SIMULTANEOUS DETERMINATION OF STABLE ISOTOPICALLY LABELED L-HISTIDINE AND UROCANIC ACID IN HUMAN PLASMA BY STABLE ISOTOPE-DILUTION MASS-SPECTROMETRY

A capillary gas chromatographic-mass spectrometric method for the simultaneous determination of stable isotopically labelled L-histidine (L-[3,3-H-2(2),1',3'-N-15(2)]histidine, L-His-[M + 4]) and urocanic acid ([3-H-2,1',3'-N-15(2)]urocanic acid, UA-[M + 3]) in human plasma was developed using DL-[2,3,3,5'-H-2(4),2'-C-13,1',3'-N-15(2)]histidine (DL-His-[M + 7]) and [2,3,5'-H-2(3),2'-C-13,1',3'-N-15(2)]lurocanic acid (UA-[M + 6]) as internal standards. L-Histidine and urocanic acid were derivatized to (alpha)N-(trifluoroacetyl)-(im)N-(ethoxycarbonyl)-L-histidine n-butyl ester and (im)N-(ethoxycarbonyl)urocanic acid n-butyl ester. Quantification was carried out by selected ion monitoring of the molecular ions of the respective derivatives of L-His-[M + 4], DL-His-[M + 7], UA-[M + 3] and UA-[M + 6]. The sensitivity, specificity, precision and accuracy of the method were demonstrated to be satisfactory for measuring plasma concentrations of L-His-[M + 4] and UA-[M + 3] following administration of trace amounts of L-His-[M + 4] to humans.