Epinephrine methylation

The intracellular signal therefore persists only as long as the hormone receptor remains occupied by epinephrine. Methyl xanthines such as caffeine and theophylline (a component of tea) inhibit the phosphodiesterase, increasing the half-life of cAMP and thereby potentiating agents that act by stimulating adenylyl cyclase. 

Amino acid-derived hormones include the catecholamines, epinephrine and norepinephrine (qv), and the thyroid hormones, thyroxine and triiodothyronine (see Thyroid AND ANTITHYROID PREPARATIONS). Catecholamines are synthesized from the amino acid tyrosine by a series of enzymatic reactions that include hydroxylations, decarboxylations, and methylations. Thyroid hormones also are derived from tyrosine iodination of the tyrosine residues on a large protein backbone results in the production of active hormone. 

Detoxifica.tlon. Detoxification systems in the human body often involve reactions that utilize sulfur-containing compounds. For example, reactions in which sulfate esters of potentially toxic compounds are formed, rendering these less toxic or nontoxic, are common as are acetylation reactions involving acetyl—SCoA (45). Another important compound is. Vadenosylmethionine [29908-03-0] (SAM), the active form of methionine. SAM acts as a methylating agent, eg, in detoxification reactions such as the methylation of pyridine derivatives, and in the formation of choline (qv), creatine [60-27-5] carnitine [461-06-3] and epinephrine [329-65-7] (50). 

The homolog of epinephrine in this series is a potent vasoconstrictor. Reaction of 3,4-dimethoxypropiophenone with butyl nitrite leads to nitrosation at the a position (36). Stepwise reduction of the nitrosoketone leads to the amino alcohol (37). Removal of the methyl ether affords racemic 38. Resolution of this last followed by separation of the (-) isomer gives levonor-defrine (38). ... 

Neural cells convert tyrosine to epinephrine and norepinephrine (Figure 31—5). While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes. Dopa decarboxylase, a pyridoxai phosphate-dependent enzyme, forms dopamine. Subsequent hydroxylation by dopamine P-oxidase then forms norepinephrine. In the adrenal medulla, phenylethanolamine-A -methyltransferase uti-hzes S-adenosyhnethionine to methylate the primary amine of norepinephrine, forming epinephrine (Figure 31-5). Tyrosine is also a precursor of triiodothyronine and thyroxine (Chapter 42). 

The conversion of tyrosine to epinephrine requires four sequential steps (1) ring hydroxylation (2) decarboxylation (3) side chain hydroxylation to form norepinephrine and (4) N-methylation to form epinephrine. The biosynthetic pathway and the enzymes involved are illustrated in Figure 42-10. 

PNMT catalyzes the N-methylation of norepinephrine to form epinephrine in the epinephrine-forming cells of the adrenal medulla. Since PNMT is soluble, it is assumed that norepinephrine-to-epinephrine conversion occurs in the cytoplasm. The synthesis of PNMT is induced by glucocorticoid hormones that reach the medulla via the intra-adrenal portal system. This special system provides for a 100-fold steroid concentration gradient over systemic arterial blood, and this high intra-adrenal concentration appears to be necessary for the induction of PNMT. 

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. 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

Epinephrine is synthesized from NE in the adrenal medulla. Norepinephrine is methylated by phenylethanolamine-N-m ethyl transferase. Neurons containing this enzyme are also found in the CNS. 

The catecholamines dopamine, norepinephrine and epinephrine are neurotransmitters and/or hormones in the periphery and in the CNS. Norepinephrine is a neurotransmitter in the brain as well as in postganglionic, sympathetic neurons. Dopamine, the precursor of norepinephrine, has biological activity in the periphery, most particularly in the kidney, and serves as a neurotransmitter in several important pathways in the CNS. Epinephrine, formed by the N-methylation of norepinephrine, is a hormone released from the adrenal gland, and it stimulates catecholamine receptors in a variety of organs. Small amounts of epinephrine are also found in the CNS, particularly in the brainstem. 

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... 

Ordinarily, low concentrations of catecholamines are free in the cytosol, where they may be metabolized by enzymes including monoamine oxidase (MAO). Thus, conversion of tyrosine to l-DOPA and l-DOPA to dopamine occurs in the cytosol dopamine then is taken up into the storage vesicles. In norepinephrine-containing neurons, the final P-hydroxylation occurs within the vesicles. In the adrenal gland, norepinephrine is N-methylated by PNMT in the cytoplasm. Epinephrine is then transported back into chromaffin granules for storage. 

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. 

Important pathways requiring SAM include synthesis of epinephrine and of the 7-methylgua-nine cap on eukaryotic mRNA, Synthesis of SAM from methionine is shown in Figure T17-3. After donating the methyl group, SAM is converted to homocysteine and remethylated in a reaction catalyzed by N-methyl THF-homocysteine methyltransferase requirii both vitamin Bj2 and N-meth d-THF. The methionine produced is once again used to make SAM. 

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. 

The coenzyme tetrahydrofolate (THF) is the main agent by which Ci fragments are transferred in the metabolism. THF can bind this type of group in various oxidation states and pass it on (see p. 108). In addition, there is activated methyl, in the form of S-adenosyl methionine (SAM). SAM is involved in many methylation reactions—e. g., in creatine synthesis (see p. 336), the conversion of norepinephrine into epinephrine (see p. 352), the inactivation of norepinephrine by methylation of a phenolic OH group (see p. 316), and in the formation of the active form of the cytostatic drug 6-mercaptopurine (see p. 402). 

Finally, N-methylation of norepinephrine yields epinephrine (adrenaline). The coenzyme for this reaction is S-adenosylme-thionine (SAM see p. 110). 

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... 

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