Regulation of hydroxyl radical-induced myofibrillar protein and iron/ascorbate-induced liposome oxidation by sodium nitrite: focus on its concentration and pH.

A hydroxyl radical generation system and a liposome oxidation system were used as separate chemical models to evaluate myofibrillar protein (MP) oxidation and lipid oxidation, respectively. The effects of sodium nitrite concentrations (25, 50, 75, 100, 125, and 150 mg/kg) on the oxidation and structural properties of MPs and lipids were evaluated at pH 5.0, 5.5, 6.0, and 6.5. Sodium nitrite promoted protein oxidation but inhibited lipid oxidation in the model system. Specifically, with increasing sodium nitrite concentration, carbonyl content significantly increased from 2.97 to 7.76 nmol/mg (P < 0.05) at pH 6.5, while sulfhydryl content significantly decreased from 24.56 to 7.70 nmol/mg (P < 0.05). The level of the thiobarbituric acid-reactive substances (TBARS) significantly decreased from 1.60 to 0.39 mg/L (P < 0.05). The content of dityrosine first increased and then decreased, peaking at 25 mg/kg. SDS-PAGE confirmed that sodium nitrite promoted protein cross-linking and aggregation. Surface hydrophobicity first rose followed by a decrease as the levels of nitrite increased, accompanied by decreased α-helix and increased β-sheet. Overall, within the two model systems employed, sodium nitrite promoted MP oxidation and structural changes while inhibiting lipid peroxidation. These findings provide mechanistic insights into the separate effects of nitrite concentration and pH on protein and lipid oxidation.