The Journal of Neuroscience, September 15, 1998, 18(18):7111-7117
Dopamine Inactivates Tryptophan Hydroxylase and Forms a
Redox-Cycling Quinoprotein: Possible Endogenous Toxin to Serotonin
Neurons
Donald M.
Kuhn1, 2 and
Robert
Arthur Jr.1
1 Cellular and Clinical Neurobiology Program,
Department of Psychiatry and Behavioral Neurosciences, and
2 Center for Molecular Medicine and Genetics, Wayne State
University School of Medicine, Detroit, Michigan 48201
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.
Key words:
tryptophan hydroxylase; dopamine; quinones; neurotoxic
amphetamines; quinoproteins; serotonin
Copyright © 1998 Society for Neuroscience 0270-6474/98/18187111-07$05.00/0
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