Identification of S-nitrosylated proteins in postmortem pork muscle using modified biotin switch method coupled with isobaric tags

Modification-Specific Proteomic Analysis Reveals Cysteine S-Nitrosylation Mediated the Effect of Preslaughter Transport Stress on Pork Quality Development

This study aimed to explore the effects of preslaughter transport stress on protein S-nitrosylation levels and S-nitrosylated proteome in post-mortem pork longissimus thoracis (LT) muscle. Pigs (N= 16) were randomly divided into 3 h transport (high-stress group, HS) and 3 h transport followed by 3 h resting treatments (low-stress control group, LS). Results demonstrated that high transport stress levels induced nitric oxide (NO) overproduction by promoting NO synthase (NOS) activity and neuronal NOS (nNOS) expression, which thereby notably increased protein S-nitrosylation levels in post-mortem muscle (p < 0.05). Proteomic analysis indicated that 133 S-nitrosylation-modified cysteines belonging to 85 proteins were significantly differential, of which 101 cysteines of 63 proteins were higher in the HS group (p < 0.05). Differential proteins including cytoskeletal and calcium-handling proteins, glycolytic enzymes, and oxidoreductase were mainly involved in the regulation of muscle contraction and energy metabolism that might together mediate meat quality development. Overall, this study provided direct evidence for changes in S-nitrosylation levels and proteome in post-mortem muscle in response to preslaughter transport stress and revealed the potential impact of S-nitrosylated proteins on meat quality.

Identification of Protein-Phenol Adducts in Meat Proteins: A Molecular Probe Technology Study

Plant polyphenols with a catechol structure can form covalent adducts with meat proteins, which affects the quality and processing of meat products. However, there is a lack of fast and effective methods of characterizing these adducts and understanding their mechanisms. This study aimed to investigate the covalent interaction between myofibrillar protein (MP) and caffeic acid (CA), a plant polyphenol with a catechol structure, using molecular probe technology. The CA-MP adducts were separated via sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and detected via Western blot and LC-MS/MS analyses. The Western blot analysis revealed that various specific adducts were successfully enriched and identified as bands around 220 kDa, 45 kDa, and two distinct bands between 95 and 130 kDa. Combined with the LC-MS/MS analysis, a total of 51 peptides were identified to be CA-adducted, corresponding to 31 proteins. More than 80% of the adducted peptides carried one adducted site, and the rest carried two adducted sites. The adducted sites were located on cysteine (C/Cys), histidine (H/His), arginine (R/Arg), lysine (K/Lys), proline (P/Pro), and N-terminal (N-Term) residues. Results showed that the covalent interaction of CA and MP was highly selective for the R side chain of amino acids. Moreover, the adducts were more likely to form via C-N bonding than C-S bonding. This study provides new insights into the covalent interaction of plant polyphenols and meat proteins, which has important implications for the rational use of plant polyphenols in the meat processing industry.

Effect of Nitric Oxide and Its Induced Protein S-Nitrosylation on the Structures and In Vitro Digestion Properties of Beef Myofibrillar Protein

This study aimed to investigate the effects of nitric oxide (NO) and its induced protein S-nitrosylation on the structures and digestion properties of beef myofibrillar protein (MP). The MP was treated with 0, 50, 250, 500, and 1000 μM concentrations of NO-donor S-nitrosoglutathione (GSNO) for 30 min at 37 °C. The results indicated that GSNO treatment significantly decreased the sulfhydryl contents whereas the carbonyl contents increased. Meanwhile, compared with the control group, the surface hydrophobicity, the intrinsic fluorescence intensity, and the α-helix content of proteins were decreased significantly with the enhancement of GSNO concentrations. In addition, 250 μM GSNO treatment increased the gastric digestibility of MP, while the gastrointestinal digestibility and the release of peptides were both inhibited by 500 and 1000 μM GSNO treatments. These data demonstrate that protein S-nitrosylation can affect the in vitro digestion properties of proteins by altering the physicochemical properties and structure of MP.

