An interference-free two-step enzyme assay with UPLC–tandem mass spectrometric product measurement for the clinical diagnosis of uridine diphosphate galactose-4-epimerase deficiency

We present a robust clinical assay for the measurement of red blood cell uridine diphosphate galactose-4-epimerase enzyme activity for the diagnostic confirmation of patients positive for a newborn screen for inherited galactosemia in whom galactose-1-phosphate uridyltransferase activity is normal. Previous assays required the use of ion-pairing reagents and frequent need for system maintenance that was not appropriate for heavy clinical use where patient results should be quickly available. We have designed a two-step enzyme assay which converts stable-isotope-labeled UDP-galactose to isotope-labeled-UDP-glucose which is converted in the second reaction to the final product of [13C6]-UDP-glucuronic acid. Measurement conditions t remove potential interference from endogenous UDP-glucose and UDP-galactose. We also report a significant ion suppression effect of the red cell preparation for which we have optimized assay sample volume to minimize this effect.