http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
Factors associated with protein turnover in skeletal muscle of bulls and steers during normal growth and postmortem storage
Six each, MARC III composite (1/4 Red Poll, 1/4 Pinzgauer, 1/4 Hereford and 1/4 Angus) bulls and steers weighing approximately 180 kg, were given ad libitum access to a typical finishing diet. At sampling periods of 0, 45, 90, 135 and 180 d, muscle biopsy and urine samples were obtained. This sampling technique allowed for correlating the relationship between myofibrillar protein turnover and the calpain and catheptic proteinase systems. Bulls exceeded steers in average daily gain throughout the duration of the study. Bulls displayed lower fractional degradation rates of myofibrillar proteins than steers at the 180 d sampling period. The direct mechanism by which castration alters protein turnover in muscle was not clear. However, the improvements in protein accretion observed in bulls appeared to be related to decreased fractional degradation rates. During the later sampling periods, bulls degraded approximately 30% less myofibrillar proteins per day than steers. Activities of the endogenous proteinase system m-calpain, m-calpain and calpastatin (calpain inhibitor) were measured at 0 and 24 h postmortem. Differences in either m- or m-calpain activities between bulls and steers were not detected. However, muscle calpastatin activity was greater for bulls than steers. Warner-Bratzier shear values, the myofibriliar fragmentation index and sensory panel tenderness ratings indicated that meat from bulls was tougher and exhibited less postmortem proteolysis than meat from steers. If calpastatin is related to protein turnover in living muscle, then an increase in calpastatin activity could possibly decrease calpain-mediated degradation and, in turn, reduce fractional degradation rate. The significant negative correlation (r= -.72) between calpastatin activity and fractional degradation rate of myofibrillar proteins (at 180 d) indicated that animals with higher calpastatin activities had lower fractional degradation rates. The depressed fractional degradation rate observed in bulls may have been the result of lower proteolytic activity of calpain proteinases due to greater inhibition by calpastatin. These results indicate that in vivo protein degradation could be mediated by the endogenous calpain inhibitor, calpastatin.