A fully automated chiral capillary electrophoresis-tandem mass spectrometry (CE-MS/MS) method was developed for enantiomeric quantification of
3,4-dihydroxyphenylalanine (
DOPA) and its precursors,
phenylalanine (Phe) and
tyrosine (Tyr). To avoid MS source contamination, a negatively charged chiral selector, sulfated β-
cyclodextrin (sulfated β-CD), that migrated away from the detector was used in combination with the partial filling technique. The six stereoisomers were simultaneously quantified in less than 12 min. Detection limits were 0.48 and 0.51 μM for l- and d-
DOPA enantiomers, respectively. Assay reproducibility values (relative standard deviations [RSDs], n=6) were 4.43, 3.15, 4.91, 5.16, 3.96, and 3.25% for l- and d-
DOPA, l- and d-Tyr, and l- and d-Phe
at 10 μM, respectively. Thanks to the high enantioseparation efficiency, detection of trace d-
DOPA in l-/d-
DOPA mixtures could be achieved. The assay was employed to study the metabolism of
DOPA, a well-known therapeutic
drug for treating
Parkinson's disease. It was found that
l-DOPA was metabolized effectively in PC-12 cells. Approximately 88% of
l-DOPA disappeared after incubation at a cell density of 2×10(6)cells/ml for 3 h. However, d-
DOPA coexisting with
l-DOPA in the incubation
solution remained intact. The enantiospecific metabolism of
DOPA in this neuronal model was demonstrated.