5-Hydroxytryptophan decarboxylation

Dietary tryptophan is the source of the formation of serotonin. Enzymes and cofactors necessary for serotonin synthesis are present in both the enterochromaf-lin cells of the gastrointestinal tract and neurons in the brain. Tryptophan is initially hydroxylated to form 5-hydroxytryptophan. Decarboxylation of the latter compound results in the formation of serotonin (Fig. 24.2). 

Pharmacological evidence was obtained several years ago that indicated that tryptophan is decarboxylated to tryptamine by both animal and bacterial enzymes. More recent studies have failed to detect this reaction, but instead have shown decarboxylation to occur only after oxidation of the indole nucleus to yield 5-hydroxytryptophan. Decarboxylation of 5-hydroxytryptophan produces 5-hydroxytryptamine, serotonin, which has important, though incompletely defined functions in animal physiology. In some animal livers there is an enzyme that decarboxylates cysteic acid to taurine. Glutamic decarboxylase has been found in animal brain, where it is responsible for the formation of 7-aminobutyric acid. This product has been implicated in nervous function as an inhibitor of synaptic transmission. ... 

The product of the hydroxylation of tryptophan, 5-hydroxytryptophan, is rapidly decarboxylated to 5-HT by a specific decarboxylase enzyme. This is generally thought to be a soluble enzyme which suggests that 5-HT is synthesised in the cytoplasm, before it is taken up into the storage vesicles. If this is the case, then considerable losses might be incurred from its metabolism by monoamine oxidase before it reaches the storage vesicles. Indeed, this could explain why 5-HT turnover seems to greatly exceed its rate of release. 

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. 

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. 

Serotonin is synthesized via tryptophan and 5-hydroxytryptophan with decarboxylation of the latter (Fig. 25-12). Within the pineal body of the brain and in the retina, serotonin is acetylated to N-acetylseroto-nin,782/783 which is then O-methylated to melatonin, the pineal hormone (Fig. 25-12). A specific inhibitor of serotonin synthesis is p-chlorophenylalanine, and studies with this and other inhibitors suggest that serotonin is required for sleep.784... 

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. 

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]. 

Aromatic L-amino acid decarboxylase catalyzes the decarboxylation of l-5-hydroxytryptophan (l-5-HTP) to serotonin (5-HT). In the assay, l-5-HTP was used as the substrate and the formation of 5-HT was measured. 

In the presence of the cofactor pyridoxyl phosphate, Dopa decarboxylase catalyzes the decarboxylation of L-dopa to dopamine. This enzyme has been shown to be the same protein as 5-hydroxytryptophan decarboxylase, and both are referred to by the name aromatic L-amino acid decarboxylase (AADC). 

Decarboxylation of L-3,4-dihydroxyphenylalanine to dopamine, and of 5-hydroxytryptophan to serotonin, is catalyzed by aromatic L-amino acid decarboxylase. A single enzyme may be responsible for both activities. This assay permits simultaneous determination of both activities. 

Specific decarboxylases for most of the common amino acids have been isolated. In mammals, a decarboxylase involved in the biosynthesis of neuroactive amines is particularly important. This enzyme decarboxylates 3,4-dihydroxyphenylalanine and 5-hydroxytryptophan (both products of tetrahydrobiopterin-dependent hydroxylations — Section 1.10.5.1) to give 3,4-dihydroxyphenethylamine and serotonin (equation 10), respectively (70MI11002). 

B. Tryptophan is hydroxylated by a mixed-function oxidase that requires tetrahydro-biopterin and 02. 5-Hydroxytryptophan is decarboxylated to form serotonin. 

Udenfriend and his colleagues (885) found in animal kidney extracts an enzyme which specifically decarboxylated 5-hydroxytryptophan (217, 218) to 5-hydroxytryptamine, but was without action on tryptophan, 7-hydroxy-... 

Treatment of biopterin and biopterin reductase deficiency consists not only of regulating the blood levels of phenylalanine but of supplying the missing form of coenzyme and the precursors of neurotransmitters, namely, dihydroxyphenylalanine and 5-hydroxytryptophan, along with a compound that inhibits peripheral aromatic decarboxylation. This compound is necessary because the amine products do not cross the blood-brain barrier. 

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... 

Finally, the decarboxylation of amino acids catalyzed by several pyridoxal phosphate-dependent enzymes has been shown to proceed by a retention of configuration at the Ca atom. The stereochemical course of the decarboxylation of 5-hydroxytryptophan to 5-hydroxytryptamine (serotonin) catalyzed by the pyridoxal phosphate-dependent aromatic L-amino acid decarboxylase (equation 15) exemplifies such studies. ... 

Tryptophan appears to be converted to a larger number of metabolites than any of the other amino acids. The degradation of tryptophan in animals occurs mainly in two pathways, I and II (Figure 4.1). The first major pathway (I), initiated by the action of tryptophan dioxygenase, involves oxidation of tryptophan to N - fc > r my I ky n urenine and the formation of a series of intermediates and byproducts, most of which appear in varying amounts in the urine, the sum of which accounts for the total metabolism of tryptophan, approximately. The second pathway (II) involves hydroxylation of tryptophan to 5-hydroxytryptophan and decarboxylation of this compound to 5-hydroxytryptamine (serotonin), a potent vasoconstrictor found particularly in the brain, intestinal tissues, blood platelets, and mast cells. A small percentage (3%) of dietary tryptophan is metabolized via the pathway (III) to indoleacetic acid. Other minor pathways also exist in animal tissues. 

