Oxidation-induced unfolding facilitates myosin cross-linking in myofibrillar protein by microbial transglutaminase

Myofibrillar protein from pork Longissimus muscle was oxidatively stressed for 2 and 24 h at 4 °C with mixed 10 μM FeCl₃/100 μM ascorbic acid/1, 5, or 10 mM H₂O₂ (which produces hydroxyl radicals) and then treated with microbial transglutaminase (MTG) (E:S = 1:20) for 2 h at 4 °C. Oxidation induced significant protein structural changes (P < 0.05) as evidenced by suppressed K-ATPase activity, elevated Ca-ATPase activity, increased carbonyl and disulfide contents, and reduced conformational stability, all in a H₂O₂ dose-dependent manner. The structural alterations, notably with mild oxidation, led to stronger MTG catalysis. More substantial amine reductions (19.8-27.6%) at 1 mM H ₂O₂ occurred as compared to 11.6% in nonoxidized samples (P < 0.05) after MTG treatment. This coincided with more pronounced losses of myosin in oxidized samples (up to 33.2%) as compared to 21.1% in nonoxidized (P < 0.05), which was attributed to glutamine-lysine cross-linking as suggested by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.