The
quinone derivative of
2,4,5-trihydroxyphenylalanine (TOPA) is a selective non-
NMDA agonist and
excitotoxin. While
3,4-dihydroxyphenylalanine (
DOPA)-containing physiological solutions have been shown to generate TOPA and
TOPA quinone (TOPA compounds), there have been no previous reports demonstrating the formation of this toxin in
biological preparations. Here, using a
pheochromocytoma catecholaminergic clonal cell line (PC12), we have identified TOPA compounds as by-products of
catecholamine synthesis. PC12 cells incubated for 45 min with 30 microM
tyrosine as a
catecholamine precursor produced 1.0 +/- 0.2 pmol/10(6) cells of total TOPA compounds. The formation of these compounds could be enhanced nearly twofold when the cells were stimulated with 56 mM KCl. Moreover, the addition of a
DOPA decarboxylase inhibitor (30 microM
NSD-1015) increased the formation of TOPA compounds in both the unstimulated and stimulated conditions to a maximum of 5.5 +/- 0.7 pmol/10(6) cells after a 45 min incubation. A time-course analysis revealed that
DOPA production above baseline levels coincided with the detectable generation of TOPA compounds. Finally, we observed an inhibition of TOPA compounds formation by 100 microM
reduced glutathione, suggesting that these
catecholamine products are formed from the extracellular autoxidation of
DOPA. We propose that
TOPA quinone may be an underestimated component of
catecholamine toxicity that could be partly responsible for the demise of neurons in several
neurodegenerative disorders, including Parkinson's and
Huntington's disease. In addition,
TOPA quinone may represent the first identified selective non-
NMDA agonist that may be synthesized in the brain.