Radioenzymatic assay of DOPA (3,4-dihydroxyphenylalanine)

We modified the single-isotope radioenzymatic assay for catecholamines [Life Sci. 21, 625 (1977)] to assay 3, 4-dihydroxyphenylalanine (DOPA). DOPA decarboxylase is used to convert DOPA to dopamine, which concurrently is converted to [3H]-3-O-methyldopamine in the presence of catechol-O-methyltransferase and [methyl-3H]-S-adenosylmethionine and assayed radioenzymatically. For assay of plasma DOPA, 50 microliter of untreated plasma is added directly into the incubation mixture. A duplicate mixture containing an internal standard requires a second 50-microliter aliquot of plasma. Because the assay measures both DOPA and endogenous dopamine, two additonal aliquots of plasma must be assayed for dopamine in the absence of the decarboxylase by the differential assay; DOPA is estimated by difference. The assay is sensitive to 25 pg (500 ng/liter of plasma). Analysis of DOPA (DOPA plus dopamine) and the concurrent differential assay of catecholamines in at least 10 samples can be done in a single working day. Plasma DOPA concentrations for 42 normotensive adults were 1430 +/- 19 ng/liter (mean +/- SEM). In contrast, do-pamine concentrations for these same subjects averaged 23 +/- 20 ng/liter. Values for the 24 women subjects (15 10 +/- 62 ng/liter) significantly (P = 0.04) exceeded those for the men (1320 +/- 75 ng/liter).