Catecholamines COMT

Two important pathways for catecholamine metaboHsm are 0-methylation by COMT, which is cytoplasmicaHy localized, and oxidative deamination by the mitochondrial localized enzyme MAO. There are large amounts of MAO in tissues such as the fiver and the heart which are responsible for the removal of most of the circulating monoamine, including some taken in from the diet. Tyramine is found in high concentrations in certain foods such as cheese, and in wine. Normally, this tyramine is deaminated in the fiver. However, if MAO is inhibited, the tyramine may then be converted into octopamine [104-14-37] which may indirecdy cause release of NE from nerve terminals to cause hypertensive crisis. Thus MAO, which is relatively nonspecific, plays an important role in the detoxification of pharmacologically active amines ingested from the diet. 

COMT O-methylates catecholamines and other compounds having a catechol structure including catecholoestrogens (Fig. 2). The two isoforms of... 

Methylphenidate like cocaine largely acts by blocking reuptake of monoamines into the presynaptic terminal. Methylphenidate administration produces an increase in the steady-state (tonic) levels of monoamines within the synaptic cleft. Thus, DAT inhibitors, such as methylphenidate, increase extracellular levels of monoamines. In contrast, they decrease the concentrations of the monoamine metabolites that depend upon monoamine oxidase (MAO), that is, HVA, but not catecholamine-o-methyltransferase (COMT), because reuptake by the transporter is required for the formation of these metabolites. By stimulating presynaptic autoreceptors, methylphenidate induced increase in dopamine transmission can also reduce monoamine synthesis, inhibit monoamine neuron firing and reduce subsequent phasic dopamine release. 

It is generally aeeepted that COMT is an extraeellular enzyme in the CNS that catalyses the transfer of methyl groups from S-adenylmethionine to the meta-hydroxy group of the eateehol nueleus. Until recently the only inhibitors of this enzyme were pyragallol and eateehol whieh were too toxic for clinical use. Now other inhibitors have been developed, e.g. entaeapone and tolcapone, but these are used mainly to protect dopa (also a catecholamine) from O-methylation, in the treatment of Parkinson s disease (Chapter 15). 

The primary mechanism used by cholinergic synapses is enzymatic degradation. Acetylcholinesterase hydrolyzes acetylcholine to its components choline and acetate it is one of the fastest acting enzymes in the body and acetylcholine removal occurs in less than 1 msec. The most important mechanism for removal of norepinephrine from the neuroeffector junction is the reuptake of this neurotransmitter into the sympathetic neuron that released it. Norepinephrine may then be metabolized intraneuronally by monoamine oxidase (MAO). The circulating catecholamines — epinephrine and norepinephrine — are inactivated by catechol-O-methyltransferase (COMT) in the liver. 

The action of catecholamines released at the synapse is modulated by diffusion and reuptake into presynaptic nerve terminals. Catecholamines diffuse from the site of release, interact with receptors and are transported back into the nerve terminal. Some of the catecholamine molecules may be catabolized by MAO and COMT. The cate-cholamine-reuptake process was originally described by Axelrod [18]. He observed that, when radioactive norepinephrine was injected intravenously, it accumulated in tissues in direct proportion to the density of the sympathetic innervation in the tissue. The amine taken up into the tissues was protected from catabolic degradation, and studies of the subcellular distribution of catecholamines showed that they were localized to synaptic vesicles. Ablation of the sympathetic input to organs abolished the ability of vesicles to accumulate and store radioactive norepinephrine. Subsequent studies demonstrated that this Na+- and Cl -dependent uptake process is a characteristic feature of catecholamine-containing neurons in both the periphery and the brain (Table 12-2). 

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). 

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. 

Some of the catecholamine will enter the target cell rather than be recaptured by the neurone. Inactivation is brought about by the second enzyme, COMT which uses S-adenosyl methionine as a methyl donor as does PNMT (involved with catecholamine... 

Monoamine oxidase (MAO) inhibitors MAO and COMT are the 2 major enzyme systems involved in the metabolism of catecholamines. Do not treat patients concomitantly with entacapone and a nonselective MAO inhibitor. 

Adrenaline and noradrenaline are unstable in aqueous solution where they are subjected to spontaneous oxidation. In vivo this mechanism is only relevant under pathophysiological conditions of an catecholamine excess, since two enzymes, the catechol-O-methyltransferase (COMT) and the monoamineoxidase (MAO), inactivate physiological amounts of the transmitters. There are at least two subtypes of the enzyme MAO, A and B, which can be inhibited selectively for therapeutic purposes, for example by moclobemide and selegiline. 

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... 

