Effect of proteolytic enzyme activity and heating on the mechanical properties of bovine single muscle fibres

The effect of proteolysis and heating on the mechanical properties of single muscle fibres isolated from bovine M. semitendinosus was investigated. Short incubations with μ-calpain (pH 7.5) and cathepsin B (pH 5.6) at room temperature reduced (P<0.01) the raw fibre strength by 50%. After subsequent heating, μ-calpain-incubated fibres were 20% weaker and cathepsin B-incubated fibres 50% weaker than heated controls. Fibres were incubated for 8 days at 2 °C under post-mortem like pH conditions (pH 5.6) or pH conditions optimal for the calpains (pH 7.5). At pH 5.6 no significant weakening of the fibres was observed when incubations were performed in 0.1, 1 or 10 mM Ca2+. In contrast, incubation in 1 or 10 mM Ca2+ at pH 7.5 decreased the strength of the fibres. Addition of the protease inhibitors E-64 or PD150,606 prevented this weakening, suggesting that the weakening is caused by proteolyses and not directly by Ca2+.