Tryptophan hydroxylase 5-hydroxytryptophan

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]. 

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. 

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). 

Serotonin, or 5-HT, is biosynthesized (3) from its dietary precursor L-tryptophan (Fig. 14.1). Serotonergic neurons contain tryptophan hydroxylase (L-tryptophan-5-monooxygenase) that converts tryptophan to 5-hydroxytryptophan (5-HTP) in what is the rate-limiting step in 5-HT biosynthesis and aromatic L-amino acid decarboxylase (previously called 5-HTP decarboxylase) that decarboxylates 5-HTP to 5-HT. This latter enzyme also is responsible for the conversion of L-DOPA to dopamine (see Chapter 12). The major route of metabolism for 5-HT is oxidative deamination by monoamine oxidase (MAO-A) to the unstable 5-hydroxyindole-3-acetaldehyde, which is either reduced to 5-hydroxytryptophol ( 15%) or oxidized to 5-hydroxyindole-3-acetic acid ( -85%). In the pineal gland, 5-HT is acetylated by 5-HT N-acetyltransferase to N-acetylserotonin, which undergoes O-methylation by 5-hydroxyindole-O-methyltransferase to melatonin. 

Brain cells take up tryptophan, which is then converted to 5-hydroxytryptophan by tryptophan hydroxylase, an enzyme whose activity is similar to that of phenylalanine hydroxylase. Aromatic amino acid decarboxylase then catalyzes the formation of the potent neurotransmitter 5-hydroxytryptamine, also called serotonin. In the blood, tryptophan is bound to serum albumin, with an affinity such that about 10% of the tryptophan is freely diffusable. The rate of tryptophan uptake by brain cells depends on the concentration of free tryptophan. In these cells, tryptophan concentration is normally well below that of the Km for tryptophan hydroxylase. Aspirin and other drugs displace tryptophan from albumin, thereby increasing the concentration of free tryptophan. 

By increasing the activity of the kynurenine pathway, estrogens could also increase the requirements for PLP and make less available to act as the coenzyme for 5-hydroxytryptophan decarboxylase, or estrogen conjugates could displace PLP from the decarboxylase coenzyme directly. Even though in nonhuman mammals tryptophan hydroxylase, not the decarboxylase, is thought to be rate-limiting in serotonin synthesis (Jl), levels of decarboxylase are said to be so low in human brain... 

Tryptophan hydroxylase (EC 1.14.16.4) is the rate-limiting enzyme that catalyses the pterin-dependent hydroxylation of tryptophan to form 5-hydroxytryptophan. Northern analysis of human pineal gland revealed the presence of two mRNA species (Austin and O Donnell 1999). The cellular concentration of tryptophan hydroxylase in pinealocytes was extremely high throughout the pineal gland. 

The majority of catecholamine and serotonin biosynthesis occurs within the nerve terminals by synthetic enzymes transported from the neuronal cell bodies. In all catecholamine neurons, the rate-limiting step in synthesis is conversion of tyrosine to dihydroxyphenylalanine by tyrosine hydroxylase. Dihydroxyphenylalanine is then converted to DA, norepinephrine, and epinephrine through a sequential process involving L-aromatic amino acid decarboxylase (conversion of dihydroxyphenylalanine to DA), dopamine-P-hydroxylase (conversion of DA to norepinephrine), and phenylethanol-amine-N-methyltransferase (conversion of norepinephrine to epinephrine). Cell-specific expression of these enzymes determines the main neurotransmitter for an individual catecholamine neuron. The synthesis pathway for serotonin involves a two-step process in which tryptophan hydroxylase first converts tryptophan to 5-hydroxytryptophan, which is then converted to... 

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. 

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