Aromatic amino acid decarboxylase

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. 

By contrast, the cytoplasmic decarboxylation of dopa to dopamine by the enzyme dopa decarboxylase is about 100 times more rapid (Am 4x 10 " M) than its synthesis and indeed it is difficult to detect endogenous dopa in the CNS. This enzyme, which requires pyridoxal phosphate (vitamin B6) as co-factor, can decarboxylate other amino acids (e.g. tryptophan and tyrosine) and in view of its low substrate specificity is known as a general L-aromatic amino-acid decarboxylase. 

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

Although the absence of paracellular transport across the BBB impedes the entry of small hydrophilic compounds into the brain, low-molecular-weight lipophilic substances may pass through the endothelial cell membranes and cytosol by passive diffusion [7]. While this physical barrier cannot protect the brain against chemicals, the metabolic barrier formed by the enzymes from the endothelial cell cytosol may transform these chemicals. Compounds transported through the BBB by carrier-mediated systems may also be metabolized. Thus, l-DOPA is transported through the BBB and then decarboxylated to dopamine by the aromatic amino acid decarboxylase [7]. 

Dopamine is formed from tyrosine by hyclroxylation with tyrosine hydroxylase and the removal of a CO2 group by aromatic amino acid decarboxylase. The catecholamine is found in high concentrations in parts of the brain—the caudate nucleus, the median eminence, the tuberculum olfactorium, and the nucleus accumbens. Dopamine appears to act as an inhibitory neurotransmitter. 

Dopamine synthesis in dopaminergic terminals (Fig. 46-3) requires tyrosine hydroxylase (TH) which, in the presence of iron and tetrahydropteridine, oxidizes tyrosine to 3,4-dihydroxyphenylalanine (levodopa.l-DOPA). Levodopa is decarboxylated to dopamine by aromatic amino acid decarboxylase (AADC), an enzyme which requires pyri-doxyl phosphate as a coenzyme (see also in Ch. 12). 

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). 

Drugs have been developed which specifically inhibit the L-aromatic amino acid decarboxylase step in catecholamine synthesis and thereby lead to a reduction in catecholamine concentration. Carbidopa and benserazide are examples of decarboxylase inhibitors which are used clinically to... 

Following the synthesis of 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, the enzyme aromatic amino acid decarboxylase (also known as 5-HTP or dopa decarboxylase) then decarboxylates the amino acid to 5-HT. L-Aromatic amino acid decarboxylase is approximately 60% bound in the nerve terminal and requires pyridoxal phosphate as an essential enzyme. 

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"...

Dopamine is the decarboxylation product of DOPA, dihydroxyphenylalanine, and is formed in a reaction catalysed by DOPA decarboxylase. This enzyme is sometimes referred to as aromatic amino acid decarboxylase, since it is relatively non-specific in its action and can catalyse decarboxylation of other aromatic amino acids, e.g. tryptophan and histidine. DOPA is itself derived by aromatic hydroxylation of tyrosine, using tetrahydrobiopterin (a pteridine derivative see Section 11.9.2) as cofactor. 

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) is formed from tryptophan by hydroxylation then decarboxylation, paralleling the tyrosine — dopamine pathway. The non-specific enzyme aromatic amino acid decarboxylase again catalyses the decarboxylation. 

Another strategy of some interest is to deplete biogenic amines such as OA by inhibiting their biosynthesis. Inhibitors of such enzymes in the biosynthetic pathway as aromatic amino acid decarboxylase which converts tyrosine to tyramine, or dopamine 3 -hydroxylase which converts tyramine to OA are known and have interesting effects in insects (e.g. see 52,53)t but a discussion of this area lies outside the scope of this paper. Nevertheless, it is a particularly interesting one since these or related enzymes are also needed to produce catecholamines for cuticular sclerotiza-tion, thus offering dual routes to the discovery of compounds with selectively deleterious actions on insects. 

Pharmacology Entacapone is a selective and reversible inhibitor of catechol-O-methyltransferase (COMT), which alters the plasma pharmacokinetics of levodopa. When entacapone is given in conjunction with levodopa and an aromatic amino acid decarboxylase inhibitor (such as carbidopa), plasma levels of levodopa are greater and more sustained than after administration of levodopa and an aromatic amino acid decarboxylase inhibitor alone. 

