Phenylalanines 3.4-dihydroxyphenylalanine

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 oxidation of cysteine, as well as other amino acids, was studied by Mudd et a/. Individual amino acids in aqueous solution were exposed to ozone the reported order of susceptibility was cysteine, methionine, tryptophan, tyrosine, histidine, cystine, and phenylalanine. Other amino acids were not affected. This order is similar to that for the relative susceptibility of amino acrids to radiation and to lipid peroxides. Evaluation of the ozonization products revealed that cysteine was converted to cysteic acid, as well as cystine methionine to methionine sulfoxide tryptophan to a variety of pioducrts, including kynurenine and N-formylkynurenine tyrosine also to a variety of products, includiitg dihydroxyphenylalanine histidine to ammonia, proline, and other compounds and cystine in part to cysteic acid. In some cases, the rate and end products depended on the pH of the solution. 

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... 

One of the best characterized physiological functions of (6R)-tetrahydrobio-pterin (BH4, 43) is the action as a cofactor for aromatic amino acid hydroxylases (Scheme 28). There are three types of aromatic amino acid hydroxylases phenylalanine hydroxylase [PAH phenylalanine monooxygenase (EC 1.14.16.1)], tyrosine hydroxylase [TH tyrosine monooxygenase (EC 1.14.16.2)] and tryptophan hydroxylase [TPH tryptophan monooxygenase (EC 1.14.16.4)]. PAH converts L-phenylalanine (125) to L-tyrosine (126), a reaction important for the catabolism of excess phenylalanine taken from the diet. TH and TPH catalyze the first step in the biosyntheses of catecholamines and serotonin, respectively. Catecholamines, i.e., dopamine, noradrenaline and adrenaline, and serotonin, are important neurotransmitters and hormones. TH hydroxylates L-tyrosine (126) to form l-DOPA (3,4-dihydroxyphenylalanine, 127), and TPH catalyzes the hydroxylation of L-tryptophan (128) to 5-hydroxytryptophan (129). The hydroxylated products, 127 and 129, are decarboxylated by the action of aromatic amino acid decarboxylase to dopamine (130) and serotonin (131), respectively. 

Aromatic //-Amino Acids - //-Phenylalanine, //-Tyrosine, and //-3,4-Dihydroxyphenylalanine... 

The favorable effect of the enamide function on asymmetric induction is indicated not only by the result with compound I, but also by later results summarized in Table I, where optical purities in the range of 70 to 80% were generally obtained for various derivatives of alanine, phenylalanine, tyrosine, and 3,4-dihydroxyphenylalanine (DOPA). The Paris group found that the Rh-(-)-DIOP catalyst yielded the unnatural R or d -amino acid derivatives, whereas l-amino acid derivatives could be obtained with a (+)-DIOP catalyst. Since the optical purity of the IV-acylamino acids can often be considerably increased by a single recrystallization (fractionation of pure enantiomer from racemate) and the IV-acetyl group can be removed by acid hydrolysis, this scheme provides an excellent asymmetric synthesis route to several amino acids. 

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.

Tyrosine, itself a degradation product of phenylalanine (Sec. 15.1), is initially converted to 3.4-dihydroxyphenylalanine (dopa), and the corresponding do pa quinone, by the copper-containing enzyme tyrosinase. Tyrosinase is found in melanocytes and is a mixed-function oxidase. It catalyzes the following reaction ... 

Histamine, serotonin and the catecholamines (dopamine, epinephrine and norepinephrine) are synthesized from the aromatic amino acids histidine, tryptophan and phenylalanine, respectively. The biosynthesis of catecholamines in adrenal medulla cells and catecholamine-secreting neurons can be simply summarized as follows [the enzyme catalysing the reaction and the key additional reagents are in square brackets] phenylalanine — tyrosine [via liver phenylalanine hydroxylase + tetrahydrobiopterin] —> i.-dopa (l.-dihydroxyphenylalanine) [via tyrosine hydroxylase + tetrahydrobiopterin] —> dopamine (dihydroxyphenylethylamine) [via dopa decarboxylase + pyridoxal phosphate] — norepinephrine (2-hydroxydopamine) [via dopamine [J-hydroxylasc + ascorbate] —> epinephrine (jV-methyl norepinephrine) [via phenylethanolamine jV-methyltransferase + S-adenosylmethionine]. 

