Patent ID: 8440169

Claim:
A stable isotope-labeled aromatic amino acid selected from those listed below: A stable isotope-labeled phenylalanine wherein a carbon atom of the phenyl group linked to a group represented by the following general formula A 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 a group represented by the following general formula A 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; −*1C(X)(Y)—*2C(Z)(15NH)(*3COOH) (A) wherein each of *1C, *2C, and *3C represents 12C or 13C atom, each of X, Y and Z represents a hydrogen or deuterium atom.