CHANGES IN THE CALPAINS AND CALPASTATIN DURING POSTMORTEM STORAGE OF BOVINE MUSCLE

Changes in activity and protein status of mu-calpain, m-calpain, and c alpastatin in bovine semimembranosus muscle during the first 7 d of po stmortem storage were monitored by using assays of proteolytic activit y, SDS-polyacrylamide gel electrophoresis, and Western blot analysis. Extractable m-calpain activity changed slightly during the first 7 d a fter death (decreased to 63% of at-death activity after 7 d), whereas extractable calpastatin activity decreased substantially (to 60% of at -death activity after 1 d and to 30% of at-death activity after 7 d of postmortem storage) during this period. Extractable mu-calpain activi ty also decreased rapidly (to 20% of at-death activity at 1 d and to l ess than 4% of its at-death activity at 7 d after death) during postmo rtem storage. Western blot analysis showed that the 80-kDa subunit of m-calpain remained undegraded during the first 7 d after death but tha t the 125- to 130-kDa calpastatin polypeptide was gone entirely at 7 d after death. Hence, the calpastatin activity remaining at 7 d origina tes from calpastatin polypeptides that are 42 kDa or smaller. The 80-k Da mu-calpain subunit was almost entirely in the 76-kDa autolyzed form at 7 d after death; this form is proteolytically active in in vitro s ystems, and it is unclear why the postmortem, autolyzed mu-calpain is not active. Over 50% of total muscle mu-calpain is tightly bound to my ofibrils 7 d after death; this mu-calpain is also nearly inactive prot eolytically. Unless postmortem muscle contains some factor that enable s mu-calpain in this muscle to be proteolytically active, it is not cl ear whether mu-calpain could be responsible for any appreciable postmo rtem myofibrillar proteolysis.