Aggregation and structural changes of silver carp actomyosin as affected by mild acidification with d-gluconic acid δ-lactone

The structural changes and aggregation properties of silver carp actomyosin acidified with d-gluconic acid-δ-lactone (GDL) were investigated. Results showed that silver carp actomyosin underwent aggregation and formation of precipitate as indicated by turbidity and centrifugation coupled electrophoresis analysis. Circular dichroism indicated that myosin rod unfolded during acidification, resulting in a gradual decrease in α-helical content. The changes in tertiary structure of actomyosin under acidic conditions were demonstrated by second-derivative UV spectra and intrinsic fluorescence. Tyrosine residues were exposed to the surface of proteins when pH was decreased to 5.5, and were buried inside the protein aggregates with further reduction in pH. In contrast, more tryptophan residues were exposed to the polar environment with decreasing pH. 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide crosslinking experiments showed that the intensity of myosin heavy chain (MHC) bands decreased sharply with decreasing pH and the actin bands decreased more slowly, suggesting that MHC is the major protein component involved in the non-covalent cross-linking and formation of aggregates during acidification of silver carp actomyosin.