1. A commercial amino acid solution supplemented with two synthetic dipeptides, l-alanyl-l-glutamine (Ala-Gln) and glycyl-l-tyrosine (Gly-Tyr), or alternatively with isonitrogenous amounts of free alanine and glycine has been continuously infused over 4 h in six apparently healthy volunteers.

2. The infusion of the solutions was not accompanied by any side effects and the volunteers reported no complaints.

3. Infusion of the alanine- and glycine-supplemented control solution resulted in an increase of the concentration of these amino acids, while no appreciable change in free glutamine concentration was observed and free tyrosine revealed a steady decrease throughout the infusion.

4. Infusion of the peptide-supplemented solution resulted in a prompt equimolar liberation of the constituent free amino acids (glutamine, alanine, tyrosine and glycine), approaching steady state after about 30 min infusion, while only trace but stable concentrations of the two dipeptides were measured throughout the infusion. No peptides were detectable in urine. The findings suggest a nearly quantitative extracellular hydrolysis of the infused dipeptides and indicate a subsequent utilization of the liberated free amino acids.

5. The estimated metabolic clearance rates and total body plasma clearances were very similar for the two dipeptides (Ala-Gln 35.9 ± 9.5 ml min−1 kg−1 and 2.9 ± 0.9 1/min, respectively; Gly-Tyr 33.7 ± 9.5 ml min−1 kg−1 and 2.7 ± 0.9 1/min, respectively); thus there is little difference in the metabolic handling of these dipeptides.

6. The study provides firm evidence that the synthetic dipeptides Ala-Gln and Gly-Tyr are quantitatively hydrolysed and that these peptides can be used as a safe and efficient source of free glutamine and tyrosine as part of a commercial solution.

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