Transport of glycyl-L-proline by human intestinal brush border membrane vesicles

This study characterizes the transport of [1-14C]glycyl-L-proline into purified brush border membrane vesicles prepared from human small intestine. Time-course uptake curves of glycyl-L-proline were similar under sodium thiocyanate or potassium thiocyanate gradient conditions (extravesicular greater than intravesicular) and did not show any overshoot phenomena. The transport of glycine and proline, however, was stimulated by the presence of sodium gradient. Measurement of peptide uptake with increasing medium osmolarity showed that glycyl-L-proline was transported into an osmotically reactive intravesicular space with insignificant binding to the surface of the vesicles. Only 2% of the glycyl-L-proline in the incubation media was hydrolyzed after 10 min of incubation. Also, there was no hydrolysis of peptide transported into the intravesicular space. The effects of increasing concentrations of glycyl-L-proline on uptake showed that uptake of the peptide was saturable and conformed to Michaelis-Menten kinetics with a Km of 4.1 +/- 0.5 mM and a Vmax of 1.53 +/- 0.07 nmol/mg protein X 0.5 min. Free amino acids did not inhibit the transport of glycyl-L-proline while dipeptides and tripeptides exerted appreciable inhibition (up to 60%). Our results show that human small intestinal brush border membrane vesicles transport glycyl-L-proline as an intact peptide by a carrier-mediated, Na+-independent process.