Tyrosine ascorbic acid

Ascorbic acid is involved in carnitine biosynthesis. Carnitine (y-amino-P-hydroxybutyric acid, trimethylbetaine) (30) is a component of heart muscle, skeletal tissue, Uver and other tissues. It is involved in the transport of fatty acids into mitochondria, where they are oxidized to provide energy for the ceU and animal. It is synthesized in animals from lysine and methionine by two hydroxylases, both containing ferrous iron and L-ascorbic acid. Ascorbic acid donates electrons to the enzymes involved in the metabohsm of L-tyrosine, cholesterol, and histamine (128). 

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

Patients of varying skin types (1-V) having striae distensae alba on the abdomen or thighs can apply topical 20% glycolic acid daily to the entire treatment area. In addition, these patients apply 10% L-ascorbic acid, 2% zinc sulfate, and 0.5% tyrosine to half of the treatment area and 0.05% tretinoin emollient cream to the other half of the treatment area. The creams are applied on a daily basis for 12 weeks. Improvement is evaluated at 4 and 12 weeks with increased elastin content within the reticular and papillary dermis [14]. 

It is not clear whether V(V) or V(IV) (or both) is the active insulin-mimetic redox state of vanadium. In the body, endogenous reducing agents such as glutathione and ascorbic acid may inhibit the oxidation of V(IV). The mechanism of action of insulin mimetics is unclear. Insulin receptors are membrane-spanning tyrosine-specific protein kinases activated by insulin on the extracellular side to catalyze intracellular protein tyrosine phosphorylation. Vanadates can act as phosphate analogs, and there is evidence for potent inhibition of phosphotyrosine phosphatases (526). Peroxovanadate complexes, for example, can induce autophosphorylation at tyrosine residues and inhibit the insulin-receptor-associated phosphotyrosine phosphatase, and these in turn activate insulin-receptor kinase. 

While esters of sulfuric acid do not play as central a role in metabolism as do phosphate esters, they occur widely. Both oxygen esters (R-0-S03 , often referred to as O-sulfates) and derivatives of sulfamic acid (R-NH-SOg, N-suIfates) are found, the latter occurring in mucopolysaccharides such as heparin. Sulfate esters of mucopolysaccharides and of steroids are ubiquitous and sulfation is the most abundant known modification of tyrosine side chains. Choline sulfate and ascorbic acid 2-sulfate are also found in cells. Sulfate esters of phenols and many other organic sulfates are present in urine. 

A combination of decarboxylation and hydroxyla-tion of the ring of tyrosine produces derivatives of o-dihydroxybenzene (catechol), which play important roles as neurotransmitters and are also precursors to melanin, the black pigment of skin and hair. Catecholamines may be formed by decarboxylation of tyrosine into tyramine (step e, Fig. 25-5) and subsequent oxidation. However, the quantitatively more important route is hydroxylation by the reduced pterin-dependent tyrosine hydroxylase (Chapter 18) to 3,4-dihydroxyphenylalanine, better known as dopa. The latter is decarboxylated to dopamine.1313 Hydroxylation of dopamine by an ascorbic acid and... 

The fundamental role of ascorbic acid in metabolic processes is not well understood. There is some evidence that it may be involved in metabolic hydroxylation reactions of tyrosine, proline, and some steroid hormones, and in the cleavage-oxidation of homogentisic acid. Its function in these metabolic processes appears to be related to the ability of vitamin C to act as a reducing agent. 

Tyrosine is converted to dopa by the rate-limiting enzyme tyrosine hydroxylase, which requires tetrahydrobiopterin, and is inhibited by a-methyltyrosine. Dopa is decarboxylated to dopamine by L-aromatic amino acid decarboxylase, which requires pyridoxal phosphate (vitamin B6) as a coenzyme. Carbidopa, which is used with levodopa in the treatment of parkinsonism, inhibits this enzyme. Dopamine is converted to norepinephrine by dopamine P-hydroxylase, which requires ascorbic acid (vitamin C), and is inhibited by diethyldithiocarbamate. Norepinephrine is converted to epinephrine by phenylethanolamine A -methyltransferase (PNMT), requiring S-adeno-sylmethionine. The activity of PNMT is stimulated by corticosteroids. 

The contribution of lipophilic antioxidants is small. Escobar et al. (E5) found that the TAC of lipophilic antoxidants in blood plasma was 16.5 1.5 pM and corresponded almost exclusively to a-tocopherol the concentration of this compound in the blood plasma, analyzed independently, was 17.6 0.3 pM. Popov and Lewin (PI9) found TAC of lipid-soluble antioxidants in blood plasma to be 28.0 8.1 /u.M, a value comparable with the concentration of a-tocopherol (20.5 6.6 /U.M). These (and other) results confirm that a-tocopherol is the main lipid-soluble antioxidant of blood plasma (II) and indicates that the contribution of the lipid-soluble antioxidants to TAC of blood plasma is in fact negligible, taking into account that TAC of human blood plasma is of the order of 1 mM (see later). The contribution of ascorbic acid is also low. This situation may differ considerably in other biological fluids and tissue homogenates. In seminal plasma, the concentration ratio of ascorbate to urate is about 1 (G3). Ascorbate and urate contribute 29% of the fast TRAP of human seminal plasma the share of proteins and polyphenolic compounds is 57%, whereas tyrosine contributes 15% of the slow TRAP (R14) (Table 7). Ascorbate and uric acid account for about half of TAC of human tears (K3). TAC of urine is determined mainly by urate and proteins (K5). 

