Relationship between skeletal muscle-specific calpain and tenderness of conditioned porcine longissimus muscle

Tenderization of skeletal muscle in meat animals has been closely linked to the postmortem activity of the calpain proteolytic enzyme system, which in cludes the specific inhibitor calpastatin. Increased understanding of the s keletal muscle-specific calpain isoform p94 has prompted suggestions as to whether it too could have a role in the tenderization process. In this stud y, two groups of pigs were identified in which shear force measurements aft er 8 d of conditioning indicated a large variation in the tenderness of lon gissimus muscle. The quantity of p94 in the muscle was monitored by immunob lotting, using a porcine-specific polyclonal antibody raised against a reco mbinant peptide fragment generated as fusion protein. The antiserum recogni zed a 94-kDa protein associated with myofibrils in skeletal but not cardiac muscle, as expected for this calpain isoform, although it could not be tes ted with the native protein because of the extreme instability of p94. In t he first experiment, the mean shear force for the tough group was 6.71 +/- .28 kg (n = 12, SEM) and that of the tender group was 3.87 +/- .12 kg (n = 12), but there was no difference in the normalized absorbance of the immuno positive 94 kDa band on Western blots from samples collected at approximate ly 40 min postmortem. In the second experiment, the stability of p94 in chi lled carcasses was investigated over 24 h, using a further two groups of 10 tough and 10 tender pigs of mean shear force values 5.36 +/- .14 kg and 2. 81 +/- .15 kg, respectively. In tough and tender animals, there was a decli ne (P < .05) in the 94-kDa immunostaining material of mean half-lives of 13 .8 and 12.9 h, respectively, although there was considerable variability. D espite this variability in half lives and shear force values, no correlatio n was seen between these factors. Thus, in porcine longissimus muscle, the variability in tenderness after 8 d of conditioning cannot be attributed to an underlying variability in p94.