Tyrosine hydroxylase catecholamine synthesis

The hydroxylation of the amino acid tyrosine to dopa (3,4-dihydroxyphenyl-alanine) is the rate-limiting step in catecholamine biosynthesis [231,257] (Table 1, p. 103). It is catalysed by the enzyme tyrosine hydroxylase. Norepinephrine synthesis is regulated at the tyrosine hydroxylase step by feed-back inhibition, i.e. tyrosine hydroxylase activity decreases as the concentration of endogenous norepinephrine increases [230]. 

Catecholamine biosynthesis begins with the uptake of the amino acid tyrosine into the sympathetic neuronal cytoplasm, and conversion to DOPA by tyrosine hydroxylase. This enzyme is highly localized to the adrenal medulla, sympathetic nerves, and central adrenergic and dopaminergic nerves. Tyrosine hydroxylase activity is subject to feedback inhibition by its products DOPA, NE, and DA, and is the rate-limiting step in catecholamine synthesis the enzyme can be blocked by the competitive inhibitor a-methyl-/)-tyrosine (31). 

As the rate-limiting enzyme, tyrosine hydroxylase is regulated in a variety of ways. The most important mechanism involves feedback inhibition by the catecholamines, which compete with the enzyme for the pteridine cofactor. Catecholamines cannot cross the blood-brain barrier hence, in the brain they must be synthesized locally. In certain central nervous system diseases (eg, Parkinson s disease), there is a local deficiency of dopamine synthesis. L-Dopa, the precursor of dopamine, readily crosses the blood-brain barrier and so is an important agent in the treatment of Parkinson s disease. 

The turnover rate of a transmitter can be calculated from measurement of either the rate at which it is synthesised or the rate at which it is lost from the endogenous store. Transmitter synthesis can be monitored by administering [ H]- or [ " C]-labelled precursors in vivo these are eventually taken up by neurons and converted into radiolabelled product (the transmitter). The rate of accumulation of the radiolabelled transmitter can be used to estimate its synthesis rate. Obviously, the choice of precursor is determined by the rate-limiting step in the synthetic pathway for instance, when measuring catecholamine turnover, tyrosine must be used instead of /-DOPA which bypasses the rate-limiting enzyme, tyrosine hydroxylase. 

The pathway for synthesis of the catecholamines dopamine, noradrenaline and adrenaline, illustrated in Fig. 8.5, was first proposed by Hermann Blaschko in 1939 but was not confirmed until 30 years later. The amino acid /-tyrosine is the primary substrate for this pathway and its hydroxylation, by tyrosine hydroxylase (TH), to /-dihydroxyphenylalanine (/-DOPA) is followed by decarboxylation to form dopamine. These two steps take place in the cytoplasm of catecholaminereleasing neurons. Dopamine is then transported into the storage vesicles where the vesicle-bound enzyme, dopamine-p-hydroxylase (DpH), converts it to noradrenaline (see also Fig. 8.4). It is possible that /-phenylalanine can act as an alternative substrate for the pathway, being converted first to m-tyrosine and then to /-DOPA. TH can bring about both these reactions but the extent to which this happens in vivo is uncertain. In all catecholamine-releasing neurons, transmitter synthesis in the terminals greatly exceeds that in the cell bodies or axons and so it can be inferred... 

The rate-limiting step in the synthesis of the catecholamines from tyrosine is tyrosine hydroxylase, so that any drug or substance which can reduce the activity of this enzyme, for example by reducing the concentration of the tetrahydropteridine cofactor, will reduce the rate of synthesis of the catecholamines. Under normal conditions tyrosine hydroxylase is maximally active, which implies that the rate of synthesis of the catecholamines is not in any way dependent on the dietary precursor tyrosine. Catecholamine synthesis may be reduced by end product inhibition. This is a process whereby catecholamine present in the synaptic cleft, for example as a result of excessive nerve stimulation, will reduce the affinity of the pteridine cofactor for tyrosine hydroxylase and thereby reduce synthesis of the transmitter. The experimental drug alpha-methyl-para-tyrosine inhibits the rate-limiting step by acting as a false substrate for the enzyme, the net result being a reduction in the catecholamine concentrations in both the central and peripheral nervous systems. 

Figure 2.16. Pathways for the synthesis and metabolism of the catecholamines. A=phenylalanine hydroxylase+pteridine cofactor+Oj B tyrosine hydroxylase+ tetrahydropteridme+Fe+ +Oj C=dopa decarboxylase+pyridoxal phosphate D= dopamine beta-oxidase+ascorbate phosphate+Cu+ +Oj E=phenylethanolamine N-methyltransferase+S-adenosylmethionine l=monoamine oxidase and aldehyde dehydrogenase 2=catechol-0-methyltransferase+S-adenosylmethionine.

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... 

Synthesis of catecholamines from Tyr begins with hydroxylation of the Tyr ring catalyzed by tyrosine hydroxylase. 

Metyrosine (Demser) is an example of this class of drugs. Chemically, metyrosine is a-methyl tyrosine. The drug blocks the action of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of catecholamines. Unlike a-methyldopa, metyrosine is not itself incorporated into the catecholamine synthetic pathway. The ultimate action of the drug is to decrease the production of catecholamines. 

Corticosteroids also affect adrenomeduUary function by increasing epinephrine production the mechanism is exertion of a stimulatory action on two of the enzymes that regulate catecholamine synthesis, tyrosine hydroxylase, the rate-Umiting enzyme, and phenyl-ethanolamine Af-methyltransferase, which catalyzes the conversion of norepinephrine to epinephrine. Steroids also influence the metabolism of circulating catecholamines by inhibiting their uptake from the circulation by noimeuronal tissues (i.e., extraneuronal uptake see Chapter 9). This effect of corticoids may explain their permissive action in potentiating the hemodynamic effects of circulating catecholamines. 

