Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in serotonin biosynthesis. A novel gene, termed TPH2, has recently been described. This gene is preferentially expressed in the central nervous system, while the original TPH1 is the peripheral gene. We have expressed human tryptophan hydroxylase 2 (hTPH2) and two deletion mutants (N⌬150 and N⌬150/ C⌬24) using isopropyl ␤-D-thiogalactopyranoside-free autoinduction in Escherichia coli. This expression system produced active wild type TPH2 with relatively low solubility. The solubility was increased for mutants lacking the NH 2 -terminal regulatory domain. The solubility of hTPH2, N⌬150, and N⌬150/ C⌬24 are 6.9, 62, and 97.5%, respectively. Removal of the regulatory domain also produced a more than 6-fold increase in enzyme stability (t1 ⁄ 2 at 37°C). The wild type hTPH2, like other members of the aromatic amino acid hydroxylase superfamily, exists as a homotetramer (236 kDa on size exclusion chromatography). Similarly, N⌬150 also migrates as a tetramer (168 kDa). In contrast, removal of the NH 2 -terminal domain and the COOHterminal, putative leucine zipper tetramerization domain produces monomeric enzyme (39 kDa). Interestingly, removal of the NH 2 -terminal regulatory domain did not affect the Michaelis constants for either substrate but did increase V max values. These data identify the NH 2 -terminal regulatory domain as the source of hTPH2 instability and reduced solubility.Tryptophan hydroxylase (TPH 2 ; EC 1.14.16.4), a tetrahydropterin (BH 4 )-dependent amino acid hydroxylase, is the key regulator of serotonin (5-hydroxytryptamine) biosynthesis. Utilizing BH 4 and O 2 as co-substrates and Fe 2ϩ as a cofactor, TPH hydroxylates tryptophan to 5-hydroxytryptophan ( Fig. 1). Subsequent decarboxylation of 5-hydroxytryptophan by amino acid decarboxylase generates serotonin (1-3). This essential monoamine has been found in a variety of tissues and implicated in a wide range of physiological functions. In the central nervous system, serotonin is synthesized primarily in the dorsal raphe nucleus and acts as a neurotransmitter; in the pineal gland, it serves as a precursor for melatonin biosynthesis (4). In the periphery, serotonin constricts large blood vessels and regulates platelet adhesion (5-7). Serotonin, produced by the enterochromaffin cells of the intestinal system, initiates peristaltic and secretory reflexes (8). Altered activity of serotonin is associated with various disorders such as depression, impulsive behaviors, aggression, suicide, drug abuse and alcoholism, sleep disorders, gastrointestinal diseases (such as irritable bowel syndrome), and cardiovascular dysfunction leading to heart failure (9).Recently, Walther et al. (10) discovered the gene responsible for nervous system TPH (TPH2). Unlike TPH1 now known to be the peripheral enzyme (11), TPH2 is neuron-specific and expressed predominantly in serotonergic neurons of the raphe nuclei (10, 12) and in the peripheral myenteric neurons in the gut (13). Human TPH1 and TPH2 display 72% sequence h...

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, is strictly controlled by several interrelated regulatory mechanisms. Enzyme synthesis is controlled by epigenetic factors, transcription factors, and mRNA levels. Enzyme activity is regulated by end-product feedback inhibition. Phosphorylation of the enzyme is catalyzed by several protein kinases and dephosphorylation is mediated by two protein phosphatases that establish a sensitive process for regulating enzyme activity on a minute-to-minute basis. Interactions between tyrosine hydroxylase and other proteins introduce additional layers to the already tightly controlled production of catecholamines. Tyrosine hydroxylase degradation by the ubiquitin-proteasome coupled pathway represents yet another mechanism of regulation. Here, we revisit the myriad mechanisms that regulate tyrosine hydroxylase expression and activity and highlight their physiological importance in the control of catecholamine biosynthesis.

Cochliobolus victoriae is a necrotrophic fungus that produces a host-selective toxin called victorin. Victorin is considered to be host selective because it has been known to affect only certain allohexaploid oat cultivars containing the dominant Vb gene. Oat cultivars containing Vb are also the only genotypes susceptible to C. victoriae. Assays were developed to screen the "nonhost" plant of C. victoriae, Arabidopsis thaliana, for victorin sensitivity. Sensitivity to victorin was identified in six of 433 bulk populations of Arabidopsis. In crosses of Col-4 (victorin-insensitive) x victorin-sensitive Arabidopsis ecotypes, victorin sensitivity segregated as a single dominant locus, as it does in oats. This Arabidopsis locus was designated LOV, for locus orchestrating victorin effects. Allelism tests indicate that LOV loci are allelic or closely linked in all six victorin-sensitive ecotypes identified. LOV was localized to the north arm of Arabidopsis thaliana chromosome I. The victorin-sensitive Arabidopsis line LOV1 but not the victorin-insensitive line Col-4 was susceptible to C. victoriae infection. Consequently, the LOV gene appears to be a genetically dominant, disease susceptibility gene.

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