Glycyl-L-leucine transport in the rat small intestine

The uptake of the peptide glycyl-L-leucine across the brush border of the rat small intestinal enterocyte was studied using everted rings. The transfer of leucine from the dipeptide into the enterocyte was greater than the glycine uptake from glycyl-L-leucine. This additional component for leucine transport was found to be abolished by the removal of sodium, by the presence of 10 mM L-alanine beta-naphthylamide, and by excess free leucine. In contrast, the uptake of glycine from glycyl-L-leucine was not sodium-dependent and was not reduced by the presence of excess free leucine. The presence of harmaline over a concentration range up to 10 mM inhibited some of the peptide-bound leucine and glycine uptake. Transport of leucine from glycyl-L-leucine in the presence of 20 mM free leucine was competitively inhibited by glycyl-L-proline, although not completely. It is concluded that, in the rat, the sodium-sensitive component of amino acid uptake from the dipeptide represents free amino acid liberated by superficial hydrolysis, while the transport of the intact peptide is not sodium dependent. In addition, it appears that there are at least two routes of entry for the intact dipeptide glycyl-L-leucine, only one of which is shared with glycyl-L-proline.