Aromatic-L-amino-acid decarboxylase

Decarboxylation of L-3,4-dihydroxyphenylalanine to dopamine, and of 5-hydroxytryptophan to serotonin, is catalyzed by aromatic L-amino acid decarboxylase. A single enzyme may be responsible for both activities. This assay permits simultaneous determination of both activities. 

The substrates and products just noted were separated on an Ultrasphere I.P. CI8 column (4.6 mm x 250 mm, 5 /tm). The mobile phase contained 75 mAf sodium phosphate (pH 2.75), 1 mM sodium octylsulfate, 500 fiM EDTA, and 13% (v/v) acetonitrile. Quantitation was by electrochemical detection of the products using 0.75 V versus an Ag/AgCl reference electrode. 

The standard incubation mixture contained 0.2 Af phosphate buffer (pH 7.5), 0.02 m Af pyridoxal phosphate, 0.1 mAf patgyline, 0.2 mAf L-dihydroxyphe-nylalanine, 0.1 mAf 5-hydroxytryptophan, and enzyme in a total volume of 200 fiL. After incubation at 37°C for 30 minutes, the reactions were terminated by addition of 800 fiL of chilled 0.1 Af perchloric acid containing 0.1 mAf sodium metabisulfite and 0.2 mAf EDTA. After centrifugation, 10 fiL aliquots were used for HPLC analysis. 

Tissue homogenates in 5 volumes of chilled 0.05 Af phosphate buffer (pH 7.4) were prepared from the liver, kidney, adrenal, brain, heart, lung, and small and large intestine of rats. 

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Together with dopamine, adrenaline and noradrenaline belong to the endogenous catecholamines that are synthesized from the precursor amino acid tyrosine (Fig. 1). In the first biosynthetic step, tyrosine hydroxylase generates l-DOPA which is further converted to dopamine by the aromatic L-amino acid decarboxylase ( Dopa decarboxylase). Dopamine is transported from the cytosol into synaptic vesicles by a vesicular monoamine transporter. In sympathetic nerves, vesicular dopamine (3-hydroxylase generates the neurotransmitter noradrenaline. In chromaffin cells of the adrenal medulla, approximately 80% of the noradrenaline is further converted into adrenaline by the enzyme phenylethanolamine-A-methyltransferase. 

Decarboxylation of histidine to histamine is catalyzed by a broad-specificity aromatic L-amino acid decarboxylase that also catalyzes the decarboxylation of dopa, 5-hy-droxytryptophan, phenylalanine, tyrosine, and tryptophan. a-Methyl amino acids, which inhibit decarboxylase activity, find appfication as antihypertensive agents. Histidine compounds present in the human body include ergothioneine, carnosine, and dietary anserine (Figure 31-2). Urinary levels of 3-methylhistidine are unusually low in patients with Wilson s disease. 

Figure 13.7 Synthesis and structure of the trace amines phenylethylamine, /)-tyramine and tryptamine. These are all formed by decarboxylation rather than hydroxylation of the precursors of the established monoamine neurotransmitters, dopamine and 5-HT. (1) Decarboxylation by aromatic L-amino acid decarboxylase (2) phenylaline hydroxylase (3) tyrosine hydroxylase (4) tryptophan hydroxylase...

Albert, V. R., Allen, J. M., and Joh, T. H. (1987). A single gene codes for aromatic L-amino acid decarboxylase in both neuronal and non-neuronal tissues. J. Biol. Chem. 262 9404-9411. 

Ichinose, H., Kurosawa, Y., Titani, K., Fujita, K., and Nagatsu, T. (1989). Isolation and characterization of a cDNA clone encoding human aromatic L-amino acid decarboxylase. Biochem. Biophys. Res. Commun. 164 1024-1030. 

Jaeger, C. B., Teitelman, G Joh, T. H., Albert, V. R Park, D. H., and Reis, D. J. (1983). Some neurons of the rat central nervous system contain aromatic L-amino acid decarboxylase but not monoamines. Science 219 1233-1235. 

Lovenberg, W Weissbach, W., and Udenfriend, S. (1962). Aromatic L-amino acid decarboxylase. J. Biol. Chem. 237 89-93. 

Nagatsu, T., Ichinose, H., Kojima, K., Kameya, T., Shimase, J., Kodama, T., and Shimosato, U. (1985). Aromatic L-amino acid decarboxylase activities in human lung tissues comparison between normal lung and lung carcinomas. Biochem. Med. 34 52-59. 

Rahman, M. K Nagatsu, T., and Kato, T. (1981). Aromatic L-amino acid decarboxylase activity in central and peripheral tissues and serum of rats with L-DOPA and L-5-hydroxytryptophan as substrates. Biochem. Pharmacol. 30 645-649. 

Thai, A. L. V., Coste, E., Allen, J. M., Palmiter, R. D., and Weber, M. J. (1993). Identification of a neuron-specific promoter of human aromatic L-amino acid decarboxylase gene. Mol. Brain. Res 17 227-238. 

