The role of endogenous proteases in the tenderisation of fast glycolysing muscle.

The activities of lysosomal cathepsins and the calpain-calpastatin system were measured to determine their involvement in the more rapid tenderisation of fast glycolysing muscle. The rate of pH fall of M. longissimus dorsi (LD) of Hereford cross Friesian heifers (n = 52, Experiment 1; n = 36, Experiment 2) was measured. Muscles were selected according to their rate of glycolysis; slow and fast (n = 16, Experiment 1; n = 10, Experiment 2). Fast glycolysing muscles were rated more tender in sensory analysis and had a lower shear force than slow glycolysing muscles. Slow glycolysing muscles had shorter sarcomere lengths. Low pH conditions in fast glycolysing muscle correlated with the enhanced release of cathepsins B and L from lysosomes. Calpain I activity was higher and calpastatin activity was lower in fast glycolysing muscle early post mortem. SDS-PAGE electrophoresis patterns showed increased proteolysis, such as the earlier appearance of the 30 kDa fragment in fast glycolysing muscle. Transmission Electron Microscopy (TEM) results showed a greater degree of ultrastructural breakdown early post mortem in fast glycolysing muscle. The results suggest that the increase in tenderness in muscles that undergo fast glycolysis early post mortem may be caused by proteolysis by released lysosomal cathepsins and calpain I uninhibited by calpastatin.