Dopamine inactivates tryptophan hydroxylase and forms a redox-cycling quinoprotein: possible endogenous toxin to serotonin neurons.

Exposure of tryptophan hydroxylase (TPH), the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin, to dopamine under mild oxidizing conditions (iron + H2O2) or in the presence of tyrosinase results in a concentration-dependent inactivation of the enzyme. Dopamine, iron, H2O2, or tyrosinase alone does not alter TPH activity. Similarly, N-acetyldopamine oxidized with one equivalent of sodium periodate causes a concentration-dependent inactivation of TPH as well. TPH is protected from dopamine-induced inactivation by reduced glutathione, ascorbic acid, and dithiothreitol but not by the radical scavengers DMSO, mannitol, or superoxide dismutase. Parallel studies with [3H]dopamine reveal a high negative correlation between inhibition of catalysis and incorporation of tritium into the enzyme. Those reducing agents and antioxidants that protect TPH from inactivation are effective in preventing the labeling of TPH by [3H]dopamine. Acid hydrolysis and HPLC with electrochemical detection (HPLC-EC) analysis of inactivated TPH revealed the formation of cysteinyl-dopamine residues within the enzyme. Exposure of dopamine-modified TPH to redox-cycling staining after SDS-PAGE confirmed the formation of a quinoprotein. These results indicate that dopamine-quinones covalently modify cysteinyl residues in TPH, leading directly to the loss of catalytic activity, and establish that TPH could be a target for dopamine-quinones in vivo after drugs (e.g., neurotoxic amphetamines) that cause dopamine-dependent inactivation of TPH. Redox cycling of a TPH-quinoprotein could also participate in the serotonin neuronal toxicity caused by these same drugs.