Proteolytic degradation of connectin, a high molecular weight myofibrillar protein, during heating of meat

Alteration of post-mortem ageing in beef by the addition of enzyme inhibitors and activators

Twenty-four hours after stunning, slices of beef M. semitendinosus were soaked in buffers at 10 °C at different pH values containing enzyme inhibitors or calcium and magnesium salts and their enzyme levels and toughness determined up to 9 days. High pH meat was tender but appeared to have the same rate of ageing as meat of normal pH. Calcium chloride accelerated ageing and produced, after completion of conditioning, more tender meat than controls. In tenderising, sodium and potassium chlorides were 43% and MgCl(2) 73% as effective as Ca salts. Cysteine proteinase inhibitors were more effective in preventing ageing than serine or aspartate inhibitors. Cysteine and aspartate inhibitors together were the most effective in preventing ageing. The inhibition of ageing by cysteine inhibitors was overcome in the presence of 30 mM CaCl(2). The results suggest a main, but not exclusive, role for calpains in meat ageing and showed a synergistic non-enzymic tenderisation by the addition of high concentrations of salts.

Studies on the structure of connectin in muscle

Enzymic hydrolysis, followed by amino acid analysis, provided no evidence for the presence of epsilon-(gamma-glutamyl) lysine or other isopeptide crosslinks in connectin. Gel elecrrophoresis in the presence of sodium dodecyl sulphate did not reveal any difference in connectin between normal and lathyritic muscle, indicating that lysyl oxidase does not initiate cross-link formation in connectin. Although connectin may be covalently crosslinked by some unknown mechanism, the available evidence suggests that the subunit of MW approximately to 900 000 is synthesised as a single polypeptide chain. In developing fetal muscle, myosin heavy chains are apparent some weeks earlier than connectin. This, together with the known susceptibility of connectin to hydrolysis, suggests that connectin exists in an exposed environment rather than as a core to the thick filament.