Absorption of two tripeptides by the human small intestine: a study using a perfusion technique.

1. A double-lumen perfusion technique has been used in man to study intestinal absorption of two tripeptides, glycyl-glycyl-glycine and L-alanyl-glycyl-glycine. Intraluminal tripeptide hydrolysis has been investigated by incubating both tripeptides in vitro with fresh uncentrifuged saline aspirates. 2. Glycine was absorbed faster from a glycyl-glycyl-glycine solution (3.3 mmol/l) than from an equivalent (10 mmol/l) glycine solution, and both L-alanine and glycine were absorbed faster from 10 mmol/l L-alanyl-glycyl-glycine than from the equivalent amino acid mixture. Free amino acids and dipeptides were aspirated during the tripeptide perfusions. 3. L-Alanyl-glycyl-glycine was hydrolysed faster than glycyl-glycyl-glycine in vitro, and results indicated that hydrolysis of the tripeptides occurred from the N-terminal end of the molecule. Intraluminal tripeptide hydrolysis was not sufficient to account for the concentrations of free amino acids and dipeptides liberated during the tripeptide perfusion experiments in vivo. 4. It is suggested that the liberated amino acids and dipeptides either diffused back into the luminal contents after hydrolysis in the brush border had taken place, or effluxed from the mucosal cell after intracellular tripeptide hydrolysis.