Ascorbic acid nitrosation inhibition

Ascorbic acid is known to inhibit the nitrosation of secondary amines. A computer model has been developed to predict the amount of nitrosamine formed under conditions that are experimentally inaccessible. The computer-calculated rates for N-nitrosomorpholine formation using rate and equilibrium constants from the literature agree well with experimental values in the absence of and presence of ascorbic acid under anaerobic conditions. In the aerobic system the inhibitory efficiency of ascorbic acid is lower, and the nature of the interactions among the various components of the mixtures is less well understood. The use of ascorbic acid for inhibition of N-nitroso compound formation both in vitro and in vivo is briefly reviewed. 

In summary, the general concept, that ascorbic acid can inhibit or prevent the nitrosation of amines, is essentially true. The specific eflFects, however, which can be expected in a given system, depend on a complex set of interactions among pH, the nature of the amine, the amount of... 

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). 

The criteria for choosing inhibitors in this study were the ability to compete with diethanolamine for the nitrite and lack of toxicity. An attempt was made to cover as broad a group as possible within the limits of feasibility. Ascorbic acid in its water soluble form and its oil soluble form, the palmitate, represent the enediols, Sorbate is a diene fatty acid which has been shown to inhibit nitrosation (10), Since the pK of sorbic acid is 4,76, at the pH of these experiments, both water soluble sorbate ion and oil soluble sorbic acid are present in significant amounts. Sodium bisulfite is a strong inorganic reducing agent which has an acceptable lack of toxicity at the concentration... 

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). 

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]. 

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. 

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. 

In addition to these aspects of the amine nitrosation reaction, the reactions of ascorbic acid with various components of the nitrite equilibria involve transformations that are also aflFected by the presence or absence of oxygen (1,23), Some of these are shown schematically in Scheme 3. If attention is then focussed on the reactions of ascorbic acid/ascorbate rather than on the nitrosation of amines, it can be seen that the amount of ascorbic acid or ascorbate available for inhibition of nitrosation can be diminished by the presence of oxygen. 

For convenience we have divided the studies into those that evaluate quantitative effects, that is, in vitro (Table III), and those in which inhibition of nitrosation is inferred from the absence of an expected toxic effect in intact animals (Table IV). It is remarkable that ascorbic acid is effective in systems as different as bacon fat and gastric juice. 

Table III. Inhibition of Nitrosation by Ascorbic Acid In Vitro...

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. 

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. °... 

Inhibition of a nitrosation reaction is possible, e. g., with ascorbic acid, which is oxidized by nitrite to its dehydro form, while nitrite is reduced to NO. Similarly, tocopherols and some other food constituents inhibit substitution reactions. Representative suitable measures to decrease exo-and endogenic nitrosamine hazards are ... 

Figure 12.28 Mechanism of catalysis and inhibition of nitrosation reaction. Y = catalyst (such as iodide), Z = reducing agent (H2A = ascorbic acid, A = dehydroascorbic acid).

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