A-Methyl norepinephrine

A number of theories have been put forward to account for the hypotensive action of a-methyldopa. Current evidence suggests that for a-methyldopa to be an antihypertensive agent, it must be converted to a-methyl-norepinephrine however, its site of action appears to be in the brain rather than in the periphery. Systemically administered a-methyldopa rapidly enters the brain, where it accumulates in noradrenergic nerves, is converted to a-methylnorepinephrine, and is released. Released a-methylnorepinephrine activates CNS a-adrenoceptors whose function is to decrease sympathetic outflow. Why a-methylnorepinephrine decreases sympathetic outflow more effectively than does the naturally occurring transmitter is not entirely clear. 

In spite of its rapid absorption and short half-life, the peak effect of methyldopa is delayed for 6 to 8 hours even after intravenons administration, and the duration of action of a single dose is nsnally about 24 hours this permits once-or twice-daily dosing. The discrepancy between the effects of methyldopa and the measured concentrations of the drug in plasma is most likely related to the time required for transport into the CNS, conversion to the active metabolite storage of a-methyl norepinephrine, and its subsequent release in the vicinity of relevant 0.2 receptors in the CNS. This is a good example of the potential for a complex relationship between a drug s pharmacokinetics and its pharmacodynamics. Patients with renal failure are more sensitive to the antihypertensive effect of methyldopa, bnt it is not known if this is due to alteration in excretion of the drng or to an increase in transport into the CNS. 

Observed rate constant (fcobsd)/s for Norepinephrine a-Methyl norepinephrine... 

Methyldopa, through its metaboHte, CX-methyInorepinephrine formed in the brain, acts on the postsynaptic tt2-adrenoceptor in the central nervous system. It reduces the adrenergic outflow to the cardiovascular system, thereby decreasing arterial blood pressure. If the conversion of methyldopa to CX-methyl norepinephrine in the brain is prevented by a dopamine -hydroxylase inhibitor capable of penetrating into the brain, it loses its antihypertensive effects. 

Figure 11.16) in which S-adenosy1 methionine transferred a methyl group to norepinephrine to give adrenaline. 

As previously mentioned, the cells of the adrenal medulla are considered modified sympathetic postganglionic neurons. Instead of a neurotransmitter, these cells release hormones into the blood. Approximately 20% of the hormonal output of the adrenal medulla is norepinephrine. The remaining 80% is epinephrine (EPI). Unlike true postganglionic neurons in the sympathetic system, the adrenal medulla contains an enzyme that methylates norepinephrine to form epinephrine. The synthesis of epinephrine, also known as adrenalin, is enhanced under conditions of stress. These two hormones released by the adrenal medulla are collectively referred to as the catecholamines. 

In cells that synthesize epinephrine, the final step in the pathway is catalyzed by the enzyme phenylethanolamine /V-methyltransferase. This enzyme is found in a small group of neurons in the brainstem that use epinephrine as their neurotransmitter and in the adrenal medullary cells, for which epinephrine is the primary hormone secreted. Phenylethanolamine N-methyltransferase (PNMT) transfers a methyl group from S-adenosylmethionine to the nitrogen of norepinephrine, forming a secondary amine [5]. The coding sequence of bovine PNMT is contained in a... 

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). 

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. 

Methyltransferases that utilize S-adenosyl-L-methionine as the methyl donor (and thus generating S-adenosyl-L-homocysteine) catalyze (a) A-methylation (e.g., norepinephrine methyltransferase, histamine methyltransferase, glycine methyltransferase, and DNA-(adenine-A ) methyltransferase), (b) O-methylation (e.g., acetylsero-tonin methyltransferase, catechol methyltransferase, and tRNA-(guanosine-0 ) methyltransferase), (c) S-methyl-ation (e.g., thiopurine methyltransferase and methionine S-methyltransferase), (d) C-methylation (eg., DNA-(cy-tosine-5) methyltransferase and indolepyruvate methyltransferase), and even (e) Co(II)-methylation during the course of the reaction catalyzed by methionine syn-thase. ... 

Metyrosine is the a-methyl derivative of tyrosine. It competitively inhibits tyrosine hydroxylase action, thns redncing the formation of epinephrine and norepinephrine. 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

P.A. Shore, D. Busfield, H.S. Alpers, Binding and release of metaraminol Mechanism of norepinephrine depletion by a-methyl-m-tyrosine and related reagents, J. Pharmacol. Exp. Ther. 146 (1964) 194-199. 

Tricyclic antidepressants potentiate the pressor effects of directly acting sympathomimetic amines, such as adrenaline (epinephrine) or noradrenaline (norepinephrine), to cause hypertension. Small amounts of these, such as may be present in local anaesthetic solutions, can be dangerous. Tricyclic antidepressants will inhibit the antihypertensive effects of the older anti hypertensive drugs, such as adrenergic neurone-blocking agents, e.g. guanethidine, a-methyl-DOPA, and clonidine. 

It is possible that the water-filled a-LTX channel, which is relatively wide ( 10A at its narrowest (Krasilnikov and Sabirov 1992 Orlova et al. 2000), can pass small molecules. Indeed, a-LTX channels inserted in the membranes of synaptosomes, NMJ nerve terminals, and receptor-transfected COS7 cells appear to pass fluorescein (Stokes-Einstein radius, Re = 4.5 A) and norepinephrine (Re < 4 A) (Davletov et al. 1998 Rahman et al. 1999 Volynski et al. 2000), shown in Figure 2 for comparison with 8-hydrated calcium ion (Rc = 4.2 A) and the toxin channel. Analysis of impermeant cations commonly used in channel studies reveals that a-LTX channels are poorly permeable (Hurlbut et al. 1994) to glucosamine H+(Re = 4.6 A) and not significantly permeable (Tse and Tse 1999) to N-methyl-D-glucamine (Re = 5.2 A), thus limiting the pore diameter by 10 A. 