Pyridoxine (vitamin Bg, 18) (Fig. 13) assists in the balancing of sodium and potassium as well as promoting red blood cell production. A lack of pyridoxine can cause anemia, nerve damage, seizures, skin problems, and sores in the mouth. It is required for the production of the monoamine neurotransmitters serotonin, dopamine, norepinephrine, and epinephrine, as it is the precursor to pyridoxal phosphate, which is the cofactor for the aromatic amino acid decarboxylase enzyme. 

If levodopa is administered alone, it is extensively metabolized by L-aromatic amino acid decarboxylase in the liver, kidney, and gastrointestinal tract. To prevent this peripheral metabolism, levodopa is coadministered with carbidopa (Sinemet), a peripheral decarboxylase inhibitor. The combination of levodopa with carbidopa lowers the necessary dose of levodopa and reduces peripheral side effects associated with its administration. 

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. 

Whilst the term biogenic amine strictly encompasses all amines of biological origin, for the purpose of this article it will be employed to refer to the catecholamine (dopamine, noradrenaline) and serotonin group of neurotransmitters. These neurotransmitters are generated from the amino acid precursors tyrosine and tryptophan, respectively, via the action of the tetrahydrobiopterin (BH4)-dependent tyrosine and tryptophan hydroxylases. Hydroxylation of the amino acid substrates leads to formation of 3,4-dihydroxy-l-phenylalanine ( -dopa) and 5-hydroxytryptophan, which are then decarboxylated via the pyridoxalphosphate-dependent aromatic amino acid decarboxylase (AADC) to yield dopamine and serotonin [4]. In noradrenergic neurones, dopamine is further metabolised to noradrenaline through the action of dopamine-jS-hydroxylase [1]. 

As a result of a high index of clinical suspicion and, on occasion, supporting biochemical data from other investigations, one of the first specialist investigations to ascertain whether a patient has an inborn error of biogenic amine metabolism is, as mentioned above, analysis of the CSF concentrations of HVA and 5HIAA. This is often performed in conjunction with the measurement of 3-methyldopa (3-MD), also known as 3-methoxytyrosine. 3-MD is formed from L-dopa via COMT activity and accumulates in conditions where aromatic amino acid decarboxylase activity is impaired. The chemical structures of HVA, 5HIAA and 3-MD are shown in Fig. 6.2.1. 

Fig. 6.2.2a-bc High-performance liquid chromatography (HPLC) with electrochemical (EC) detection of neurotransmitter metabolites, a standard mixture b cerebrospinal fluid (CSF) sample - control c CSF sample - aromatic amino acid decarboxylase (AADC) deficiency. Peak identification 1 = 5HIAA (7.7 min), 2 = 3-MD (9.6 min), 3 = HVA (11.7 min)... 

Table 6.2.2 Typical CSF profiles of HVA, 5HIAA and 3-methyldopa (3-MD) for the inborn errors of metabolism associated with a disruption of biogenic amine metabolism. A downward-pointing arrow indicates that a particular metabolite is below the established reference range. An upward pointing arrow is indicative that a metabolite is above the established reference range. WR indicates that the concentration of the metabolite is likely to be within the reference range. AADC Aromatic amino acid decarboxylase, PNPO pyridox(am)ine-5 -phosphate oxidase...

Hyland K, Clayton P (1992) Aromatic amino acid decarboxylase deficiency diagnostic methodology. Clin Chem 38 2405-2410... 

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. 

The general scheme of the biosynthesis of catecholamines was first postulated in 1939 (29) and finally confirmed in 1964 (Fig. 2) (30). Although not shown in Figure 2, in some cases the amino acid phenylalanine [63-91-2] can serve as a precursor it is converted in the liver to (-)-tyrosine [60-18-4] by the enzyme phenylalanine hydroxylase. Four enzymes are involved in E formation in the adrenal medulla and certain neurons in the brain tyrosine hydroxylase, dopa decarboxylase (also referred to as L-aromatic amino acid decarboxylase), dopamine-P-hydroxylase, and phenylethanolamine iV-methyltransferase. Neurons that form DA as their transmitter lack the last two of these enzymes, and sympathetic neurons and other neurons in the central nervous system that form NE as a transmitter do not contain phenylethanolamine N-methyl-transferase. The component enzymes and their properties involved in the formation of catecholamines have been purified to homogeneity and their properties examined. The human genes for tyrosine hydroxylase, dopamine- 3-oxidase and dopa decarboxylase, have been cloned (31,32). It is anticipated that further studies on the molecular structure and expression of these enzymes should yield interesting information about their regulation and function. 

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