3.4- Dihydroxyphenylalanine, formation

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. 

Vitamin C is essential for the formation of collagen, the principal structural protein in skin, bone, tendons, and ligaments, being a cofactor in the hydroxylation of the amino acids proline to 4-hydroxyproline, and of lysine to 5-hydroxylysine. These hydroxyamino acids account for up to 25% of the collagen structure. Vitamin C is also associated with some other hydroxylation reactions, e.g. the hydroxylation of tyrosine to dopa (dihydroxyphenylalanine) in the pathway to catecholamines (see Box 15.3). Deficiency leads to scurvy, a condition characterized by muscular pain, skin lesions, fragile blood vessels, bleeding gums, and tooth loss. Vitamin C also has valuable antioxidant properties (see Box 9.2), and these are exploited commercially in the food industries. 

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 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... 

A particularly well-studied ligand is L-3,4-dihydroxyphenylalanine (l-DOPA) this may coordinate like alaninate or a pyrocatechol 700 Zn11 appears to favour binding to l-DOPA as to pyrocatechols.701 Formation constants have been measured for the ternary complexes Zn11 dopamine alanine/pyrocatechol702 and Zn11 l-DOPA L (L = penicillamine, L-alanine, glycine, 2,2 -bipyridine, citric add, tartaric acid or sulfosalicylic add). 3... 

Synthesis of norepinephrine Tyrosine is transported by a Na+-linked carrier into the axoplasm of the adrenergic neuron, where it is hydroxylated to dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase1. This is the rate-limiting step in the formation of norepinephrine. DOPA is decarboxylated to form dopamine. 

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

The concept of inhibition via p elimination of fluoride ion has now been extended to the irreversible inhibition of a-amino acid decarboxylases. Ornithine decarboxylase (ODC), which catalyzes the decarboxylation of ornithine to putrescine is irreversibly inhibited by a-difluoromethylornithine (IX Fig. 9) (28). In this case, the carbanion formation which precedes P elimination is generated by loss of CO2, and not by proton abstraction (Fig. 9). Similarly, aromatic amino acid decarboxylase is irreversibly inhibited by C-difluoromethyl-3,4-dihydroxyphenylalanine (29) while histidine decarboxylase, ornithine decarboxylase and aromatic amino acid decarboxylase have been inhibited by the corresponding <=d-monof luoromethylanri.no acids, respectively (29). 

The most attractive detailed hypotheses (Fig. 11) suggest the formation of the erythrinane skeleton by oxidation of LXXXVIII, a symmetrical intermediate derived from two molecules of tyrosine or dihydroxyphenylalanine. The two additional bonds necessary might be formed in either order. In one hypothesis (1, 57) oxidation of one aromatic ring to the o-quinone (LXXXIX) is followed by nucleophilic addition of the amino group and further oxidation to XC (or the related o-quinone). This sequence is exactly analogous to the in vitro oxidation of dihydroxyphenylalanine itself to the quinone dopachrome (3S). Nucleophilic or radical addition of the second phenolic ring to the quinoid system would complete the spiro skeleton of XCI. 

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. 

L-DOPA dihydroxyphenylalanine, a drug used to treat Parkinson s disease, lacrimation promoting tear formation. 

The product is applied for the treatment of Parkinsonism that is caused by a lack of l-dopamine and its receptors in the brain. L-Dopamine is synthesized in organisms by decarboxylation of L-3,4-dihydroxyphenylalanine (L-dopa). Since L-dopamine cannot pass the blood-brain barrier L-dopa is applied in combination with dopadecarbox-ylase-inhibitors to avoid formation of L-dopamine outside the brain. Ajinomoto produces L-dopa by this lyase-biotransformation with suspended whole cells in a fed batch reactor on a scale of 250 t a-1. Much earlier, Monsanto has successfully scaled up the chemical synthesis of L-dopa (Fig. 19-38). 

Recent studies have identified shell precursors from S. mansoni and Fasciola hepatica with molecular masses of 14-48 kDa (5,6) further suggesting a role for sclerotinization in eggshell formation. A 31 kDa precursor from F. hepatica is particularly unique in that it is rich in dihydroxyphenylalanine (DOPA), a potential precursor of quinone crosslinking (11). A 35 kDa eggshell precursor, identified by pulse labeling S. mansoni with [ CJtyrosine, is converted into an approximately 100 kDa protein in shells of newly laid eggs most likely due to a quinone-mediated erosslinking mechanism (12). 

Commercial preparations of pig heart glutamate-oxaloacetate transaminase have been screened for their ability to transaminate various a-keto acids with l-[ N]glutamate (32). In addition to l-[ N]aspartate, enzyme preparations were able to catalyze the formation of labeled tyrosine, phenylalanine, leucine, and dihydroxyphenylalanine, as demonstrated by HPLC (17). However, these amino acids have not yet been obtained in radiopure form by this method. The -keto acid analogs of valine and tryptophan were not transaminated by the enzyme preparations. Glutamate-oxaloacetate transaminases obtained from several commercial sources have varying abilities to transaminate the -keto acid... 

Martin RB, Zwitterion formation upcm deprotonaticHi in L-3,4-dihydroxyphenylalanine and other phenolic amines, /. Phy. Chem., 75,2657-2661 (1971). Qted in Szuiczewski DH, Hong W-H and Epinephrine, APDS, 7,193-229 (1978) ottier refierences were also given. 

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