Differences in Characteristics between Glycine and Glycylglycine Transport in Guinea Pig Small Intestine

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Abstract
The properties of glycylglycine and glycine transport across the mucosal border of the epithelium were studied in isolated guinea pig ileum. The initial influxes of both substances obeyed simple Michaelis-Menten kinetics, but the values of the kinetic parameters were greatly different between 2 substances; Jmax [maximal influx] and Kt [transport constant] for glycine were 290 nmol .cntdot. cm-2 .cntdot. min-1 and 27 mM, and those for glycylglycine were 57.4 nmol .cntdot. cm-2 .cntdot. min-1 and 3.4 mM, respectively. Total replacement of Na+ in the incubation medium with mannitol completely suppressed the cellular uptake of glycine, while it reduced glycylglycine uptake only slightly. The omission of Na+ caused a slight increase in Kt for glycylglycine (from 3.4 mM at 100 mM Na+ to 5.2 mM at 0 mM Na+) without changing the value of Jmax. Glycylglycine taken up by the epithelial cells was completely hydrolyzed into glycine within the 2 min incubation period. There apparently are entirely distinct transport systems for glycine and its dimer; the membrane digestion and subsequent transport as the monomer may not be involved in the absorption of glycylglycine.