Modelling post-mortem tenderisation-IV: Role of calpains and calpastatin in conditioning

A generalised model, based on published data, was developed quantifying the mechanism by which the activities of calpains I and II are responsible for post-mortem tenderisation. Tenderisation is proposed to result from the activities of 'free' activated-calpains, the activities of which are controlled by the changes in the calcium ion concentration, the binding of calpains to calpastatin, the inactivations of 'free' activated-calpains and their proteolysis of calpastatin. At the low myoplasmic ('free') calcium ion concentrations prevailing soon after slaughter, calpains are largely 'inert' and little tenderisation occurs. As the pH declines, the 'free' calcium ion concentration rises and activates calpain I: however, most of this activated-calpain I is then bound to calpastatin. With a futher decline in pH, the binding of activated-calpain I to calpastatin is reduced and the level of 'free' activated-calpain I rises and tenderisation increases. A comparable process occurs with calpain II at higher 'free' calcium ion concentrations which occur as the pH declines further. As the level of 'free' activated-calpains rises, their proteolysis of calpastatin also increases, causing a lowering of levels of calpastatin and reducing the inhibition of calphins. Concurrently, 'free' activated-calpains are inactivated. This balance between inhibition, inactivation and activity of calpains and their decrease as the pH declines maintains the proteolytic activity of calpains and produces the gradual process of tenderisation, initially by calpain I and at the later stages mainly by calpain II. Eventually, the activities of calpains decline to zero and tenderisatin stops. Equations were derived to describe the changes from stunning to the completion of conditioning, and parameters were calculated to determine the activities of calpains and tenderisation in beef M. longissimus dorsi.