Metabolic Fate of β-Aspartyl-(14)C-glycine in Normal Young Rats

The metabolic fate of α- and β-L-aspartyl-[U-(14)C]glycine was investigated in normal young rats in vivo and in vitro. The radioactive dipeptides were synthesized from L-aspartic acid and [U-(14)C]glycine in our laboratory. When labeled β-aspartylglycine was given intraperitoneally, about 66% of the dose was excreted in the urine and 8% was recovered in the expired carbon dioxide over a 24-hr period. More than 90% of the urinary radioactivity was present in the β-aspartylglycine fraction of the urine. When the labeled α-aspartylglycine was given, 3% and 22% of the dose were recovered, respectively, in the urine and expired carbon dioxide. In slice experiments with kidney, liver and small intestine from normal rats, α-aspartylglycine was rapidly and almost completely hydrolyzed, and a large amount of free (14)C-glycine was released. In contrast, β-aspartylglycine was hardly hydrolyzed to the corresponding amino acids in liver and small intestine, and only slightly in kidney. These results suggest that in normal young rats most β-aspartylglycine, which may originate from endogenous tissue proteins, is hardly hydrolyzed and rapidly excreted into the urine.