Functional Characterization of Dipeptide Transport System in Human Jejunum

Siamak A. Adibi; Mohammad R. Soleimanpour

doi:10.1172/JCI107685

Published May 1, 1974 - More info

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Abstract

The present studies were performed to determine whether dipeptide absorption in human jejunum exhibits the characteristics of carrier-mediated transport. 15-cm jejunal segments from human volunteers were perfused with test solutions containing varying amounts of either glycylglycine, glycylleucine, glycine, leucine, glycylglycine with leucine or glycine, glycylglycine with glycylleucine, or glycylleucine with an equimolar mixture of free glycine and leucine. Jejunal absorption rates of both glycylglycine and glycylleucine followed the kinetics of a saturable process. The K_m value in millimoles/liter of glycylglycine was significantly greater than the K_m value of glycylleucine (43.3±2.6 vs. 26.8±5.9, P < 0.05); and the K_m value of glycine was also significantly greater than the K_m value of leucine (42.7±7.5 vs. 20.4±5.4, P < 0.05). While overlapping occurred among the K_m values of free amino acids and dipeptides, the transport kinetics of dipeptides were characterized by higher V_max values (in micromoles per minute per 15 centimeters) than those of free amino acids. For example, the V_max values for glycylglycine and glycine were 837±62 and 590±56, respectively (P < 0.02). While jejunal absorption rates of glycylglycine were not significantly affected by free leucine or free glycine, they were competitively inhibited by glycylleucine. The jejunal absorption rate of glycylleucine was not significantly altered by an equimolar mixture of free glycine and leucine. The selective absorption of dipeptides was investigated by infusing three equimolar mixtures, each containing two different dipeptides. Among the three dipeptides examined, glycylglycine was the least absorbed. There was no significant difference between the absorption of glycylleucine and leucylglycine.

The above studies suggest that absorption of both glycylglycine and glycylleucine is mediated by a carrier which is not shared with free neutral amino acids; and that both COOH- and NH₂-terminal amino acids appear to be influential in imposing the affinity of a dipeptide for the absorption sites.