There is a long-standing myth that ascorbate is required for the hydroxy-lation of tyrosine to dihydroxyphenylalanine (see Figure 13.4) and the similar reactions of phenylalanine and tryptophan hydroxylases. This belief arose as a result of early studies of a nonenzymic reaction to synthesize the hydroxy-lated amino acids for further study. It became established that ascorbate was required for these hydroxylations, and it is stUl common to include it in the incubation buffer. So far from requiring ascorbate, the addition of relatively low concentrations of ascorbate to preparations of tyrosine hydroxylase that has been activated by cAMP-dependent protein kinase results in irreversible loss of activity, although the unactivated form of the enzyme is unaffected by ascorbate (WUgus and Roskoski, 1988). As discussed in Section 10.4.1, these enzymes are biopterin-dependent, and require dUiydrobiopterin reductase and NADPH for activity. There is, however, evidence that, in some nerve cell lines in culture, tyrosine hydroxylase may be induced by ascorbate (Seitz et al., 1998). 

Manganese toxicity has been observed in miners exposed to high levels of Mn02 dust. The neurological symptoms mimic Parkinson s disease. Major changes were observed in the biogenic amines, dihydroxyphenylalanine (DOPA), and phenylalanine. Restoration of appropriate levels of these bioamines alleviated the symptoms. Chelation therapy has not been demonstrated as an effective strategy. ... 

Luse and M(iLaren (1963) have reviewed published research on the photolysis products and quantum yields tor the destruction of amino acids and have attributed the photochemical inactivation of the enzymes chymo-trypsin, lysozyme, ribonuclease, and trypsin by UV light at 254 m i primarily to destruction of the cystyl and tryptophyl residues. The destruction of these residues in proteins was suggested to be a function of the product of the number of residues present, the molecular extinction coefficient, and the quantum yield for destruction of each residue. Cysteine and tryptamine were identified among the irradiation products from cystine and tryptophan, respectively. Tyrosine, histidine, and phenylalanine were also shown to be degraded by UV, histidine yielding histamine, urocanic acid, and other imidazole derivatives, and phenylalanine yielding tyrosine and dihydroxyphenylalanine. Destruction of these three amino acids was not considered to contribute appreciably to the enzyme inactivation. 

Phenylalanine can be hydroxylated to form tyrosine, which subsequently can be hydroxylated to form dopa (3,4-dihydroxyphenylalanine). Both hydroxylation reactions are catalyzed by mixed-function oxidases, which require tetrahydrobiopterin as a cofactor. 

Tyrosine is produced by hydroxylation of the essential amino acid phenylalanine and is hydroxylated to form dihydroxyphenylalanine (dopa), which is subsequently decarboxylated to form dopamine. 

Decarboxylation of amino acids is a typical feature of the bacterial decomposition of proteins. Both phenylethylamine and tyramine were isolated from putrid meat by Barger and Walpole (30), who considered it extremely probable that they were derived from phenylalanine and tyrosine, respectively. No cell-free preparation of phenylalanine decarboxylase appears to have been reported, but decarboxylation by a crude Streptococcus faecalis preparation provides a valuable method of phenylalanine assay (887). Bacterial tyrosine decarboxylase has been studied in detail (495), especially by Gale and co-workers (summarized in 284). It requires pyridoxal phosphate as coenzyme (26, 326, 327) and, unlike mammalian tyrosine decarboxylase, also attacks dihydroxyphenylalanine. Decarboxylation normally only occurs in acid media and is considered primarily to be a protective mechanism tending to restore the pH to neu-... 

Treatment of biopterin and biopterin reductase deficiency consists not only of regulating the blood levels of phenylalanine but of supplying the missing form of coenzyme and the precursors of neurotransmitters, namely, dihydroxyphenylalanine and 5-hydroxytryptophan, along with a compound that inhibits peripheral aromatic decarboxylation. This compound is necessary because the amine products do not cross the blood-brain barrier. 

