Changes in the levels of free amines in amino acid residues are also a
marker of protein oxidation. Carbonyls can be formed in proteins
through oxidation of the side chains from lysine, threonine, arginine
and proline (Estévez, 2011). The MP of sausages with higher NaNO2
(100, 200 and 400 mg/kg) had significantly higher free amines compared
to the MP of sausages without NaNO2 (Fig. 1 B). The loss of free
amines on day 0 could be explained by that free amines of amino acid
residues can be deaminated and replaced with a carbonyl moiety.
Study reported that the amino group of lysine was converted into a carbonyl
during attack with hypochlorite (Hazell, Van den Berg, & Stocker,
1994). Furthermore, the formation of active carbonyl–NH2 interactions
also contributed to the loss of free amines (Fig. 1 B), as evidenced by
SDS-PAGE (Fig. 2). This may explain the loss of free amines during storage
(Fig. 1 B).