Further mechanistic studies showed that no free peroxynitrite is formed during the reactions of NO with the oxy-forms of these proteins, and that nitrate is formed quantitatively, at both pH 7.0 and pH 9.0 [18]. Analysis of the proteins after ten cycles of oxidation by NO and reduction by ascorbic acid indicated that fewer than 1% of the tyrosine residues are nitrated. These results show that when peroxynitrite is coordinated to the heme of myoglobin or hemoglobin, it rapidly iso-merizes to nitrate, and thus cannot nitrate the tyrosine residues of the globin. 

Figure 4. Permeability of a phospholipid/cholesterol film for different solutes ascorbic acid (1), tyrosine (2), uric acid (3), acetaminophen (4), cysteine (5), desipramine (6), perphenazine (7), trimipramine (8), promethazine (9), and chlorpromazine (10). (Adapted from ref. 13.)...

Figure 8. Electrochemical behaviors of pharmaceutical drugs at PPy-GOD film electrodes and at GC electrode (a) At 20 A PPy-GOD film electrode in 0.5 mM ascorbic acid with 0.05 M PB (pH 7.4) solution, (b) At 20 A PPy-GOD film electrode in 0.5 mM desipramine with 0.05 M PB (pH 7.4) solution. 1 at GC electrode, 2 at PPy-GOD film electrode, (c) At 1000 A PPy-GOD film in 0.05 M PB solution (pH 7.4) with (1) 0.5 mM uric acid, (2) 0.5 mM tyrosine, (3) 1 mM ascorbic acid, (4) 0.5 mM promethazine, and (5) 1 mM K4Fe(CN)6. Scan rate 50 mV/s.

Ascorbic acid is a vitamin in primates. In most other animals, it can be synthesized by a branch of the glucoronic acid pathway (Chapter 18). It is apparently not changed into any coenzyme in the human being and participates as a vitamin in a reducing capacity in several biochemical reactions. These include the post-translational hydroxylation of proline in collagen biosynthesis (Chapter 8) and in tyrosine metabolism (Chapter 20). Ascorbic acid is oxidized to dehydroascorbic acid, a diketo derivative of ascorbate. Scurvy is a deficiency disease caused by a shortage of dietary ascorbic acid. In children, this results in defective bone formation in adults, extensive bleeding occurs in a number of locations. Scurvy is to be suspected if serum ascorbic acid levels fall below 1 jug/mL. 

Additional errors of phenylalanine and tyrosine metabolism include tyrosinosis, or hereditary tyrosinemia, neonatal tyrosinemia, and alcaptonuria. In the first case, there is a probable defect in p-hydroxyphenylpyruvate oxidase. In neonatal tyrosinemia, the problem is transient and may be solved by the administration of ascorbic acid. Ascorbic acid is apparently a cofactor for p-hydroxy-phenylpyruvate oxidase. Alcaptonuria is a benign disorder in which homogen-tisic acid oxidase is inoperative and homogentisic acid is excreted in the urine. Air oxidizes the homogentisic acid to a pigment, giving urine a black color. This pigment also accumulates in the patient s tissues. 

Seitz G, Gebhardt S, Beck JF, Bohm W, Lode HN, Niethammer D, and Bruchelt G (1998) Ascorbic acid stimulates DOPA synthesis and tyrosine hydroxylase gene expression in the human neuroblastoma cell line SK-N-SH. Neuroscience Letters 244,33-6. 

The nature of the active site in beta-amylase is not unambiguously known for enzymes from different sources. Early experiments on purified barley and on malted barley first indicated, from studies of the modification of the enzyme with nitrous acid and ketene, that free tyrosine and sulfhydryl groups are essential for activity, whereas free a-amino groups are not. The importance of the sulfhydryl groups was emphasized by the partial recovery of activity of the modified or oxidized enzyme (that is, treated with nitrous acid, iodine, phenyl mercuribenzoate, ferricyanide, and cupric ions) when it was treated with hydrogen sulfide or cysteine. Barley feeto-amylase (not highly purified) has been reported to contain 12—15 sulfhydryl groups per molecule by titration with p-chloromercuribenzoate, and the loss of free sulfhydryl content by treatment with L-ascorbic acid in the presence of cupric ions was found to be directly related to the loss of activity. 

Tyrosine monooxygenase uses biopterin as a cofactor. Biopterin is made in the body and is not a vitamin. Its structure resembles that of folic acid. Dopa decarboxylase is a vitamin B -requiring enzyme. Dopamine hydroxylase is a copper metalloenzyme. The active form of the enzyme contains copper in the reduced state (cuprous, Cu+). With each catalytic event, the copper is oxidized to the cupric state (Cu ). The enzyme uses ascorbic acid as a cofactor for converting the cupric copper back to cuprous copper. Thus, each catalytic event also results in the conversion of ascorbic acid to semidehydroascorbate. The semidehydroascorbate, perhaps by disproportionation, is converted to ascorbate and dehydroascorbate. The catalytic cycle of dopamine hydroxylase is shown in Figure 9,86. Dopamine hydroxylase, as well as the stored catecholamines, are located in special vesicles... 

FIGURE 9.85 Biosynthesis of catecholstntnes. Tyrosine is used for the synthesis of various small molecules, which are used as hormones and neuiotransmitters. The nutritional biochemist mi t be especially interested in the pathway of epinephrine biosynthesis, as it requires the participation of four separate cofactors. These are fl) biopterin 2) pyridoxal phosphate 3) ascorbic acid and (4) S-adenosyl-mcthionine. 

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