Add time:07/19/2019         Source:sciencedirect.com

A specific method is described for the quantitative analysis of deuterated and non-deuterated phenylalanine and tyrosine in human plasma by gas chromatography-mass spectrometry using selective ion monitoring. From the several derivatives investigated, the N- or N,O-trifluoroacetyl methyl esters were found to be the most suitable for our purposes. dl-Phenylalanine-4-d1 and l-tyrosine-d7 were used as internal standards. The sensitivity of this method permits the measurement of amounts as small as ca. 2.5 ng/ml in plasma for both phenylalanine and tyrosine. The coefficients of variation were found to be ca. 1.6% (n = 12) for phenylalanine and 3.0% (n = 12) for tyrosine.Using this method, an in vivo determination of phenylalanine-4-monooxygenase activity in humans is possible by loading the subjects with deuterated l-phenylalanine-d5 (accepted as substrate by phenylalanine-4-monooxygenase E.C. 1.14.16.1) and the subsequent measuring of deuterated l-tyrosine-d4 formed and residual l-phenylalanine-d5.

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