Removal of glycylglutamine from plasma by individual tissues: mechanism and impact on amino acid fluxes in postabsorption and starvation

A possible source of glutamine, for inclusion in the parenteral solutions, is glycylglutamine. The aim of this article is to review briefly the information on metabolism of glycylglutamine when administered intravenously. The fact that there is efficient utilization of intravenously infused glycylglutamine was evident with very little excretion in the urine. Although all the tissues examined, except brain, participated in the removal of glycylglutamine from plasma, kidney predominated in this regard. This may be related to the presence of carrier-mediated systems for cellular uptake of glycylglutamine in the kidney and the lack of them in other tissues. Starvation did not alter the metabolic clearance of glycylglutamine, although it reduced the removal by the kidney. Renal metabolism of glycylglutamine resulted in the release of constituent amino acids that were largely utilized by the liver in the postabsorptive state and by skeletal muscle in starvation. This alteration was accompanied by a selective inhibition of muscle release of amino acids that are substrates for enhanced hepatic gluconeogenesis and renal ammoniagenesis in starvation. Because there was no change either in plasma glucose level or ammonia excretion during the infusion of glycylglutamine in starved human subjects, apparently the amino acid residues of glycylglutamine fulfilled the substrate needs for these functions. These results provide a metabolic basis for further investigations of the possible nutritional benefit of including glycylglutamine in parenteral nutrition.