Oxidation of l-DOPA

Some enzymes are able to differentiate between D and L isomers of substrates. Dopa, with a chiral center, exists in enantiomeric pairs, D and L. The naturally occurring dopa molecule has the L configuration, so it might be expected that the enzyme-catalyzed oxidation of L-dopa is more facile than that of D-dopa. The relative reactivity can be determined in a quantitative way by comparing Ku or Vmix values. Does tyrosinase exhibit stereoselectivity ... 

Selective oxidations of optically active substrates by iron(II) complexes were observed in the oxidation of L-dopa and L-adrenaline by [Fe(tetpy)(OH)2]+ complex ions (14) anchored to poly(L-glutamate) (FeTL) or poly(D-glutamate) (FeTD)74). 

Oxidation of L-DOPA to L-dopaquinone is an important biological process, and dopaquinone is known to play a key role in the oxidative conversion of tyrosine into melanins, the primary pigments of skin and hair. It has been found that under biomimetic conditions both resorcinol and phloroglucinol inhibit this action of L-DOPA, and the compound 1 was isolated from the enzyme-catalysed L-DOPA/phloroglucinol reaction. Compound 1 could also be prepared by fenicyanide oxidation of a mixture of L-DOPA and phloroglucinol. 

Oxidation of terminal olefins to methyl ketones by aqueous palladium chloride and oxygen is very slow, but addition of micellar sodium lauryl sulphate increases the rate of formation of 2-octanone from 1-octene twentyfold at 50 °C. There is weaker catalysis by the non-ionic surfactant Brij-35 and inhibition by cationic surfactants. " Oxidation of diosphenol (35) in basic aqueous tetradecyltrimethylammonium chloride is faster and more effective than in water, giving a higher yield of (36). Two attempts at effecting the enantioselective reduction of aromatic ketones, one in micelles of R-dodecyl-dimethyl-a-phenylethylammonium bromide and the other in sodium cho-late micelles, both give optical yields of less than 2%. Rather more success was obtained in the catalysed oxidation of L-Dopa, 3,4-dihydroxyphenyI-alanine. In the presence of the Cu complex of N-lauroyl-L-histidine in cetyl-trimethylammonium bromide micelles reaction was 1.42 (pH 6.90, 30 °C) to... 

It is clear from the above experiments that the enzymatic oxidation of tyrosine is rather complex. The observed sequence of reactions is summarized in Figure 5. Tyrosine [1] is first slowly oxidized to L-Dopa [2]. The oxidation of L-Dopa to dopaquinone [3] occurs somewhat more... 

Mediated oxidation of L-dopa at a PVP-IrCle - coated electrode... 

Linear sweep voltammetry yields a current peak at a potential of 0.73 V vs. SCE for the oxidation of L-dopa in aqueous CFsCOONa (0.1 M, pH = 2) at a bare glassy-carbon electrode. This potential is close to that where IrCle " is oxidized and only a single, composite wave comprising currents from both the mediated and direct oxidation of L-dopa was observed in rotating-disc experiments when the glassy-carbon electrode was coated with a PVP-film in which IrCle had been incorporated. At potentials more positive than that required to convert IrCle to IrCle , current plateaus were recorded which were dependent on both electrode rotation rate, w, and on the L-dopa substrate concentration,... 

In the present case this was determined to be is = 0.88 mA cm 2 for a substrate concentration of 1 mM from the intercepts of Koutecky-Levich plots for the oxidation of L-dopa at electrodes coated with identical PVP films to those used for the mediation experiments but which contained the non-mediating surrogate anion Fe(CN)63 instead of IrCle - in an effort to preserve morphology. The values of is for other substrate concentrations were calculated from the relevant equation in Figure 2. In favourable cases (see the mediated oxidation of catechol at a Nafion-Fe(bipy)3 coated electrode below), two distinct waves corresponding to direct and mediated reactions may be observed which greatly facilitates the deter- mination of is. 

Chapter 2 to 6 have introduced a variety of reactions such as asymmetric C-C bond formations (Chapters 2, 3, and 5), asymmetric oxidation reactions (Chapter 4), and asymmetric reduction reactions (Chapter 6). Such asymmetric reactions have been applied in several industrial processes, such as the asymmetric synthesis of l-DOPA, a drug for the treatment of Parkinson s disease, via Rh(DIPAMP)-catalyzed hydrogenation (Monsanto) the asymmetric synthesis of the cyclopropane component of cilastatin using a copper complex-catalyzed asymmetric cyclopropanation reaction (Sumitomo) and the industrial synthesis of menthol and citronellal through asymmetric isomerization of enamines and asymmetric hydrogenation reactions (Takasago). Now, the side chain of taxol can also be synthesized by several asymmetric approaches. 

The distal extradiol cleavage of L-dopa 12, catalyzed by an iron-dependent dioxygenase, gives an alanyl muconic semialdehyde derivative 18 which, on cyclization and lactol oxidation, yields stizolobic acid 16. The pyrone ring is then ammonolyzed24 to give 3-(6-carboxy-2-oxo-4-pyridyl)alanine 17 (Scheme 3). 

