Transport of glycyl-L-proline into intestinal and renal brush border vesicles from rabbit.

Transport of labeled glycyl-L-proline has been shown to occur with highly purified brush border membrane vesicles from the epithelial cells of rabbit small intestine and renal cortex. With 1-min incubation, transport occurs mainly as the intact dipeptide since less than 10% of the dipeptide in the medium is hydrolyzed within the period. The properties of the dipeptide transport system are similar in both small intestinal and renal brush border membrane vesicles. The steady state transport varies inversely with medium osmolarity. Extrapolation to infinite medium osmolarity indicates that transport occurs predominantly into an osmotically reactive intravesicular space rather than binding to the membranes. The affinity constants (Kt) for glycyl-L-proline transport in small intestinal and renal brush border membrane vesicles are comparable (0.9 mM in intestine and 1.1 mM in kidney). Under conditions in which presence of a Na+ gradient between external and intravesicular media stimulated L-alanine transport, glycyl-L-proline transport remains unaffected. Other dipeptides strongly inhibit the transport of glycyl-L-proline but amino acids have no effect. The selective inhibition of glycyl-L-proline transport by other dipeptides is observed in the presence as well as in the absence of a Na+ gradient. Harmaline inhibits Na+-stimulated L-alanine transport but it has no effect on glycyl-L-proline transport even in the presence of Na+. In these respects, dipeptide transport seems to differ from amino acid transport. It is proposed that the Na+ gradient hypothesis of sugar and amino acid transport is not applicable for dipeptide transport. These data provide additional evidence for the distinct nature of amino acid and dipeptide transport systems.