Determination of multiply labeled serine and glycine isotopomers in human plasma by isotope dilution negative‐ion chemical ionization mass spectrometry

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Abstract
A gas chromatography/negative‐ion chemical ionization mass spectrometry method is presented to measure the isotopic enrichment of multiple serine and glycine isotopomers. The amino acid N‐heptafluorobutyryl n‐propyl ester derivatives were used. The method had good analytical linearity between 10 and 800 μg mL−1 and both precision and accuracy were 5% for plasma amino acids. Sensitivity permitted analysis of 100 pg amino acid on column. The method was applied to metabolic studies of the serine to glycine interconversion in humans. (α‐15N)serine and (1,2‐13C2)glycine were given as a single intravenous bolus to six healthy male subjects. Plasma concentration of glycine and serine were determined after addition of (α‐15N,1,2,3‐13C3) serine and (α‐15N,1,2‐13C2)glycine as internal standards to 500 μL of plasma. Since glycine and serine are rapidly interconverted by hepatic serine hydroxymethyl transferase, the resultant tracer spectrum requires deconvolution of the enrichment of four isotopomers of each amino acid. Deconvolution of the ion abundance ratios to yield tracer‐to‐tracee ratios for each isotopomer was done using Brauman's least squares approach.