Tyrosinase inhibitors prevent browning in foodbecause they inhibit the oxidation caused by the enzyme tyrosinase. Cuminaldehyde is identified as a potent mushroom tyrosinase monophenol monooxygenase inhibitor from cumin seeds, ft inhibits the oxidation of L-3,4-dihydroxyphenylalanine (l-DOPA) by mushroom tyrosinase with an ID50 of 7.7g/ml (0.05 mM). Its oxidized analogue, cumic acid (p-isopropylbenzoic acid), also inhibits this oxidation with an 1D50 of 43g/ml (0.26mM). These two inhibitors affect mushroom tyrosinase activity in different ways (Kubo and Kinst-Hori, 1998). 

A synthetic phaeomelanin can be easily obtained by the tyrosinase-catalyzed oxidation of L-tyrosine or L-dopa in the presence of excess L-cysteine at pH 6.8 followed by chromatography of the acid-soluble fraction on a Sephadex column. This procedure leads to the isolation of four major reddish brown pigments that are similar to natural... 

The enzymatic synthesis reaction of l-DOPA is carried out in a batchwise system with cells of E. herbicola with high TPL activity. Since pyruvate, one of the substrates, was unstable in the reaction mixture at a high temperature, low temperature was favored for the synthesis of l-DOPA. The reaction was carried out at 16 °C for 48 h in a reaction mixture containing various amounts of sodium pyruvate, 5 g of ammonium acetate, 0.6 g of pyrocatechol, 0.2 g of sodium sulfite, 0.1 g of EDTA, and cells harvested from 100 ml of broth, in a total volume of 100 ml. The pH was adjusted to 8.0 by the addition of ammonia. At 2-h intervals, sodium pyruvate and pyrocatechol were added to the reaction mixture to maintain the initial concentrations. The maximum synthesis of l-DOPA was obtained when the concentration of sodium pyruvate was kept at 0.5%. The substrates, pyrocatechol and pyruvate, were added at intervals to prevent the denaturation of TPL and to prevent byproduct formation. The addition of sodium sulfite is effective in maintaining the reactor in a reductive state and in preventing the oxidation of the l-DOPA produced. l-DOPA is insoluble in the reaction medium, so it appears as a crystalline precipitate during the reaction, at final amounts reaching 110 g/1 [19-21]. 

FIGURE 20-7 Pharmacological preservation of L-DOPA and striatal dopamine. The principal site of action of inhibitors of catechol-O-methyltransferase (COMT) (such as tolcapone and entacapone) is in the peripheral circulation. They block the O-methylation of levodopa (l-DOPA) and increase the fraction of the drug available for delivery to the brain. Tolcapone also has effects in the CNS. Inhibitors of MAO-B, such as low-dose selegiline and rasagiline, will act within the CNS to reduce oxidative deamination of DA, thereby enhancing vesicular stores. AAD, aromatic L-amino acid decarboxylase DA, dopamine DOPAC, 3,4-dihydroxyphenylacetic acid MAO, monoamine oxidase 3MT, 3-methoxyl-tyramine 3-O-MD, 3-O-methyl DOPA. 

Serotonin, or 5-HT, is biosynthesized (3) from its dietary precursor L-tryptophan (Fig. 14.1). Serotonergic neurons contain tryptophan hydroxylase (L-tryptophan-5-monooxygenase) that converts tryptophan to 5-hydroxytryptophan (5-HTP) in what is the rate-limiting step in 5-HT biosynthesis and aromatic L-amino acid decarboxylase (previously called 5-HTP decarboxylase) that decarboxylates 5-HTP to 5-HT. This latter enzyme also is responsible for the conversion of L-DOPA to dopamine (see Chapter 12). The major route of metabolism for 5-HT is oxidative deamination by monoamine oxidase (MAO-A) to the unstable 5-hydroxyindole-3-acetaldehyde, which is either reduced to 5-hydroxytryptophol ( 15%) or oxidized to 5-hydroxyindole-3-acetic acid ( -85%). In the pineal gland, 5-HT is acetylated by 5-HT N-acetyltransferase to N-acetylserotonin, which undergoes O-methylation by 5-hydroxyindole-O-methyltransferase to melatonin. 

Based on knowledge of a biosynthetic pathway one can select certain steps which could be of interest for bioconversion of (a) readily available precursor(s). This could, for example, be stereospecific reactions, like the reduction of quinidinone in quinine or quinidine and the epoxidation of atropine to scopolamine. For the bioconversion one can consider using plant cells [e.g., the production of L-dopa from tyrosine by immobilized cells of Mucuna pruriens (10)] or isolated enzymes from the plant itself. An interesting example of the latter is the (5)-tetrahydroprotoberberine oxidase (STOX) enzyme, which oxidizes (5)-reticuline but not its stereoisomer (11). This feature can be used in the production of (i )-reticuline from a racemic mixture (see below). Immobilized strictosidine synthase has been successfully used to couple secologanin and tryptamine. The gene for this enzyme has been isolated from Rauvolfia (6) and cloned in Escherichia coli, in which it is expressed, resulting in the biosynthesis of active enzyme (7). The cultured bacteria produced 20 times more enzyme... 

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