Studies on a wide-spectrum intestinal dipeptide uptake system in the monkey and in the human

Abstract

1. The intestinal transport of glycine and leucine residues of glycyl-L-leucine was studied in the monkey and in the human in vitro. Uptake of both [¹⁴C]glycyl-L-leuine and glycyl-L-[¹⁴C]leucine show similar K_t values, but there is a marked difference in the V_max. values. Preliminary studies suggest that this anomalous difference in the V_max. values may be due to the greater efflux rate of glycine from the tissue. 2. Arrhenius plots of both [¹⁴C]glycyl-L-leucine uptake and glycyl-L-[¹⁴C]leucine uptake in the monkey intestine show a discontinuity at about 20°C. The activation energies above and below the discontinuity are similar for both [¹⁴C]glycyl-L-leucine uptake and glycyl-L-[¹⁴C]leucine uptake. These similarities in uptake characteristics suggest that the dipeptide glycyl-L-leucine is transported as one unit. 3. In the monkey intestine, glycyl-L-leucine uptake is inhibited by a wide variety of dipeptides, including those containing acidic and basic amino acids. The inhibition was shown to be competitive by using four representative dipeptides namely: L-alanyl-L-alanine, L-alanyl-L-leucine, L-glutamyl-L-glutamic acid and L-lysyl-L-lysine. The results strongly suggest that in the monkey intestine there may be a dipeptide-uptake system with an extremely broad specificity. These results were also confirmed in the human in a limited way.