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To study the relationship between oxidative stress and mitochondrial damage during post-slaughter chilling, M. longissimus dorsi from Tan sheep was treated with the antioxidant N-acetyl-L-cysteine (NAC) solution or physiological saline and stored at 0–4 ℃. After 0, 2, 4, 6 and 8 days, the opening degree of mitochondrial permeability transition pore (MPTP), membrane potential, reactive oxygen species (ROS) content, complex I, II, III and IV activities, the activities of succinate dehydrogenase (SDH), catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) and malondialdehyde (MDA) content were determined, and the mitochondrial structure of myoblasts was observed under transmission electron microscopy (TEM). The results showed that the opening degree of MPTP in both the NAC and control groups gradually increased (P < 0.05) during the postmortem maturation process, and so did the ROS and MDA levels (P < 0.05). The mitochondrial membrane potential and the activities of complexes I, II, III and IV, SDH, CAT, SOD and GSH-Px showed a gradual decreasing trend (P < 0.05). It can be concluded that oxidative stress occurred in the muscle tissue of Tan sheep after slaughter, resulting in a significant increase in ROS content. Excessive ROS, exceeding the ability of the antioxidant defense system to scavenge it, could lead to an imbalance in the redox system, which could in turn accelerate the increase of ROS levels, thereby triggering the opening of MPTP and a decrease in the mitochondrial membrane potential, finally resulting in serious mitochondrial damage. However, NAC alleviated this phenomenon. Increased ROS levels impaired the normal operation of the mitochondrial electron transport chain, thus leading to increased mitochondrial damage. In addition, the lipid oxidation product MDA also damaged the function and structure of mitochondria, causing mitochondrial structural damages such as swelling and deformation, and finally leading to biological dysfunctions.

The aim of this study was to assess the effect of nitric oxide (NO)-mediated mitochondrial apoptosis on Tan sheep meat tenderness during postmortem aging. The hind leg muscle of Tan sheep was injected with 0.8% NaCl (control), 20 mmol/L aminoguanidine hydrochloride (AH) or 90 mmol/L L-arginine (L-Arg) and aged at 4 ℃ for 0, 6, 12, 24, 48, 96, and 192 h. NO and inducible nitric oxide synthase (iNOS) contents, cytochrome c (Cyt c) redox state, oxidative stress level, mitochondrial apoptotic pathways, and tenderness were measured during postmortem aging. The results indicated that at 12 and 24 h after slaughter, NO and iNOS levels were significantly higher, and ATP levels were significantly lower in the L-Arg group than in the control group (P < 0.05). At 24 and 48 h, Cyt c reduction levels and superoxide dismutase (SOD) activity were significantly higher, while the level of reactive oxygen species (ROS) was significantly lower in the AH group than in the control group (P < 0.05). From 12 to 96 h, ATP contents were significantly lower in the L-Arg group than in the control group (P < 0.05). In addition, at 24 h, Ca2+ concentration significantly increased in the L-Arg group compared with the control group (P < 0.05). At all time points except 24 h, the caspase-3 activity in the L-Arg group was significantly higher than that in the control group (P < 0.05). Moreover, it was found that the Bax/Bcl-2 ratio, pH, and myofibril fragmentation index (MFI) in the L-Arg group were significantly higher than those in the control group at all time points after slaughter (P < 0.05). Shear force was significantly lower in the L-Arg group but significantly higher in the AH group than in the control group (P < 0.05). In conclusion, NO exacerbates mitochondrial oxidative stress and directly upregulates ROS levels during the postmortem aging of Tan sheep meat, leading to a decrease in SOD activity, irreversible opening of mitochondrial permeability transition pore (MPTP), and ultimately mitochondrial damage. In addition, by controlling the redox state of Cyt c, NO indirectly activates caspase-3 activity to accelerate apoptotic energy metabolism, deplete ATP and consequently maintain myocyte homeostasis. Furthermore, NO affects the release of Ca2+, up-regulates the Bax/Bcl-2 ratio, and induces the occurrence of mitochondrial apoptotic cascade, ultimately resulting in an increase in MFI and a decrease in shear force and, consequently, improving the tenderness of Tan sheep meat.

In order to investigate the regulatory effect of B-cell lymphoma-2 (Bcl-2) on cell apoptosis and water retention in postmortem Qinchuan beef, the longissimus dorsi muscle of Qinchuan cattle was injected with either 200 μmol/L glycochenodeoxycholic acid (GCDA) or 10 μmol/L 2-ethyl-3-methylpentanamide (ABT-737) solution and aged at 4 ℃ postmortem. After 0, 6, 12, 24, 48, 72 and 120 h, the changes in Bcl-2 and Bcl-2-associated X protein (Bax) contents, the concentrations of endoplasmic reticulum and mitochondrial calcium ion, voltage-dependent anion channel protein 1 (VDAC1) expression, calpain content, the mitochondrial apoptosis pathway and water retention were determined. The results showed that at 6 h after slaughter, the Bcl-2 content of the GCDA-treated group was significantly higher than that of the control group (P < 0.01), the calcium ion concentration in the endoplasmic reticulum was significantly lower than that of the control group (P < 0.01), and cooking loss was significantly lower in both treatment groups than the control group (P < 0.05). At 12 and 24 h post-slaughter, the Bcl-2 content of the ABT-737-treated group was significantly lower than that of the control group (P < 0.01). At 12 and 120 h post-slaughter, the endoplasmic reticulum calcium ion, mitochondrial calcium ion and calpain levels in the ABT-737-treated group were significantly greater than those in the control group (P < 0.01). During 0–120 h post-slaughter, the total expression of VDAC1 in both treatment groups was lower than that in the control group. In summary, during the postmortem aging of Qinchuan beef, under the regulation of Bcl-2, calcium overload occurs through VDAC1-facilitated calcium transport, resulting in the activation of calpain, and the overloading of Ca2+ in mitochondria promotes the activation of caspase-9 and caspase-3, thus inducing cell apoptosis. The higher the degrees of cell apoptosis and caspase activation, the higher the degree of degradation of myofibrillar proteins, and the poorer the water-holding capacity of the meat.