An investigation of the assay of dopamine using trinitrobenzensulphonic acid.

The reaction between 2,4,6-trinitrobenzene-1-sulphonic acid with 3,4-dihydroxy-phenylethylamine (dopamine) to form trinitrophenyl dopamine (TNP-dopamine) is found to be complex in that the TNP-dopamine is unstable. The course of the reaction shows TNP-dopamine rising to a steady state concentration equal to about half the initial dopamine concentration within 20 min and then decomposing with a half-time of about 1 hr. Continuous extraction into benzene to separate the TNP-dopamine from TNP-DOPA, which interferes with the spectrophotometric assay, also prevents the decomposition of TNP-dopamine so that quantitative recoveries are obtained. The method has been used to follow the course of a DOPA decarboxylase-catalysed reaction satisfactorily.