Comparing Heat Shock Proteins in Angus and Brahman Cattle and their Effect On Tenderness

ObjectivesHeat shock proteins (HSP) are biomarkers of stress and perform chaperoning functions to fold, unfold, and refold proteins after heat stress. Brahman are more heat tolerant than Angus, while Angus beef has been associated with greater palatability than Brahman. The objectives were to determine if HSP content in the longissimus lumborum differs between Angus and Brahman and examine how HSP content relates to the eating quality of beef.Materials and MethodsAngus and Brahman steers (n = 12 per breed) were finished during summer in Florida and harvested at approximately 17 mo of age. Samples of longissimus lumborum were collected at 1 h after exsanguination and were immediately immersed in liquid nitrogen. Samples were then pulverized, diluted in extraction buffer, and homogenized. The protein samples were assayed to assess protein concentration and subsequently diluted to equal concentrations for loading into acrylamide gels. Proteins were separated by gel electrophoresis, and western blotting was used to evaluate content of aβ-crystallin, HSP27, HSP60, HSP70, and HSP90. Target bands were detected and quantified using LI-COR Odyssey and target signal was normalized to total protein stain. Tenderness was evaluated in 14d-aged steaks using Warner-Bratzler shear force (WBSF) and a trained sensory panel. Data were analyzed using one-way ANOVA and Pearson correlations were conducted for content of HSPs and objective and subjective tenderness.ResultsHSP27, HSP60, and HSP70 did not differ between breeds (P > 0.05); however, HSP90 and aβ-crystallin were greater (P = 0.005) in the longissimus lumborum of Angus compared to Brahman. Even though WBSF did not differ (P = 0.29) between breeds, breed affected (P < 0.0001) sensory tenderness. Content of aβ-crystallin was associated with sensory tenderness (r2 = 0.52, P = 0.0098).ConclusionLongissimus dorsi from the Angus were contained more aβ-crystallin and HSP90 than Brahman. Elevated concentrations of both aβ-crystallin and HSP90 could be breed related or may have been influenced by the season they were harvested. While WBSF was not affected by breed, panelists rated Angus steaks as more tender after aging for 14d. Content of aβ-crystallin is associated with tenderness; however further work is necessary to determine if this small HSP affects proteolysis.

Early Postmortem Metabolism and Protease Activation in Bovine Muscles

ObjectivesThe conversion of muscle to meat is largely controlled by postmortem energy metabolism and pH decline. These biochemical changes influence activity of enzymes implicated in proteolysis and meat tenderization. Therefore, our objective was to investigate pH decline, muscle energy metabolism, and protease activation in functionally distinct bovine muscles.Materials and MethodsSteers (n = 6) were harvested at approximately 18.5 mo and 630 kg live weight. Samples from the longissimus lumborum (LL) and diaphragm (Dia) were taken at 1, 3, and 24h postmortem, immediately frozen using liquid nitrogen, and stored in ultra-freezer until analysis. Muscle pH was obtained using a pH meter at the same time points. Myosin heavy chain composition (I, IIa, and IIx) was determined using gel electrophoresis. Substrate (residual glycogen), as well as glycolytic metabolites, glucose, glucose-6-phosphate, and lactate, were quantified by enzymatic methods; muscle ATP at 1 and 3h was also determined. Western blotting was used to evaluate protease activation (calpain-1 and caspase-3). Data were analyzed using a randomized block design, with slaughter date as block. Animal within slaughter date was considered as random effect and fixed effects of muscle, time, and the interaction tested. Time was considered a repeated measure.ResultsDiaphragm contained a greater percentage of slow myosin heavy chain compared to LL (80% vs. 12%, respectively). Consistent with fiber type, LL contained greater glycogen than Dia at 1h (P < 0.05), but not at subsequent times postmortem. Overall, a greater decline in glycogen occurred in LL. Accordingly, lactate concentration increased markedly in LL postmortem and to a lesser extent in Dia (interaction effect; P < 0.01). Although muscles exhibited similar lactate content at 1h, at 24h the LL showed elevated lactate relative to Dia (88 vs. 53 µmol/g tissue, respectively). Accumulation of glucose and glucose-6-phosphate were affected by muscle (P < 0.01) and time (P < 0.01), with greater final content in LL compared to Dia. Muscles exhibited different patterns of postmortem pH decline (muscle × time, P < 0.0001). Initially, pH of LL was higher than Dia (P < 0.01) and remained different at 3h (P < 0.05); but by 24h, pH values were similar. Content of ATP was influenced by muscle (P < 0.01) and time (P < 0.01). Initial ATP was greater (P < 0.01) in LL than in Dia and remained greater (P = 0.002) at 3h postmortem. From 1 to 24h, the pattern of calpain autolysis differed between muscles (interaction effect; P = 0.01). Calpain-1 autolysis was similar at all times in Dia, whereas autolysis increased in LL from 3h to 24h postmortem. Caspase-3 was identified by one band (32 kDa) that represents the zymogen (procaspase-3). Procaspase-3 content is affected by muscle (P < 0.01), with Dia containing greater content than LL.ConclusionAlthough the Dia is considered a slow muscle, it exhibited a more rapid pH decline and lower ATP levels than LL early postmortem. These parameters were expected to coincide with more rapid calpain-1 autolysis in Dia, but this was not the case. Further work is necessary to understand the interaction between pH decline, muscle type, and postmortem proteolysis.

