A sensitive radioenzymatic assay for dopamine, norepinephrine, and epinephrine in plasma and tissue

A double-isotope, radioenzymatic assay for measuring dopamine, norepinephrine, and epinephrine in one sample is described. The assay procedure includes incubation, solvent extraction, and thin-layer chromatography. Dopamine, norepinephrine, and epinephrine were incubated with catechol-O-methyl transferase (COMT) and [3H]S-acenosyl methionine ([3H]SAM) and were converted to the O-methylated tritiated derivatives: [3H]methoxytyramine, [3H]normetanephrine, and [3H]metanephrine, respectively. After several extraction steps the O-methylated products were purified by means of two-dimensional, thin-layer chromatography using silica gel. The thin-layer chromatographic system resulted complete separation of the three O-methylated compounds with an overlap of only 1-2%. The assay was linear from 0 to 5 ng for each catecholamine and had a sensitivity of 10-30 pg. The addition of large amounts of plasma reduced the activity of COMT, but increasing the magnesium concentration in the incubation mixture and the addition of EGTA to plasma samples improved the recoveries. Each sample was corrected for losses incurred during extraction and chromatography by using [14C]methoxytyramine, [14C]normetanephrine, and [14Ci1metanephrine that were added at the end of incubation. Several catechol compounds known to be O-methylated by COMT were examined for crossreactivity. Of the substances tested, only dihydroxyphenylalanine (DOPA) exhibited cross-reactivity. However, the apparent 30% cross-reactivity of DOPA with dopamine was due to the presence of decarboxylase activity in the COMT preparation. As little as 50 mul of trunk plasma from decapitated rats was sufficient for the determination of the three catecholamines.