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The role of some small peptides in the transfer of amino nitrogen across the wall of vascularly perfused intestine.

The characteristics have been investigated of the transfer into the vascular bed of L‐leucine and glycine from free amino acids or peptides in the intestinal lumen of Rana pipiens. Over the concentration range 0‐5‐10 mM the transfer of L‐leucine is but little affected by the presence of equimolar concentrations of glycine but the transfer of glycine, in contrast, is greatly inhibited by the presence of L‐leucine. 2. With glycyl‐L‐leucine in the intestinal lumen, the rate of transfer of glycine into the vascular bed is much greater than from the mixture of the two amino acids and is equal to that of the L‐leucine. From L‐leucyl‐glycine the rates of transfer of leucine and of glycine are also higher than from the mixture of the two amino acids but the rate of transfer of glycine is somewhat lower than that of leucine. There is no evidence of the presence of the dipeptides in the effluent from the portal vein. 3. When the peptide glycyl‐L‐leucine is added to the lumen in the presence of 10 mM concentrations of the free amino acids, additional amounts of L‐leucine and of glycine are transferred in approximately equimolar quantities into the vascular bed. This additional transfer exhibits saturation with respect to concentration of peptide in the intestinal lumen. An additional transfer of amino acids was also found when L‐leucyl‐glycine was added to the lumen in the presence of saturating concentrations of the two amino acids. 4. Evidence is presented that the presence of the dipeptides in the intestinal lumen had little effect on the transfer of free amino acids from the lumen into the vascular bed. Although the transfer of free amino acids from the lumen into the vascular bed is significantly, but not completely, abolished when the Na in the intestinal lumen is replaced by K, the transfer of the amino acids from the dipeptides is but little affected. 5. The findings are discussed in relation to the view that the dipeptides are transported into the mucosal epithelium by a process that is distinct from those promoting uptake of the individual amino acids. Being completely hydrolysed, there is no evidence for an accumulative uptake of the peptides; it is suggested that this may be related to the fact that the peptide uptake occurs in the absence of Na in the intestinal lumen.