One of the most common causes of unacceptability in meat quality is toughness. Toughness is attributed to a range of factors including the amount of intramuscular connective tissue, intramuscular fat, and the length of the sarcomere. However, it is apparent that the extent of proteolysis of key proteins within muscle fibres is significant determinant of ultimate tenderness. The objective of this manuscript is to describe the main endogenous proteolytic enzyme systems that have the potential to be involved in muscle post-mortem proteolysis and whether the experimental evidence available supports this involvement.

No abstract

The objective of this study was to investigate the protease family caspases in skeletal muscle and their potential contribution to postmortem proteolysis and meat tenderization. Ten Large White gilts were slaughtered, and samples of LM were taken at 0, 2, 4, 8, 16, 32, and 192 h after slaughter and immediately snap frozen in liquid nitrogen. Samples were subsequently analyzed for caspase 3/7 and caspase 9 activity, protein levels of known caspase substrates, alpha II spectrin and poly (ADP-ribose) polymerase (PARP), as well as, at 192 h, shear force. Specific degradation products of alpha II spectrin and PARP, which are known indicators of caspase activity, and apoptosis were detected on immunoblots of muscle samples taken over the postmortem period. The relationships between the changes observed in caspase activities and protein levels of PARP and spectrin across the entire postmortem conditioning period were investigated (n = 70). Protein levels of alpha II spectrin cleavage products across the conditioning period were found to correlate positively to caspase 3/7 activity (r = 0.38, P = 0.003) and caspase 9 activity (r = 0.32, P = 0.012), indicating that caspase-mediated cleavage was occurring in situ. There was a negative relationship between shear force and the 0 to 32 h ratio of caspase 3/7 (r = -0.62, P = 0.053) and caspase 9 activities (r = -0.68, P = 0.044). In addition, there was also a negative relationship between shear force and the level of the caspase-generated alpha II spectrin 120 kDa degradation product (r = -0.75, P = 0.012). The findings of this study indicate that changes in caspase activity and caspase-mediated cleavage take place in muscle during the conditioning period, and this could be associated with the development of tender meat.

Transient kinetic studies of Mg2+-dependent heavy-meromyosin ATPase (adenosine triphosphatase) were done by monitoring the release of both ADP and Pi into the reaction medium by using linked assay systems. The release of Pi was monitored by its quantitative transfer to ADP, with concomitant reduction of NAD+ in the presence of D-glyceraldehyde 3-phosphate, D-glyceraldehyde 3-phosphate dehydrogenase and phosphoglycerate kinase. The dissociation rates of the products, ADP and Pi, from heavy meromyosin were shown to be faster than the rate-controlling process, which occurs after the initial bond cleavage of ATP. The chromophoric ATP analogue, 6-mercapto-9-f-D-ribofuranosylpurine 5'-triphosphate (thioATP) was used as a substrate and spectral changes associated with a single turnover of heavy meromyosin could be assigned to elementary processes ofthe mechanism. It was shown that the dissociation rate ofthioADP was not the rate-controlling process of the thioATPase, whose catalytic-centre activity was 7.6 times that of the ATPase at pH8. The dissociation rate of ADP from heavy meromyosin was measured by using thioATP as displacing agent and was found to be 2.3 s-1, which is about 50 times the catalytic-centre activity of the ATPase at pH8. Transient kinetic studies with chromophoric adenosine phosphate analogues have general application for kinases and ATPases both in characterizing the chemical states of the intermediates and in delineating the elementary processes of the enzyme mechanism.