Effect of wet-aging on meat quality and exudate metabolome changes in different beef muscles.

The aim of this study was to evaluate meat quality and changes in the meat exudate metabolome of different beef muscles (5 d postmortem, longissimus lumborum and psoas major muscles) during wet-aging (additional 3, 7, 14, 21, and 28 d of aging). Shear force of meat declined significantly (P < 0.001) with aging, meanwhile, increased myofibril fragmentation index, lipid and protein oxidation with aging were observed (P < 0.01). Psoas major (PM) showed significantly higher (P < 0.05) purge loss, centrifugal loss, and cooking loss, as well as higher tenderness and more severe lipid and protein oxidation (P < 0.01) than longissimus lumborum (LL) during aging. Principal component analysis of the metabolomic profiles revealed distinct clusters according to the period of aging and the type of muscle simultaneously. Overabundant amino acids, peptides, oxidized fatty acids, and hydroxy fatty acids were found in long-term aged meat exudates, and forty metabolites were significantly correlated with meat quality characteristics. Fifty-nine metabolites were significantly affected by muscle type. These results demonstrated the potential possibility of evaluating meat quality using meat exudate metabolomics.

The carbon monoxide prodrug oCOm-21 increases Ca2+ sensitivity of the cardiac myofilament.

Patients undergoing cardiopulmonary bypass procedures require inotropic support to improve hemodynamic function and cardiac output. Current inotropes such as dobutamine, can promote arrhythmias, prompting a demand for improved inotropes with little effect on intracellular Ca2+ flux. Low-dose carbon monoxide (CO) induces inotropic effects in perfused hearts. Using the CO-releasing pro-drug, oCOm-21, we investigated if this inotropic effect results from an increase in myofilament Ca2+ sensitivity. Male Sprague Dawley rat left ventricular cardiomyocytes were permeabilized, and myofilament force was measured as a function of -log [Ca2+ ] (pCa) in the range of 9.0-4.5 under five conditions: vehicle, oCOm-21, the oCOm-21 control BP-21, and levosimendan, (9 cells/group). Ca2+ sensitivity was assessed by the Ca2+ concentration at which 50% of maximal force is produced (pCa50 ). oCOm-21, but not BP-21 significantly increased pCa50 compared to vehicle, respectively (pCa50 5.52 vs. 5.47 vs. 5.44; p < 0.05). No change in myofilament phosphorylation was seen after oCOm-21 treatment. Pretreatment of cardiomyocytes with the heme scavenger hemopexin, abolished the Ca2+ sensitizing effect of oCOm-21. These results support the hypothesis that oCOm-21-derived CO increases myofilament Ca2+ sensitivity through a heme-dependent mechanism but not by phosphorylation. Further analyses will confirm if this Ca2+ sensitizing effect occurs in an intact heart.

Non-Canonical Localization of Cardiac Troponins: Expanding Functions or Causing Pathologies?

The troponin complex-consisting of three subunits: troponin C (TnC), cardiac troponin I (cTnI) and cardiac troponin T (cTnT)-plays a key role in the regulation of myocardial contraction. Troponins are preferentially localized in the cytoplasm and bind to myofibrils. However, numerous, albeit scattered, studies have shown the presence of troponins in the nuclei of muscle cells. There is increasing evidence that the nuclear localization of troponins may be functionally important, making troponins an important nuclear player in the pathogenesis of various diseases including cancer and myopathies. Further studies in this area could potentially lead to the development of treatments for certain pathologies. In this review, we collected and discussed recent data on the properties of non-canonically localized cardiac troponins, the molecular mechanisms leading to this non-canonical localization, and the possible functions or pathological effects of these non-canonically localized troponins.

A novel imaging method (FIM-ID) reveals that myofibrillogenesis plays a major role in the mechanically induced growth of skeletal muscle.

An increase in mechanical loading, such as that which occurs during resistance exercise, induces radial growth of muscle fibers (i.e. an increase in cross-sectional area). Muscle fibers are largely composed of myofibrils, but whether radial growth is mediated by an increase in the size of the myofibrils (i.e. myofibril hypertrophy) and/or the number of myofibrils (i.e. myofibrillogenesis) is not known. Electron microscopy (EM) can provide images with the level of resolution that is needed to address this question, but the acquisition and subsequent analysis of EM images is a time- and cost-intensive process. To overcome this, we developed a novel method for visualizing myofibrils with a standard fluorescence microscope (fluorescence imaging of myofibrils with image deconvolution [FIM-ID]). Images from FIM-ID have a high degree of resolution and contrast, and these properties enabled us to develop pipelines for automated measurements of myofibril size and number. After extensively validating the automated measurements, we used both mouse and human models of increased mechanical loading to discover that the radial growth of muscle fibers is largely mediated by myofibrillogenesis. Collectively, the outcomes of this study offer insight into a fundamentally important topic in the field of muscle growth and provide future investigators with a time- and cost-effective means to study it.

