Properties of Synemin, a Protein Important in Maintaining the Structural Integrity of Muscle Cells

Intermediate filaments (IFs), composed primarily of the protein desmin, link together the myofibrils in skeletal muscle. We have shown that the protein synemin is a novel IF protein and that it interacts with desmin. Together, these proteins are important in maintaining the structural integrity of muscle cells

Early postmortem biochemical factors influence tenderness and water-holding capacity of three porcine muscles

The objective of this study was to determine whether differences in pork tenderness and water-holding capacity could be explained by factors influencing calpain activity and proteolysis. Halothane-negative (HAL-1843 normal) Duroc pigs (n = 16) were slaughtered, and temperature and pH of the longissimus dorsi (LD), semimembranosus (SM), and psoas major (PM) were measured at 30 and 45 min and 1, 6, 12, and 24 h postmortem. Calpastatin activity; μ-calpain activity; and autolysis and proteolysis of titin, nebulin, desmin, and troponin-T were determined on muscle samples from the LD, SM, and PM at early times postmortem. Myofibrils from each muscle were purified to assess myofibril-bound μ-calpain. Percentage drip loss was determined, and Warner-Bratzler shear (WBS) force was analyzed. Myosin heavy-chain (MHC) isoforms were examined using SDS-PAGE. The pH of PM was lower (P \u3c 0.01) than the pH of LD and SM at 30 and 45 min and 1 h postmortem. The PM had a higher (P \u3c 0.01) percentage of the MHC type IIa/IIx isoforms than the LD. The LD had the greatest proportion of (P \u3c 0.01) MHC IIb isoforms of any of the muscles. The PM had the lowest (P \u3c 0.01) percentage of MHC IIb isoforms and a greater (P \u3c 0.05) percentage of type I MHC isoforms than the LD and SM. The PM had less (P \u3c 0.01) drip loss after 96 h of storage than the SM and LD. The PM had more desmin degradation (P \u3c 0.01) than the LD and SM at 45 min and 6 h postmortem. Degradation of titin occurred earlier in the PM than the LD and SM. At 45 min postmortem, the PM consistently had some autolysis of μ-calpain, whereas the LD and SM did not. At 6 h postmortem, some autolysis of μ-calpain (80-kDa subunit) was observed in all three muscles. The rapid pH decline and increased rate of autolysis in the PM paralleled an earlier appearance of myofibril-bound μ-calpain. The SM had higher calpastatin activity (P \u3c 0.05) at 45 min, 6 h, and 24 h and had higher WBS values at 48 h (P \u3c 0.01) and 120 h (P \u3c 0.05) postmortem than the LD. At 48 and 120 h postmortem, more degradation of desmin, titin, and nebulin were observed in the LD than in the SM. These results show that μ-calpain activity, μ-calpain autolysis, and protein degradation are associated with differences in pork tenderness and water-holding capacity observed in different muscles