Erythrocyte galactose 1-phosphate quantified by isotope-dilution gas chromatography-mass spectrometry.

BACKGROUND Measurements of alpha-D-galactose 1-phosphate (Gal-1-P) in erythrocytes are used to monitor the adequacy of dietary therapy in the treatment of galactosemia. We have devised a gas chromatography-mass spectrometry (GC/MS) isotope-dilution method for quantification of Gal-1-P. METHODS We prepared trimethylsilyl (TMS) derivatives and used alpha-D-[2-(13)C]Gal-1-P as the internal standard for GC/MS. Results obtained with this method were compared with those determined by the established enzymatic method for samples from 23 healthy individuals (11 children and 12 adults), 9 suspected patients with galactosemia, 12 galactosemic patients on diet therapy, and 2 newly diagnosed toxic neonates. RESULTS The method was linear up to 2.5 mmol/L with a lower limit of detection of 2.1 nmol (0.55 mg/L). Intra- and interassay imprecision (CVs) was 2.2-8.8%. In the 23 healthy individuals, values ranged from nondetectable to 9.2 micromol/L (2.4 mg/L of packed erythrocytes). Galactosemic patients on diet therapy had values of 10.9-45 mg/L of packed erythrocytes, whereas the newly identified patients had values of 166 and 373 mg/L. CONCLUSIONS The GC/MS method is precise and useful over the wide range of concentrations needed to assess the galactose burden in patients with galactosemia.