Ascorbate nitrite reduction

Indeed, given an improperly designed or understood system, a blocking agent, like ascorbic acid, could be catalytic toward nitrosamine formation. For example, if the source of nitrosatlng agent is nitrite ion and the susceptible amine is in the lipid phase, conceivably ascorbic acid could cause the rapid reduction of nitrite ion to nitric oxide which could migrate to the lipid phase. Subsequent oxidation of NO to NO in the lipid phase could cause nitrosation. 

Ascorbic acid has been found to be the most effective and useful inhibitor of amine nitrosation [23]. Ascorbic acid inhibits the formation of DMN from oxytetracycline and nitrite, and also from aminophenazone (aminopyrine) and nitrite. Tannins are present in a variety of foods, competing with secondary amines for nitrite and thus leading to a reduction in the amount of nitrosamine formed [24]. 

Principally the same, but chemically simpler, sequence was used to prepare arylnitro anion-radicals from arylamines, in high yields. For instance, aqueous sodium nitrite solution was added to a mixture of ascorbic acid and sodium 3,5-dibromo-4-aminobenzenesulfonate in water. After addition of aqueous sodium hydroxide solution, the cation-radical of sodium 3,5-dibromo-4-nitro-benzenesulfonate was formed in the solution. The latter was completely characterized by its ESR spectrum. Double functions of the nitrite and ascorbic acid in the reaction should be underlined. Nitrite takes part in diazotization of the starting amine and trapping of the phenyl a-radical formed after one-electron reduction of the intermediary diazo compound. Ascorbic acid produces acidity to the reaction solution (needed for diazotization) and plays the role of a reductant when the medium becomes alkaline. The method described was proposed for ESR analytical determination of nitrite ions in water solutions (Lagercrantz 1998). 

However, the latter residue is in no sense equivalent to the Tyr 25 of the P. pantotrophus enzyme. The Tyr 10, which is not an essential residue (19), is provided by the other subunit to that in which it is positioned close to the di heme iron (Fig. 6). In other words, there is a crossing over of the domains. A reduced state structure of the P. aeruginosa enzyme has only been obtained with nitric oxide bound to the d heme iron (20) (Fig. 6). As expected, the heme c domain is unaltered by the reduction, but the Tyr 10 has moved away from the heme d iron, and clearly the hydroxide ligand to the d heme has dissociated so as to allow the binding of the nitric oxide (Fig. 6). This form of the enzyme was prepared by first reducing with ascorbate and then adding nitrite. 

The major health concern regarding use of curing salts is the possibility of nitrosamine formation in the cured products. Nitrite ion appears to be the precursor compound required for nitrosamine formation, rather than NO. Inclusion of reductants such as ascorbate, now required in bacon, lowers nitrite level in the product and increases the level of NO available for cured meat color formation and stability. 

Ascorbate, cysteine, hydroquinone, and NADH are capable of acting as re-ductants for NOMb formation in model systems containing sodium nitrite and Mb (Fox and Ackerman, 1968). Ascorbate, cysteine, and hydroquinone all form nitroso-reductant intermediates which released NO, forming a NO-MMb complex which was then reduced to NOMb. Release of NO from the reductant-NO complex was rate limiting in production of NOMb. For NADH as reductant, reduction of NOMMb to NOMb was the rate limiting step. In summary, two reduction steps were required, the reduction of nitrite (as nitrous acid or its anhydride, N2O3) to NO, and reduction of NOMMb to NOMb. 

In contrast to fresh muscle, meat has low levels of NAD (Madhavi and Carpenter, 1993). Thus, NAD-dependent enzymatic pathways for NOMb formation ate relatively unimportant in meat curing. In commercial practice, nitrite is reduced to NO by nonenzymatic means, including use of reductants such as ascorbate and erythorbate. Although meat has sufficient reducing ability to obtain a slow conversion of nitrite to NO, ascorbate or its isomer, erythorbate, is commonly added to curing brines or sausage emulsions to obtain faster NO production and thus a more rapid development of cured meat color. Care must be taken... 

Cassens (1997) has reported a dramatic decline in the residual nitrite levels in cured meat products in the United States. The current residual nitrite content of cured meat products is about 10 ppm. In 1975 an average residual nitrite content in cured meats was reported as 52.5 ppm. This reduction of nitrite levels by about 80 percent has been attributed to lower ingoing nitrite, increased use of ascorbates, improved process control, and altered formulations. 

The synthesis of NO from dietary nitrite, under the acid conditions of the stomach and the presence of reductants such as ascorbic acid, has been proposed to have an antimicrobial function. 

The inhibitive action of nitrite is pH dependent, which led to the assumption that undissociated nitrous acid is the active substance. However, this is a hypothetical acid from which equimolar parts of H20, NO, and N02 are formed. In the 1960s it became clear that nitric oxide (NO) is the active species. Nitric oxide can also be generated through reduction of nitrite, e.g., by ascorbate or isoascorbate (Wirth, 1985). 

The blue copper center in cupredoxins is also found in multidomain, multicopper enzymes " such as ascorbate oxidase (AO), laccase (Lc), human ceruloplasmin (Cp), " and a subfamily of copper-containing nitrite reductase. Nitrite reductase (NiR) catalyzes reduction of nitrite (N(32 ) to nitric oxide (NO), a step in the biological dinitrification cycle. Two distinct types of NIR are known multiheme NiR (see Chapter 8.29) and multicopper NiR. The multicopper... 

Others have fabricated PPy nanowires by template-free potentiostatic polymerization in LiC104 and Na2C03 [3,4], which leads to the formation of disordered polymer mats at the electrode surface. These electrodes were used for the electrocatalytic reduction of nitrite and the oxidation of ascorbic acid. However, no comparisons were made between the nanostructured surfaces and bulk electropolymerized films. 

The salts associated with cured meats, nitrates and nitrites, are assayed by aqueous extraction and colorimetric determination. The ISO methods involve extraction in hot water containing borax solution and deproteination of a portion of the extract with Carrez reagents. Nitrite (ISO 2918 1975) is determined directly by formation of an azo dye with sulfanilamide and N-(l-naphthyl)ethylenediamine dihydrochloride which is measured at 538 mn. Nitrate (including nitrite) (ISO 3091 1975) is determined by cadmium reduction to nitrite and azo dye formation. Reduction of nitrate to nitrite may be carried out on a special column (as in the ISO method) or using spongy cadmium. Care must be taken to prevent interference from ascorbic acid if it is present in the sample at relatively high levels (>20pgml ). 

Phosphate is mostly determined photometrically through the reaction with ammonium heptamolyb-date in acidic medium in the presence of potassium antimonyl tartrate as a catalyst. 12-Molybdophos-phoric acid is formed that upon reduction with, for example, ascorbic acid has a blue color. The lower limit of detection is lOpgl. Chromium(VI), nitrite, and high concentrations of chloride and iron interfere. The lower recoveries caused by iron are eliminated by addition of hydrogensulfite. 

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