Isomers of l-DOPA and dopamine with a nine-atom 19F atom tag were exposed to aromatic acid decarboxylase (AADC) in the lysate of Escherichia coli JM109 that had been transformed with the plasmid pKKAADCII. The resulting samples were analyzed with HPLC. The first study investigated the conversion of the tagged l-DOPA into tagged dopamine, using the tagged dopamine as a standard. A second study was undertaken to identify the source of peaks seen in the enzymatic assays. l-DOPA with the tag bonded at position 5 served as the best substrate for AADC. Isomers that fit into the active site of AADC are likely to follow the biosynthetic path for dopamine in vivo and are potentially useful in magnetic resonance studies. The enzymatic assay described here provides an efficient and cost-effective tool for screening new compounds for use in the fluorine imaging of neural pathways.

A novel compound for use in magnetic resonance (MR) imaging was created by covalently bonding multiple 19F atom tags to L-DOPA. Tagging L-DOPA permits bypassing the rate-limiting factor in the biosynthesis of dopamine (DA), the conversion of tyrosine into L-DOPA. The next step in the biosynthetic pathway, the removal of the carboxyl group on the molecule by the enzyme L-aromatic acid decarboxylase (AADC), happens rapidly after L-DOPA is taken up into neurons. In order to be useful as a tool in MR imaging, the novel compound and/ or its perfluoro-tagged metabolites must accumulate in vesicles in dopaminergic neurons. We administered L-DOPA with a nine 19F atom tag (250 mg/kg) to mice pups, waited 1.5 or 3 hr, and used high pressure liquid chromatography (HPLC) to examine neural tissue samples for tagged L-DOPA and tagged DA. The isomer of L-DOPA with the tag bonded at the 5 position yielded the highest conversion to tagged DA at 1.5 hr after an i.p. injection. This study provides the first direct evidence that L-DOPA, tagged with nine fluorine atoms, is taken up into mammalian brain dopaminergic neurons where it is converted to perfluro-tagged DA. The use of these tagged compounds may make it feasible to investigate the uptake and conversion of important neurotransmitter in vivo with fluorine imaging.