
2. l-Valyl-l-valine was capable of complete competitive inhibition of mediated uptake of glycylsarcosine. Free l-valine did not inhibit mediated uptake of glycylsarcosine. Glycylsarcosine could inhibit mediated uptake of l-valyl-l-valine only partially, but a mixture of glycylsarcosine and l-valine was capable of producing complete inhibition of mediated uptake of l-valyl-l-valine.
3. Investigation of the kinetics of uptake of l-valyl-l-valine indicated two mediated components. Component (a), which disappeared in the presence of free l-leucine, probably represented uptake of free l-valine after hydrolysis of the peptide. Component (b) probably represented peptide uptake.
4. The estimates of Kt obtained for uptake of intact l-valyl-l-valine were many times greater than Ki for inhibition of uptake of glycylsarcosine by l-valyl-l-valine. A possible explanation of the discrepancy is the existence of two pathways for uptake of l-valyl-l-valine and glycylsarcosine, for one of which l-valyl-l-valine has a low Kt (i.e. a high affinity) not readily demonstrable by kinetic analysis.
5. The results suggest that mediated uptake of l-valyl-l-valine is the result of at least two processes, uptake of intact peptide by a mechanism or mechanisms shared with glycylsarcosine and also hydrolysis followed by uptake of free l-valine; estimates of the proportions of intact valine and of free valine taken up by mediated transport suggest that at pH 5 uptake of intact peptide varies from 25% at low concentrations to 55% at high concentrations. They do not explain why l-valyl-l-valine is a stronger inhibitor of uptake of glycylsarcosine than the more lipophilic l-leucyl-l-leucine, but do suggest how such a situation could arise.