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Is there someone on the list who either is a biochemist or knows one who might be interested in answering the following question about the regulation of dopamine synthesis. I looked in the book 'Biochemistry' by Stryer and in 'Biochemistry' by Voet and Voet and got the following information, which is the starting point for my question about dopamine. The rate limiting step in the synthesis of the neurotransmitter, dopamine, from the amino acid, tyrosine, in the brain is: tyrosine + O2 + tetrahydrobiopterin ---> dopa + H2O + quinonoid dihydrobiopterin. This reaction is catalyzed by tyrosine hydroxylase, a monooxidase like phenylalanine hydroxylase. Dopa (i.e., Levodopa) is an abbreviation for 3,4-dihydroxyphenylalanine. Tetrahydrobiopterin, the cofactor that activates molecular oxygen in this reaction, is regenerated by the reaction: quinonoid dihydrobiopterin + NADH + H+ ---> tetrahydrobiopterin + NAD+ + H2O, which is catalyzed by dihydropteridine reductase. The overall net reaction catalyzed by tyrosine hydroxylase and dihydropteridine reductase is tyrosine + O2 + NADH + H+ ---> dopa + NAD+ + H2O. The next step is: dopa ---> dopamine, which is catalyzed by dopa decarboxylase. Dopamine is degraded in the reaction: dopamine ---> 3-o-methyldopamine, which is catalyzed by catechol-o-methyltransferase (COMT), and in the reaction: dopamine ---> 3,4-dihydroxyphenylethylaldehyde, which is catalyzed by monoamineoxidase-B (MAO-B). My question is: How is the level of dopamine regulated? Does dopamine inhibit tyrosine hydroxylase or does it inhibit the expression of the enzyme? What is the specific mechanism for the inhibition reaction? That is, what are the specific reactions that accomplish the regulation? Baldwin Robertson ... [log in to unmask]