Catecholamines can be transported into effector cells extraneuronal uptake). These cells generally contain both COMT and MAO. The combined processes of extraneuronal uptake and 0-methylation are believed to be a minor but functionally significant, site of irreversible loss of catecholamines. The precise role of extraneuronal MAO in transmitter inactivation remains unknown. 

C. Structural modification by placing the hydroxy groups at positions 3 and 5 of the phenyl ring has resulted in compounds that are not substrates for COMT, resulting in lower rates of metabolism and enhanced oral bioavailability compared to catecholamines. 

The Other enzyme in catecholamine catabolism is catecholamine 0-methyltransferase (COMT), a cytoplasmic enzyme that uses S-adenosyl-methionine to methylate the 3-OH of catecholamines and render them inactive. The methylated compounds are not taken up into the synapse. 

Dopamine metabolism was covered in the discussion of general catecholamine biochemistry. Dopamine is stored in synaptic vesicles, and this storage can be manipulated. Although the reuptake of released DA is the major deactivating mechanism, MAO and COMT act enzymatically on DA in the same way as on NE. However, following the degradative pathway of NE, DA will finally be metabolized to homovanillic acid (3-methoxy-4-hydroxy-phenylacetic acid), since it lacks the P-hydroxyl group. 

Catecholamines are absorbed from the intestines, but are rapidly degraded in gut and liver by enzymes MAO and COMT. Thus they are inactive on oral administration. 

Metabolism of catecholamines by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO). 

Phenylephrine was discussed previously when describing the actions of a relatively pure 04 agonist (Table 9-2). Because it is not a catechol derivative (Figure 9-4), it is not inactivated by COMT and has a longer duration of action than the catecholamines. It is an effective mydriatic and decongestant and can be used to raise the blood pressure (Figure 9-6). 

The kinetic mechanism of the methylation reaction of human COMT has been studied exhaustively using recombinant enzymes [19]. The mechanism is sequential ordered AdoMet binding first, then Mg2+ and the catechol substrate as the last ligand. Human S-COMT and MB-COMT have similar kinetic properties. The main difference is the one-order lower Km value of MB-COMT for dopamine as substrate (S-COMT 207 pMand MB-COMT 15 pAT). The COMT enzyme is a rather slow enzyme with a low catalytic number. At saturating substrate levels S-COMT has a double efficiency compared with MB-COMT (kcat=37 and kcat =17, respectively). At low substrate concentrations (<10 iM) the MB-COMT seems to methylate catecholamines more rapidly than S-COMT. 

Dopamine, norepinephrine and epinephrine are products of the metabolism of dietary phenylalanine. This is an interesting sequence of reactions in that we will be discussing not only the three neurotransmitters formed but also considering the DOPA precursor and its use in the treatment of Parkinson s Disease. These molecules are also called catecholamines. Catechol is an ortho dihydroxyphenyl derivative. Degradation of the final product in the pathway, epinephrine, can be accomplished by oxidation (monoamine oxidase - MAO)or methylation (catecholamine 0-methyl transferase - COMT). The diagram on the next page illustrates the scheme of successive oxidations which produce the various catecholamines. 

Degradation of catecholamines The catecholamines are inacti vated by oxidative deamination catalyzed by monoamine oxidase (MAO), and by O-methylation carried out by catechol-O-methyl-transferase (COMT, Figure 21.15). The two reactions can occur in either order. The aldehyde products of the MAO reaction are axi dized to the corresponding acids. The metabolic products of these reactions are excreted in the urine as vanillylmandelic acid, metanephrine, and normetanephrine. 

Catalytic dyad 620 Catalytic subunits 348 Catalytic triad 134, 611-614, 619, 635 of serine proteases 611—614 Cataracts 169 Catechol 838 Catecholamine(s) 553 Catechol O-methyltransferase (COMT) 591 Cathepsin(s) 619,621 G 610... 

COMT Eq. 12-3), which is found in postsynaptic membranes as well as in liver, kidney, and other tissues. It apparently provides the principal means of inactivating circulating catecholamines. In a process... 

There are two enzymes capable of metabolizing catecholamines. The first is monoamine oxidase (MAO), a mitochondrial enzyme that oxidatively deaminates catecholamines, tyramine, serotonin, and histamine. MAO is further subclassified as either monoamine oxidase A, which metabolizes norepinephrine and is inhibited by tranylcypromine, and monoamine oxidase B, which metabolizes dopamine and is inhibited by 1-deprenyl. Catechol-O-methyltransferase (COMT), a soluble enzyme present mainly in the liver and kidney, is also found in postsynaptic neuronal elements. About 15% of norepinephrine is metabolized postsynaptically by COMT. 

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