5- Hydroxytryptamine oxidation

After an overview of neurotransmitter systems and function and a consideration of which substances can be classified as neurotransmitters, section A deals with their release, effects on neuronal excitability and receptor interaction. The synaptic physiology and pharmacology and possible brain function of each neurotransmitter is then covered in some detail (section B). Special attention is given to acetylcholine, glutamate, GABA, noradrenaline, dopamine, 5-hydroxytryptamine and the peptides but the purines, histamine, steroids and nitric oxide are not forgotten and there is a brief overview of appropriate basic pharmacology. 

The problem of selectivity is the most serious drawback to in vivo electrochemical analysis. Many compounds of neurochemical interest oxidize at very similar potentials. While this problem can be overcome somewhat by use of differential waveforms (see Sect. 3.2), many important compounds cannot be resolvai voltammetrically. It is generally not possible to distinguish between dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) or l tween 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA). Of even more serious concern, ascorbic acid oxidizes at the same potential as dopamine and uric acid oxidizes at the same potential as 5-HT, both of these interferences are present in millimolar concentrations... 

Enteric nerves control intestinal smooth muscle action and are connected to the brain by the autonomic nervous system. IBS is thought to result from dysregulation of this brain-gut axis. The enteric nervous system is composed of two gan-glionated plexuses that control gut innervation the submucous plexus (Meissner s plexus) and the myenteric plexus (Auerbach s plexus). The enteric nervous system and the central nervous system (CNS) are interconnected and interdependent. A number of neurochemicals mediate their function, including serotonin (5-hydroxytryptamine or 5-HT), acetylcholine, substance P, and nitric oxide, among others. 

Sensitive electrochemical techniques have also been developed to directly measure the release of oxidizable neurotransmitters such as catecholamines (CAs) and serotonin (5-hydroxytryptamine, 5-HT). Current flows in the circuit when the potential of the electrode is positive enough to withdraw electrons from, i.e. oxidize, the released neurotransmitter. The technique is very sensitive and readily detects the release of individual quanta of neuro transmitter resulting from the fusion of single secretory vesicles to the plasmalemma (Fig. 10-2). 

Klemm, P., Hecker, M., Stockhausen, H., Wu, C. C., Thiemermann, C., Inhibition by N-acetyl-5-hydroxytryptamine of nitric oxide synthase expression in cultured cells and in the anaesthetized rat, Br. 

The use of HPLC to analyze biogenic amines and their acid metabolites is well documented. HPLC assays for classical biogenic amines such as norepinephrine (NE), epinephrine (E), dopamine (DA), and 5-hydroxytryptamine (5-HT, serotonin) and their acid metabolites are based on several physicochemical properties that include a catechol moiety (aryl 1,2-dihydroxy), basicity, easily oxidized nature, and/or native fluorescence characteristics (Anderson, 1985). Based on these characteristics, various types of detector systems can be employed to assay low concentrations of these analytes in various matrices such as plasma, urine, cerebrospinal fluid (CSE), tissue, and dialysate. 

MAO Inhibitors. MAO is an enzyme which oxidizes a variety of monoamines. Among the substrates of this enzyme are tyramine, tryptamine, 5-hydroxytryptamine, histamine, and short chain aliphatic monoamines ( ). Oxidation of histamine to imidazoleacetaldehyde can be carried out by DAO as well as MAO. is also responsible for the conversion of N -MH, the product of HMT, to N -MIAA. Many MAO inhibitors have been identified they are conventionally divided into hydrazides, hydrazines and amines (44). Some MAO inhibitors, e.g. the hydrazines, are non-selective they also inhibit DAO. 

McDuffie JE, Coaxum SD, Maleque MA. 5-Hydroxytryptamine evokes endothelial nitric oxide synthase activation in bovine aortic endothelial cell cultures. Proc Soc Exp Biol Med 1999 221 386-390. 

Hydrogenation of the latter gave the corresponding diprimary amine (X R = Me), which on demethylation gave the phenol (X R = H). Ferricyanide oxidation then gave 5-hydroxytryptamine in 25% over-all yield from 2,5-dimethoxybenzaldehyde (109). 

Serotonin (5-hydroxytryptamine) is oxidized [via MAO and aldehyde dehydrogenase] to 5-hydroxyindoleacetic acid (5-HIAA). 

