Postmortem proteolysis of structural muscle proteins in the psoas major (PM) is dramatically faster than in the longissiumus dorsi (LD). Previous work has attributed this difference to an increased rate of pH decline and subsequent earlier activation of μ-calpain. However, the mechanisms responsible for the differences in pH decline between muscles are not well understood. Therefore, we hypothesized that early postmortem muscle energy metabolism controls the rate of pH decline. To test this, LD and PM muscle samples (10 g) were harvested at 3, 45, and 1440 min post-stunning from market-weight pigs (108.8 ± 3.8 kg). Muscle metabolites were measured with high-performance liquid chromatography (HPLC). Total muscle phosphate levels and muscle pH were also measured. Titin proteolysis was quantified with vertical agarose gel electrophoresis (VAGE). At 3 min post-stunning, the PM contained less (P < 0.05) ATP and creatine phosphate (CP), but more (P < 0.05) ADP and IMP. At 45 min postmortem, only IMP content was greater (P < 0.05) in the PM and no differences existed at 1440 min. Muscle pH was not different at 3 min postmortem, but was lower (P < 0.05) in the PM at 45 min, and higher (P < 0.05) in the PM at 1440 min. Greater titin proteolysis occurred (P < 0.05) in PM by 1440 min and no differences were detected in total phosphate levels. These data show that energy metabolism and proteolysis in the PM differs from that in LD and suggests that a more rapid pH decline may activate calpain-dependent proteolysis, resulting in increased proteolysis of the PM and a more tender product.
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