Kinetics of Nitrosation of the Amino Acids Proline, Hydroxyproline, and Sarcosine23

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Abstract
The kinetics of nitrosation of the amino acids proline (Pro), hydroxyproline (HyPro), and sarcosine (Sar) were studied by 2 methods which gave similar results. The pH optimum was 2.5, compared with 3.4 for simple secondary amines. At pH 2.5 and 25° C, pH-dependent rate constant k1 was 0.037 (Pro), 0.31 (HyPro), and 0.23 (Sar) M-2 sec-1. Equations were developed for pH-independent rate constant k2, based on the complex ionization of amino acids and the dissociation constants of the amino acid methyl esters. These satisfactorily explained the pH dependency of k1. The value of k2 [1.4 (Pro), 2.1 (HyPro), and 2.6 (Sar) ×105 M-2 sec-1 at 25° C] varied k2 for 5 simple secondary amines. L-Prolyglycine and prolylleucylglycylamide were nitrosated 7–10 times more readily than Pro, with pH optima of 3.0 and 3.4, respectively. Thiocyanate increased the rate of Sar nitrosation 10- to 400-fold, especially at low pH, and the kinetics of this effect were examined. Citrate-perchlorate buffers at low pH and high concentrations of chloride were weakly inhibitory. The possibility of using these results to predict approximate nitrosamino acid formation in stored food or in the stomach is discussed.

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