ABSTRACTMitochondria and sarcoplasmic reticulum were isolated from prerigor bovine muscle, and their anaerobic calcium uptake activities and stabilities were determined at temperatures, pH values, and ATP concentrations selected to simulate conditions in postmortem muscle. Both organelles were relatively active at 1 mM ATP and showed optimum activity at 3 mM. The pH optimum was 7.2 for mitochondria and 6.0–6.5 for sarcoplasmic reticulum; activities of both increased with increasing temperatures. Under simulated postmortem conditions both organelles initially increased in activity, but mitochondria were first to decline below their initial value. Stabilities declined with decreasing, pH values and increasing temperatures, especially above 25°C for mitochondria and above 40°C for sarcoplasmic reticu
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