The HPLC method described here allows the assay of both isoforms in the same run by using 5-hydroxytryptamine and 3-methoxy-4-hydroxybenzyl-amine as highly selective substrates for the A and B isoforms, respectively. The product of 5-hydroxytryptamine that is quantitated is 5-hydroxyindole-2-carboxylic acid, which is obtained by including aldehyde dehydrogenase in the assay mixture to oxidize the intermediate aldehyde. The product of monoamine oxidase B is 3-methoxy-4-hydroxybenzaldehyde. The internal standards used are 5-hydroxyindole-2-carboxylic acid and 3,4-dihydroxyben-zoic acid. 

Figure 9.125 Relationship between amine concentration and oxidation product formation at a fixed level of ceruloplasmin. Oxidation of adrenaline ( ) was monitored at 300 nm oxidation of 5-hydroxytryptamine (O) was monitored at 315 nm. (From Richards, 1983.)...

Apart from the relatively small amounts that are required for synthesis of the neurotransmitter serotonin (5-hydroxytryptamine), and for net new protein synthesis, essentially the whole of the dietary intake of tryptophan is metabolized by way of the oxidative pathway shown in Figures 8.4 and 9.4, which provides both a mechanism for total catabolism by way of acetyl coenzyme A and a pathway for synthesis of the nicotinamide nucleotide coenzymes (Section 8.3). 

Bertoldi M and Voltattorni CB (2001) Dopa decarboxylase exhibits low pH halftransaminase and high pH oxidative deaminase activities toward serotonin (5-hydroxytryptamine). Protein Science 10, 1178-86. 

As shown in Figure 8.2, NAD(P) can be synthesized from the tryptophan metabolite quinolinic acid. The oxidative pathway of tryptophan metabolism is shown in Figure 8.4. Under normed conditions, almost aU of the dietary intake of tryptophan, apart from the smedl amount that is used for net new protein synthesis, is metabolized by this pathway, and hence is potentially available for NAD synthesis. About 1% of tryptophan metabolism is byway of 5-hydroxylation and decarboxylation to 5-hydroxytryptamine (serotonin), which is excreted mainly as 5-hydroxyindoleacetic acid. 

These mushrooms contain the psychoactive compound psilocybin and, in some cases, also the lesser active substance psilocin. Psilocybin is highly stable and is not destroyed by cooking or drying. Psilocin is rapidly destroyed by oxidation. Psilocybin can be extracted from the mushroom by boiling the mushroom in water. The exact mechanism of action of psilocybin has not been determined but as an indo-leamine it is thought to act similarly to LSD, as an agonist at 5-hydroxytryptamine receptors in the central nervous system. 

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. 

Wrona, M. Z. and Dryhurst, G., Interactions of 5-hydroxytryptamine with oxidative enzymes, Biochem. Pharmacol., 41(8), 1145, 1991. 

As well as the microsomal enzymes involved in the oxidation of amines, there are a number of other amine oxidase enzymes which have a different subcellular distribution. The most important are the monoamine oxidases and the diamine oxidases. The monoamine oxidases are located in the mitochondria within the cell and are found in the liver and also other organs such as the heart and central nervous system and in vascular tissue. They are a group of flavoprotein enzymes with overlapping substrate specificities. Although primarily of importance in the metabolism of endogenous compounds such as 5-hydroxytryptamine they may be involved in the metabolism of foreign compounds. The enzyme found in the liver will deaminate secondary and tertiary aliphatic amines as well as primary amines, although the latter are the preferred substrates... 

MAO catalyzes the oxidative deamination of catecholamines, 5-hydroxytryptamine (serotonin), and other monoamines, both primary such as NE, and secondary such as EP. It is one of several oxidase-type enzymes whose coenzyme is the flavin-adenine-dinucleotide (FAD) covalently bound as a prosthetic group (Fig. 9-3). The isoalloxazine ring system is viewed as the catalytically functional component of the enzyme. In a narrow view N-5 and C-4a is where the redox reaction takes place (i.e., +H+, +le or -H+, -le), although the whole chromophoric N-5-C-4a-C-4-N-3-C-2-N-l region undoubtedly participates. Figure 9-3 is a proposed structure of MAO isolated from pig brain (Salach et al., 1976).4... 

Some of the physiological and biochemical actions of the barbiturates are similar to those exhibited by chlorpromazine and the other tranquillizers. Thus, the ascending reticular system is inhibited and there is evidence that barbiturates both stimulate and inhibit the hypothalamus. Their greater hypnotic action is probably due to the fact that, like all potential anaesthetics, they cause a general depression of the central nervous system. Like chlorpromazine, some barbiturates uncouple oxidative phosphorylation in vitro (p. 301). On the other hand, the barbiturates do not exert any antagonism in vitro towards histamine, 5-hydroxytryptamine, noradrenaline or acetylcholine nor is their adininistration accompanied by signs of extra-pyramidal stimulation. 

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