AMSA/ASAS Meat Science and Muscle Biology: Pork Quality

The pH-dependent ATPase activity of myofibrils from muscles varying in fiber type is thought to modulate postmortem muscle metabolism. The objective of this study was to determine the effects various conditions, simulating postmortem conditions, have on myofibrillar ATPase activity of red semitendinosus (RST) and white semitendinosus (WST) porcine muscle. The ATPase activity of myofibrils was measured at 39◦C, 10-4 M Ca2+, and pH ranging from 5.0 to 7.5 at 0.25 unit intervals. To test if decreased pH irreversibly alters myofibrillar ATPase activity, activity was measured using myofibrils incubated 90 min at 39◦C at pH 7.0, then pH 5.0 or 6.0, and back to pH 7.0. Maximum activity was at approximately pH 7.0 with WST myofibrils exhibiting greater (P < .0001) ATPase activity than RST myofibrils (0.621 vs. 0.445 nmole Pi/mg protein/min). Activity of both RST and WST myofibrils declined from maximum rates to approximately zero between pH 6.25 and 5.5. Activity declined to half-maximal levels at pH 5.95 in both RST and WST myofibrils. After incubation at pH 5.0, activity was not recovered in RST or WST myofibrils. Following incubation at pH 6.0, ATPase activity recovered to a higher (P < 0.05) percent of maximum activity in RST than WST myofibrils. These data suggest that postmortem muscle pH and muscle fiber type affect overall myofibrillar ATPase activity and that low pH may irreversibly alter myofibrillar proteins causing a loss in ATPase activity, with this pH sensitivity being more pronounced in WST than RST.