Post-mortem changes in cytoskeletal proteins of muscle

Progressive changes have been identified in the solubility of muscle-cell proteins during post-mortem muscle ageing, particularly the cytoskeletal proteins, desmin and connectin. Ox sternomandibularis muscle was sampled immediately post mortem and up to six days later. It was homogenised and separated into three salt-soluble fractions: phosphate soluble, concentrated KI soluble and guanidine-HCl soluble. Proteins in each fraction were analysed on sodium dodecylsulphate polyacrylamide gels. Changes reported previously by other authors were confirmed. In addition desmin, which was restricted to the guanidine fraction, disappeared, apparently due to proteolysis during storage. Connectin was also partly lost from the guanidine fraction, possibly through increased solubility in the KI fraction. In this respect an unidentified polypeptide of 110 000 D appeared during storage. Desmin extracted from ox muscle was partially purified and identified by amino acid analysis. It apparently occurs in vivo as a network of linked collars around the Z-discs and its loss during post-mortem storage probably accounts for the ease with which stored muscle disintegrates into individual myofibrils on homogenisation. The disintegration of the cytoskeletal network can account for the post-mortem changes in the physical properties of muscle for the increased tenderness after cooking of stored meat. However, factors other than those related to changes in cytoskeletal proteins are responsible for the toughness of cooked, cold-shortened muscle since cold shortening prior to storage did not affect the distribution of proteins among the three salt fractions, nor the patterns obtained in gel analyses.