These results must also be

These results must also be addressed in light of porcine AMPKγ3R200Q (RN−) mutation. Pigs harboring this mutation produce meat with an abnormally low ultimate pH (pH<5.4), usually referred to as Veliparib meat (Copenhafer et al., 2006, Lundström et al., 1998, Milan et al., 2000). This extremely low pH is very close to the isoelectric point of major muscle proteins, thereby causing greater exudate and a lighter colored product, quite unsavory to most consumers (Copenhafer et al., 2006, Scheffler et al., 2011). We have extensively used AMPKγ3R200Q pigs as a model for exaggerated pH decline because we believe that understanding how ultimate pH is reached in their muscle may help us prevent it from occurring in normal muscle. The low ultimate pH has been attributed to the high muscle glycogen of these pigs (Monin & Sellier, 1985). However, we and others have shown that additional substrate above a critical threshold contributes little to additional glycolytic flux (England et al., 2016, Henckel et al., 2002, Lundström et al., 1998, Scheffler et al., 2013). Instead, we have recently shown that low ultimate pH of muscle from AMPKγ3R200Q pigs is mainly due to increase in glycolytic flux, and to a lesser extent to low buffering capacity (Matarneh, England, Scheffler, Oliver, & Gerrard, 2015). Therefore, the underlying mechanism for this condition must work through increasing glycolytic flux. England, Matarneh, Scheffler, Wachet, and Gerrard (2015) showed that muscle from these mutant pigs had greater activity and abundance of the enzyme AMP deaminase, which results in greater AMP levels postmortem. AMP is a potent activator for rate limiting enzymes in the glycolytic pathway (Greaser, 1986), which in turn allows for extended postmortem glycolysis. Additionally, muscle from AMPKγ3R200Q pigs exhibit greater mitochondrial content and enhanced oxidative capacity compared to wild-type pigs (Estrade et al., 1994, Scheffler et al., 2014). Therefore, results of the current study strongly argue that mitochondria play a significant role in determining ultimate pH of the AMPKγ3R200Q pigs. Curiously, however, red muscle (high mitochondria) does not exhibibit this problem but does not exhibit elevated glycolytic enzymes either (England et al., 2016). Collectively, lower ultimate pH of muscle from the AMPKγ3R200Q pigs is likely a function of lower AMP deaminase activity, lower buffering capacity, and greater mitochondrial content.
Implications The results of these studies demonstrate that mitochondria can significantly contribute to postmortem metabolism. Mitochondria enhanced glycogen degradation, lactate accumulation, and pH decline. Our data also indicated that the majority of mitochondrial effect was through increasing the rate of ATP hydrolysis. Further, mitochondria maintained their effect when added after 240min, while myosin ATPase failed to produce similar effect. These data expand our knowledge of how ultimate pH is determined and may change the way postmortem metabolism has been classically viewed. Additionally, the accuracy of modelling to predict postmortem pH decline might be improved by incorporation of the abundance of mitochondria in the model. Finally, AMPKγ3R200Q pig longissimus muscle contains greater mitochondria abundance than wild-type pigs, which may contribute to the lower ultimate pH muscle from these animals. While a promising step forward, further in vivo investigation is necessary to confirm these findings, given the inherent limitation associated of in vitro studies. To that end, we have updated our working model (England et al., 2015, England et al., 2016, England et al., 2014, Matarneh et al., 2015, Matarneh et al., 2017) of those factors controlling the rate and the extent of postmortem pH decline (Fig. 8). The rate of postmortem ATP hydrolysis drives the rate of pH decline (Scopes, 1974), while the ultimate pH is determined by the amount of glycogen present in the muscle at the time of harvest, provided glycogen levels are ≤53μmol/g of muscle (Henckel et al., 2002). If glycogen levels are above this threshold, however, ultimate pH is determined by the amount of substrate traversing PFK-1 before complete inactivation around pH5.5 (England et al., 2014). Further, lower AMPD activity (England et al., 2015) and greater glycolytic capacity (England et al., 2016) and mitochondria (F1 ATPase) content can extend pH decline by promoting more substrate to pass PFK-1. Finally, buffering capacity of a muscle may help to explain variations in ultimate pH between muscles with similar lactate levels (Matarneh et al., 2015).