Mitochondrial Function in Oxidative and Glycolytic Bovine Skeletal Muscle Postmortem

ObjectivesMitochondrial function in postmortem muscle is affected by decreasing oxygenation. Functional properties relating to energy production and integrity of mitochondria may influence development of meat quality characteristics. Therefore, the objective was to evaluate changes in mitochondrial function in oxidative and glycolytic muscles during the first 24h postmortem.Materials and MethodsSteers (n = 6) of primarily Angus (80 to 100%) genetics were harvested at approximately 18.5 mo and 630 kg live weight. Samples from the longissimus lumborum (LL) and diaphragm (Dia) were collected at 1, 3, and 24h postmortem. Fresh-preserved muscle samples were permeabilized using saponin, and muscle bundles (2–4 mg) were transferred to a high-resolution oxygraph for respiration measurements (oxygen consumption rate, OCR, pmol/sec/mg of tissue). Samples were assessed in duplicate under hyperoxia. First, pyruvate and malate were added to support the TCA cycle and assess leak respiration. Then, ADP was added to support electron flow through complex I. The influence of glutamate on NADH production (complex I) was tested, followed by complex II activation by succinate. Integrity of the mitochondria outer membrane was tested with cytochrome c. Next, an uncoupler (FCCP) was added to force the electron transport system (ETS) to maximum capacity. Citrate synthase (CS) activity (nmol/min/mg tissue) was determined in frozen samples and used as a marker of mitochondria content. Subsequently, respiration data were normalized to CS activity (pmol/sec/U CS) to account for differences in mitochondria content. Coupling efficiency of oxidative phosphorylation was calculated as 1– (Leak/ADP-stimulated oxidative phosphorylation capacity). Raw and normalized OCR were analyzed in a randomized block design, with slaughter date as block and fixed effects of muscle, time, and the interaction. Time was considered a repeated measure.ResultsMuscle type affected (P = 0.0002) leak OCR, with Dia showing a higher rate than LL. After ADP was added, mitochondria from Dia exhibited higher OCR at all times tested and at all steps, with OCR being 4 times higher after FCCP addition. Mitochondrial content, evidenced by greater (P < 0.0001) CS activity in Dia, largely explained differences in OCR between muscles. After OCR was normalized to CS activity, the 1 and 3h postmortem OCR from Dia and LL were similar (P > 0.05). However, at 24h postmortem, OCR after ADP, glutamate, and FCCP additions were greater (P < 0.05) in Dia mitochondria. Time, but not muscle, affected cytochrome c response. At 1h postmortem, cytochrome c increased OCR by 6.6%, supporting that mitochondria outer membrane integrity is not compromised. However, cytochrome c response at 3h postmortem increased 52.4%, indicating outer membrane damage. Coupling efficiency is different between muscles (P = 0.005) with Dia exhibiting greater efficiency.ConclusionDespite inherent metabolic differences between the LL and Dia, mitochondria from both muscles are intact and coupled at 1h postmortem. However, by 24h postmortem, functional properties of LL mitochondria are reduced compared to Dia. Declining mitochondrial function may be associated with calcium overload, mitochondrial fragmentation, and protease activation.