A potential role for cyclized quinones derived from dopamine, DOPA, and 3,4-dihydroxyphenylacetic acid in proteasomal inhibition.

We examined the ability of oxidation products of dopamine, DOPA, and 3,4-dihydroxyphenylacetic acid (DOPAC) to inhibit proteasomal activity. Dopamine, DOPA, and DOPAC underwent tyrosinase-catalyzed oxidation to generate aminochrome, dopachrome, and furanoquinone, respectively. In these studies, the oxidation of dopamine by tyrosinase generated product(s) that inhibited the proteasome, and proteasomal inhibition correlated with the presence of the UV-visible spectrum of aminochrome. The addition of superoxide dismutase and catalase did not prevent proteasomal inhibition. The addition of NADH and the quinone reductase NAD(P)H:quinone oxidoreductase 1 (NQO1) protected against aminochrome-induced proteasome inhibition. Although NQO1 protected against dopamine-induced proteasomal inhibition, the metabolism of aminochrome by NQO1 led to oxygen uptake because of the generation of a redox-labile cyclized hydroquinone, further demonstrating the lack of involvement of oxygen radicals in proteasomal inhibition. DOPA underwent tyrosinase-catalyzed oxidation to form dopachrome, and similar to aminochrome, proteasomal inhibition correlated with the presence of a dopachrome UV-visible spectrum. The inclusion of NQO1 did not protect against proteasomal inhibition induced by dopachrome. Oxidation of DOPAC by tyrosinase generated furanoquinone, which was a poor proteasome inhibitor. These studies demonstrate that oxidation products, including cyclized quinones derived from dopamine and related compounds, rather than oxygen radicals have the ability to inhibit the proteasome. They also suggest an important protective role for NQO1 in protecting against dopamine-induced proteasomal inhibition. The ability of endogenous intermediates formed during dopaminergic metabolism to cause proteasomal inhibition provides a potential basis for the selectivity of dopaminergic neuron damage in Parkinson's disease.