The present invention herein provides, for instance, a stable isotope-labeled phenylalanine wherein a carbon atom of the phenyl group linked to an amino acid residue is 13C, 2 to 4 carbon atoms of the remaining 5 carbon atoms constituting the phenyl group are 12C atoms to which deuterium atoms are bonded, and the remaining carbon atoms are 13C atoms to which hydrogen atoms are linked, and a stable isotope-labeled tyrosine wherein a carbon atom of the phenyl group linked to an amino acid residue is 13C, the carbon atom bonded to the hydroxyl group (OH group) of the phenyl group is 12C or 13C, 2 to 4 carbon atoms of the remaining 4 carbon atoms constituting the phenyl group are 12C atoms to which deuterium atoms are bonded, and the remaining carbon atoms are 13C atoms to which hydrogen atoms are linked. The stable isotope-labeled amino acid permits the elimination of such a conventional problem concerning the complexity of the NMR signals ascribed to aromatic rings, the complexity being a principal cause of making the NMR analysis difficult, encountered when using the conventional uniformly labeled amino acid residue. Moreover, the isotope-labeled amino acid likewise permits the substantial improvement of the sensitivity thereof to the NMR spectroscopic analysis.
