Monoamine oxidase, amino acids

The synthesis and metabolism of trace amines and monoamine neurotransmitters largely overlap [1]. The trace amines PEA, TYR and TRP are synthesized in neurons by decarboxylation of precursor amino acids through the enzyme aromatic amino acid decarboxylase (AADC). OCT is derived from TYR. by involvement of the enzyme dopamine (3-hydroxylase (Fig. 1 DBH). The catabolism of trace amines occurs in both glia and neurons and is predominantly mediated by monoamine oxidases (MAO-A and -B). While TYR., TRP and OCT show approximately equal affinities toward MAO-A and MAO-B, PEA serves as preferred substrate for MAO-B. The metabolites phenylacetic acid (PEA), hydroxyphenylacetic acid (TYR.), hydroxymandelic acid (OCT), and indole-3-acetic (TRP) are believed to be pharmacologically inactive. 

Trace Amines. Figure 1 The main routes of trace amine metabolism. The trace amines (3-phenylethylamine (PEA), p-tyramine (TYR), octopamine (OCT) and tryptamine (TRP), highlighted by white shading, are each generated from their respective precursor amino acids by decarboxylation. They are rapidly metabolized by monoamine oxidase (MAO) to the pharmacologically inactive carboxylic acids. To a limited extent trace amines are also A/-methylated to the corresponding secondary amines which are believed to be pharmacologically active. Abbreviations AADC, aromatic amino acid decarboxylase DBH, dopamine b-hydroxylase NMT, nonspecific A/-methyltransferase PNMT, phenylethanolamine A/-methyltransferase TH, tyrosine hydroxylase. 

Recently Turner and coworkers have sought to extend the deracemization method beyond a-amino acids to encompass chiral amines. Chiral amines are increasingly important building blocks for pharmaceutical compounds that are either in clinical development or currently licensed for use as drugs (Figure 5.7). At the outset of this work, it was known that type II monoamine oxidases were able to catalyze the oxidation of simple amines to imines in an analogous fashion to amino acid oxidases. However, monoamine oxidases generally possess narrow substrate specificity and moreover have been only documented to catalyze the oxidation of simple, nonchiral... 

Figure 9.4 The synthesis and metabolism of 5-HT. The primary substrate for the pathway is the essential amino acid, tryptophan and its hydroxylation to 5-hydrox5dryptophan is the rate-limiting step in the synthesis of 5-HT. The cytoplasmic enzyme, monoamine oxidase (MAOa), is ultimately responsible for the catabolism of 5-HT to 5-hydroxyindoleacetic acid...

FIGURE 29-2. Levodopa absorption and metabolism. Levodopa is absorbed in the small intestine and is distributed into the plasma and brain compartments by an active transport mechanism. Levodopa is metabolized by dopa decarboxylase, monoamine oxidase, and catechol-O-methyltransferase. Carbidopa does not cross the blood-brain barrier. Large, neutral amino acids in food compete with levodopa for intestinal absorption (transport across gut endothelium to plasma). They also compete for transport across the brain (plasma compartment to brain compartment). Food and anticholinergics delay gastric emptying resulting in levodopa degradation in the stomach and a decreased amount of levodopa absorbed. If the interaction becomes a problem, administer levodopa 30 minutes before or 60 minutes after meals. 

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). 

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"...

Monoamine oxidases are enzymes that catalyze the racemization of ot-amino acids 186). Both l- and D-selective monoamine oxidases are known (i.e., they catalyze the racemization of either S)- or (R)-enantiomers of amino acids). This property has been exploited to obtain enantiomerically pure (R) and S) amino acids by using an appropriate achiral reducing agent such as NaBH4, NaB(CN)H3, or H3N BH3 in combination with an l- or D-selective monoamine oxidase 187). 

Levodopa, the metabolic precursor of dopamine, is the most effective agent in the treatment of Parkinson s disease but not for drug-induced Parkinsonism. Oral levodopa is absorbed by an active transport system for aromatic amino acids. Levodopa has a short elimination half-life of 1-3 hours. Transport over the blood-brain barrier is also mediated by an active process. In the brain levodopa is converted to dopamine by decarboxylation and both its therapeutic and adverse effects are mediated by dopamine. Either re-uptake of dopamine takes place or it is metabolized, mainly by monoamine oxidases. The isoenzyme monoamine oxidase B (MAO-B) is responsible for the majority of oxidative metabolism of dopamine in the striatum. As considerable peripheral conversion of levodopa to dopamine takes place large doses of the drug are needed if given alone. Such doses are associated with a high rate of side effects, especially nausea and vomiting but also cardiovascular adverse reactions. Peripheral dopa decarboxylase inhibitors like carbidopa or benserazide do not cross the blood-brain barrier and therefore only interfere with levodopa decarboxylation in the periphery. The combined treatment with levodopa with a peripheral decarboxylase inhibitor considerably decreases oral levodopa doses. However it should be realized that neuropsychiatric complications are not prevented by decarboxylase inhibitors as even with lower doses relatively more levodopa becomes available in the brain. 

M.G. Palfreyman, I.A. McDonald, J.R. Fozard, Y. Mely, A.J. Sleight, M. Zreika, J. Wagner, P. Bey, P.J. Lewis, Inhibition of monoamine oxidase selectively in brain monoamine nerves using the bioprecursor (MDL 72394), a substrate for aromatic L-amino acid decarboxylase, J. Neurochem. 45 (1985) 1850-1860. 

