Ascorbic acid reactions involving

Iron Absorption. A very important effect of ascorbic acid is the enhancement of absorption of nonheme iron from foods. Ascorbic acid also enhances the reduction of ferric iron to ferrous iron. This is important both in increasing iron absorption and in its function in many hydroxylation reactions (140,141). In addition, ascorbic acid is involved in iron metaboHsm. It serves to transfer iron to the Hver and to incorporate it into ferritin. 

The striking increases in the formation of tritiated water and tritiated hydroxyproline on in vitro addition of ascorbate are consistent with a function of this vitamin in hydroxylation—probably at step 3. The present results do not support a systemic ascorbic acid-mediated effect, the belief that ascorbic acid functions in the maintenance of collagen, or acts by stimulating maturation of the fibroblasts in the system under study here. The present data do not support the possibility that intermediates containing hydroxyproline accumulate in scurvy. The proposal that ascorbic acid is involved in the hydroxylation reaction itself is consistent with studies on the nonenzymatic hydroxylation of proline (4) and on enzymatic hydroxylation of other compounds (5, 20, 21, 44, 51). 

The role of transition metals in the catalysis of redox reactions of l-ascorbic acid frequently involves a step in which the transition metal ion is itself reduced. 

Kurata, T. Miyake, N. Otsuka, Y. Formation of L-threonolactone and oxalic acid in the autoxidation reaction of L-ascorbic acid - Possible involvement of singlet oxygen. Biosci. Biotech. Biochem. 1996b, 60, 1212-1214. 

In its reactions with free radicals, vitamin E is converted to the a-tocopheroxyl radical. By regenerating vitamin E, ascorbic acid becomes involved in protecting the membranes indirectly. The mechanism whereby water-soluble ascorbic acid regenerates fat-soluble vitamin E has not yet been elucidated. Regeneration reactions would have to take place at the interface between hydrophilic and hydrophobic sites (Lambe-let et aL, 1985). It is suggested that ascorbate functions to recycle vitamin E... 

Further studies (see Vol. 22, p. 167) have been reported on the photo oxygenation of L-ascorbic acid derivatives, involving the reactions of 3-0-methyl-L-ascorbic acid and the analogues (42). 

The first term is the law observed in the absence of catalyst. The effect is dramatic, however, with associated rate constants ko and ki being 50.3 1 mol s and 3.43 x 10 1 mol s respectively, and may be ascribed to the formation of a Mo -BrOa" complex with no evidence for any change in the oxidation state of the catalyst during the course of the reaction. The kinetics and mechanism of the bromate-ascorbic acid reaction have also been reported, and the effects of phosphate on the oxidation of iodide by chlorate in the presence of catalytic concentrations of vanadium(iv) have been described, the latter systems being considered to involve vanadium(iv)-phosphate complexes. 

It is of interest to note that when solutions of ascorbic acid and the polymer which have been treated with a sequestering resin are allowed to react, the rate is much lower, indicating the importance of the role of the metal ions. A probable inference from this is that most if not all studies of oxidation effects of L-ascorbic acid have involved metal-ion catalysis (presumably by iron or copper ions). The function of the ascorbic acid in such systems would then be to regenerate the Fe or Cu ions which are the actual catalysts. In the reaction of iron(iii) with penicillamine, redox reactions involving molecular oxygen are observed in basic media with the formation of red complexes. In acid conditions, however, a less stable blue species is observed. 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

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). 

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

The kinetic results reported by Jameson and Blackburn (11,12) for the copper catalyzed autoxidation of ascorbic acid are substantially different from those of Taqui Khan and Martell (6). The former could not reproduce the spontaneous oxidation in the absence of added catalysts when they used extremely pure reagents. These results imply that ascorbic acid is inert toward oxidation by dioxygen and earlier reports on spontaneous oxidation are artifacts due to catalytic impurities. In support of these considerations, it is worthwhile noting that trace amounts of transition metal ions, in particular Cu(II), may cause irreproducibilities in experimental work with ascorbic acid (13). While this problem can be eliminated by masking the metal ion(s), the masking agent needs to be selected carefully since it could become involved in side reactions in a given system. 

Group-transfer reactions often involve vitamins3, which humans need to have in then-diet, since we are incapable of realizing their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid) and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of C02, pantothenate for acyl group transfer, thiamine (vitamin as thiamine pyrophosphate) for transfer of aldehyde groups and folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins such as pyridoxine (vitamin B6, as pyridoxal phosphate), vitamin B12 and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

Analysis of antioxidant activity by performing a FRAP assay was proposed by Benzie and Strain [23]. It involves colorimetric determination of the reaction mixture in which the oxidants contained in the sample reduce Fe ions to Fe. At low pH, Fe(in)-TPTZ (ferric-tripyridltria-zine) complex is reduced to the ferrous (Fe ) form and intense blue colour at 593 nm can be observed. The FRAP reagent is prepared by mixing 2.5 ml of TPTZ (2,4,6-tris (l-pyridyl)-5-triazine) solution (10 mM in 40mM HCl), 25 ml acetate buffer, pH 3.6, and 2.5 ml FeCl3 H20 (20 mM). The colour of Fe(II)(TPTZ)2 which appears in the solution is measured colorimetri-cally after incubation at 37°C. The measurement results are compared to those of a blank sample, which contains deionised water instead of the analysed sample. The duration of the assay differs from one study to another 4 min [23, 24], 10 min [25] to 15 min [26]. The analysis results are converted and expressed with reference to a standard substance, which can be ascorbic acid [26], FeS04 [23, 25], Trolox [27,18]. 

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