Characteristics of different amino acids as protein precursors in mouse brain: advantages of certain carboxyl-labeled amino acids

We have studied the metabolism of the carboxyl-labeled amino acids leucine, tyrosine, and valine in mouse brain and the incorporation of these isotopes into protein. When metabolized into compounds other than protein each of these amino acids is decarboxylated so that the radioactive portion of the molecule is lost as ¹⁴CO₂. During the first 16 min after intravenous administration, when the maximal rate of protein labeling occurs, radioactive metabolites from these amino acids do not build up. Nearly 90% of the label is in the precursor amino acid itself at 8 min and over 80% at 16 min. Because metabolites of the carboxyl-labeled amino acids do not accumulate, the trichloroacetic acid-soluble fraction is a simple and reliable measure of the radioactivity of the precursor amino acid available for protein synthesis. In addition, because radioactive metabolites are essentially absent they cannot label protein. The use of these carboxyl-labeled amino acids simplifies the technical and theoretical complications that arise in measuring brain protein synthesis and allows studies impractical with many other radioactive amino acids. In contrast, the corresponding tritiated amino acids—leucine-4,5-³H, tyrosine-3,5-³H, and valine-2,3-³H—give rise to large amounts of labeled metabolic products, including large quantities of tritiated water. Tritiated water makes up 50% of the total TCA-soluble fraction only 4-min after injection of tritiated leucine. Valine-2,3-³H gives rise to considerably fewer metabolic products than leucine-4,5-³H or tyrosine-3,5-³H apparently because the linear pathway for valine catabolism eliminates the tritium from valine before its products enter into central metabolism. All three amino acids, leucine, valine, and tyrosine, are rapidly incorporated into brain proteins. The rate of incorporation is nearly identical for leucine and tyrosine and somewhat slower for valine.