Mechanisms of glycyl‐L‐leucine uptake by guinea‐pig small intestine: relative importance of intact‐peptide transport.

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Abstract
1. Characteristics of glycyl‐L‐leucine influx across the mucosal border of isolated guinea‐pig ileum have been investigated. The influx of the peptide was measured with glycine‐labelled or leucine‐labelled compounds (Gly*‐Leu or Gly‐Leu*) and compared with that of a constituent amino acids under various experimental conditions 2. Gly‐Leu* influx over a wide range of peptide concentrations. The latter obeyed simple Michaelis‐‐Menten kinetics whereas the former could be described in terms of two saturable components. 3. Total replacement of medium Na with mannitol had no effect on Gly*‐Leu influx, while it markedly reduced Gly‐Leu* influx to a level slightly greater than Gly*‐Leu influx. L‐Leucine influx was partially dependent on Na in contrast to glycine influx which was absolutely dependent on Na. 4. Gly*‐Leu influx was not inhibited by the simultaneous presence of glycine or L‐isoleucine, while Gly‐Leu* influx was strongly inhibited by L‐leucine and L‐isoleucine. Gly‐Leu* influx under submaximal inhibition by L‐isoleucine was about the same as Gly*‐Leu influx. Di‐ or tri‐glycine did not inhibit glycyl‐L‐leucine influx, while glycyl‐L‐leucine markedly inhibited diglycine influx, the inhibition being not competitive but of the mixed type. 5. A Michaelis‐‐Menten type relation was observed for the increment in the transmural potential induced by glycyl‐L‐leucine, L‐leucine or the mixture of the dipeptide and L‐leucine. In all cases, the values of the maximum potential change were identical, suggesting that a single electrogenic transfer mechanism was operating in these cases. 6. It is concluded that about a half of glycyl‐L‐leucine influx is mediated by a carrier system for intact glycyl‐L‐leucine which is independent of sodium, and the other half is transported as L‐leucine after membrane surface hydrolysis, part of this component being sodium‐dependent and electrogenic. It is also suggested that the carrier sites for glycyl‐L‐leucine and glycylglycine are very closely located but separated.