Noradrenaline methylation

Pittalnga A, Pattarini R, Severi P, Raiteri M (1996) Hnman brain N-methyl-D-aspartate receptors regnlating noradrenaline release are positively modnlated by HIV-1 coat protein gpl20. Aids 10(5) 463 68... 

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 8.8 The metabolic pathway(s) for noradrenaline. MAO is responsible for the oxidative deamination of noradrenaline derivatives while COMT 0-methylates noradrenaline. Most intraneuronal metabolism involves MAO while COMT is mainly found extraneuronally. However, both these enz5unes can act on each other s products, yielding a complex cocktail of metabolites. The reasons for this complex network of metabolites are not known...

The enzyme /i-phenylethanolamine-A-methyl transferase, which is required to convert noradrenaline (NA) to adrenaline (Ad), is present in the CNS and there is histofluoro-metric evidence (positive staining with antibodies to that enzyme and to tyrosine hydroxylase and dopamine /i-hydroxylase as well) for adrenergic cell bodies in two groups (nuclei) alongside NA neurons of the locus coeruleus (EC) but ventral and lateral (Ci) and dorsal and medial (C2) to it. Projections go to the hypothalamus and in... 

Analogues Chemicals with similar molecular structures (e.g., adrenaline differs from noradrenaline by the addition of one methyl group to the N atom). 

OCTs) tyramine, tryptamine, N-methylnico-tineamide (NMN), choline, spermine, spermidine quinine, d-tubocurarine, procanamide, dopamine, noradrenaline, serotonin, histamine, corticosterone, l-methyl-4-phenylpyridinium (MPP), despramine... 

Figure 5. Schematic of nerve ending and effector cell. 1, liberation of noradrenaline (NA) 2, reuptake 3, combining with a-adrenergic receptor 4, diffusion 5, degradation of NA. MAO = monoamineoxydase, COMT = cat echol-O-methyl-transferase.

The first step is catalysed by the tetrahydrobiopterin-dependent enzyme tyrosine hydroxylase (tyrosine 3-monooxygenase), which is regulated by end-product feedback is the rate controlling step in this pathway. A second hydroxylation reaction, that of dopamine to noradrenaline (norepinephrine) (dopamine [3 oxygenase) requires ascorbate (vitamin C). The final reaction is the conversion of noradrenaline (norepinephrine) to adrenaline (epinephrine). This is a methylation step catalysed by phenylethanolamine-jV-methyl transferase (PNMT) in which S-adenosylmethionine (SAM) acts as the methyl group donor. Contrast this with catechol-O-methyl transferase (COMT) which takes part in catecholamine degradation (Section 4.6). 

Synthesis of noradrenaline (norepinephrine) is shown in Figure 4.7. This follows the same route as synthesis of adrenaline (epinephrine) but terminates at noradrenaline (norepinephrine) because parasympathetic neurones lack the phenylethanolamine-N-methyl transferase required to form adrenaline (epinephrine). Acetylcholine is synthesized from acetyl-Co A and choline by the enzyme choline acetyltransferase (CAT). Choline is made available for this reaction by uptake, via specific high-affinity transporters, within the axonal membrane. Following their synthesis, noradrenaline (norepinephrine) or acetylcholine are stored within vesicles. Release from the vesicle occurs when the incoming nerve impulse causes an influx of calcium ions resulting in exocytosis of the neurotransmitter. 

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. 

The diagram below shows the pathway of pain transmission from the peripheral nerves to the cerebral cortex. There are three levels of neuronal involvement and the signals may be modulated at two points during their course to the cerebral cortex. Descending inhibitory pathways arise in the midbrain and pass to the dorsal horn as shown. Multiple different neurotransmitters are involved in the pathway and include gamma-aminobutyric acid (GABA), N-methyl-D-aspartate (NMDA), noradrenaline and opioids. 

Phenylalanine (Phe or F) (2-amino-3-phenyl-propanoic acid) is a neutral, aromatic amino acid with the formula HOOCCH(NH2)CH2C6H5. It is classified as nonpolar because of the hydrophobic nature of the benzyl side chain. Tyr and Phe play a significant role not only in protein structure but also as important precursors for thyroid and adrenocortical hormones as well as in the synthesis of neurotransmitters such as dopamine and noradrenaline. The genetic disorder phenylketonuria (PKU) is the inability to metabolize Phe. This is caused by a deficiency of phenylalanine hydroxylase with the result that there is an accumulation of Phe in body fluids. Individuals with this disorder are known as phenylketonurics and must abstain from consumption of Phe. A nonfood source of Phe is the artificial sweetener aspartame (L-aspartyl-L-phenylalanine methyl ester), which is metabolized by the body into several by-products including Phe. The side chain of Phe is immune from side reactions, but during catalytic hydrogenations the aromatic ring can be saturated and converted into a hexahydrophenylalanine residue. ... 

