IS Z-DISK DEGRADATION RESPONSIBLE FOR POSTMORTEM TENDERIZATION

A number of studies have suggested that Z-disk degradation is a major factor contributing to postmortem tenderization. These conclusions see m to have been based largely on experimental findings showing that the calpain system has a major role in postmortem tenderization, and that when incubated with myofibrils or muscle strips, purified calpain rem oves Z-disks. Approximately 65 to 80% of all postmortem tenderization occurs during the first 3 or 4 d postmortem, however, and there is lit tle or no ultrastructurally detectable Z-disk degradation during this period. Electron microscope studies described in this paper show that, during the first 3 or 4 d of postmortem storage at 4 degrees C, both costameres and N-2 lines are degraded. Costameres link myofibrils to t he sarcolemma, and N-2 lines have been reported to be areas where titi n and nebulin filaments, which form a cytoskeletal network linking thi ck and thin filaments, respectively, to the Z-disk, coalesce. Filament ous structures linking adjacent myofibrils laterally at the level of e ach Z-disk are also degraded during the first 3 or 4 d of postmortem s torage at 4 degrees C, resulting in gaps between myofibrils in postmor tem muscle. Degradation of these structures would have important effec ts on tenderness. The proteins constituting these structures, nebulin and titin (N-2 lines); vinculin, desmin, and dystrophin (three of the six to eight proteins constituting costameres); and desmin (filaments linking adjacent myofibrils) are all excellent substrates for the calp ains, and nebulin, titin, vinculin, and desmin are largely degraded wi thin 3 d postmortem in semimembranosus muscle. Electron micrographs of myofibrils used in the myofibril fragmentation index assay show that these myofibrils, which have been assumed to be broken at their Z-disk s, in fact have intact Z-disks and are broken in their I-bands.