Approximately 45% of human body mass is made of skeletal muscle. These muscles contract and relax to provide the mechanical forces needed for breathing, moving, keeping warm and performing many other essential processes. Both sedentary and active adults lose approximately 30-40% of this muscle mass by the age of 80, increasing their risk of disease, disability and death. As a result, there is much interest in developing therapies that can restore, maintain and increase muscle mass in older individuals. Muscles are made of multiple fibers that are in turn largely composed of smaller units known as myofibrils. Previous studies have shown that performing resistance training or other exercise that increases the mechanical loads placed on muscles stimulates muscle growth. This growth is largely due to increased girth of the existing muscle fibers. However, it remained unclear whether this was due to myofibrils growing in size, increasing in number, or a combination of both. To address this question, Jorgenson et al. developed a fluorescence imaging method called FIM-ID to count the number and measure the size of myofibrils within cross-sections of skeletal muscle. Using FIM-ID to study samples of mouse and human muscle fibers then revealed that increasing mechanical loads on muscles increased the number of myofibrils and this was largely responsible for muscle fiber growth. FIM-ID mostly relies on common laboratory instruments and free open-source software is used to count and measure the myofibrils. Jorgenson et al. hope that this will allow as many other researchers as possible to use FIM-ID to study myofibrils in the future. A better understanding of how the body controls the number of myofibrils may lead to the development of therapies that can mimic the effects of exercise on muscles to maintain or even increase muscle mass in human patients.

Protein Kinase D Plays a Crucial Role in Maintaining Cardiac Homeostasis by Regulating Post-Translational Modifications of Myofilament Proteins.

Protein kinase D (PKD) enzymes play important roles in regulating myocardial contraction, hypertrophy, and remodeling. One of the proteins phosphorylated by PKD is titin, which is involved in myofilament function. In this study, we aimed to investigate the role of PKD in cardiomyocyte function under conditions of oxidative stress. To do this, we used mice with a cardiomyocyte-specific knock-out of Prkd1, which encodes PKD1 (Prkd1loxP/loxP; αMHC-Cre; PKD1 cKO), as well as wild type littermate controls (Prkd1loxP/loxP; WT). We isolated permeabilized cardiomyocytes from PKD1 cKO mice and found that they exhibited increased passive stiffness (Fpassive), which was associated with increased oxidation of titin, but showed no change in titin ubiquitination. Additionally, the PKD1 cKO mice showed increased myofilament calcium (Ca2+) sensitivity (pCa50) and reduced maximum Ca2+-activated tension. These changes were accompanied by increased oxidation and reduced phosphorylation of the small myofilament protein cardiac myosin binding protein C (cMyBPC), as well as altered phosphorylation levels at different phosphosites in troponin I (TnI). The increased Fpassive and pCa50, and the reduced maximum Ca2+-activated tension were reversed when we treated the isolated permeabilized cardiomyocytes with reduced glutathione (GSH). This indicated that myofilament protein oxidation contributes to cardiomyocyte dysfunction. Furthermore, the PKD1 cKO mice exhibited increased oxidative stress and increased expression of pro-inflammatory markers interleukin (IL)-6, IL-18, and tumor necrosis factor alpha (TNF-α). Both oxidative stress and inflammation contributed to an increase in microtubule-associated protein 1 light chain 3 (LC3)-II levels and heat shock response by inhibiting the mammalian target of rapamycin (mTOR) in the PKD1 cKO mouse myocytes. These findings revealed a previously unknown role for PKD1 in regulating diastolic passive properties, myofilament Ca2+ sensitivity, and maximum Ca2+-activated tension under conditions of oxidative stress. Finally, we emphasized the importance of PKD1 in maintaining the balance of oxidative stress and inflammation in the context of autophagy, as well as cardiomyocyte function.

Structural maturation of myofilaments in engineered 3D cardiac microtissues characterized using small angle x-ray scattering.

Understanding the structural and functional development of human-induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs) is essential to engineering cardiac tissue that enables pharmaceutical testing, modeling diseases, and designing therapies. Here we use a method not commonly applied to biological materials, small angle x-ray scattering, to characterize the structural development of hiPSC-CMs within three-dimensional engineered tissues during their preliminary stages of maturation. An x-ray scattering experimental method enables the reliable characterization of the cardiomyocyte myofilament spacing with maturation time. The myofilament lattice spacing monotonically decreases as the tissue matures from its initial post-seeding state over the span of 10 days. Visualization of the spacing at a grid of positions in the tissue provides an approach to characterizing the maturation and organization of cardiomyocyte myofilaments and has the potential to help elucidate mechanisms of pathophysiology, and disease progression, thereby stimulating new biological hypotheses in stem cell engineering.

Differential utilisation of subcellular skeletal muscle glycogen pools: a comparative analysis between 1 and 15 min of maximal exercise.