The hereditary absence of phenylalanine hydroxylase, which is found principally in the liver, is the cause of the biochemical defect phenylketonuria (Chapter 25, Section B).430 4308 Especially important in the metabolism of the brain are tyrosine hydroxylase, which converts tyrosine to 3,4-dihydroxyphenylalanine, the rate-limiting step in biosynthesis of the catecholamines (Chapter 25), and tryptophan hydroxylase, which catalyzes formation of 5-hydroxytryptophan, the first step in synthesis of the neurotransmitter 5-hydroxytryptamine (Chapter 25). All three of the pterin-dependent hydroxylases are under complex regulatory control.431 432 For example, tyrosine hydroxylase is acted on by at least four kinases with phosphorylation occurring at several sites.431 433 4338 The kinases are responsive to nerve growth factor and epidermal growth factor,434 cAMP,435 Ca2+ + calmodulin, and Ca2+ + phospholipid (protein kinase C).436 The hydroxylase is inhibited by its endproducts, the catecholamines,435 and its activity is also affected by the availability of tetrahydrobiopterin.436... 

Muscle-derived differentiation factor (MDF) induces tyrosine hydroxylase expression in a variety of central nervous system neurons, including those of striatum, cerebellum, and cortex. Normally, i.e., without MDF, these neurons do not express this enzyme of catecholamine synthesis. Further in vitro studies revealed that MDF enhances TH-mRNA 40-fold in fetal mesencephalic neurons. In vivo studies, employing infusion of partially isolated MDF, reported this molecule to enhance tyrosine hydroxylase activity in dopamine-depleted striata of 6-OHDA-lesioned animals. Furthermore, an increase of striatal dopamine concentrations and a partial compensation of rotational asymmetry were observed. In contrast, dopaminergic parameters were not affected by administration of MDF in control animals, suggesting that adult dopaminergic neurons may regain sensitivity toward differentiation factors after lesion. 

Dopamine, a catecholamine, is synthesized in the terminals of dopaminergic neurons from tyrosine, which is transported across the blood-brain barrier by an active process (Figure 23.7). The rate-limiting step in the synthesis of dopamine is the conversion of 1-tyrosine to 1-dihydroxy-phenyta-lanine (1-dopa), catalyzed by the enzyme tyrosine hydroxylase, which is present within catechola-minergic neurons. 

Dopamine is synthesized from the amino acid tyrosine by the enzymes tyrosine hydroxylase (TH which forms L-3,4-dihydroxylphenylalanine, l-DOPA) and L-amino acid decarboxylase (AADC) in dopaminergic neurons. The mRNA expression of the TH (which is the rate-limiting enzyme in the synthesis of dopamine) is abundant throughout the human mesencephalon (Fig. 1) TH is a phenotypic marker for all catecholamines, dopamine, noradrenaline and adrenaline. Evidence for the presence of TH has been documented in the mesencephalon from at least 4.5 to 7 weeks of human fetal life... 

Figure 13.4. Synthesis of the catecholamines. Tyrosine hydroxylase, EC 1.14.16.2 (see also Fignre 10.10) aromatic amino acid decarboxylase, EC 4.1.1.26 and dopamine -hydroxylase, EC 1.14.17.1.

Metirosine (a-methyl-p-t5n osine) is a competitive inhibitor of the eruyme tyrosine hydroxylase, which converts t5U-osine to dopa as dopa is further converted to noradrenaline and adrenaline they are similarly depleted by metirosine. It is used as an adjuvant (with phenoxybenzamine) to treat phaeo-chromocytomas that caimot be removed surgically. Catecholamine synthesis is reduced by up to 80% over 3 days. It also readily penetrates the CNS and depletes brain noradrenaline and dopamine causing reserpine-like side effects (see above). Hence, in patients whose life expectancy is threatened more by tumour invasion than by mild or moderate hypertension, the need for the drug should be weighed carefully. 

NE is synthesized by tyrosine hydroxylation (meta ring position) followed by decarboxylation and side chain p carbon hydroxylation. The synthesis of this catecholamine is regulated by tyrosine hydroxylase. Tyrosine hydroxylation is also a key step in the synthesis of two other important catecholamines, dopamine and epinephrine. NE is packaged via active transport into synaptic (or chromaffin) vesicles prior to release by neuronal depolarization. The effects of NE are mediated by adrenergic receptors (a or P) which are G protein coupled resulting in either increases or decreases in smooth muscle tone as well as increases in cardiac rate and contractility. These effects arise out of receptor mediated increases in intracellular Ca and activation or inhibition of various protein kinases. The effects of NE are terminated essentially as a result of its active transport into the presynaptic nerve ending via an energy and Na" dependent process which utilizes the norepinephrine transporter (NET). Ultimately, NE and other catecholamines are metabolized by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). 

Cytosolic Ca can modulate the activity of serine/threonine CaMKs via calmodulin. For instance, neuronal CaMK stimulates phosphorylation and activates tyrosine hydroxylase, the limiting enzyme in the synthesis of catecholamine neurotransmitters. Ca also regulates the activation of the conventional PKC isoforms. Increased Ca " promotes binding of Ca " to... 

Catecholamines are synthesized from the amino acid tyrosine, and serotonin from tryptophan as shown in Figure 29-2. The rate-limiting step in catecholamine biosynthesis involves conversion of tyrosine to 3,4-dihydroxyphenylalanine (L-dopa) by the enzyme, tyrosine hydroxylase. A related enzyme, tryptophan hydroxylase, catalyzes conversion of tryptophan to 5-hydroxytryptophan in the first step of serotonin synthesis. 

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