Sarcoplasmic and myofibril-bound calpains during storage of pork longissimus muscle: New insights on protein degradation

The subcellular distribution of calpain-1 and -2 and the proteolytical activity of myofibril-bound calpains in pork were investigated during 12 days cold storage. The content of sarcoplasmic calpain-1 decreased during storage while myofibril-bound calpain-1 content first increased (P < 0.05) to 17% of that of 12 h-sarcoplasmic calpain-1 on day 6 followed by a gradual decrease with subsequent storage, suggesting that calpain-1 gradually translocated from sarcoplasm to myofibrils during the initial 6 days of postmortem storage. Intact desmin decreased (P < 0.05) after incubation of myofibrils with 0.05 mM Ca2+, and this was more pronounced with 5 mM Ca2+ (P < 0.05). Ca2+ titration curves of day 6 myofibrils showed two distinct proteolytic activities becoming activated in the range 0.03 to 0.06 mM and 0.4 to 0.8 mM Ca2+, respectively. The results suggest that both calpain-1 and calpain-2 binds to myofibrils during storage and subsequently degrade structural proteins including desmin.