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Effect of Protein Intake on Tissue Amino Acid Levels and the Enzymes of Serine Biosynthesis in the Rat¹

Departments of Medicine, Biochemistry and Pathology, University of North Carolina School of Medicine, Chapel Hill, North Carolina

Enzymes of serine metabolism including 3-P-glycerate dehydrogenase, P-serine phosphatase, D-glycerate dehydrogenase and serine dehydratase were measured in several tissues of the rat under varying dietary conditions. The enzymes of the phosphorylated pathway of serine biosynthesis were present in liver, kidney and brain, but not in heart, muscle or intestine. Low protein diets resulted in marked increases in the activity of 3-P-glycerate dehydrogenase and P-serine phosphatase in liver, but not in kidney or brain. This increased enzyme activity in liver was correlated with a depression in the tissue concentration of cystine, methionine and taurine. Supplementation of the diet with methionine or cystine reversed the enzyme changes and partially reversed the decrease in sulfur-containing amino acid levels in the liver. No changes in amino acid content occurred in kidney and brain with the diets or supplements studied. The data suggest the probable association of the concentration of specific amino acids with levels of 3-P-glycerate dehydrogenase in various tissues. A depression of hepatic serine, glycine, and alanine occurred when the low protein diet was supplemented with methionine. Methionine completely prevented the rise of 3-P-glycerate dehydrogenase in the protein-depleted rats. This observation demonstrates the importance of the phosphorylated pathway of serine biosynthesis in maintaining hepatic serine biosynthesis in rats fed low protein diets.

Manuscript received 9 May 1968.