Another dmg closely similar to DOPA but used for different applications is a-methyl-DOPA (Figure 10.19a). This molecule acts in the peripheral autonomous system but also enters the brain, by the same route as DOPA. It is converted by DOPA decarboxylase to the false transmitter a-methyl-dopamine. Like dopamine or norepinephrine, a-methyl-dopamine is accumulated inside the transmitter vesicles, and released in response to action potentials. While it has no strong effect on postsynaptic a,-receptors, it does activate 0C2-receptors. It will therefore inhibit the further release of transmitter without stimulating the postsynaptic neuron. The effect of methyl-DOPA is augmented by the fact that it is fairly resistant to monoamine oxidase. Its mode of action resembles that of clonidine (which accomplishes the same in a less roundabout manner). 

Metyrosine (23, a-methyl-L-tyrosine), a norepinephrine biosynthesis inhibitor, is in limited clinical use to help control hypertensive episodes and other symptoms of catecholamine overproduction in patients with the rare adrenal tumor pheochromocytoma (10). Metyrosine, a competitive inhibitor of tyrosine hydroxylase, inhibits the production of catecholamines by the tumor. Although metyrosine is useful in treating hypertension caused by excess catecholamine biosynthesis... 

The SAR for norepinephrine uptake inhibition by amphetamine analogs is similar to that for inhibition of 5-HT reuptake. The protypical unsubstituted derivative, 2-phenethyl-amine, is a weak uptake inhibitor in isolated rat heart membranes (IDgo = 1.1 jM) (143). Introduction of a methyl group at the Cl position adjacent to the amino group results in a 10-fold increase in potency (i.e., dexamphetamine, (8), 1D = 0.18 pAO (143). Sibutra-mine, a tertiary amine, shows moderate NE uptake activity iK = 350 nAO, but its des-methyl and di-desmethyl metabolites, (JD-BTS 54 354 (44) and (R)-BTS 54 505 (46), exhibit potent activity values <20 nM) (see Table 15.10) (71). 

The additional presence of phenylethanolamine N-methyltransferase in adrenal medullary chromaffin cells leads to further conversion of norepinephrine to epinephrine (Figure 29-2). Since phenylethanolamine N-methyitransferase is a cytosolic enzyme, this step depends on leakage of norepinephrine from vesicular storage granules into the ceU cytoplasm and the transfer of a methyl group from S-adenosylmethionine to norepinephrine. Epinephrine is then translocated into chromaffin granules where the amine is stored, awaiting release. 

Duloxetine (LY-248686), (S)-(-i-)-N-methyl-3-(l-naphthyloxy)-3-(2-thienyl)propyl-amine, is expected to be not only a new potent antidepressant but also a NE (norepinephrine) reuptake inhibitor, a 5-HT (serotonin) reuptake inhibitor, and a new treatment drug for stress urinary incontinence [18]. In order to produce an enantiopure key intermediate for the synthesis of the (S)-amine, the Eli Lilly group proposed various strategies [19]. As a result, they selected the enantioseparation of racemic 3-(dimethylamino)-l-(2-thienyl)propan-l-ol with (S)-mandelic acid by diastereomeric salt formation as the most economic and suitable process for industrial-scale production with efficient supporting techniques such as the racemization of the antipode and recycling the recovered materials [20]. However, in the process of demethylation for the preparation of (S)-Duloxetine from (S)-3-(di-methylamino)-l-(2-fhienyl)propan-l-ol, there are some critical problems, such as low yield and considerable decomposition to give impurities. Thus, a direct synthesis of (S)-Duloxetine starting from (S)-3-(methylamino)-l-(2-thienyl)propan-l-ol is expected to be a new route for the production of (S)-Duloxetine. 

Interesting central anti-amphetamine effects are described for tyros ie hydroxylase inhibitors such as a-methyl-tyrosine. Since norepinephrine depletors do not antagonize amphetamine and tyrosine hydroxylase inhibitors have little direct sedative effect, it is suggested that small but critical levels of norepinephrine at receptors are necessary for amphetamine to exert both its stimulant and anorexigenic effects. Whether this applies to other anorectic drugs remains to be determined. 

Recent studies on the mechanism of action of a-methyldihydroxyphenyl-alanine point to the possibility that it is decarboxylated to produce a-methyl-dihydroxydopamine and this substance by beta oxidation is converted into a-methyldihydroxynorepinephrine. These agents in turn are thought to bring about release of norepinephrine from storage sites (18, 19),... 

Fig. 13. Formation of vanillylmandelic acid from norepinephrine and epinephrine [adapted from Gidow et ti. (G12)]. (a) Methylation of the amino group of norepinephrine (h) methylation through catechol-O-methyltransferase and (c) monoamine oxidation + aldehyde oxidation.

The catecholamine-synthesizing enzymes are not only able to synthesize dopamine and norepinephrine from a physiologically occurring substrate such as L-dopa, but also from exogenous substrates such as alpha-methyldopa, which is converted to alpha-methyldopamine and in turn to alpha-methylnorepinephrine. Alpha-methyldopamine and alpha-methyl-norepinephrine are called false transmitters and, in general (except for alpha-methylnorepinephrine), are weaker agonists. Alpha-methyldopa is used in the management of hypertension. 

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