Studies on the Post-Mortem Fragmentation of Myofibrils

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Abstract
There was a close relationship between the fragmentation of rnyofibrils and the tension developed during post-mortem contraction of muscle. The extent of fragmentation was at its maximum when the sarcomeres attained a length of 2·0 to 2·2 μm. The rate of fragmentation of myofibrils depended upon the calcium ion concentration within a range of 10−5 to 2 × 10−2 M, with a minimum at pH 6·5. The fragmentation of myofibrils free from muscle fibers was not affected by 10 mi iodoacetate, an irreversible inhibitor of calcium-activated factor (CAF). Incubation of myofibrils with 10 nmi CaCl2 caused the release of about 12% of the total myofibrillar proteins after homogenization. The protein solution contained little α-actinin, and considerable amounts of 54,000- and 76,000-dalton components which seem to originate from the Z-line. SDS-polyacrylamide gels of troponin prepared from the incubated myofibrils did not change with time of incubation. These findings are in contrast with the proteolytic degradation of Z-lines by CAF treatment, in which α-actinin and 87,000-dalton component are released. These data directly demonstrate that the in vitro fragmentation of post-mortem muscle (i.e. during its conversion into myofibrils upon mechanical homogenization) is different from that induced by CAF. The possible role of calcium ions during in vitro fragmentation of myofibrils is discussed.