Inhibition of Nitrosamine Formation by Ascorbate

Ascorbate ctin act as a radictd-trapping tintioxideint, reacting with superoxide eind a proton to yield hydrogen peroxide, or with the hydroxy radical to yield water. In each case, the product is monodehydroascorbate. 

As well as its antioxidemt role, tiscorbate can be a source of hydroxyl and superoxide radicals. At high concentrations, it can reduce molecular oxygen to superoxide, being oxidized to monodehydroascorbate. At lower concentrations of ascorbate, both Fe + and Cu + ions are reduced by ascorbate, yielding monodehydroascorbate. Fe + and Cu+ are readily reoxidized by reaction with hydrogen peroxide to yield hydroxide ions and hydroxyl radicals. Cu also reacts with molecular oxygen to yield superoxide. 

It seems likely that the prooxidant actions of ascorbate are of relatively little importance in vivo. Except in cases of iron overload, there are almost no treinsition metal ions in free solution. They are ali bound to proteins, and because the renal transport system is readily saturated, plasma and tissue concentrations of ascorbate EU e unlikely to rise to a sufficient extent to lead to radiceil formation (Halliwell, 1996 CeuT emd Frei, 1999a). 

In addition to dietary sources, a significant amount of nitrate is formed endogenously by the metabolism of nitric oxide - 1 mg per kg of body weight per day (about the same as the average dietary intake), increasing 20-fold in response to inflammation and immune stimulation. There is considerable secretion of nitrate in saliva, and up to 20% of this may be reduced to nitrite by oral bacteria. Under the acidic conditions of the stomach, nitrite can react with amines in foods to form carcinogenic N-nitrosamines, although it is not known to what extent this occurs in vivo. 

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Tannenbaum SR, WishnokJS. Inhibition of nitrosamine formation by ascorbic acid. Ann NY Acad Sci 1987 498 354-363. 

Scavengers Inhibit nitrosamine formation by competing with the amine for the nltrosatlng agent. Ascorbic acid Is a typical exan le of the competition by rapid reduction of the nltrosatlng agent as shown. 

Inhibitors of nitrosation generally function by competing with the amine for the nitrosating agent. An inhibitor would thus react with nitrite at a faster rate than with amines. The inhibition reaction has recently been reviewed ( 35). The ability of ascorbate to act as a potent inhibitor of nitrosamine formation has resulted in the use of the vitamin in nitrite-preserved foods and pharmaceuticals. Furthermore, the effectiveness of ascorbate in inhibiting nitrosamine formation is dependent on (1) the concentration of ascorbate (an excess is required) (2) pH (ascorbate is nitrosated 240 times more rapidly than ascorbic acid) (3) the reactivity of the amine toward nitrosation and (4) the extent of oxygenation of the system. 

Antioxidant activity involving the transfer of two electrons between the ascorbate/dehy-droascorbate redox couple or donation of one electron to inactivate highly reactive free radicals, e.g., protection of vitamin E by reduction of the tocopheryl radical Competitive inhibition in substrate binding reactions, including inhibition of the formation of carcinogenic nitrosamines... 

Several ascorbic acid derivatives were examined by Pensabene et al. (570) for their ability to inhibit nitrosation of pyrrolidine in a model system developed to simulate the lipid-aqueous-protein composition of bacon. While sodium ascorbate was quite effective in the aqueous phase, a combination of an ascorbyl ester with sodium ascorbate gave a better effect in the lipid phase (Table XX). The use of ascorbates and tocopherol as inhibitors of nitrosamine formation and oxidation in foods of the aqueous and lipid type has been reviewed by Newmark and Mergens (326). These compounds in combination could be markedly useful in preventing food contamination with nitrosamines and/or nitros-amides in cured meats such as bacon. 

Review of Ascorbic Acid Mechanisms of Action. Ascorbic acid and AP have antioxidant activity in fats, oils, vitamin A, and carotenoids. In these systems AP is a better antioxidant than are the phenolic antioxidants BHT and BHA, both from these data and others (29,35). Ascorbic acid protects against oxidation of flavor compounds in wine, beer, fruits, artichokes, and cauliflower (29) presumably by oxygen scavenging. The well-known formation of nitric oxide from nitrites by ascorbic acid is used not only for inhibition of nitrosamine fortnation, but also to promote... 

N-Nitrosamines are formed in foods by the reaction of secondary and tertiary amines with a nitrosating agent, usually nitrous anhydride, which forms from nitrite in acidic, aqueous solution. NDMA is the most common volatile amine found in food. Food constituents and the physical make-up of the food can affect the extent of nitrosamine formation. Ascorbic acid and sulfur dioxide have been used to inhibit the formation of nitrosamines. 

Inhibition of nitrosation is generally accompHshed by substances that compete effectively for the active nitrosating iatermediate. /V-Nitrosamine formation in vitro can be inhibited by ascorbic acid [50-81-7] (vitamin C) and a-tocopherol [59-02-9] (vitamin E) (61,62), as well as by several other classes of compounds including pyrroles, phenols, and a2iridines (63—65). Inhibition of iatragastric nitrosation ia humans by ascorbic acid and by foods such as fmit and vegetable juices or food extracts has been reported ia several instances (26,66,67). 

N-Nitrosamine inhibitors Ascorbic acid and its derivatives, andDC-tocopherol have been widely studied as inhibitors of the N-nitrosation reactions in bacon (33,48-51). The effect of sodium ascorbate on NPYR formation is variable, complete inhibition is not achieved, and although results indicate lower levels of NPYR in ascorbate-containing bacon, there are examples of increases (52). Recently, it has been concluded (29) that the essential but probably not the only requirement for a potential anti-N-nitrosamine agent in bacon are its (a) ability to trap NO radicals, (b) lipophilicity, (c) non-steam volatility and (d) heat stability up to 174 C (maximum frying temperature). These appear important requirements since the precursors of NPYR have been associated with bacon adipose tissue (15). Consequently, ascorbyl paImitate has been found to be more effective than sodium ascorbate in reducing N-nitrosamine formation (33), while long chain acetals of ascorbic acid, when used at the 500 and lOOO mg/kg levels have been reported to be capable of reducing the formation of N-nitrosamines in the cooked-out fat by 92 and 97%, respectively (49). 

The promising contribution of adding tocopherol to bacon, along with sodium ascorbate, to inhibit nitrosamine formation undertaken by Mergens and associates is reported in detail in the Mergens et al, reference (1978). 

In the presence of a nucleophilic anion such as I, Br, CP, SCN, acetate or phthalate, nitrous acid can be converted into more active nitrosating species. A -nitrosamines formation can be accelerated by certain microorganisms at acid pH values. On the other hand, the nitrosation reactions can be inhibited by compounds such as ascorbic acid, sulfamic acid, tocopherol, and others. °... 

More recently, results obtained in the U.S.A. have indicated that the formation of nitrosamines in bacon can be Inhibited by presence of ascorbic acid. If this can be confirmed and extended to other meat products, it would resolve a major part of the nitrite problem . 

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