The relationship between tenderness, proteolysis, muscle contraction and dissociation of actomyosin

The right side m. longmissius thoracis et lumborum (LTL) of 18 10-month-old male lambs was injected with the protease inhibitor E-64, and the left sid e was injected with isotonic saline within 30 min of stunning. Carcasses we re hung during chilling by one of three methods; Achilles tendon; pubic sym physis or pubic symphysis with 2-kg weights attached to each hindleg. The L TL section was divided into three portions and aged for either 1, 3 or 7 da ys. The tenderness, myofibrillar fragmentation and dissociation of extracte d actomyosin by pyrophosphate were determined for each portion of LTL. The protease inhibitor increased shear force values by 57%, with a decrease in shear force in the cranial-caudal direction along the LTL. Muscle from Achi lles hung carcasses was the toughest. For sarcomere length hanging method h ad the greatest effect, with muscle from tenderstretched/weighted carcasses having the longest sarcomeres. Injection of an inhibitor caused a signific ant reduction in myofibrillar fragmentation (P < 0.001) and hanging method and ageing period also significantly affected this characteristic (P < 0.05 ). Osmolality of samples aged for 1 day was unaffected by any main effect. The amount of actomyosin (mg/g of muscle sample) extracted from muscles inj ected with the inhibitor was significantly less (P < 0.001) and there was a lso a significant effect (P < 0.05) of portion on this variable. The relati onship between pyrophosphate and the percentage of myosin dissociated from the actomyosin complex was modelled using an exponential function; Y = A - B exp(-kx). Analysis of A, B and k for the 69 samples fitted with an expone ntial function showed that there was no significant effect (P > 0.05) on A and k of any main effect or their interactions. There was, however, a signi ficant effect (P < 0.05) of portion on B and also a significant interaction between injection and hanging method (P < 0.05). It was found that B did n ot explain any additional variance in shear force or myofibrillar fragmenta tion over the main effects and interactions previously found significant. T he inhibitor E-64 was effective at preventing tenderisation indicating the role of cysteine proteases in proteolytic degradation. There was no apparen t effect of dissociation on tenderness as measured in this experiment and t herefore a causal relationship cannot be identified. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.