Purified bovine longissimus muscle myofibrils were prepared from muscle at death and from muscle samples stored at 2') 25") or 37'C for 1, 3, and 7 days postmortem. Tbe myofibrils were analyzed by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. Titin migrated as a closely spaced doublet of very high molecular weight (M, 'L 1 x 106) in myofibrils from at-death muscle samples. With increased storage time and temperature, the top band of the titin doublet gradually disappeared. the lower doublet band (putative breakdown product of upper band) remained after 7 days storage at 2" or 25"C, but disappeared by 3 days of postmortem storage at 37°C. Thus, titin is degraded in postmortem muscle, and the rate of degradation is enhanced by increases in storage time and temperature.

This study examined the in vitro effects of calcium-activated protease (CAF) on bovine myofibrillar proteins and structure under postmortem-like conditions of pH and temperature. Effects usually associated with this enzyme under optimal conditions were reduced as temperature and/or pH were lowered. However, significant activity remained at 15°C and pH 6.5, and some activity was detectable at even lower pH's and temperatures. Effects observed included: solubihzation of myofibrillar protein, degradation of the myofibrillar protein titin and others, and an increase in the degree of myofibrillar fragmentation. These results suggest that CAF is able to hydrolyze proteins that are important to structural integrity under conditions mimicking those present in postmortem muscle.

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Evidence suggests that desmin, titin and nebulin, three recently discovered proteins, have cytoskeletal roles in muscle cells. The three proteins have been purified from mature skeletal muscle and partially characterized. Properties of the three proteins are described, with special regard to their probable roles and importance in maintaining muscle cell integrity. Results will be shown that demonstrate ability of purified desmin to self‐assemble into synthetic 10‐nm (intermediate) diameter filaments. Taken together with immunoelectron microscope results (Richardson et al. 1981), it is evident that desmin is the major component of 10‐nm filaments of mature skeletal muscle cells and that the desmin filaments link adjacent myofibrils at their Z‐line levels and seemingly tie the myofibrils into the cell cyto‐skeleton. Desmin is degraded at about the same rate as is the highly susceptible troponin‐T in bovine semitendinosus muscle postmortem. Alterations in desmin and other recently discovered cytoskeletal proteins would be expected to disrupt muscle cell integrity and to have marked effects on properties of muscle important to its use as food.

No abstract