5-Hydroxytryptophan hydroxylase

Serotonin is synthesized from tryptophan in two steps. Tryptophan is hydroxylated by tryptophan hydroxylase, and 5-hydroxytryptophan is decarboxylated to give serotonin. Most serotonin in the body is found in the enterochromaffin cells of the intestinal tract and the pineal gland. Platelets take up and store serotonin but do not synthesize it. 

Serotonergic neurons contain the enzyme L-tryptophan-5-monooxygenase (EC 1.14.16.4), more commonly termed tryptophan hydroxylase, which converts tryptophan to 5-hydroxytryptophan (5-HTP) (Fig. 13-5). Tryptophan hydroxylase contains 444 amino acids, corresponding to a molecular weight of about 51 Da. This enzyme is synthesized in serotonergic cell bodies of the raphe nuclei and is found only in cells that synthesize 5-HT. Therefore its distribution in brain is similar to that of 5-HT itself. The Km of tryptophan hydroxylase for tryptophan is approximately 30-60 pmol/1, a concentration comparable to that of tryptophan in brain. If the concentration of tryptophan in serotonergic neurons is assumed to be comparable to that in whole brain, the enzyme would not be saturated with substrate, and the formation of 5-HT in brain would be expected to rise as the brain concentration of tryptophan increases. This has been found to occur in response to raising the dietary intake of tryptophan specifically. 

Serotonin is an indolamine neurotransmitter, derived from the amino acid L-tryptophan. Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase. 5-HTP is converted to 5-hydroxytryptamine (serotonin, 5-HT) by aromatic amino acid decarboxylase. In the pineal gland, 5-HT may be further converted to /V-acetyl serotonin by 5-HT /V-acetyltransferase and then to melatonin by 5-hyroxyindole-O-methyltransferase. 5-HT is catabolized by monoamine oxidase, and the primary end metabolite is 5-hydroxyindoleacetic acid (5-HIAA). 

Following the synthesis of 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, the enzyme aromatic amino acid decarboxylase (also known as 5-HTP or dopa decarboxylase) then decarboxylates the amino acid to 5-HT. L-Aromatic amino acid decarboxylase is approximately 60% bound in the nerve terminal and requires pyridoxal phosphate as an essential enzyme. 

Whilst the term biogenic amine strictly encompasses all amines of biological origin, for the purpose of this article it will be employed to refer to the catecholamine (dopamine, noradrenaline) and serotonin group of neurotransmitters. These neurotransmitters are generated from the amino acid precursors tyrosine and tryptophan, respectively, via the action of the tetrahydrobiopterin (BH4)-dependent tyrosine and tryptophan hydroxylases. Hydroxylation of the amino acid substrates leads to formation of 3,4-dihydroxy-l-phenylalanine ( -dopa) and 5-hydroxytryptophan, which are then decarboxylated via the pyridoxalphosphate-dependent aromatic amino acid decarboxylase (AADC) to yield dopamine and serotonin [4]. In noradrenergic neurones, dopamine is further metabolised to noradrenaline through the action of dopamine-jS-hydroxylase [1]. 

Serotonin, also called 5-hydroxytryptamine, is synthesized and stored at several sites in the body (Figure 21.18). By far the largest amount of serotonin is found in cells of the intestinal mucosa. Smaller amounts occur in platelets and in the central nervous system. Serotonin is synthesized from tryptophan, which is hydroxy-lated in a reaction analogous to that catalyzed by phenylalanine hydroxylase. The product, 5-hydroxytryptophan, is decarboxylated to serotonin. Serotonin has multiple physiologic roles, including pain perception, affective disorders, and regulation of sleep, temperature, and blood pressure. 

The hereditary absence of phenylalanine hydroxylase, which is found principally in the liver, is the cause of the biochemical defect phenylketonuria (Chapter 25, Section B).430 4308 Especially important in the metabolism of the brain are tyrosine hydroxylase, which converts tyrosine to 3,4-dihydroxyphenylalanine, the rate-limiting step in biosynthesis of the catecholamines (Chapter 25), and tryptophan hydroxylase, which catalyzes formation of 5-hydroxytryptophan, the first step in synthesis of the neurotransmitter 5-hydroxytryptamine (Chapter 25). All three of the pterin-dependent hydroxylases are under complex regulatory control.431 432 For example, tyrosine hydroxylase is acted on by at least four kinases with phosphorylation occurring at several sites.431 433 4338 The kinases are responsive to nerve growth factor and epidermal growth factor,434 cAMP,435 Ca2+ + calmodulin, and Ca2+ + phospholipid (protein kinase C).436 The hydroxylase is inhibited by its endproducts, the catecholamines,435 and its activity is also affected by the availability of tetrahydrobiopterin.436... 

FIGURE 5—34. Serotonin (5-hydroxytryptamine [5HT ) is produced from enzymes after the amino acid precursor tryptophan is transported into the serotonin neuron. The tryptophan transport pump is distinct from the serotonin transporter (see Fig. 5—35). Once transported into the serotonin neuron, tryptophan is converted into 5-hydroxytryptophan (5HTP) by the enzyme tryptophan hydroxylase (TryOH) which is then converted into 5HT by the enzyme aromatic amino acid decarboxylase (AAADC). Serotonin is then stored in synaptic vesicles, where it stays until released by a neuronal impulse. 

