The synthesis of glycine and serine by the rat

Abstract

Amino acid or casein diets containing 0.01, 0.1, 0.5 or 2% [alpha-Cl4] glycine or 0.7% L- [beta-C14] serine were fed to groups of rats over periods extending from 20 to 39 days. Food consumption and changes of body wt. were measured. Using diets containing 2% [alpha-C14] glycine it was shown that the specific radioactivity of the glycine of the visceral proteins, which was about 40% of that of the fed amino acid, was not affected by the rate of growth nor by variation of the feeding period within the limits mentioned. Radioactivity of the glycine of the carcass increased with increasing growth rate. It was assumed that the amino acid present in the visceral protein at the beginning of the expt. was replaced almost completely and that the dietary glycine or serine and the glycine or serine synthesized by the animal are metabolically indistinguishable. The avg. daily amt. of glycine or serine synthesized was calculated and found to be constant and independent of the level of glycine in the diet, the mean being 2.5 m-moles/100 g. body wt./day. The corresponding figure for serine was 3.5, suggesting that endogenous glycine is largely derived from serine. The conversion of the alpha-C atom of glycine into the beta-C atom of serine was greatly diminished when the level of glycine in the diet was decreased, indicating that this pathway may be concerned with the disposal of excess glycine. It is concluded that the constant level of glycine in the tissues is maintained by regulation of the rate of degradation and not by variations in the rate of synthesis.