Monoamine oxidase, substrates

Oxazolidinones and dihydrofuranones as inactivators and substrates of monoamine oxidase B, approaches to the design of antiparkinsonian agents 97F343. 

Monoamine Oxidases and their Inhibitors. Table 1 Substrate specificity of the two forms of rat liver and brain monoamine oxidase... 

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. 

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... 

Upon mutagenesis of the monoamine oxidase from Aspergillus niger (MAO-N) within several rounds of directed evolution [65], variant biocatalysts were identified with largely expanded substrate acceptance, enabling also the deracemization of tertiary amines incorporating straight-chain and cyclic structural motifs [66]. 

Just as the synthesis of DA and NA is similar so is their metabolism. They are both substrates for monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). In the brain MAO is found in, or attached to, the membrane of the intraneuronal mitochondria. Thus it is only able to deaminate DA which has been taken up into nerve endings and blockade of DA uptake leads to a marked reduction in the level of its deaminated metabolites and in particular DOPAC. The final metabolite, homovanillic... 

Monoamine oxidase exists in two forms, MAOa and MAOb. The former is more active against NA and 5-HT than it is against DA, which is a substrate for both, even though, like S-phenylethylamine, it is more affected by MAOb. H seems likely that MAOb is the dominant enzyme in human brain and inhibitors of it, such as selegiline, have some value in the treatment of Parkinson s disease by prolonging the action of the remaining endogenous DA as well as that formed from administered levodopa. 

After reuptake into the cytosol, some noradrenaline may be taken up into the storage vesicles by the vesicular transporter and stored in the vesicles for subsequent release (see above). However, it is thought that the majority is broken down within the cytosol of the nerve terminal by monoamine oxidase (MAO ECl.4.3.4). A second degradative enzyme, catechol-O-methyl transferase (COMT EC2.1.1.6), is found mostly in nonneuronal tissues, such as smooth muscle, endothelial cells or glia. The metabolic pathway for noradrenaline follows a complex sequence of alternatives because the metabolic product of each of these enzymes can act as a substrate for the other (Fig 8.8). This could enable one of these enzymes to compensate for a deficiency in the other to some extent. 

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...

Buckholtz, N., Boggan, W. Monoamine oxidase inhibition in brain and liver produced by b-carbohnes structure activity relationships and substrate specihcity. Biochem. Pharmacol. 26 1991, 1977. 

Biosynthesis is performed in one step by the enzyme L-histidine decarboxylase (HDC, E.C. 4.1.1.22). Histamine metabolism occurs mainly by two pathways. Oxidation is carried out by diamine oxidase (DAO, E.C. 1.4.3.6), leading to imidazole acetic acid (IAA), whereas methyla-tion is effected by histamine N-methyltransferase (HMT, E.C. 2.1.1.8), producing fe/e-methylhistamine (t-MH). IAA can exist as a riboside or ribotide conjugate. t-MH is further metabolized by monoamine oxidase (MAO)-B, producing fe/e-methylimidazole acetic acid (t-MIAA). Note that histamine is a substrate for DAO but not for MAO. Aldehyde intermediates, formed by the oxidation of both histamine and t-MH, are thought to be quickly oxidized to acids under normal circumstances. In the vertebrate CNS, histamine is almost exclusively methylated... 

No importance of metabolic processes to the mechanism of action has yet been demonstrated. The phenethylamines generally are not good inhibitors of monoamine oxidase (MAO), although more active compounds may not be good substrates for this enzyme (MAO) (36). However, no extensive studies of phenethylamines have been reported, as either inhibitors or substrates of MAO. For... 

In contrast to the flavin-dependent monoamine oxidases, SSAO/VAP-1 has evolved to hydroxylate a tyrosine residue in the active site which is further oxidized to the quinone state by oxygen in the presence of copper ion releasing hydrogen peroxide [28-30]. The primary amine in the substrate (R-NH2, Scheme 1) forms a Schiff-base with the quinone carbonyl group, which through a series of steps ultimately releases the aldehyde product. 

In contrast, much is known about the catabolism of catecholamines. Adrenaline (epinephrine) released into the plasma to act as a classical hormone and noradrenaline (norepinephrine) from the parasympathetic nerves are substrates for two important enzymes monoamine oxidase (MAO) found in the mitochondria of sympathetic neurones and the more widely distributed catechol-O-methyl transferase (COMT). Noradrenaline (norepinephrine) undergoes re-uptake from the synaptic cleft by high-affrnity transporters and once within the neurone may be stored within vesicles for reuse or subjected to oxidative decarboxylation by MAO. Dopamine and serotonin are also substrates for MAO and are therefore catabolized in a similar fashion to adrenaline (epinephrine) and noradrenaline (norepinephrine), the final products being homo-vanillic acid (HVA) and 5-hydroxyindoleacetic acid (5HIAA) respectively. 