The answer is e. (Murray, pp 307-346. Scriver, pp 1667—1724. Sack, pp 121-138. Wilson, pp 287—3177) In humans, tyrosine can be formed by the hydroxylation of phenylalanine. This reaction is catalyzed by the enzyme phenylalanine hydroxylase. A deficiency of phenylalanine hydroxylase results in the disease called phenylketonuria [PKU(261600)]. In this disease it is usually the accumulation of phenylalanine and its metabolites rather than the lack of tyrosine that is the cause of the severe mental retardation ultimately seen. Once formed, tyrosine is the precursor of many important signal molecules. Catalyzed by tyrosine hydroxylase, tyrosine is hydroxylated to form L-dihydroxyphenylalanine (dopa), which in turn is decarboxylated to form dopamine in the presence of dopa decarboxylase. Then, norepinephrine and finally epinephrine are formed from dopamine. All of these are signal molecules to some degree. Dopa and inhibitors of dopa decarboxylase are used in the treatment of Parkinson s disease, a neurologic disorder. Norepinephrine is a transmitter at smooth-muscle junctions innervated by sympathetic nerve libers. Epinephrine and dopamine are catecholamine transmitters synthesized in sympathetic nerve terminals and in the adrenal gland. Tyrosine is also the precursor of thyroxine, the major thyroid hormone, and melanin, a skin pigment. 

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. 

In this particular problem, a set of 27 samples containing different amounts of L-phenylalanine, L-3,4-dihydroxyphenylalanine (DOPA), 1,4-dihydroxybenzene and L-tryptophan have been measured by fluorescence spectroscopy [Baunsgaard 1999, Riu Bro 2002], The data are important as a model system for many biological systems, e.g. in food and environmental analysis. The goal here is to develop a PARAFAC model of the measured data because a PARAFAC model will ideally resolve the pure spectra as well as the relative concentrations of the analytes [Bro 1997, Bro 1998, Bro 1999, Leurgans et al. 1993, Leurgans Ross 1992, Ross Leurgans 1995]. 

DDC catalyzes the conversion of L-3,4-dihydroxyphenylalanine (l-DOPA) into dopamine (Figure 10), a neurotransmitter found in the nervous system and peripheral tissues of both vertebrates and invertebrates and also in plants where it is implicated in the biosynthesis of benzylisoquinoline alkaloids. " DDC also catalyzes the decarboxylation of tryptophan, phenylalanine, and tyrosine and of 5-hydroxy-L-tryptophan to give 5-hydroxytryptamine (serotonin), and, therefore, is also referred to as aromatic amino acid decarboxylase. Inhibitors of DDC, for example, carbiDOPA and benserazide, are currently used in the treatment of Parkinson s disease to increase the amount of l-DOPA in the brain. 

Methyldopa (a-methyl-3,4-dihydroxy-L-phenylalanine), an analog of 3,4-dihydroxyphenylalanine (DOPA), is metabolized by the L-aromatic amino acid decarboxylase in adrenergic neurons to a-methyldopamine, which then is converted to a-methylnorepinephrine. a-Methylnorepi-nephrine is stored in the secretory vesicles of adrenergic neurons, substituting for norepinephrine (NE) itself. Thus, when the adrenergic neuron discharges its neurotransmitter, a-methylnorepinephrine is released instead of norepinephrine. 

Gliotoxin.—Gliotoxin (46) is a metabolite of Trichoderma viride and Penicillium terlikowskii. Recently, phenylalanine but not w-tyrosine (in contrast to earlier work ) or o-tyrosine or 2,3-dihydroxyphenylalanine has been shown to be incorporated. The incorporation was sufficiently high to allow the use of... 

Note that L-dihydroxyphenylalanine (L-dopa) is an intermediate in the conversion of tyrosine. Lower-than-normal levels of L-dopa are involved in Parkinson s disease. Tyrosine or phenylalanine supplements might increase the levels of dopamine, though L-dopa, the immediate precursor, is usually prescribed because L-dopa passes into the brain quickly through the blood-brain barrier. 

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