2.5- Dihydroxyphenylalanine, conversion

Figure 8.5 The synthetic pathway for noradrenaline. The hydroxylation of the amino acid, tyrosine, which forms dihydroxyphenylalanine (DOPA) is the rate-limiting step. Conversion of dopamine to noradrenaline is effected by the vesicular enzyme, dopamine-P-hydroxylase (DpH) after uptake of dopamine into the vesicles from the cell cytosol...

Vitamin Ba (pyridoxine, pyridoxal, pyridoxamine) like nicotinic acid is a pyridine derivative. Its phosphorylated form is the coenzyme in enzymes that decarboxylate amino acids, e.g., tyrosine, arginine, glycine, glutamic acid, and dihydroxyphenylalanine. Vitamin B participates as coenzyme in various transaminations. It also functions in the conversion of tryptophan to nicotinic acid and amide. It is generally concerned with protein metabolism, e.g., the vitamin B8 requirement is increased in rats during increased protein intake. Vitamin B6 is also involved in the formation of unsaturated fatty acids. 

Mechanism of Action A tyrosine hydroxylase inhibitor that blocks conversion of tyrosine to dihydroxyphenylalanine, the rate limiting step in the biosynthetic pathway of catecholamines. Therapeutic Effect Reduces levels of endogenous catecholamines. 

With tyrosinase, on the contrary, a two-electron oxidation occurs, as no EPR signal was detected in the catechol oxidation at pH 5.3 Melanins are polymerization products of tyrosine, whereby tyrosinase catalyses the first steps the formation of dopa (3,4-dihydroxyphenylalanine) and of dopaquinone, leading to an indolequi-none polymer The peroxidase mechanism for the conversion of tyrosine into dopa in melanogenesis was not substantiated In natural and synthetic melanins free radicals of a semiquinone type were detected by EPR 4-10 x 10 spins g of a hydrated suspension (the material was modified on drying and the number of free spins increased). The fairly symmetrical EPR signal had a g-value of 2.004 and a line-width of 4-10 G The melanins seem to be natural radical scavengers. 

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

In a similar context, 6-18F-3,4-dihydroxyphenylalanine (6-18F-DOPA) was synthesized by direct fluorination of L-3-(3-hydroxy-4-pivaloxyphenyl)alanine (ra-P-DOPA) with Ac018F in acetic acid resulting in the 2- and 5-18F isomers. Hydrolysis of the reaction mixture in hydrochloric acid followed by HPLC separation gave 6-18F-DOPA (equation 23)44. Another application of Ac018F was reported in the synthesis of a trimethyl tin precursor of 2-oxoquazepam, 7-chloro-1 -(2,2,2-trifluoroethyl)-1,3--dihydro-5-(2-fluorophenyl)-2//-l,4-benzodiazepin-2-one, a benzodiazepine agonist, and its conversion to [18F]-2-oxoquazepam by reaction with AcQ18F (equation 24)45. 

Dihydroxyphenylalanine (DOPA) (99) is produced by tyrosine hydroxylase-catalyzed hydroxylation of Tyr. Recent interest in the use of [18F]-6-F-DOPA (100) as a PET scanning agent for regional dopaminergic brain function is based on its conversion, in the brain, to [,8F]-6-F-dopamine (101)161. The fact that fluorine in the 6-position of DOPA, dopamine and other catecholamines retards methylation by catechol-O-methyl transferase presumably increases the biological half-life of the tracer. In contrast, fluorine in the 5-position increases the rate of methylation162. 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

The synthesis of dopamine originates from the precursor the amino acid L-tyrosine, which must be transported across the blood-brain barrier into the dopaminergic neuron. The rate limiting step in the synthesis is the conversion of L-tyrosine to L-dihydroxyphenylalanine (L-DOPA) by the enzyme tyrosine hydroxylase (TH). L-DOPA is subsequently converted to dopamine by aromatic L-amino acid decarboxylase. The latter enzyme turns over so rapidly that L-DOPA levels in the brain are negligible under normal conditions.1... 

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. 

The conversion of tyrosine to 3,4-dihydroxyphenylalanine occurs both in vivo in man (590) and in vitro by the action of tissue tyrosinase (205, 688). Mammals can decarboxylate both tyrosine (402,407) and dihydroxyphenyl-alanine (406), tyrosine decarboxylase and dihydroxyphenylalanine (dopa) decarboxylases being quite distinct and separable (405), though both are dependent on pyridoxal phosphate (73, 758, and review 72). In mammals dihydroxyphenylalanine is the most readily decarboxylated of all amino acids, and it is therefore not unreasonable to assume that this is the substrate normally decarboxylated in adrenaline biosynthesis cf. 74, 75). Support for this concept derives from the fact that both the substrate and the product of the reaction (3,4-dihydroxyphenylethylamine diagram 11) can or do occur in the adrenal (298, 299, 802), and the amine is moreover, like adrenaline and noradrenaline, a normal urinary excretion product (245, 404). 

Melanin is the normal pigment of the skin and mammalian hair. Carcinomatous growths in which abnormal melanin formation occurs are known as melanomas. A congenital metabolic defect in w hich skin pigmentation does not occur is known as albinism, and is inherited as a recessive Mende-lian character (cf. 40). Albinos occur in many species besides man (e.g., the pink-eyed white rabbit). As adrenaline formation is apparently unimpaired in albinos, the metabolic block presumably lies in the conversion of dihydroxyphenylalanine to melanin, as shown in diagram 6, rather than in the conversion of tyrosine to dihydroxyphenylalanine. However, the exact nature of the block has not been established. 

