To examine the origin of urinary hydroxyproline peptides, the metabolism of the radioactive tripeptide, glycyl-¹⁴C-prolylhydroxyproline, was investigated in normal young rats in vivo. The radioactive tripeptide was synthesized from glycine, L-(U-¹⁴C)proline and hydroxy-L-proline in our laboratory. The distributions of the radioactivity in body protein, lipid and soluble fractions were 23.7, 1.8 and 0.12% of the injected dose, respectively, 56 hr after the intraperitoneal injection of the ¹⁴C-tripeptide. The excretions of the radioactivity into expired carbon dioxide and urine were 29.6 and 34.2% of the injected dose, respectively, and large proportions of both the ¹⁴C excretions occurred during the first 12hr.
The results suggest that not a small amount of the glycylprolylhydroxyproline peptide injected is hydrolyzed in tissues of animals and the free proline derived is used for protein synthesis and/or further degraded to expired carbon dioxide.

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