ENZYMATIC-CHEMICAL PREPARATION OF QUINOXALINE DERIVATIVES FROM L-AMINO-ACIDS FOR GAS CHROMATOGRAPHIC-MASS SPECTROMETRIC ANALYSES

We report on an enzymatic-chemical method for the specific preparation of L-amino acid-derived quinoxalinols suitable for the sensitive estimation of C-13- or H-2-label enrichment by gas chromatography-mass spectrometry: Amino acid fractions are isolated from physiological fluids by ion-exchange chromatography. The compound of interest is converted to the corresponding a-ore acid by treatment with an L-amino acid dehydrogenase of the desired specificity. Reaction of the a-ore acid with o-phenylenediamine yields the quinoxaline derivative. Isotopic label enrichment is then determined by gas chromatographic-mass spectrometric analysis of the O-trimethylsilyl derivative using an ammonia-chemical ionization mode and selected ion monitoring of the quasi-molecular ions [MH](+) and [MH + n](+) (n = number of labeled positions). Details for application of various generally available L-amino acid dehydrogenases (L-ala DH, L-glu DH, L-leu DH, L-phe DH) are presented. The method was used, e.g., for serum analysis in in vivo studies on the decarboxylation rates of branched-chain L-[1-C-13]-amino acids. (C) 1995 Academic Press, Inc.