1. A jejunal perfusion technique has been used in normal volunteer subjects to study jejunal absorption of amino acid residues from a partial enzymic hydrolysate of casein in which about 50% of the amino acids existed as small peptides, and also from an equivalent mixture of free amino acids.

2. The effect of a high concentration of the dipeptide glycylglycine on the absorption of amino acid residues from these preparations was studied to quantify the importance of mucosal uptake of intact peptides during absorption of the partial hydrolysate of casein.

3. The results were unexpected. Glycylglycine significantly inhibited absorption of several amino acid residues (aspartic acid + asparagine, serine, glutamic acid + glutamine, proline, alanine, phenylalanine, threonine and isoleucine) from the free amino acid mixture, whereas it significantly inhibited the absorption of only two (serine, glutamic acid + glutamine) from the peptide-containing partial casein hydrolysate.

4. The effect of glycylglycine on absorption of amino acids from the mixture of free amino acids was apparently due to inhibition of amino acid uptake by free glycine liberated from the dipeptide during perfusion. The reason for the failure of glycylglycine to cause extensive inhibition of absorption from the partial hydrolysate is not clear. It may be due to glycylglycine being only a weak inhibitor of peptide uptake, but the possibility that some peptides are taken up by a system unavailable to glycylglycine has to be considered.

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