5-hydroxytryptamine tryptophan hydroxylase

Knapp, S. Mandell, A.J. and Geyer, M.A. Effects of amphetamines on regional tryptophan hydroxylase aetivity and synaptosomal conversion of tryptophan to 5-hydroxytryptamine in rat brain. J Pharmacol Exp Ther 189 676-689, 1974. 

Sanders-Bush, E. Bushing, J.A. and Sulser, F. Long-term effects of p-chloroamphetamine on tryptophan hydroxylase activity and on levels of 5-hydroxytryptamine and 5-hydroxyindole aeetie acid in brain. 

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). 

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. 

Carcinoid is a tumor of the enterochromaffin cells that normally synthesize 5-hydroxytrytophan and 5-hydroxytryptamine. The carcinoid syndrome is seen when there are significant metastases of the primary tumor in the liver. It is characterized by increased gastrointestinal motility and diarrhea, as well as by regular periodic flushing. These symptoms can be attributed to systemic release of large amounts of serotonin and can be controlled with inhibitors of tryptophan hydroxylase, such as p-chlorophenylalanine. The synthesis of 5-hydroxytryptamine in advanced carcinoid syndrome may be so great that as much as 60% of the body s tryptophan metabolism proceeds by this pathway, compared with about 1% under normal conditions. A significant number of... 

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. 

Not only is methamphetamine administration toxic to the dopaminergic system, but the serotonergic system in the various brain areas is also altered. Hotchkiss and Gibb (1980) reported that methamphetamine, administered as described above, decreased tryptophan hydroxylase (TPH) activity in the serotonergic nerve terminal of rat brain and spinal cord. Similarly, the content of 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindole acetic acid (5-HIAA) were also severely depressed. In contrast to the effects in the dopaminergic system, these serotonergic parameters were decreased by methamphetamine within 15 minutes after a single dose... 

Tryptophan hydroxylase uses 02 and the electron donor BH4 to hydroxylate C-5 of tryptophan. The product, called 5-hyroxy-tryptophan, then undergoes a decarboxylation catalyzed by 5-hydroxy tryptophan decarboxylase, a pyridoxal phosphate-requiring enzyme. Serotonin, often referred to as 5-hydroxytryptamine, is the product of this reacdon. 

Serotonin, or 5-hydroxytryptamine (5-HT), is another monoamine whose important central effects have only been recognized recently. It had previously been known as a vasoconstrictor in the plasma. Once identified chemically, it was found to be widely distributed in the body. After determination of significant brain levels in the hypothalamus, medulla, midbrain, and other areas (Table 12-3), and after establishing its biosynthetic paths, serotonin became recognized as a neurotransmitter. 5-HT is presently less well understood than are the catecholamines. Figure 12-3 outlines its biosynthesis from the essential amino acid tryptophan. Try enters the brain by active transport (as is L-dopa) and is hydroxylated there by tryptophan hydroxylase, which is an enzyme similar to if not identical to, tyrosine hydroxylase. 

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. 

Neckers, L. M., Bertilsson, L., Koslow, S. H., and Meek, J. L., 1976, Reduction of tryptophan hydroxylase activity and 5-hydroxytryptamine concentration in certain rat brain nuclei after / -chloroamphetamine, /. Pharmacol. Exp. Ther. 196 333-338. 

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. 

I Tryptophan 5-hydroxylase 5-Hydroxy tryptophan I Aromatic L-amino acid decarboxylase 5-Hydroxytryptamine (serotonin)... 

SYNTHESIS, STORAGE, AND RELEASE OF CATECHOLAMINES Synthesis—The steps in the synthesis of DA, NE (known outside the U.S. as noradrenaline), and Epi (known as adrenahne) are shown in Eigure 6-A. Tyrosine is sequentially 3-hydroxylated and decarboxylated to form DA. DA is 3-hydroxylated to yield NE (the transmitter in postganglionic nerves of the sympathetic branch of the ANS), which is N-methylated in chromaffin tissue to give Epi. The enzymes involved are not completely specific consequently, other endogenous substances and some drugs are also substrates. 5-hydroxytryptamine (5-HT, serotonin) can be produced from 5-hydroxy-L-tryptophan by aromatic L-amino acid decarboxylase (AAD or dopa decarboxylase). AAD also converts dopa into DA, and methyldopa to a-methyl-DA, which is converted to a-methyl-NE by dopamine /3-hydroxylase (Dj3H Table 6-4). 

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... 

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