Abstract

Tryptophan hydroxylase, the initial and rate limiting enzyme in the biosynthesis of serotonin (5-HT), is inactivated by the quinone of l-DOPA. l-DOPA itself has no effect on enzyme activity. The inactivation of tryptophan hydroxylase could be prevented by glutathione (GSH), dithiothreitol, cysteine, and ascorbic acid but not by scavengers of hydrogen peroxide (catalase), hydroxyl radical (DMSO), or superoxide (superoxide dismutase). All cysteinyl residues within tryptophan hydroxylase are modified after treatment with l-DOPA-quinone as revealed by loss of DTNB-reactivity and formation of cysteinyl–DOPA residues. l-DOPA-quinone also converts tryptophan hydroxylase to a redox-cycling quinoprotein. These results suggest a possible mechanism of 5-HT neuronal damage in Parkinson's Disease by a redox-cycling quinoprotein.

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