The caspase proteolytic system in callipyge and normal lambs in longissimus, semimembranosus, and infraspinatus muscles during postmortem storage1

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Abstract
The objective of this experiment was to determine whether the caspase proteolytic system has a role in postmortem tenderization. Six ewes and 6 wethers that were noncarriers and 6 ewes and 6 wethers that were expressing the callipyge gene were used for this study. Caspase activities were determined in LM at 7 different time points during the postmortem storage period: 0 h, 4 h, 8 h, 24 h, 2 d, 7 d, and 21 d and in semimembranosus (SM) and infraspinatus (IS) muscles at 0 h, 8 h, 24 h, and 7 d from callipyge and noncallipyge (normal) lambs. Calpastatin activity was determined at 0 h, 2 d, 7 d, and 21 d and slice shear force measured at 2, 7, and 21 d in the LM. Calpastatin activity and slice shear force were greater in LM from callipyge lambs than normal lambs at each time point (P < 0.001 and P < 0.0001, respectively). Caspases 3 and 7 are executioner caspases, and their combined activity was found to decrease during the postmortem storage period in LM, SM, and IS muscles from callipyge and normal lambs. Similarly, activity of the initiator caspase (caspase 9) decreased (P < 0.05) in all 3 muscles across the postmortem storage period in callipyge and normal lambs, and its decrease in activity preceded that of the executioner caspases 3/7. A positive relationship also was detected between caspase 9 and caspase 3/7 in LM, SM, and IS muscles (P < 0.0001, r = 0.85, r = 0.86, r = 0.84, respectively), which is consistent with caspase 9 being responsible for the cleavage and activation of the executioner caspases (caspase 3/7) downstream. Caspase 3/7 and caspase 9 activities at 8 h in SM were greater in normal lamb than callipyge lamb (P < 0.05), with a trend for caspase 3/7 activity to be greater at 24 h postmortem (P = 0.0841). There also was a trend for caspase 3/7 activity to be greater in LM at 21 d in normal lamb than in callipyge lamb (P = 0.053), although there were no differences detected in caspase activities between genotypes in the IS muscle, which is not affected by the callipyge gene. A negative relationship also was detected between peak caspase 3/7 activity at 8 h in LM from normal lambs and calpastatin activity at 0 and 2 d (r = −0.65, r = −0.68, respectively, P < 0.05). This relationship was not observed in LM from callipyge lambs, suggesting that caspase 3/7 may be cleaving calpastatin in normal lambs but the level of calpastatin in callipyge lambs is such that caspase 3/7 cannot degrade it sufficiently to overcome the increased content of calpastatin, and thus, calpastatin activity is the overriding factor in postmortem proteolysis in these animals. There was no direct evidence from this study that caspases have a significant role in postmortem tenderization, but they may have some role through calpastatin degradation.

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