tryptophan hydroxylase
Summary
Tryptophan hydroxylase is the rate-limiting enzyme in serotonin biosynthesis that converts L-tryptophan to 5-hydroxytryptophan (5-HTP). It exists as two isoforms: TPH1 (peripheral tissues) and TPH2 (central nervous system). This enzyme is crucial for mood regulation and is a target for antidepressant therapy.
Detail
Tryptophan hydroxylase catalyzes the first and rate-limiting step in the biosynthesis of serotonin (5-HT) from the essential amino acid L-tryptophan. The enzyme requires tetrahydrobiopterin (BH4) as a cofactor, along with iron and oxygen. Two distinct isoforms exist: TPH1, found primarily in peripheral tissues including the gut, pineal gland, and skin, and TPH2, which is predominantly expressed in the brainstem raphe nuclei and other CNS serotonergic neurons. The reaction produces 5-hydroxytryptophan (5-HTP), which is subsequently decarboxylated by aromatic L-amino acid decarboxylase to form serotonin. TPH activity is subject to feedback inhibition by serotonin itself. Genetic polymorphisms in TPH2 have been associated with depression, suicide risk, and response to SSRI therapy. The enzyme's activity can be modulated by various factors including stress hormones, inflammatory cytokines, and circadian rhythms. Understanding TPH regulation is clinically relevant for depression treatment, as decreased enzyme activity may contribute to serotonin deficiency states.
Sources
- Lippincott Illustrated Reviews: Biochemistry
- Katzung & Trevor's Basic & Clinical Pharmacology
- Stahl's Essential Psychopharmacology
- Harper's Illustrated Biochemistry
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