A rapid radioenzymatic assay for dopamine and N-acetyldopamine

Dopamine (DA) was measured in various tissue extracts as [ 3H]methoxy- N-acetyldopamine after incubation with two partially purified enzymes, catechol- O-methyl transferase (EC 2.1.1.1) and N-acetyltransferase (EC 2.3.1.5), in the presence of [ 3H]adenosylmethionine and acetyl-CoA. This product can be separated quantitatively from labeled products of norepinephrine and epinephrine by solvent extraction. N-Acetyl-DA can be assayed by omitting the acetylating system from the incubation mixture. The procedure is rapid, convenient for processing large numbers of samples, and has a sensitivity of approximately 0.1 pmol. It has been used to measure DA in ganglia and in individual neurons from gastropod mollusks.