Protein S-Nitrosylation Regulates Postmortem Beef Apoptosis through the Intrinsic Mitochondrial Pathway

The objective of the present study was to investigate the regulatory mechanism of protein S-nitrosylation on early postmortem beef muscle apoptosis. Beef semimembranosus (SM) muscles at 45 min postmortem were treated with nitric oxide (NO) donor, control (NaCl solution), or nitric oxide synthase (NOS) inhibitor for 24 h at 4 °C. Bcl-2 expression and mitochondrial membrane potential were significantly increased by the NO donor treatment at 6 h postmortem, while the NOS inhibitor group exhibited a lower Bcl-2 level and mitochondrial membrane potential in comparison with the control (P < 0.05). The cytochrome c expression analysis highlighted that NO donor incubation repressed cytochrome c release from mitochondria to the cytoplasm. Further, S-nitrosylation levels of caspase-3 and caspase-9 were elevated after incubation with the NO donor (P < 0.05), leading to decreased caspase-3 and caspase-9 activities (P < 0.05). The aforementioned findings imply that protein S-nitrosylation mediates postmortem apoptosis of beef SM through the mitochondrial apoptotic pathway.

Tandem Mass Tag Labeling-Based Analysis to Characterize Muscle-Specific Proteome Changes during Postmortem Aging of Bison Longissimus Lumborum and Psoas Major Muscles

The objective of the study was to examine the variations in sarcoplasmic proteomes of bison longissimus lumborum (LL) and psoas major (PM) muscles during postmortem aging utilizing tandem mass tag (TMT) isobaric labeling coupled with liquid chromatography mass-spectrometry (LC-MS/MS) for the categorization of muscles with muscle-specific inherent color stability. A total of 576 proteins were identified (P < 0.05) in both bison LL and PM muscles, where 97 proteins were identified as differentially abundant (fold change > 1.5, P < 0.05) from the three comparisons between muscles during postmortem aging periods (PM vs LL at 2 d, 7 d and 14 d). Among those proteins, the most important protein groups based on functions are related to electron transport chain (ETC) or oxidative phosphorylation, tricarboxylic acid cycle (TCA), ATP transport, carbohydrate metabolism, fatty acid oxidation, chaperones, oxygen transport, muscle contraction, calcium signaling, and protein synthesis. In PM, most of the proteins from ETC, TCA cycle, fatty acid oxidation, ATP and oxygen transport, and muscle contraction were more abundant or exhibited increased expression during aging compared to LL. On the other hand, the proteins involved in carbohydrate metabolism, chaperone function and protein synthesis mostly exhibited decreased expression in PM muscle relative to LL. These results clearly demonstrate that the proteins associated with oxidative metabolism showed increased expression in PM muscles. This indicates that oxidative damage or subsequent color deterioration resulted in bison PM muscles being attacked by the reactive oxygen species produced during those metabolic process. In contrast, proteins involved in glycolysis and chaperone activity exhibited a decrease in expression in bison PM muscles, resulting decline in color stability compared with LL. Because glycolytic enzymes and chaperones maintain oxidative and/or color stability by producing reducing equivalents in glycolytic pathway and with the protein folding ability of chaperones, respectively in LL muscles.

Meat tenderness: advances in biology, biochemistry, molecular mechanisms and new technologies

Meat tenderness is an important quality trait critical to consumer acceptance, and determines satisfaction, repeat purchase and willingness-to-pay premium prices. Recent advances in tenderness research from a variety of perspectives are presented. Our understanding of molecular factors influencing tenderization are discussed in relation to glycolysis, calcium release, protease activation, apoptosis and heat shock proteins, the use of proteomic analysis for monitoring changes, proteomic biomarkers and oxidative/nitrosative stress. Each of these structural, metabolic and molecular determinants of meat tenderness are then discussed in greater detail in relation to animal variation, postmortem influences, and changes during cooking, with a focus on recent advances. Innovations in postmortem technologies and enzymes for meat tenderization are discussed including their potential commercial application. Continued success of the meat industry relies on ongoing advances in our understanding, and in industry innovation. The recent advances in fundamental and applied research on meat tenderness in relation to the various sectors of the supply chain will enable such innovation.

