Evaluation of role of the peptide transport system in absorption of dipeptides in the rat small intestine in chronic experiments in vivo

Abstract

Hydrolysis and absorption of glycylgycine and glycyl-L-leucine as well as absorption of glycine and leucine were studied in chronic experiments on rats with their isolated small intestine loop. Values of the true kinetic constants (with taking into account effect of the preepithelial layer) were determined to be as follows: 1) Kt = 46.7 +/- 4.0 and 2.15 +/- 0.59 mM; Jmax = 0.74 +/- 0.15 and 0.16 +/- 0.03 micromol x min(-1) x cm(-1) (for the transport of free glycine and leucine, respectively); 2) Kt = 4.4 +/- 0.6 and 4.8 +/- 0.9 mM, Jmax = 0.24 +/- 0.02 and 0.23 +/- 0.02 micromol x min(-1) x cm(-1) (for the transport of glycylgycine and glycyl-L-leucine, respectively); 3) Km = 5.4 +/- 1.0 and 38.2 +/- 4.4 mM, Vmax = 0.09 +/- 0.02 and 0.24 +/- 0.07 micromol x min(-1) x cm(-1) (for membrane hydrolysis of these dipeptides, respectively). According to our calculations, in the wide range of the initial glycylgycine concentrations (2.5-40 mM) the part of the peptide component in its total absorption accounts for 0.77-0.80. In the case of glycyl-L-leucine the part of the peptide component in the total glycine absorption decreases from 0.89 to 0.84, while in the total leucine absorption--from 0.86 to 0.71, the initial dipeptide concentration rising from 5 to 40 mM. The obtained results show that the peptide component prevails in absorption of the studied dipeptides in the rat small intestine, but its role is much lesser that what many authors believe. In the case of glycyl-L-leucine, the peptide component can achieve saturation in the range of high substrate concentrations, its part decreasing essentially to become compared with absorption of free amino acids formed as a result of the dipeptide membrane hydrolysis.