The hydroxylation of tryptophan produces 5-hydroxytryptophan, which can then be decarboxylated, catalyzed by tryptophan decarboxylase, a PALP-requiring enzyme, to 5-hydroxy tryptamine, also known as serotonin. Serotonin is an important compound in normal brain function and tranquility. Therefore, any disturbance of tryptophan metabolism via this pathway can lead to mental disturbances. Serotonin can be destroyed by the enzyme monoamine oxidase (a flavo protein), which catalyzes the formation of ammonia and 5-hydroxyindole acetaldehyde in an irreversible reaction. The aldehyde is rapidly oxidized enzymatically, utilizing NAD+ to form 5-hydroxy indoleacetate, which is then usually excreted. The formation and turnover of serotonin can be estimated by 5-hydroxy indoleacetate output in the urine. 

Thus there is much evidence to suggest that the histidine decarboxylase having its maximum activity in the pH range 8-0-9-5 is a single enzyme which can decarboxylate not only L-histidine, but also L-/S-(3,4-dihydroxyphenyl)-alanine and L-5-hydroxytryptophan. Enzyme preparations which decarboxylate one or more of these three compounds have been found also to decarboxylate the substances listed in Table 4.3, thus providing support for the existence of a general aromatic amino acid decarboxylase. It is this enzyme which will be referred to as the non-specific histidine decarboxylase. 

The other physiologically important monoamine is 5-hydroxytryptamine (serotonin or 5-HT). It is formed from tryptophan via 5-hydroxytryptophan (5-HTP) Figure 5.2). The nature and properties of tryptophan-5-hydroxy-lase is still obscure, though the hydroxylation of tryptophan in vivo has been demonstrated. There is no clear evidence that this conversion occurs in brain tissue. The decarboxylation of 5-HTP, however, takes place in brain and the decarboxylating enzyme is found in all cerebral areas which contain 5-hydroxytryptamine. 5-HTP decarboxylase is closely related to, if not identical with, DOPA decarboxylase - and agents which inhibit dopan ine formation similarly inhibit the production of 5-hydroxytryptamine. There... 

In most animals, 5-hydroxytryptamine appears to cross the blood-brain barrier with difficulty (the mouse provides an exception ) but the behavioural effects of 5-hydroxytryptophan (which is decarboxylated in the brain to 5-hydroxytryptamine) have been extensively studied in a number of species. Although the various reports show some differences in detail, it is... 

Fig (2). Aromatic L-amino acid decarboxylase also known as tryptophan decarboxylase, catalyses the synthesis of 5-bydnoxyiriplaminc (serotonin) from 5-hydroxytryptophan. lire reaction consists of a decarboxylation activity that is found in many human tissue cells... 

The enzyme AADC is involved in different metabolic pathways synthesizing two important neurotransmitters dopamine and serotonin [24]. AADC decarboxylates L-dihydroxy-phenylalanine to form dopamine and 5-hydroxytryptophan to produce serotonin. Tryptophan decarboxylase activity is detected in many brain neurons and non-nervous tissue cells. 

The enzyme that converts L-5-hydroxytryptophan to 5-HT, aromatic L-amino acid decarboxylase, is the same enzyme that decarboxylates L-dopa in catecholamine synthesis (see Chapter 6). 5-HT is stored in secretory granules by a vesicular transporter and released by exocytosis. The action of released 5-HT is terminated by neuronal uptake mediated by a specific 5-HT transporter localized in the membrane of serotonergic axon terminals (where it terminates the action of 5-HT in the synapse) and in the membrane of platelets (where it takes up 5-HT from the blood). This uptake system is the only way that platelets acquire 5-HT (they lack the enzymes required to synthesize 5-HT). The 5-HT transporter, SERT, has been cloned (see Chapters 2 and 12). 

In the second step. L-dopa is decarboxylated to dopamine by aromatic L-amino acid decarboxylase, another cytoplasmic enzyme. Aromatic L-amino acid decarboxylase was discovered in the 1930s and originally named dopa decarboxylase. Researchers subsequently discovered that dopa decarboxylase is not specific for dopa and decarboxylates other aromatic amino acids having the L (or S) absolute configuration, such as 5-hydroxytryptophan, tryptophan, and tyrosine. Nevertheless, the enzyme... 

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. 

Serotonin S-hydroxytryptamine, M, 176.2, a plant and animal hormone. It is produced by hydroxylation of L-tryptophan to 5-hydroxytryptophan, followed by decarboxylation. The synthesis occurs in the central nervous system, lung, spleen and argentaffine light cells of the intestinal mucosa. S. is stored in thrombocytes and mast cells of the blood. It acts as a Neuro-transmitter (see), stimulates peristalsis of the intestine, and causes a dose-dependent constriction of smooth muscle. It stimulates the release from arterial endothelium of a dilator substance which counteracts its primary constricting effect [T.M. Cocks X A. Angus Nature 305 (1983) 627-630]. S. is a precursor of the hormone Melatonin (see). It is inactivated and degraded by monoamine oxidases and aldehyde oxidases to 5-hydroxy-indoleacetic acid. 

Little is known about factors which limit the production of amines. The metabolic reaction which limits the biosynthesis of dopamine and serotonin is believed not to be the decarboxylation of their biochemical precursors dopa and 5-hydroxytryptophane by the ubiquitous decarboxylase, but to be the hydroxylation of the parent compounds tyrosine, and tryptophane. 140-3,189) Under comparable experimental conditions the production of serotonin from 5-hydroxytryptophane took place 30-40 times faster than from tryptophane,and the production of dopamine and noradrenaline respectively from labelled dopa was 70-100 times faster than from labelled tyrosine. In the biosynthesis of noradrenaline, the hydroxylation of dopamine, catalysed by dopamine-j -oxidase, takes place fairly slowly and is considered to be a rate-limiting reaction. ... 

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