Effects of Small Peptides as Intraluminal Substrates on Transport Carriers for Amino Acids and Peptides.

The effects of small peptides on brush-border membrane enzyme activities and on transport carriers for amino acids and dipeptides were investigated by measurement of these enzyme activities and the transmural potential difference in the small intestines of guinea pigs fed elemental diets containing either small peptides (SP) or amino acids (AA) as the nitrogen source. Brush-border membrane aminopeptidase activities were significantly higher in the group fed the diet containing SP. In addition, the transmural potential differences induced by L-leucine and glycyl-L-leucine (Gly-Leu) were significantly higher in the SP group. The maximum potential differences for L-leucine and Gly-Leu were significantly higher in the SP group, whereas the half saturation values did not differ between the two groups. The same effect of a SP diet was seen in rats, when the uptake of L-[U-14C]leucylglycine (L-[14C]Leu-Gly) into intestinal brush-border membrane vesicles was measured. Some 70% of L-[14C]Leu-Gly was not hydrolyzed by aminopeptidase, but was found as a dipeptide in the brush-border membrane vesicles. These results indicate that small peptides as intraluminal substrates increase brush-border membrane aminopeptidase activities and the activity of carriers for amino acids and dipeptides.