Metabolic Fate of Free and Peptide-Linked Phenylalanine During Infusion into Portal or Femoral Vein of Rats

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Abstract
[14C]Phenylalanine, either as free amino acid or as glycylphenylalanine, was continuously infused over 3 h into the portal or femoral vein of anaesthetized Wistar rats. Radioactive phenylalanine and metabolites, as well as labelled proteins, were determined in plasma during infusion and at the end of the experiments in kidney, liver, brain, heart, and femoral muscle. No differences in plasma and tissue pools were observed, if the results after portal versus femoral infusion are compared. The infusion of peptide-linked phenylalanine results in a distribution of radioactivity similar to that occurring after infusion of the free amino acid. Peptidase activity toward glycyl-phenylalanine was found in plasma as well as tissues. The unhydrolyzed di-peptide could not be detected in plasma or in the tissues. These data suggest that administration of nutrients into the peripheral vein does not alter their metabolic fate.

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