Synthesis and back exchange of 18O labeled amino acids for use as internal standards with mass spectrometry.

Amino acids with two oxygen-18 atoms in the carboxyl moiety can be synthesized easily from the amino acids and H2 18O through exchange. The exchange conditions are simply solution of the amino acid (1-4 mg) in acidic H2 18O (0.1-0.2ml) followed by heating at 60-70 degrees C for several days. Eighteen oxygen-18 amino acids with incorporations typically greater than 90% 18O2 have been achieved and characterized by mass spectrometry. The exchange conditions do not lead to racemization. Loss of the oxygen-18 atoms through back exchange was found to be pH and temperature dependent, but yet slow enough to allow these isotopimers to be used in most studies as internal standards for quantitative mass spectrometry. The 18O carboxylic atoms were not exchanged upon incubation in plasma for three days at 37 degrees C, PH 7.4. The exchange reaction is general enough to allow the simultaneous synthesis of several [18O] amino acids.