POSTMORTEM PROTEASES ACTIVITY IN RELATION TO MUSCLE PROTEIN-TURNOVER IN BELGIAN BLUE BULLS WITH DIFFERENT GROWTH-RATES

Post mortem protease activities (calpains 1 and 2, calpastatin, cathep sins L and B) in longissimus dorsi muscle were related to in vivo musc le protein (MP) turnover and animal growth in young Belgian Blue beef bulls subjected to restricted feed intake (growth period) followed by compensatory growth (fattening period). Control animals (Group +) show ed normal growth rates during the two periods: 1.13 and 1.44 kg.d-1 re spectively. In the experimental group (Group -) average daily growth w as about 0.34 kg.d-1 during the first period and it increased sharply upon refeeding (with maxima approaching 2 kg.d-1). In vivo muscle prot ein measurements, based on urinary 3-methylhistidine excretion and nit rogen balance, showed lower values during feed restriction, especially for MP accretion: 122 vs 372 g MP.d-1 (P < .01). Both synthesis and d egradation rates were lowered: 360 vs 795 and 238 vs 423 g MP.d-1 resp ectively. Refeeding resulted in increases in synthesis, degradation an d accretion rates. One month before slaughter, MP turnover had reached the same values as in the control group: 769 and 764 g MP degraded pe r day. Cathepsin B and L activities measured 1 d and 8 d post mortem ( pm) showed no differences between groups. In contrast, pm calpain 1 an d 2 activities in longissimus dorsi (1 h pm) were respectively lower a nd higher in the compensatory growth group (5.6 and 37.2 vs 13.1 and 2 5.3 U/g) and were apparently not related to MP turnover recorded one m onth before slaughter. Differences in pm calpain activities could be r elated to a higher catch up cumulative MP turnover during compensatory growth but possibly also to age and compositional differences at slau ghter. However, pm calpastatin levels were not different between group s, suggesting that extent of in vivo protein degradation (shortly befo re slaughter) might be monitored by calpastatin activity rather than b y the actual calpain levels. Finally, pm calpastatin levels could also be the main determinant factor in pm proteolysis: this might explain the lack of meat texture differences between groups, although calpain levels were different.