Tryptophan decarboxylase metabolism

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. 

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. 

Solatium tuberosum Catharanthus roseus Tryptophan decarboxylase (TDC) Redirection of shikimate metabolism reduced phenylalanine increased disease susceptibility... 

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. 

Figure 1. Biosynthetic pathways for biogenic amines. In Drosophila and vertebrates decarboxylation of DOPA and 5-hydroxy-tryptophan is catalyzed by the same enzyme, DDC. In vertebrates this enzyme is called amino acid decarboxylase (AADC). Only vertebrates further metabolize dopamine to norepinephrine and epinephrine. TH, tryosine hydroxylase DDC, DOPA decarboxylase DBH, dopamine b-hydroxylase PNMT, phenylethanolamine N-methyltransferase. Tryp-OH tryptophan hydroxylase.

Figure 2.18. The major pathway leading to the synthesis and metabolism of 5-hydroxytryptamine (5-HT). Metabolism of tryptophan to tryptamine is a minor pathway which may be of functional importance following administration of a monoamine oxidase (MAO) inhibitor. Tryptamine is a trace amine. L-Aromatic amino acid decarboxylase is also known to decarboxylate dopa and therefore the term "L-aromatic amino acid decarboxylase" refers to both "dopa decarboxylase"...

Vitamin B6 occurs naturally in three related forms pyridoxine (6.26 the alcohol form), pyridoxal (6.27 aldehyde) and pyridoxamine (6.28 amine). All are structurally related to pyridine. The active co-enzyme form of this vitamin is pyridoxal phosphate (PLP 6.29), which is a co-factor for transaminases which catalyse the transfer of amino groups (6.29). PLP is also important for amino acid decarboxylases and functions in the metabolism of glycogen and the synthesis of sphingolipids in the nervous system. In addition, PLP is involved in the formation of niacin from tryptophan (section 6.3.3) and in the initial synthesis of haem. 

GH has been shown to induce a number of enzymes concerned with amino acid metabolism in the liver of the hypophysectomized rat (in vivo or in perfused liver [78]). Induced enzymes include tyrosine aminotransferase, tryptophan oxygenase and ornithine decarboxylase. The effects are complex, particularly in relation to interaction with glucocorticoids, and in some experiments GH lowered enzyme levels induced by glucocorticoids, although given alone it led to induction of these enzymes. 

Figure 8.4. Pathways of tryptophan metabolism. Tryptophan dioxygenase, EC 1.13.11.11 formylkynurenine formamidase, EC 3.5.1.9 kynurenine hydroxylase, EC 1.14.13.9 kynttreninase, 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 add 167.1. CoA, coenzyme A...

Some women suffer from mental deprc.ssion when taking estrogen-containing oral contraceptives, and this depression could be due to another malfunction in tryptophan metabolism, leading to S-hydroxytryptamine (serotonin). Some evidence indicates that the decarboxylation of S-hydroxytrypto-phan is inhibited (in vitro) by estfogen conjugates competing with pyridoxal phosphate for the decarboxylase apoenzyme. 

Thiram and other dithiocarbamates are metabolic poisons. The acute effects of thiram are very similar to that of carbon disulfide, supporting the notion that the common metabolite of this compound is responsible for its toxic effects. The exact mechanism of toxicity is still unclear, however it has been postulated that the intracellular action of thiram involves metabolites of carbon disulfide, causing microsome injury and cytochrome P450 disruption, leading to increased heme-oxygenase activity. The intracellular mechanism of toxicity of thiram may include inhibition of monoamine oxidase, altered vitamin Bg and tryptophan metabolism, and cellular deprivation of zinc and copper. It induces accumulation of acetaldehyde in the bloodstream following ethanol or paraldehyde treatment. Thiram inhibits the in vitro conversion of dopamine to noradrenalin in cardiac and adrenal medulla cell preparations. It depresses some hepatic microsomal demethylation reactions, microsomal cytochrome P450 content and the synthesis of phospholipids. Thiram has also been shown to have moderate inhibitory action on decarboxylases and, in fish, on muscle acetylcholinesterases. 

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... 

Figure 12-3. Synthesis and metabolism of serotonin. TH = tryptophane hydroxylase AAD = l-aromatic amino acid decarboxylase MAO = monoamine oxidase ADH = aldehyde dehydrogenase. (Broken arrow indicates possible aberrant pathway.)...

A variety of other enzymes involved in amino acid catabolism have been detected in both protozoa and helminths. These include deaminases such as histidase, decarboxylases, some of which are involved in biosynthesis of amines and related compounds, and hydroxylases of proline, tryptophan and tyrosine. These additional enzymes have mostly been reported in helminths (1). L-Amino acid oxidases and D-amino acid oxidases are also present and the availability of the latter would allow D-amino acids to be metabolized in the absence of amino acid racemases. 

The uses of GC to study enzymatic reactions in the amino acid metabolism have also been common. In such studies, measuring either a decreasing substrate concentration or an increase of the reaction product, GC can frequently offer greater sensitivity than other analytical methods. Alternatively, stereospecificity of some enzymatic reactions can be distinguished [505] if the resolution of optical isomers through GC is employed. Recent examples of the enzyme activity determinations are those concerning tryptophan pyrolase [506] and glutamic acid decarboxylase [507] in brain tissue. 

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. 

Acetylcholine is formed from choline (which is also an important constituent of phospholipids) and acetyl CoA under the catalytic influence of choline acetyl-ase. It is hydrolised by acetylcholinesterase or choline esterase. Two important steps in the formation of noradrenaline from tyr dopa decarboxylase and dopamine hydroxylase. Adrenaline is formed from noradrenaline by phenyl ethanolamine A -methyltransferase. Both noradrenaline and adrenaline are metabolised by catechol 0-methyl transferase or monoamine oxidase. Some later steps in their metabolism involve aldehyde dehydrogenase and alcohol dehydrogenase (aldehyde reductase), After hydroxylation to its 5-hydroxy derivative, tryptophan is converted by 5-hydroxytryptophan decarboxylase to 5-hydroxytryptamine (serotonin). The major routes of serotonin metabolism involve either monoamine oxidase or hydroxyindole 0-methyltransferase. Histamine is synthesised from histidine by histidine decarboxylase, and is metabolised by either diamine oxidase or histamine Af-methyltransferase. Gamma aminobutyric acid is formed by glutamate decarboxylase and metabolised by... 

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. ... 

Fig. 1.1. Biosynthesis and regeneration of tetrahydrobiopterin including possible metabolic defects and catabolism of phenylalanine. l.l=phenylalanine-4-hydroxylase (PAH) 1.2/1.6 = GTP cyclohydrolase I (GTPCH), 1.3 = 6-pyruvoyl-tetra-hydropterin synthase (PTPS), 1.4 = dihydropteridine reductase (DHPR), 1.5 = pterin-4a-carbinolamine dehydratase (PCD), 1.7 = sepiapterin reductase SR, carbonyl reductase (CR), aldose reductase (AR), dihydrofolate reductase (DHFR), aromatic amino acid decarboxylase (AADC), tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), nitric oxide synthase (NOS). Pathological metabolites used as specific markers in the differential diagnosis are marked in squares. n.e.=non-enzymatic...

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