THE EFFECTS OF HYDROXYL RADICAL ATTACK ON DOPA, DOPAMINE, 6-HYDROXYDOPA, AND 6-HYDROXYDOPAMINE

High pressure liquid chromatography with electrochemical detection (HP LC-ED) was employed in conjugation with a sensitive and specific salic ylate hydroxylation assay to evaluate the immediate effects of hydroxy l radical ((OH)-O-.) attack on four catechol intermediates of eumelani n, dopamine (3,4-dihydroxyphenylethylamine), its precursor dopa (3,4-d ihydroxyphenylalanine), and their respective neurotoxic trihydroxyphen yl derivatives, 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine, 6 -OHDA) and 6-hydroxydopa(2,4,5-trihydroxyphenylalanine, TOPA). Semiqui none and quinone species were identified as the initial products of th e oxidation of these four catechol substrates. The enhanced oxidations of the catechols when exposed to (OH)-O-. attack was accompanied by m arked decreases in the level of each semiquinone species. Quinone leve ls were elevated in reactions involving (OH)-O-. attack on dopamine an d 6-OHDA, but absent in reactions involving radical attack on dopa or TOPA, suggesting that dopaquinone (DOQ) and TOPA p-quinone (TOPA p-Q) are oxidized more rapidly by (OH)-O-. than are the quinones of dopamin e and 6-OHDA. The formation of 6-OHDA p-quinone (6-OHDA p-Q) in incuba tions involving DA and (OH)-O-. suggest that the (OH)-O-.-mediated hyd roxylation of DA may be a mechanism for generating this potentially cy totoxic trihydroxyphenyl. The results of this study demonstrate for th e first time that semiquinone and quinone intermediates of eumelanin a re the initial products derived from the (OH)-O-.-mediated oxidations of dopa, DA, TOPA, and 6-OHDA. These observations suggest that if (OH) -O-. is generated beyond the capabilities of cytoprotective mechanisms , the radical can rapidly oxidize catechol precursors, augment melanog enesis, and generate additional cytotoxic quinoid intermediates of eum elanin.