COMPARISON OF METABOLISM OF GLYCINE INJECTED INTRAVENOUSLY IN FREE AND DIPEPTIDE FORMS

Dipeptides, like diglycine, when injected intravenously in rats, are quickly taken up by tissues and hydrolyzed. Metabolism of glycine residue released by diglycine hydrolysis was investigated in this study. Radioactive labeling of nonprotein [trichloroacetic acid (TCA) soluble] and protein fractions of pancreas, intestinal mucosa, liver, kidney cortex, lung, gastrocnemius muscle, brain and plasma, as well as expired CO2, were determined after injection of 14C-diglycine. Results were compared with those obtained after equivalent 14C-glycine injection. The effect of tissue dipeptide hydrolysis on distribution of radioactivity was examined after injection of 14C-glycylsarcosine under similar experimental conditions. Generally, intravenous injection of diglycine was at least as effective as that of glycine in labeling of nonprotein and protein fractions; labeling of kidney TCA-soluble pool was significantly higher and only that of muscle was lower after diglycine injection. Tissue capacities for transport and protein synthesis appeared to determine the extent of nonprotein and protein labeling, respectively, after both 14C-glycine and 14C-diglycine injections. Radioactive labeling of nonprotein pools and proteins were influenced by tissue capacities for dipeptide hydrolysis; both were higher in kidney and lower in pancreas and liver when glycylsarcosine was injected as compared to diglycine. Oxidation of injected glycine (labeling of expired CO2) was significantly enhanced when glycine was given in the form of diglycine but was reduced when given as glycylsarcosine as compared to free-glycine injection. In conclusion, glycine residue of a readily hydrolyzable peptide, like diglycine, is as effective as free glycine when injected intravenously for incorporation into tissue proteins (except muscle) and for oxidative energy metabolism, while injection of an hydrolysis-resistant peptide like glycylsarcosine favors incorporation of glycine residue into kidney protein at the expense of other tissues. © 1979.