L-DOPA INHIBITS COMPLEX-IV OF THE ELECTRON-TRANSPORT CHAIN IN CATECHOLAMINE-RICH HUMAN NEUROBLASTOMA NB69 CELLS

L-3,4-Dihydroxyphenylalanine (L-DOPA) is toxic for human neuroblastoma cells NB69 and its toxicity is related to several mechanisms includin g quinone formation and enhanced production of free radicals related t o the metabolism of dopamine via monoamine oxidase type B. We studied the effect of L-DOPA on activities of enzyme complexes in the electron transport chain (ETC) in homogenate preparations from the human neuro blastoma cell line NB69. As a preliminary step we compared the activit y of ETC in cellular homogenates with that of purified mitochondria fr om NB69 cells and rat brain. Specific activities for complex I, comple x II-III, and complex IV in NB69 cells were, respectively, 65, 96, and 32% of those in brain mitochondria. Complex I activity was inhibited in a dose-dependent way by 1-methyl-4-phenylpyridinium ion with an EC( 50) of similar to 150 mu M. Treatment with 0.25 mM L-DOPA for 5 days r educes complex IV activity to 74% of control values but does not chang e either complex I or citrate synthase. Ascorbic acid (1 mM), which pr otects NB69 cells from L-DOPA-induced neurotoxicity, increases complex IV activity to 133% of the control and does not change other ETC comp lexes. Ascorbic acid also reverses L-DOPA-induced reduction of complex IV activity in NB69 cells. This observation might indicate that the p rotection observed with ascorbic acid is related to complex IV activat ion. In vitro incubation with L-DOPA (0.125-4 mM) for 2 min produced a dose-dependent reduction of complex IV without change in complex I an d II-III activities.