1. The absorption of lysine and two lysine-containing dipeptides was studied in normal and cystinuric subjects by jejunal perfusion. Absorption of each dipeptide was compared with that of its constituent amino acids.
2. in normal subjects lysine was better absorbed from the dipeptide glycyl-l-lysine than from the equivalent amino acid mixture. in cystinuric patients lysine was absorbed very poorly in its free form but was absorbed normally from the dipeptide. in cystinuric patients lysine was also absorbed faster as l-lysyl-l-lysine than as free lysine.
3. During perfusion of both dipeptides significantly greater quantities of free lysine were found within the intestinal lumen in cystinuric subjects than in normal subjects. This is thought to be due to back-diffusion after hydrolysis of dipeptide by the mucosa.
4. These observations confirm the existence of dipeptide transport systems independent of those for free amino acids.