A mitochondrial protein increases glycolytic flux in muscle postmortem

Abstract Text: Development of meat quality attributes is prominently influenced by the rate and extent of postmortem pH decline. Abnormally low ultimate pH (pH < 5.4) adversely impacts meat color, texture, water holding capacity, and protein content. Postmortem muscle acidification is usually explained by the degradation of muscle glycogen via glycogenolysis and glycolysis to yield lactate and hydrogen ions, which accumulate in the muscle and cause a drop in pH. The role of mitochondria in this process is not well-established; therefore, the purpose of this study was to determine the role of mitochondria in postmortem metabolism. Muscle samples were excised from the longissimus lumborum muscle of six market weight pigs within 5 min of exsanguination. Samples were either immediately frozen in liquid nitrogen or designated for mitochondria isolation. Frozen muscle samples were homogenized into a reaction buffer that mimics postmortem glycolysis. Either 0 or 0.5 mg/ml isolated mitochondria were incorporated into the buffer with or without mitochondrial inhibitors for complexes I, IV, and V. Aliquots were removed at 0, 30, 120 240, and 1440 min for pH and metabolite analysis. Data were analyzed with a mixed model in SAS-JMP and changes over time were determined by repeated-measures. Mitochondria lowered (P < 0.05) pH values at 240 and 1440 min regardless of inhibitors. The reduction in pH was coupled with enhanced (P < 0.05) glycogen degradation, glucose 6-phosphate formation and lactate accumulation. These data indicate that the effect of mitochondria was through increasing the rate of glycolytic flux and not through mitochondrial electron transport chain as indicated by the lack of inhibitors effect; however, the mechanism by which mitochondria enhanced glycolytic flux is still unknown. To study this phenomenon further, mitochondrial samples were homogenized with a polytron then centrifuged and the resulting supernatants were transferred to another tube while pellets were re-suspended again. Mitochondrial supernatants or pellets were added to the in-vitro model to test which compartment was responsible for the effect. Mitochondrial supernatants produced the same effect as preparations including total mitochondria. No effect was observed with the mitochondrial pellet. To further narrow our target of investigation, mitochondrial supernatant proteins were precipitated with perchloric acid (PCA). Supernatants were centrifuged and neutralized before re-testing in the in-vitro system. The effect of mitochondrial supernatant was lost after PCA treatment. These data indicate that a mitochondrial-based protein is capable of increasing glycolytic flux in postmortem muscle.  Additional research in needed to identify the causative agent.

Keywords: Postmortem metabolism, mitochondria, glycolytic flux.