Enrichment of glycine pool in plasma and tissues by glycine, di-, tri-, and tetraglycine

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Abstract
Very little information is available on metabolism of oligopeptides in vivo. The present studies were performed to investigate the metabolic fate of diglycine, triglycine, and tetraglycine when injected into a central vein in rats. These peptides disappeared rapidly from plasma without any significant loss in urine. Plasma and tissue concentrations of glycine were measured when the same amount of glycine was injected in free or peptide form. Two minutes after the injection of glycine (1.l0 mumol/g body wt), there was over a tenfold increase in plasma glycine concentration. This increase was diminished when diglycine instead of glycine was injected. Each increase in the number of glycine residues resulted in further reduction in the initial rise in plasma glycine concentration was increased by each injection. This was more pronounced in the kidney than in the liver. Injection of triglycine and tetraglycine resulted in greater glycine concentration in the kidney than injection of either glycine or diglycine. Furthermore, unlike liver and muscle, each increase in the number of glycine residues resulted in greater recovery of glycine peptides from the kidney. These results suggest that with each increase in the number of glycine residues a greater amount of injected glycine peptide is taken up by the kidney for hydrolysis to glycine.