AADC Amino acid decarboxyiase MAO Monoamine oxidase... 

At behavlorally effective lntraperitoneal doses (10 mg/kg), SNA has been reported to Increase serotonin (5-HT) concentrations In rat brain.60 5-Hydroxylndoleacetlc acid concentrations are first decreased and then Increased. It has been claimed that SNA causes a decrease followed by a compensatory increase in 5-HT turnover. However, these studies have been criticized on experimental grounds,56 and later studies showed that SNA produces a decrease In 5-HT.58 No effects on aromatic amino acid decarboxylase and monoamine oxidase, two enzymes Involved In 5-HT formation and destruction, were observed SNA has been shown to reduce the concentration of the 5-HT precursor tryptophan. The uptake of 5-HT Into rat brain preparations in vitro and Into the brain stem In vivo Is somewhat Inhibited. There also seems to be a strain difference In response to SNA.61 In general, acute administration of SNA does not seem consistently to cause marked changes In brain serotonin content or turnover. 

Many of the biochemical processes that generate chemical energy for the cell take place in the mitochondria. These organelles contain the biochemical equipment necessary for fatty acid oxidation, di- and tricarboxylic acid oxidation, amino acid oxidation, electron transport, and ATP generation. In this experiment, a mitochondrial fraction will be isolated from beef heart muscle. The mitochondria will be analyzed for protein content and fractionated into submitochondrial particles. Each fraction will be analyzed for malate dehydrogenase and monoamine oxidase activities. 

That amines formed from naturally occurring amino acids are partly responsible for chronic hypertension is a rather attractive hypothesis first suggested by the experiments of Holtz (35). Besides the normal metabolic enzymes of amino acids, tissues, especially kidney, liver, and brain, contain amino acid decarboxylases, some of them specific for certain amino acids, some less so. These are anaerobic enzymes. After decarboxylation, certain monoamines are deaminated by amine oxidases which are sensitive to oxygen tension. The best known of these oxidases is the enzyme of Blaschko, Richter, and Schlossmann (9), which may be a mixture of three or more (29), and which is specific for many nonsubstituted vasoactive amines found in the body, with the notable exception of histamine. 

Monoamine oxidase, tyrosine hydroxylase, and L-amino acid oxidase generate hydrogen peroxide as their reaction product. Hydrogen peroxide is also produced by auto-oxidation of catecholamines in the presence of vitamin C. Moreover, phospholipase A2 (PLA2), cyclooxygenase (COX), and lipoxygenase (LOX), the enzymes associated with arachidonic acid release and the arachidonic acid cascade,... 

FIGURE 23.7 Dopamine (DA) is synthesized within neuronal terminals from the precursor tyrosine by the sequential actions of the enzymes tyrosine hydroxylase, producing the intermediary L-dihydroxyphenylalanine (Dopa), and aromatic L-amino acid decarboxylase. In the terminal, dopamine is transported into storage vesicles by a transporter protein (T) associated with the vesicular membrane. Release, triggered by depolarization and entry of Ca2+, allows dopamine to act on postsynaptic dopamine receptors (DAR). Several distinct types of dopamine receptors are present in the brain, and the differential actions of dopamine on postsynaptic targets bearing different types of dopamine receptors have important implications for the function of neural circuits. The actions of dopamine are terminated by the sequential actions of the enzymes catechol-O-methyl-transferase (COMT) and monoamine oxidase (MAO), or by reuptake of dopamine into the terminal. 

The excretion of amines is unusual in animals. Amines are highly toxic and one method employed by vertebrates to detoxify them is via monoamine oxidase, an enzyme which has been detected in H. diminuta (569). Amines can arise from the decarboxylation of the appropriate amino acid, e.g. glycine and alanine can give rise to methylamine and ethylamine, respectively. Another possible source of amines may be the reduction of azo or nitro compounds (39) and azo- and nitro-reductase activity has been reported from M. expansa (180, 181). Furthermore, the physiologically active amines octopamine, dopamine, adrenalin and serotonin (5-hydroxytryptamine) have been demonstrated in cestodes (283, 296, 435, 681, 682, 758, 859), where they probably function predominantly as neurotransmitters (see Chapter 2). 

List of Abbreviations Ach, acetylcholine AMPA, a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid CNS, central nervous system COMT, catechol-O-methyltransferase DA, dopamine DRP-2, dihy-dropyrimidinase-related protein 2 DSM, diagnostic and statistical manual of mental disorders GNAS1, guanine nucleotide-binding protein (G-protein) alpha stimulating activity polypeptide 1 5-HIAA, 5-hydroxyindole acetic acid 5-FIT, serotonin (5-hydroxytryptamine) MAO, monoamine oxidase MHPG, 3-methoxy-4-hydroxyphenylglycol NE, norepinephrine NMDA, N-methyl-D-aspartate PCP, phencyclidine SSRI, selective serotonin reuptake inhibitor SDS, schedule for the deficit syndrome... 

Fig. 1. A. Chemical structure of key molecules involved in the key steps in intracerebral synthesis and metabolism of dopamine. The successive steps are regulated by the enzymes tyrosine hydroxylase (TH), aromatic amino acid decarboxylase (AADC), monoamine oxidase (MAO) and dopamine-p-hydroxylase (DBH). B. Structure of key toxins and other drugs acting on dopamine neurones, including 6-hydroxydopamine (6-OHDA), a-methyl tyrosine, and amphetamine. For further details see Iversen and Iversen (1981) or Cooper et al. (1996).

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