Myoglobin Post-Translational Modifications Influence Color Stability of Beef Longissimus Lumborum

Post-translational modifications (PTM) of proteins play critical roles in biological processes. PTM of muscle proteins influence meat quality. Nonetheless, myoglobin (Mb) PTM and their impact on fresh beef color stability have not been characterized yet. Therefore, our objectives were to identify Mb PTM in beef longissimus lumborum muscle during postmortem aging and to characterize their influence on color stability. The longissimus lumborum muscles from 9 (n = 9) beef carcasses (24 h postmortem) were subjected to wet aging for 0, 7, 14, and 21 d. At the end of each wet-aging period, steaks were fabricated. One steak for analyses of PTM was immediately frozen at −80°C, whereas other steaks were assigned to refrigerated storage in the darkness under aerobic packaging. Instrumental color and biochemical attributes were evaluated on day 0, 3, or 6 of storage. Mb PTM were analyzed using two-dimensional electrophoresis and tandem mass spectrometry. Surface redness (a* value), color stability, and Mb concentration decreased (P &lt; 0.05) upon aging. Gel image analyses identified 6 Mb spots with similar molecular weight (17 kDa) but different isoelectric pH. Tandem mass spectrometry identified multiple PTM (phosphorylation, methylation, carboxymethylation, acetylation, and 4-hydroxynonenal alkylation) in these 6 isoforms. The amino acids susceptible to phosphorylation were serine (S), threonine (T), and tyrosine, whereas other PTM were detected in lysine (K), arginine (R), and histidine residues. Additionally, distal histidine (position 64), critical to heme stability, was found to be alkylated. Overall, Mb PTM increased with aging. The aging-induced PTM, especially those occurring close to hydrophobic heme pocket, could disrupt Mb tertiary structure, influence heme affinity, and compromise oxygen binding capacity, leading to decreased color stability of fresh beef. Furthermore, PTM at K45, K47, and K87 were unique to Mb from non-aged beef, whereas PTM at R31, T51, K96, K98, S121, R139, and K147 were unique to Mb from aged counterparts, indicating that these Mb PTM could be used as novel biomarkers for fresh beef color stability.

Effect of nitric oxide on myofibrillar proteins and the susceptibility to calpain-1 proteolysis

This study was designed to investigate the nature of modification of myofibrillar proteins by nitric oxide (NO) and the extent to which S-nitrosylation alters their susceptibility to calpain-1 proteolysis. Isolated myofibrils from porcine semimembranosus muscle were incubated with the NO donor S-nitrosoglutathione (GSNO) at 0, 20, 50, 250, 1000 µM for 30 min at 37 °C and then incubated with purified calpain-1. GSNO treatment decreased the thiol content of myofibrillar proteins and increased their intensity and amount of S-nitrosylation. GSNO caused the formation of proteins cross-linkage through intermolecular disulfide. More desmin and titin (T2, the degraded fragment of original titin) were degraded by calpain-1 when myofibrils were incubated with 1000 µM GSNO. Incubation with 250 and 1000 µM GSNO suppressed calpain-1-catalyzed cleavage of troponin-T. The data suggest that NO could change the redox state of myofibrillar proteins and subsequently affect the extent of proteolysis by calpain-1 in a protein-dependent manner.

A bioinformatics study on characteristics, metabolic pathways, and cellular functions of the identified S-nitrosylated proteins in postmortem pork muscle

This study aimed to determine the characteristics, metabolic pathways and cellular functions of S-nitrosylated proteins from pork postmortem muscle using bioinformatics analysis. The results showed that S-nitrosylated proteins had a broad range of molecular weight and pI value and were mainly located in the functional region of secondary structure. The motif revealed the lysine (K) positioned at -5, -7, +1 and +5 through the S-nitrosocysteine while "C-X-X-C" was identified as the motif for non-S-nitrosylation-modified cysteine. The proteins were widely localized in cell compartments and mostly belonged to enzymes participating in the metabolic process. Glycolysis was the most significant pathways of S-nitrosylated proteins in postmortem muscle. The cell death of muscle cells was predicted to be inhibited by S-nitrosylation with the potential influence on the apoptosis. Those identified pathways and cellular functions of S-nitrosylation are proposed to have a profound influence on meat quality and should be highly regarded.