Tyrosine also has an important role in the central nervous system and melanocyte and is the precursor of both melanins and catecholamines (epinephrine and norepinephrine). The conversion to these products takes place in the appropriate tissues, usually melanocyte, the central nervous system, or the adrenal gland. In each of these tissues, the enzyme tyrosinase catalyzes the conversion of tyrosine to dihydroxyphenylalanine (DOPA) by hydroxylating the ring adjacent to the parahydroxy group. This is a catechol ring. If this were an amine instead of an amino acid, it would be a catecholamine. The DOPA is a precursor of catecholamines in the adrenal gland and central nervous system. In melanocyte, the DOPA is converted to melanine. In the disease albinism, the tyrosinase in the... 

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. 

A different approach to cyclodopa (alkyl 5,6-dihydroxy-2,3-dihydroindole-2-carboxylates has been devised (ref. 40) since the described process (ref.38) only results in acceptable yields when operated in very dilute solution. In the modified method. Scheme 5b, racemic 3,4-dihydroxyphenylalanine (dopa) methyl ester hydrochloride (C) was iodinated with potassium iodate in a two phase system, then treated with sodium dithionite and acetylated to afford methyl 5,6-diacetoxy-7-iodo-2,3-dihydroindole-2-carboxylate. Reductive treatment of this with Pd-C, and hydrogen in ethanolic solution containing triethylamine and conversion to the hydrochloride gave the product (methyl ester diacetate hydrochloride)in 40% overall yield. 

The classic example of this approach involves the use of levodopa (l-3,4-dihydroxyphenylalanine, Figure 8.13) to treat Parkinson s disease [58]. Parkinson s disease is distinguished by the marked depletion of dopamine— an essential neurotransmitter—in the basal ganglia. Direct dopamine replacement is not possible, because dopamine does not permeate through the blood-brain barrier. However, the metabolic precursor of dopamine, levodopa, is transported across brain capillaries by the neutral amino acid transporter (see Table 5.5 and the related discussion). Peripheral administration of levodopa, therefore, produces an increase in levodopa concentration within the central nervous system some of these molecules are converted into dopamine due to the presence of decarboxylate enzymes in the brain tissue, but decar-boxylate activity is also present in the intestines and blood. To prevent conversion of levodopa into dopamine before entry to the brain, levodopa is usually administered with decarboxylase inhibitors. 

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. 

In Scheme 13.40 and as noted above, the action of the iron-containing enzyme tyrosine 3-monooxygenase (EC 1.14.16.2) is shown to effect the conversion of tyrosine (Tyr, Y) and oxygen (O2) to 3,4-dihydroxyphenylalanine (L-dopa), while the cofactor tetrahydrobiopterin undergoes oxidation to 4a-hydroxytetrahydrobiop-terin. Then, the general aromatic-L-amino acid decarboxylase (EC 4.1.1.28), an enzyme that uses pyridoxal as a cofactor, effects the decarboxylation of the bisphe-noUc add to the corresponding amine, dopamine [3,4-dihydroxyphenethylamine, 2-(3,4-dihydroxyphenyl)ethanamine]. 

The majority of catecholamine and serotonin biosynthesis occurs within the nerve terminals by synthetic enzymes transported from the neuronal cell bodies. In all catecholamine neurons, the rate-limiting step in synthesis is conversion of tyrosine to dihydroxyphenylalanine by tyrosine hydroxylase. Dihydroxyphenylalanine is then converted to DA, norepinephrine, and epinephrine through a sequential process involving L-aromatic amino acid decarboxylase (conversion of dihydroxyphenylalanine to DA), dopamine-P-hydroxylase (conversion of DA to norepinephrine), and phenylethanol-amine-N-methyltransferase (conversion of norepinephrine to epinephrine). Cell-specific expression of these enzymes determines the main neurotransmitter for an individual catecholamine neuron. The synthesis pathway for serotonin involves a two-step process in which tryptophan hydroxylase first converts tryptophan to 5-hydroxytryptophan, which is then converted to... 

Conversion of 3,4-dihydroxyphenylalanine (dopa) to hydroxytyramine and norepinephrine, and of hydroxytyramine to the latter by homogenates of adrenal medulla has also been reported by Demis et al. (246) and Hagen (247). By using the soluble protein fraction of the adrenal medulla the enzyme fraction was freed of amine oxidase, which otherwise strongly attacks these amines. Conversion to hydroxytyramine was promoted by the addition of PLP. 

FIGURE 12 Conversion of tyrosine to melanin, catalyzed in part by tyrosinase (T). DOPA, Dihydroxyphenylalanine. Only part of melanin is shown. 

Tyrosine is first oxidized to give dihydroxyphenylalanine (= dopa), an intermediate in the formation of melanin (cf. Chapt. VIII-11), which is subsequently decarboxylatcd to dopamin . Dopamine already possesses some biologic activity it is one of the tissue hormones (cf. Section 12). The next step is hydroxylation of the side chain. The resulting norepinephrine is finally methylated by active methionine. The first step in the inactivation of the hormone is the methylation of the 3-OH group this is followed by conversions on the side chains. 

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