A SENSITIVE ENZYMATIC‐RADIOISOTOPIC ASSAY FOR 3,4‐DIHYDROXYPHENYLACETIC ACID

Abstract

Abstract— An assay for 3,4‐dihydroxyphenylacetic acid (DOPAC) capable of detecting as little as 1 pmole of DOPAC is described. The basis of the assay is the O ‐methylation of DOPAC utilizing S ‐[methyl‐ 3 H]adenosyl‐l‐methionine and a partially purified catechol‐ O ‐methyl transferase to form [methyl‐ 3 H]homovanillic acid. The [methyl‐ 3 H]homovanillic acid is purified with ion exchange resin and either solvent partitions or TLC. Utilizing this assay, the effect of either chlorpromazine or reserpine upon the content of DOPAC both in the caudate nucleus and in the substantia nigra was determined. Both of these drugs increase the level of DOPAC in the caudate nucleus but have minimal effects upon the level of DOPAC in the substantia nigra.