Early postmortem metabolism and protease activation in contrasting bovine muscles

Abstract

Muscle to meat conversion is influenced by muscle properties and metabolism. Fiber type profile impacts glycolytic capacity as well as protein turnover rate in vivo. Our objective was to investigate protease content and activation during the early postmortem period using muscles with known divergent metabolism. Samples from longissimus lumborum (LL) and diaphragm (Dia) were taken from predominantly Angus steer carcasses (n = 6) at 1, 3, and 24h postmortem and frozen. Myosin heavy chain (MyHC) isoforms, ATP, glycogen, glucose, glucose-6-phosphate (G6P), and lactate concentrations were determined. Procaspase-3, calpain-1, calpastatin, desmin, and troponin-T were assessed by immunodetection. Fixed effects of muscle (m), time postmortem (t) and the interaction (m × t) were investigated, and least square means were separated by Bonferroni test at 5% significance. Muscles showed contrasting MyHC profiles, with LL represented primarily by IIx and IIa isoforms (~ 88%) whereas Dia contained mostly (80%) type I isoform. Glycogen degradation was more pronounced in LL and coincided with more rapid accumulation of glucose and lactate (P < 0.01). Procaspase-3 content was influenced by muscle (m: P < 0.01), being greater in Dia. Fragments indicating activation of procaspase-3 postmortem were not detected. Calpain-1 autolysis and intact calpastatin (135 kDa) content were influenced by muscle and time (m × t: P < 0.01 and P < 0.01, respectively). Calpastatin fragmentation postmortem was not associated with greater procaspase-3 content. Fast glycolytic LL displayed faster protease activation and greater proteolysis during the first 24h postmortem.