Competitive inhibitors bind to specific groups in the enzyme active site to form an enzyme-inhibitor complex. The inhibitor and substrate compete for the same site, so that the substrate is prevented from binding. This is usually because the substrate and inhibitor share considerable stmctural similarity. Catalysis is diminished because a lower proportion of molecules have a bound substrate. Inhibition can be relieved by increasing the concentration of substrate. Some simple examples are shown below. Thus, sulfanilamide is an inhibitor of the enzyme that incorporates j9-aminobenzoic acid into folic acid, and has antibacterial properties by restricting folic acid biosynthesis in the bacterium (see Box 11.13). Some phenylethylamine derivatives, e.g. phenelzine, provide useful antidepressant drags by inhibiting the enzyme monoamine oxidase. The cA-isomer maleic acid is a powerful inhibitor of the enzyme that utilizes the trans-isomer fumaric acid in the Krebs cycle. 

In order to extend the approach to include deracemization of chiral secondary amines, this group carried out directed evolution on the monoamine oxidase (MAO) enzyme MAO-N (Scheme 2.32). A new variant was identified with improved catalytic properties towards a cyclic secondary amine 64, the substrate used in the evolution experiments. This new variant had a single point mutation, lle246Met, and was found to have improved catalytic properties towards a number of other cyclic secondary amines. The new variant was used in the deracemization of rac-64 yielding (R)-64 in high yield and enantiomeric excess [34]. 

Monoamine oxidase exists in the human body in two molecular forms, known as type A and type B. Each of these isozymes has selective substrate and inhibitor characteristics. Neurotransmitter amines, such as norepinephrine and serotonin, are preferentially metabolized by MAO-A in the brain. MAO-B is more likely to be involved in the catabolism of human brain dopamine, although dopamine is also a substrate for MAO-A. 

Fig. 1. Influence of substituents in the aromatic ring on the selectivity of 3-(2-aminoethoxy)-1,2-benzisoxazole derivatives as monoamine oxidases (MAO) A or B inhibitors. Ratio of selectivity was calculated from IC50 values of MAO A and B determined by an in vitro assay of mouse brain mitochondria using 5-hydroxy tryptamine (5-HT) and 2-phenylethylamine (PEA) as specific substrates, respectively (see Ref. [9]). Smaller values indicate that inhibitors are MAO A selective [71].

C.H. Williams, Monoamine-oxidase. 1. Specificity of some substrates and inhibitors, Biochem. Pharmacol. 23 (1974) 615-628. 

R.B. Silverman, W.P. Flawe, SAR studies of fluorine-substituted benzylamines and substituted 2-phenylethylamines as substrates and inactivators of monoamine-oxidase-B, J. Enzyme Inhib. 9 (1995) 203-215. 

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. 

Monoamine oxidase A (MAO A) acts selectively on the substrates norepinephrine and serotonin, whereas monoamine oxidase B (MAO B) preferentially affects phenylethylamine. Both MAO A and MAO B oxidize dopamine and tyramine. MAO A inhibition appears to be most relevant to the antidepressant effects of these drugs. Drugs that inhibit both MAO A and MAO B are called non-selective. The MAOI antidepressants currently available in the United States are nonselective inhibitors. Because tyramine can be metabolized by either MAO A or MAO B, drugs that selectively inhibit one of these enzymes but not the other do not require dietary... 

Monoamine oxidase (MAO) (E.C. 1.4.3.4) is an enzyme found in all tissues and almost all cells, bound to the outer mitochondrial membrane. Its active site contains flavine adenine dinucleotide (FAD), which is bound to the cysteine of a -Ser-Gly-Gly-Cys-Tyr sequence. Ser and Tyr in this sequence suggest a nucleophilic environment, and histidine is necessary for the activity of the enzyme. Thiol reagents inhibit MAO. There are at least two classes of MAO binding sites, either on the same molecule or on different isozymes. They are designated as MAO-A, which is specific for 5-HT (serotonin) as a substrate, and MAO-B, which prefers phenylethylamine. Similarly, MAO inhibitors show a preference for one or the other active site, as discussed below. 

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