Tyrosine hydroxylase, catecholamine

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). 

Together with dopamine, adrenaline and noradrenaline belong to the endogenous catecholamines that are synthesized from the precursor amino acid tyrosine (Fig. 1). In the first biosynthetic step, tyrosine hydroxylase generates l-DOPA which is further converted to dopamine by the aromatic L-amino acid decarboxylase ( Dopa decarboxylase). Dopamine is transported from the cytosol into synaptic vesicles by a vesicular monoamine transporter. In sympathetic nerves, vesicular dopamine (3-hydroxylase generates the neurotransmitter noradrenaline. In chromaffin cells of the adrenal medulla, approximately 80% of the noradrenaline is further converted into adrenaline by the enzyme phenylethanolamine-A-methyltransferase. 

A. Tyrosine Hydroxylase Is Rate-Limiting FOR Catecholamine Biosynthesis ... 

Tyrosine is the immediate precursor of catecholamines, and tyrosine hydroxylase is the rate-limiting enzyme in catecholamine biosynthesis. Tyrosine hydroxylase is found in both soluble and particle-bound forms only in tissues that synthesize catecholamines it functions as an oxidoreductase, with tetrahydropteridine as a cofactor, to convert L-tyrosine to L-dihydroxyphenylalanine (L-dopa). 

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. 

Bacopoulus, NG and Bhatnager, RK (1977) Correlation between tyrosine hydroxylase activity and catecholamine concentration or turnover in brain regions. J. Neurochem. 29 631-643. 

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... 

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

Graham, D.G. Oxidative pathways for catecholamines in the genesis of neuromelanin and cytotoxic quinones. Mol Pharmacol 14 633-643. 1978. Hotchkiss, A.J., and Gibb, J.W. Long-term effects of multiple doses of methamphetamine on tryptophan hydroxylase and tyrosine hydroxylase activity in rat brain. J Pharmacol Exp Ther 214 257-262, 1980. 

Jones, B. E. Beaudet, A. (1987). Distribution of acetylcholine and catecholamine neurons in the cat brainstem a choline acetyltransferase and tyrosine hydroxylase immunohistochemical study. J. Comp. Neurol. 261, 15-32. 

Ramsey A., Fitzpatrick P. (1998). Effects of phosphorylation of serine 40 of tyrosine hydroxylase on binding of catecholamines evidence for a novel regulatory mechanism. Biochemistry 37, 8980-6. 

Dopamine is formed from tyrosine by hyclroxylation with tyrosine hydroxylase and the removal of a CO2 group by aromatic amino acid decarboxylase. The catecholamine is found in high concentrations in parts of the brain—the caudate nucleus, the median eminence, the tuberculum olfactorium, and the nucleus accumbens. Dopamine appears to act as an inhibitory neurotransmitter. 

The answers are 327-a, 328-c 329-d. (Hardman, pp 120, 250, 582-583.) Epinephrine is made from tyrosine in a series of steps through L-dopa, dopamine, NE, and finally epinephrine. The conversion of tyrosine to dopa by tyrosine hydroxylase is the rate-limiting step in this pathway. Epinephrine constitutes about 80% of the catecholamines in the adrenal medulla. The enzyme that synthesizes epinephrine from NE is also found in certain areas of the CNS. 

Tyrosine hydroxylase is the rate-limiting enzyme for the biosynthesis of catecholamines 212... 

Tyrosine hydroxylase is the rate-limiting enzyme for the biosynthesis of catecholamines. Tyrosine hydroxylase (TH) is found in all cells that synthesize catecholamines and is a mixed-function oxidase that uses molecular oxygen and tyrosine as its substrates and biopterin as its cofactor [1], TH is a homotetramer, each subunit of which has a molecular weight of approximately 60,000. It catalyzes the addition of a hydroxyl group to the meta position of tyrosine, thus forming 3,4-dihydroxy-L-phenylalanine (l-DOPA). 

Tyrosine hydroxylase is present in neurons in the CNS and PNS that synthesize catecholamines. The enzyme is also present at high concentrations in cells that are devel-opmentally related to catecholaminergic neurons, such as... 

The regulation of phosphorylation of tyrosine hydroxylase is affected by stimuli that increase Ca2+ or cAMP concentrations in neurons, including nerve impulse conduction and certain neurotransmitters in well-defined regions of the nervous system, in the adrenal medulla and in cultured pheochromocytoma cells. In addition, tyrosine hydroxylase phosphorylation is stimulated by nerve growth factor in certain cell types, possibly via the activation of ERKs. These changes in the phosphorylation of tyrosine hydroxylase have been shown to correlate with changes in the catalytic activity of the enzyme and in the rate of catecholamine biosynthesis. 

A second, more extensive experiment involved oral administration of three daily doses (100 mg/kg) of parachlorophenylalanine (PCPA). This tryptophan hydroxylase inhibitor (47), like reserpine, enhanced the behavioral effects of LSD (13) moreover, hypersensitivity occurred when 5-HT, but not other monoamine, concentrations were below normal in both forebrain and hindbrain (13). That is, effects were observed at 5 and 12 days (when 5-HT was depleted to 10-20% and 60-70% of normal) but not at 21 days (when 5-HT had returned to normal). Control experiments (13) indicated that (a) the interaction of PCPA, 5-HT, and LSD was probably not caused by generalized hyperactivity or hyperirritability sometimes seen after PCPA (73) (b) PCPA does not affect threshold doses of other psychoactive but nonserotonergic compounds, such as d-amphetamine (0.3 mg/kg) and (c) pretreatment with a-methylparatyrosine, a tyrosine hydroxylase inhibitor which depletes catecholamines rather than indoleamines, does not alter sensitivity to LSD. 

Peters, D. A. V., and Tang, S. (1977) The effects of repeated D-lysergic acid diethylamide injections on catecholamine levels and tyrosine hydroxylase activity in rat brain regions. J. Neurochem., 28 59-62. 

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). 

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. 

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