Fonlupt P, Rey C, Pacheco H. 1987. Comparison of basal and noradrenaline stimulated methylation of chloroform-extractable products in synaptosomal preparations from the rat brain. Biochem Pharmacol 36 1527-1729. 

Approximately 30 years ago, Schildkraut postulated that noradrenaline may play a pivotal role in the aetiology of depression. Evidence in favour of this hypothesis was provided by the observation that the antihypertensive drug reserpine, which depletes both the central and peripheral vesicular stores of catecholamines such as noradrenaline, is likely to precipitate depression in patients in remission. The experimental drug alpha-methyl-paratyrosine that blocks the synthesis of noradrenaline by inhibiting the rate-limiting enzyme tyrosine hydroxylase was also shown to precipitate depression in patients during remission. While such findings are only indirect indicators that noradrenaline plays an important role in human behaviour, and may be defective in depression, more direct evidence is needed to substantiate the hypothesis. The most obvious approach would be to determine the concentration of noradrenaline and/or its major central... 

Rapid relapse following administration of the tyrosine hydroxylase inhibitor alpha-methyl-tyrosine to depressed patients who respond to a noradrenaline reuptake inhibitor such as desipramine... 

The noradrenaline normally contained in the storage granules can be partly or completely replaced by structurally related sympathomimetic amines, either by injection of the amine itself, or of suitable precursors such as a-methyl-DOPA or a-methyl-w-tyrosine. These amines can be depleted from the heart by guanethidine in the same way as the noradrenaline which they had replaced. a-Methylnoradrenaline [337] and metaraminol [338] are depleted less readily than noradrenaline from rabbit or rat hearts, whereas dopamine, octopamine and w-octopamine are depleted more readily than noradrenaline [339]. The more rapid depletion of these last three compounds was attributed to weaker binding in the storage granules [339], but could equally well be due to their greater susceptibility to destruction by monoamine oxidase, since both a-methyl-noradrenaline and metaraminol are resistant to attack by monoamine oxidase. 

Note that in nature, these are all enzyme-catalysed reactions. This makes the reactions totally specific. It means possible competing Sn2 reactions involving attack at either of the two methylene carbons in SAM are not encountered. It also means that where the substrate contains two or more potential nucleophiles, reaction occurs at only one site, dictated by the enzyme. The enzymes are usually termed methyltransferases. Thus, in animals an A-methyltransferase is responsible for SAM-dependent A-methylation of noradrenaline (norepinephrine) to adrenaline (epinephrine), whereas an O-methyltransferase in plants catalyses esterification of salicylic acid to methyl salicylate. 

Methylations are catalyzed by a family of relatively specific methyl-transferases involving the transfer of methyl groups to hydroxyl groups (0-methylation as in norepinephrine [noradrenaline]) or to amino groups (N-methylation of norepinephrine, histamine, or serotonin). 

The hydroxyl groups of the phenyl ring are a prerequisite for the activation of all adrenoceptors, if both are absent the molecule has only an indirect sympathomimetic effect (see Fig. 5). Indirect sympathomimetics only have a -, a2 and -adrenoceptor activity since they act via an increase of the noradrenaline concentration in the synaptic cleft. If the methyl-group at the N-position of adrenaline is substituted by a longer or more bulky moiety the molecule gains affinity for the and loses affinity for O -adrenoceptors. An isopropyl moiety is already the optimum for the affinity towards 0-adrenoceptors (isoprenaline), larger substituents enhance only the binding to the 2-subtype (for example fenoterol). 

While the inhibition of noradrenaline re-uptake exerts predominantly an a-adrenergic effect, a selective jS-adrenergic effect can not be obtained by such an indirect mechanism. All selective /3-sympathomi-metics activate the receptors, P -, P2- or both sub-types, directly. The first pure jS-sympathomimetic in clinical use was isoproterenol which is structurally identical to adrenaline except the methyl-moiety at the N-position in the side-chain is replaced by an isopropyl-group. All effects produced by isoproterenol are due to either P -or 62-adrenoceptor stimulation tachycardia, increased stroke volume, decreased vascular resistance, broncho dilatation and, in pregnancy, uterus relaxation. The metabolic effects of isoproterenol are less pronounced than those of adrenaline. 

N.E. Anden, On the mechanism of noradrenaline depletion by ot-methyl metatyrosine and metaraminol, Acta Pharmacol. Toxicol. 21 (1964) 260-271. 

A. Carlsson, B. Waldeck, Different mechanisms of dmg-induced release of noradrenaline and its congeners a-methyl-noradrenaline and metaraminol, Eur. J. Pharmacol. 4(1968) 165-168. 

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