Oxygen tyrosine hydroxylase

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

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). 

Free tryptophan is transported into the brain and nerve terminal by an active transport system which it shares with tyrosine and a number of other essential amino acids. On entering the nerve terminal, tryptophan is hydroxylated by tryptophan hydroxylase, which is the rate-limiting step in the synthesis of 5-HT. Tryptophan hydroxylase is not bound in the nerve terminal and optimal activity of the enzyme is only achieved in the presence of molecular oxygen and a pteridine cofactor. Unlike tyrosine hydroxylase, tryptophan hydroxylase is not usually saturated by its substrate. This implies that if the brain concentration rises then the rate of 5-HT synthesis will also increase. Conversely, the rate of 5-HT synthesis will decrease following the administration of experimental drugs such as para-chlorophenylalanine, a synthetic amino acid which irreversibly inhibits the enzyme. Para-chloramphetamine also inhibits the activity of this enzyme, but this experimental drug also increases 5-HT release and delays its reuptake thereby leading to the appearance of the so-called "serotonin syndrome", which in animals is associated with abnormal movements, body posture and temperature. 

Figure 19-2. Aromatic amino acid hydroxylase reaction. Aromatic amino acids are hy-droxylated by a common mechanism catalyzed by a family of hydroxylases.The enzyme family consists of phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. In addition to substrate, all three enzymes require molecular oxygen and the cofactor tetrahydrobiopterin.Tetrahydrobiopterin is consumed in this reaction and converted into pterin 4cx-carbinolamine. DOPA, dihydroxyphenylalanine.

Parkinson s disease is caused by the oxidative stress-induced loss of dopaminergic neurons and can be effectively treated with levo-dopa in combination with dopa decarboxylase inhibitors such as carbidopa or catechoi-0-methyltransferase inhibitors such as tolca-pone. Levodopa is well known to increase the life spans of patients with Parkinson s disease. It may do this by enhancing brain dopamine levels and inhibiting tyrosine hydroxylase, which produces oxygen radicals. Several dopamine receptor agonists are available for use in Parkinson s disease and are extensively used in patients suffering from the adverse effects of levodopa. Anticholinergics such as trihexyphenidyl are also used in Parkinson s disease. 

Epinephrine (adrenaline) (Figure 32-7) is synthesized from tyrosine by conversion of tyrosine to 3,4-dihydro-xyphenylalanine (dopa) by tyrosine-3-monooxygenase (tyrosine hydroxylase) in the cytosol. The mixed-function oxidase requires molecular oxygen and tetrahydro-biopterin, which is produced from dihydrobiopterin by NADPH-dependent dihydrofolate reductase. In the reaction, tetrahydrobiopterin is oxidized to dihydrobiopterin, which is reduced to the tetrahydro form by NADH-dependent dihydropteridine reductase. These reactions are similar to the hydroxylations of aromatic amino acids (phenylalanine and tryptophan), in which an obligatory biopterin electron donor system is used (Chapter 17). 

There are three hydroxylation reactions on the ubiquinone biosynthetic pathway. All three oxygens are derived from molecular oxygen 1198] and heme is not required [199]. This suggests that these hydroxylations proceed by a mechanism analogous to that of the flavin dependent tyrosine hydroxylase (Fig. 42) [14]. 

List of Abbreviations HIF, hypoxia-inducible factor HREs, hypoxia response elements Hsps, heat-shock proteins iNOS, inducible nitric oxide synthase NE, noradrenabne ODG, oxygen-dependent genes ROS, reactive oxygen species TH, tyrosine hydroxylase... 

Fates of tyrosine. Tyrosine can be degraded by oxidative processes to ace-toacetate and fumarate which enter the energy generating pathways of the citric acid cycle to produce ATP as indicated in Figure 38-2. Tyrosine can be further metabolized to produce various neurotransmitters such as dopamine, epinephrine, and norepinephrine. Hydroxylation of tyrosine by tyrosine hydroxylase produces dihydroxyphenylalanine (DORA). This enzyme, like phenylalanine hydroxylase, requires molecular oxygen and telrahydrobiopterin. As is the case for phenylalanine hydroxylase, the tyrosine hydroxylase reaction is sensitive to perturbations in dihydropteridine reductase or the biopterin synthesis pathway, anyone of which could lead to interruption of tyrosine hydroxylation, an increase in tyrosine levels, and an increase in transamination of tyrosine to form its cognate a-keto acid, para-hydroxyphenylpyruvate, which also would appear in urine as a contributor to phenylketonuria. 

Tyrosine hydroxylase uses BH4 to activate O,. One oxygen atom is attached to tyrosine s aromatic ring, while the other atom oxidizes the coenzyme. DOPA, the product of the reaction, is used in the synthesis of the other catecholamines. 

Tyrosine hydroxylase is an iron-containing mixed function oxidase that utilizes tetrahydrobiopterin, oxygen, and L-tyrosine as cosubstrates. Molecular oxygen is transferred from O2 to the aromatic ring of tyrosine. In recent years the molecular properties and the mechanisms that regulate the activity of this enzyme have been studied in depth by several laboratories (Kuhn and Lovenberg, 1983). 

Mimoun and Roch used hydrazobenzene (PhNHNHPh) for oxygenation of cyclohexane, cyclohexene, and toluene." The most active complex was formed from FCCI2 and carboxylic acid in the presence of hydrazobenzene. Davis et al. used Fepy4Cl2 and PbC02H for the same reaction and proposed the hydroperoxide complex [Fe "(OOH) PhNNHPh)] as an active species."" Sheu et al. used Fe" complexes (Fe(PA) or Fc(DPAH)2) (PA picolinato) for the monooxygenation of saturated hydrocarbons, especially ketonization of methylenic carbons. "" This system was applied to the hydroxylation of aromatic hydrocarbons as reaction mimic for tyrosine hydroxylase."" With phenol as a reactant, the dominant product was catechol. 

Czyzyk-Krzeska MF, Paulding WR, Beresh JE, Kroll SL. Posttranscriptional regulation of tyrosine hydroxylase gene expression by oxygen in PC12 cells. Kidney Int 1997 ... 

Brain tryptophan-S-hydroxylase is a monooxygenase requiring molecular oxygen as a cosubstrate and an external electron donor, tetrahydrobiopterin, the cofactor also required by phenylalanine and tyrosine hydroxylases. 

Figure 2 shows the influence of oxygen on dopaminergic (marked by tyrosine hydroxylase - TH) differentiation of human fetal neural stem cells derived firom the midbrain and forebrain. Proliferating cells were labeled with Ki-67, neuroblasts with MAP2, and piITtubulin antibodies [13]. 

Rarely, phenylketonuria results from a defect in the metabolism of biopterin, a cofactor for the phenylalanine hydroxylase pathway. The electron donor for phenylalanine hydroxylase is tetrahydrobiopterin (BH4), which transfers electrons to molecular oxygen to form tyrosine and dihydrobiopterin (QH2 Fig. 40-2 reaction 2). BH4 is regenerated from QH2 in an NADH-dependent reaction that is catalyzed by dihydropteridine reductase (DHPR), which is widely distributed. In the brain, this... 

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