Selective ion monitoring of tryptophan, N-acetyltryptophan and kynurenine in human serum. Application to the in vivo measurement of tryptophan pyrrolase activity.

A specific method is described for the determination of deuterated and non-deuterated N-acetyltryptophan, tryptophan and kynurenine in human serum and urine using gas chromatography-mass fragmentography. N-Acetyltryptophan was analysed as the N-trimethylsilyl methyl ester derivative; tryptophan and kynurenine were converted into their N-pentafluoropropionyl methyl esters. N-Acetyl-DL-tryptophan-d11, tryptophan-d8 and kynurenine-d2 were used as internal standards. The coefficients of variation were found to be about 8% (n = 9) for tryptophan and N-acetyltryptophan and about 2.4% (n = 9) for kynurenine. Using this method, an in vivo determination of the tryptophan pyrrolase activity [L-tryptophan oxygen 2,3-oxidoreductase (decyclizing), E.C. 1.13.11.11] is possible by loading the subjects with deuterated L-tryptophan-d5 and subsequently measuring the deuterated L-kynurenine-d4 formed and the residual L-tryptophan-d5.