Degradation of ubiquitin in beef during storage.

Sarcoplasmic proteins were prepared from the quadriceps femoris muscle immediately after slaughter (2.5 hr) and from stored muscle samples at 10 days post mortem for SDS-PAGE analysis and Western blotting. Characterization with ubiquitin antiserum (Sigma, St. Louis, MO, USA) showed clear and strong recognition of ubiquitin (8.6 kDa) and another minor band (17 kDa) in purified ubiquitin (Sigma, St. Louis, MO, USA). Among the sarcoplasmic proteins prepared, this antiserum also reacted with the bands corresponding to purified ubiquitin (8.6 kDa and 17 kDa) and a small amount of some other higher-molecularmass proteins which were considered to be ubiquitin-protein conjugates. However, at 10 days post mortem, both ubiquitin and the ubiquitin-protein conjugates had almost disappeared, suggesting their degradation by proteinases.