Properties of myofibril-bound calpain activity in longissimus muscle of callipyge and normal sheep

Properties of the calpain bound to myofibrils in longissimus muscle from callipyge or noncallipyge sheep were examined after 0, 1, 3, and 10 d of postmortem storage at 4°C. Western analysis has shown that most of this calpain is μ-calpain, although the sensitivity of the antibodies used in the earlier studies could not eliminate the possibility that up to 10% of the calpain was m-calpain. The calpain is bound tightly, and very little is removed by washing with the detergent Triton X-100; hence, it is not bound to phospholipids in the myofibril. Over 25% of total μ-calpain was bound to myofibrils from at-death muscle, and this increased to ∼40% after 1 d postmortem. The amount of myofibril-bound μ-calpain increased only slightly between 1 and 10 d of postmortem storage. The percentage of autolyzed μ-calpain increases with time postmortem until after 10 d postmortem, when all myofibril-bound μ-calpain is autolyzed. The specific activity of the myofibril-bound calpain is very low and is only 6 to 13% as high as the specific activity of extractable μ-calpain from the same muscle. It is unclear whether this low specific activity is the result of unavailability of the active site of the myofibril-bound calpain to exogenous substrate. The myofibril-bound calpain degrades desmin, nebulin, titin, and troponin T in the myofibrils, and also releases undegraded α-actinin and undergoes additional autolysis when incubated with Ca²⁺; all these activities occurred slowly considering the amount of myofibril-bound calpain. Activity of the myofibril-bound calpain was partly (58 to 67%) inhibited by the calpain inhibitors, E-64 and iodoacetate; was more effectively inhibited by a broader-based protease inhibitor, leupeptin (84 to 89%); and was poorly inhibited (43 to 45%) by calpastatin. Release of undegraded α-actinin and autolysis are properties specific to the calpains, and it is unclear whether some of the myofibril-bound proteolytic activity originates from proteases other than the calpains or whether the active site of myofibril-bound calpain is shielded from the inhibitors. Activities and properties of the myofibril-bound calpain were identical in longissimus muscle from callipyge and normal sheep, although previous studies had indicated that the "normal" longissimus was much more tender than the callipyge longissimus. Hence, it seems unlikely that the myofibril-bound calpain has a significant role in postmortem tenderization of ovine longissimus.