L-dopa is effective in the treatment of Parkinson s disease, a disorder characterised by low levels of dopamine, since L-dopa is metabolised into dopamine. However, this biosynthesis normally occurs in both the peripheral nervous system (PNS) and the central nervous system CNS. The related drug carbidopa inhibits aromatic L-amino acid decarboxylase only in the periphery, since it does not cross the blood-brain barrier. So, when carbidopa is given with L-dopa, it reduces the biosynthesis of L-dopa to dopamine in the periphery and, thus, increases the bioavailability of L-dopa for the dopaminergic neurons in the brain. Hence, carbidopa increases the clinical efficacy of L-dopa for Parkinsonian patients. 

Carbidopa An inhibitor of aromatic L-amino acid decarboxylase used with L-dopa in the treatment of Parkinson s disease. 

The answer is c. (idardman, p 510.) Carbidopa is an inhibitor of aromatic L-amino acid decarboxylase. It cannot readily penetrate the central nervous system (CNS) and, thus, decreases the decarboxylation of L-clopa in the peripheral tissues. This promotes an increased concentration of L-clopa in the nigrostriatum, where it is converted to dopamine. In addition, the effective dose of L-dopa can be reduced... 

The other enzyme involved in the synthesis of 5-HT, aromatic L-amino acid decarboxylase (AADC) (EC 4.1.1.28), is a soluble pyridoxal-5 -phosphate-dependent enzyme, which converts 5-HTP to 5-HT (Fig. 13-5). It has been demonstrated that administration of pyridoxine increases the rate of synthesis of 5-HT in monkey brain, as revealed using position emission tomography (this technique is discussed in Ch. 58). This presumably reflects a regulatory effect of pyridoxine on AADC activity and raises the interesting issue of the use of pyridoxine supplementation in situations associated with 5-HT deficiency. 

Eaton, M. J., Gudehithlu, K. P., Quach, T., Sivia, C. P., Hadjiconstantinou, M. and Neff, N. H. Distribution of aromatic L-amino acid decarboxylase mRNA in mouse brain by in situ hybridization histology. J. Comp. Neurol. 337 640-654,1993. 

AADC aromatic L-amino acid decarboxylase BDZ benzodiazepine... 

Synthesis of norepinephrine begins with the amino acid tyrosine, which enters the neuron by active transport, perhaps facilitated by a permease. In the neuronal cytosol, tyrosine is converted by the enzyme tyrosine hydroxylase to dihydroxyphenylalanine (dopa), which is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase, sometimes termed dopa-decarboxylase. The dopamine is actively transported into storage vesicles, where it is converted to norepinephrine (the transmitter) by dopamine (3-hydroxylase, an enzyme within the storage vesicle. 

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. 

J. R. Fozard (1982). Highly potent irreversible inhibitors of aromatic L-amino acid decarboxylase. Trends Pharmacol. Sci. 3 429. 

It is an indole ethylamine formed in biological systems from the amino acid L-tryptophan by hydroxylation with tryptophan hydroxylase enzyme, followed by the decarboxylation by the nonspecific aromatic L-amino acid decarboxylase. 5-HT is then taken up into secretory granules and stored. 

Shen H, Kannari K, Yamato H, et al. Effects of benser-azide on L-DOPA-derived extracellular dopamine levels and aromatic L-amino acid decarboxylase activity in the striatum of 6-hydroxydopamine-lesioned rats. Tohoku J Exp Med. 2003 199 149-159. 

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. 

Aromatic L-amino acid decarboxylase 5-Hydroxytryptamine (serotonin)... 

Sanchez-Pernaute, R. et al. (2001). Functional effect of adeno-associated virus mediated gene transfer of aromatic L-amino acid decarboxylase into the striatum of 6-OHDA-lesioned rats. Mol. Ther. 4(4), 324-330. 

Shen, Y. et al. (2000). Triple transduction with adeno-associated virus vectors expressing tyrosine hydroxylase, aromatic-L-amino-acid decarboxylase, and GTP cyclohydrolase I for gene therapy of Parkinson s disease. Hum. Gene Ther. 11(11), 1509-1519. 

In clinical practice, L-dopa is conventionally administered in combination with a peripherally acting inhibitor of aromatic L-amino acid decarboxylase (e.g., carbidopa). If L-dopa is administered alone, the drug is largely decarboxylated by enzymes in the intestinal mucosa and other peripheral sites. Inhibition of peripheral decarboxylase by carbidopa markedly increases the fraction of orally administered L-dopa that remains unmetabolized and available to enter the brain (i.e., its bioavailability). 

Balan IS, Ugrumov MV, Calas A, Mailly P, Krieger M, Thibault J (2000) Tyrosine Hydroxylase-expressing and/ or aromatic L-amino acid decarboxylase-expressing neurons in the mediobasal hypothalamus of perinatal rats differentiation and sexual dimorphism. J Comp Neurol 25 167-176. 

Kitahama K, Ikemoto K, Jouvet A, Nagatsu I, Sakamoto N, Pearson J (1998) Aromatic L-amino acid decarboxylase- and tyrosine hydroxylase-immunohistochemistry in the adult human hypothalamus. J Chem Neuroanat 75 43-55. 

Finally, the decarboxylation of amino acids catalyzed by several pyridoxal phosphate-dependent enzymes has been shown to proceed by a retention of configuration at the Ca atom144. The stereochemical course of the decarboxylation of 5-hydroxy tryptophan to 5-hydroxytryptamine (serotonin) catalyzed by the pyridoxal phosphate-dependent aromatic L-amino acid decarboxylase (equation 15) exemplifies such studies145. 

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