In case 4, a positron emission tomography scan using fluorine 18 L-3,4- dihydroxyphenylalanine
([.sup.18]F-DOPA PET) was performed in Frankfurt, Germany.
Levodopa had been discontinued at least 3 days beforehand, and baseline plasma dihydroxyphenylalanine
(DOPA) concentrations were within normal limits.
This report described a liquid chromatographic electrochemical detection (LCED) (4) method for measuring plasma concentrations of catecholamines as well as dihydroxyphenylglycol (DHPG), the main deaminated metabolite of norepinephrine, dihydroxyphenylalanine
(DOPA), the immediate precursor of dopamine, and dihydroxyphenylacetic acid (DOPAC), the deaminated metabolite of dopamine.
In melanogenesis, L-tyrosine is hydroxylated to dihydroxyphenylalanine
(L-DOPA), and then L-dOpA is oxidized to DOPA-quinone with two initial steps .
Neurotransmitters, such as gamma-aminobutyric acid (GABA) and dihydroxyphenylalanine
(DOPA), have been suggested to mimic the function of natural signal molecules (Morse et al., 1979; Morse, 1985; Bonar et al., 1990; Boettcher and Targett, 1998), but the peripheral or central neural site (or sites) of action by these mimetics is still unclear.
Alternatively, AADC in the carcinoid tumor may convert dihydroxyphenylalanine
into dopamine, thus providing substrate for enhanced catecholamine synthesis outside the tumor.