Aggregation and conformational changes of tilapia actomyosin as affected by calcium ion during setting

The effect of CaCl2 on aggregation and conformational changes of tropical tilapia (Oreochromis niloticus) actomyosin incubated at 4 and 40 °C was investigated. Aggregation of tilapia actomyosin incubated at 40 °C for 30 min increased with addition of 10–100 mM CaCl2. Formation of higher molecular weight protein (HMP) at 40 °C was enhanced by addition of >10 mM Ca2+ ion, but suppressed by 2 mM N-ethylmaleimide (NEM) and 1 mM phenylmethnesulfonyl fluoride (PMSF), suggesting the involvement of endogenous transglutaminase (TGase). Moreover, addition of 10–100 mM CaCl2 destabilized actomyosin as evident by an increase in aniline naphthalenesulfonate surface hydrophobicity (S0-ANS) and loss of α-helical structure at 40 °C. However, CaCl2 only increased S0-ANS of actomyosin incubated at 4 °C without disturbing its secondary structure. Both ε-(γ-glutamyl)lysine isopeptide bonds and hydrophobic interactions appeared to be involved in HMP aggregates formed at 40 °C. Breaking force and deformation of actomyosin gels incubated at 40 °C for 30 min increased with added CaCl2 level and reached the maximum at 100 mM CaCl2, corresponding to an increased intensity of HMP observed on 5% SDS-PAGE. Ca2+ improved gelation during setting at 40 °C by not only activating endogenous TGase but also promoting hydrophobic interactions among unfolded actomyosin. Setting was also induced to a lesser extent at 4 °C in the presence of >10 mM CaCl2.