Radioenzymatic assay for plasma dihydroxyphenylglycol (DHPG), dihydroxymandelic acid (DOMA) and dihydroxyphenylacetic acid (DOPAC).

An adaptation of the single-isotope radioenzymatic catecholamine assay technique allows simultaneous quantitation of free dihydroxyphenylglycol (DHPG), dihydroxymandelic acid (DOMA), and dihydroxyphenylacetic acid (DOPAC) in small volumes of plasma. Incubation of sample with catechol-O-methyltransferase and S-adenosylmethionine is followed by acidic extractions of metabolites and thin-layer chromatography. O-methylated products of the beta-hydroxylated metabolites DHPG and DOMA are further subjected to periodate cleavage to improve sensitivity. Quantitation by liquid scintillation counting with internal standardization results in a sensitivity of approximately 8, 30 and 70 pg for DHPG, DOMA and DOPAC. Normal values for these three metabolites in resting humans are (mean +/- S.D., pg/ml): 1010 +/- 280 for DHPG, 2090 +/- 720 for DOMA and 3270 +/- 1470 for DOPAC.