One of the best characterized physiological functions of (6R)-tetrahydrobio-pterin (BH4, 43) is the action as a cofactor for aromatic amino acid hydroxylases (Scheme 28). There are three types of aromatic amino acid hydroxylases phenylalanine hydroxylase [PAH phenylalanine monooxygenase (EC 1.14.16.1)], tyrosine hydroxylase [TH tyrosine monooxygenase (EC 1.14.16.2)] and tryptophan hydroxylase [TPH tryptophan monooxygenase (EC 1.14.16.4)]. PAH converts L-phenylalanine (125) to L-tyrosine (126), a reaction important for the catabolism of excess phenylalanine taken from the diet. TH and TPH catalyze the first step in the biosyntheses of catecholamines and serotonin, respectively. Catecholamines, i.e., dopamine, noradrenaline and adrenaline, and serotonin, are important neurotransmitters and hormones. TH hydroxylates L-tyrosine (126) to form l-DOPA (3,4-dihydroxyphenylalanine, 127), and TPH catalyzes the hydroxylation of L-tryptophan (128) to 5-hydroxytryptophan (129). The hydroxylated products, 127 and 129, are decarboxylated by the action of aromatic amino acid decarboxylase to dopamine (130) and serotonin (131), respectively. 

PAH, a nonheme iron-containing enzyme, is a member of a larger BI Independent amino acid hydroxylase family. In addition to PAH, the enzyme family includes tyrosine hydroxylase and tryptophan hydroxylase. The enzymes in this family participate in critical metabolic steps and are tissue specific. PAH catabolizes excess dietary PA and synthesizes tyrosine. In adrenal and nervous tissue, tyrosine hydroxylase catalyzes the initial steps in the synthesis of dihydrox-yphenylalanine. In the brain, tryptophan is converted to 5-hydroxytryptophan as the first step of serotonin synthesis. Consequently, these enzymes are highly regulated not only by their expression in different tissues but also by reversible phosphorylation of a critical serine residue found in regulatory domains of the three enzymes. Since all three enzymes are phosphorylated and dephosphorylated by different kinases and phosphatases in response to the need for the different synthetic products, it is not unexpected that the exact regulatory signal for each member of the enzyme family is unique. 

Histamine is synthesized from the amino acid histidine by simple decarboxylation catalysed by histidine decarboxylase. Serotonin is synthesized primarily in platelets, the gastro-intestinal (GI) tract and the brain from the indolyl amino acid tryptophan tryptophan —> 5-hydroxytryptophan [via tryptophan hydroxylase + tetrahydrobiopterin] —> 5-hydroxy-tryptamine (serotonin) [via 5-hydroxytryptophan decarboxylase]. 

Figure 8.4. Pathways of tryptophan metaholism. Tryptophan dioxygenase, EC 1.13.11.11 formylkynurenine formamidase, EC 3.5.1.9 kynurenine hydroxylase, EC 1.14.13.9 kynureninase, EC 3.7.1.3 3-hydroxyanthranilate oxidase, EC 1.10.3.5 picolinate carboxylase, EC 4.1.1.45 kynurenine oxoglutarate aminotransferase, EC 2.6.1.7 kynurenine glyoxylate aminotransferase, 2.6.1.63 tryptophan hydroxylase, EC 1.14.16.4 and 5-hydroxytryptophan decarboxylase, EC 4.1.1.26. Relative molecular masses (Mr) tryptophan, 204.2 serotonin, 176.2 kynurenine, 208.2 3-hydroxykynurenine, 223.2 kynurenic acid, 189.2 xanthurenic acid, 205.2 and quinolinic acid 167.1. CoA, coenzyme A.

Fig. 5 Tryptophrin is converted to 5-hydroxytryptophan by tryptophan hydroxylase (1). The production of serotonin in the next step is catalysed by 5-HTP decaiboxylase (a PLP-dependent enzyme). Serotonin is then acetylated to N-acetyl-serotonin by N-acetyltransferase (2). Melatonin is then produced via the action of hydroxyindole-O-methyltransferase (3)...

Serotonin or 5-hydroxytryptamine (5-HT), a monoamine, is widely distributed in many cells of the body and about 1-2% of the entire serotonin body content is found in the CNS. Serotonin is synthesized by the enzyme amino acid decarboxylase from 5-hydroxytryptophan (which is derived from tryptophan via tryptophan hydroxylase). The rate-limiting step is the production of 5-hydroxytryptophan by tryptophan hydroxylase. Serotonin is removed from the synapse by a high-affinity serotonin uptake site that is capable of transporting serotonin in either direction, depending on the concentration. 

The synthesis of 5-HT from tryptophan in serotonergic neurons occurs in two steps. First, the enzyme tryptophan hydroxylase catalyzes the conversion of tryptophan to 5-hydroxytryptophan (5-HTP). Then, the enzyme aromatic amino acid decarboxylase catalyzes the conversion of 5-FlTP to serotonin. 

