Measurement of phenylalanine hydroxylating activity in vivo : (18)O-tyrosine in plasma after exposing rats to(18)O 2.

A gas chromatographic-mass spectrometric method was developed for analyzing phenylalanine and tyrosine (Tyr) in plasma and brain. With this procedure, we were able to show that(18)O2 is incorporated into Tyr in vivo and that the presence of(18)O-Tyr in plasma is a relative measure of phenylalanine hydroxylating activity. Treatment withp-chlorophenylalanine decreases the(18)O incorporation into Tyr. Because of the simplicity of the procedure and its easy adaptability to human studies, the incorporation of(18)O2 into endogenous constituents might serve as a useful diagnostic procedure for some metabolic disorders, such as phenylketonuria. These results also indicate that labeling with(18)O cannot be used to measure the turnover rate of brain catecholamines as previously proposed.