Catecholamines are synthesized from the amino acid tyrosine, and serotonin from tryptophan as shown in Figure 29-2. The rate-limiting step in catecholamine biosynthesis involves conversion of tyrosine to 3,4-dihydroxyphenylalanine (L-dopa) by the enzyme, tyrosine hydroxylase. A related enzyme, tryptophan hydroxylase, catalyzes conversion of tryptophan to 5-hydroxytryptophan in the first step of serotonin synthesis. 

Patients with aromatic L-amino acid decarboxylase deficiency present with clinical and biochemical manifestations that overlap those of the tyrosine hydroxylase deficiency states described previously. However, this deficiency state is characterized by additional decreases in CSF and urinary levels of 5-HIAA. Also, levels of L-dopa in urine, plasma, and cerebrospinal fluid are increased, and not decreased as in the other deficiency states. Levels of 5-hydroxytryptophan are similarly increased. 

Abnormal indole derivatives in the urine and low levels of serotonin (a product of tryptophan metabolism) in blood and brain point to a defect in tryptophan metabolism in PKU. 5-Hydroxytryptophan decarboxylase, which catalyzes the conversion of 5-hydroxytryptophan to serotonin, is inhibited in vitro by some of the metabolites of phenylalanine. Phenylalanine hydroxylase is similar to the enzyme that catalyzes the hydroxylation of tryptophan to 5-hydroxytryptophan, a precursor of serotonin. In vitro, phenylalanine is also found to inhibit the hydroxylation of tryptophan. The mental defects associated with PKU may be caused by decreased production of serotonin. High phenylalanine levels may disturb the transport of amino... 

D. Because of the DHPR deficiency, the activities of Phe hydroxylase and Tyr hydroxylase are low, hence the synthesis of catecholamine neu-rotransmitters are depressed. The synthesis of the indoleamine neurotransmitter serotonin is also depressed because BH is required for the hydroxylation of tryptophan. The best treatment is to decrease the Phe load by a low-Phe diet and provide the precursors for the catecholamine and indoleamine neurotransmitters that occur after the enzymes affected by the deficiency of BH, which would be L-dopa and 5-hydroxytryptophan. 

HT is formed by the hydroxylation of tryptophan, by tryptophan hydroxylase, to 5-hydroxytryptophan (5-HTP), followed by decarboxylation to 5-HT using 5-HTP decarboxylase, which is the same enzyme as DOPA decarboxylase (see Table 1.2). Importantly, brain tryptophan hydroxylase is unsatrtrated at normal concerrtrations of tryptophan in the brain, hence altering availability of brain tryptophan will alter brain... 

Tetrahydrobiopterin (BH4) is required in the synthesis of neurotransmitters such as norepinephrine and serotonin, which are produced in brain. BH4 is required in the reaction catalyzed by tyrosine hydroxylase in which tyrosine is hydrox-ylated to form L-dopa (a precursor of several neurotransmitters including dopamine and norepinephrine), and the reaction catalyzed by tryptophan hydroxylase in which tryptophan is hydroxylated to form 5-hydroxytryptophan. L-Dopa and 5-hydroxytryptophan must be supplied to individuals lacking the capacity to synthesize BH4 because the latter molecule does not cross the blood-brain barrier. 

The synthesis of serotonin from tryptophan is carried out in two steps controlled by two enzymes tryptophan hydroxylase (TPH) and aromatic L-amino acid decarboxylase (AADC). The second enzyme, A ADC, is also known as DOPA carboxylase or 5-hydroxytryptophan carboxylase when it acts specifically in 5-HT synthesis. In the first step, the TPH adds a hydroxyl chemical group (OH) to tryptophan to make 5-hydroxytryptophan, Fig (1). In the second step, AADC removes the carboxyl group (-COOH) from 5-hydroxy tryptophan to make serotonin. Fig (2). 

A small subset of patients with hyperphenylalaninemia show an appropriate reduction in plasma phenylalanine levels with dietary restriction of this amino acid however, these patients still develop progressive neurologic symptoms and seizures and usually die within the first 2 years of life ("malignant" hyperphenylalaninemia). These infants exhibit normal phenylalanine hydroxylase (PAH) activity but have a deficiency in dihy-dropteridine reductase (DHPR), an enzyme required for the regeneration of tetrahydro-biopterin (BH4), a cofactor of PAH (see Fig. 39.18). Less frequently, DHPR activity is normal but a defect in the biosynthesis of BH4 exists. In either case, dietary therapy corrects the hyperphenylalaninemia. However, BH4 is also a cofactor for two other hydroxy-lations required in the synthesis of neurotransmitters in the brain the hydroxylation of tryptophan to 5-hydroxytryptophan and of tyrosine to L-dopa (see Chapter 48). It has been suggested that the resulting deficit in central nervous system neurotransmitter activity is, at least in part, responsible for the neurologic manifestations and eventual death of these patients. 

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