In skeletal muscle, glycogen particles are distributed both within and between myofibrils, as well as just beneath the sarcolemma. Their precise localisation may influence their degradation rate. Here, we investigated how exercise at different intensities and durations (1- and 15-min maximal exercise) with known variations in glycogenolytic rate and contribution from anaerobic metabolism affects utilisation of the distinct pools. Furthermore, we investigated how decreased glycogen availability achieved through lowering carbohydrate and energy intake after glycogen-depleting exercise affect the storage of glycogen particles (size, numerical density, localisation). Twenty participants were divided into two groups performing either a 1-min (n = 10) or a 15-min (n = 10) maximal cycling exercise test. In a randomised, counterbalanced, cross-over design, the exercise tests were performed following short-term consumption of two distinct diets with either high or moderate carbohydrate content (10 vs. 4 g kg-1 body mass (BM) day-1) mediating a difference in total energy consumption (240 vs. 138 g kg-1 BM day-1). Muscle biopsies from m. vastus lateralis were obtained before and after the exercise tests. Intermyofibrillar glycogen was preferentially utilised during the 1-min test, whereas intramyofibrillar glycogen was preferentially utilised during the 15-min test. Lowering carbohydrate and energy intake after glycogen-depleting exercise reduced glycogen availability by decreasing particle size across all pools and diminishing numerical density in the intramyofibrillar and subsarcolemmal pools. In conclusion, distinct subcellular glycogen pools were differentially utilised during 1-min and 15-min maximal cycling exercise. Additionally, lowered carbohydrate and energy consumption after glycogen-depleting exercise altered glycogen storage by reducing particle size and numerical density, depending on subcellular localisation. KEY POINTS: In human skeletal muscle, glycogen particles are localised in distinct subcellular compartments, referred to as intermyofibrillar, intramyofibrillar and subsarcolemmal pools. The intermyofibrillar and subsarcolemmal pools are close to mitochondria, while the intramyofibrillar pool is at a distance from mitochondria. We show that 1 min of maximal exercise is associated with a preferential utilisation of intermyofibrillar glycogen, and, on the other hand, that 15 min of maximal exercise is associated with a preferential utilisation of intramyofibrillar glycogen. Furthermore, we demonstrate that reduced glycogen availability achieved through lowering carbohydrate and energy intake after glycogen-depleting exercise is characterised by a decreased glycogen particle size across all compartments, with the numerical density only diminished in the intramyofibrillar and subsarcolemmal compartments. These results suggest that exercise intensity influences the subcellular pools of glycogen differently and that the dietary content of carbohydrates and energy is linked to the size and subcellular distribution of glycogen particles.

Number of denatured rigor cross-bridges determines the intracellular volume shrinkage in porcine muscle fibre under PSE-inducing condition.

Earlier onset of rigor mortis is a critical physiological progress occurring in the development of pale soft and exudative (PSE) meat. However, how rigor cross-bridges denature under different physiological conditions and their impacts on water-holding capacity remains unclear. To address this scientific question, we firstly established a method to quantify the extent of rigor cross-bridge denaturation using skinned fibres prepared from porcine longissimus thoracis et lumborum muscle. Effects of pH and temperature on the kinetics of rigor cross-bridge denaturation, actomyosin denaturation and shrinkage of muscle fibre were studied. We then manipulated the number of rigor cross-bridges before the denaturation condition was initiated (pH 5.5, 38 °C). Results suggested that the loss of water-holding capacity in PSE meat is determined by the number of denatured rigor cross-bridges. Physiochemical analysis on myofibrils demonstrated that increase in protein oxidation, surface hydrophobicity and loss of electrostatic repulsive force between myofibrils may be involved in the mechanism.

Myofilament-based physiological regulatory compensation preserves diastolic function in failing hearts with severe Ca2+ handling deficits.

Severe dysfunction in cardiac muscle intracellular Ca2+ handling is a common pathway underlying heart failure. Here we used an inducible genetic model of severe Ca2+ cycling dysfunction by the targeted temporal gene ablation of the cardiac Ca2+ ATPase, SERCA2, in otherwise normal adult mice. In this model, in vivo heart performance was minimally affected initially, even though Serca2a protein was markedly reduced. The mechanism underlying the sustained in vivo heart performance in the weeks prior to complete heart pump failure and death is not clear and is important to understand. Studies were primarily focused on understanding how in vivo diastolic function could be relatively normal under conditions of marked Serca2a deficiency. Interestingly, data show increased cardiac troponin I (cTnI) serine 23/24 phosphorylation content in hearts upon Serca2a ablation in vivo. We report that hearts isolated from the Serca2-deficient mice retained near normal heart pump functional responses to ß-adrenergic stimulation. Unexpectedly, using genetic complementation models, in concert with inducible Serca2 ablation, data show that Serca2a-deficient hearts that also lacked the central ß-adrenergic signaling-dependent Serca2a negative regulator, phospholamban (PLN), had severe diastolic dysfunction that could still be corrected by ß-adrenergic stimulation. Notably, integrating a serines 23/24-to-alanine PKA-refractory sarcomere incorporated cTnI molecular switch complex in mice deficient in Serca2 showed blunting of ß-adrenergic stimulation-mediated enhanced diastolic heart performance. Taken together, these data provide evidence of a compensatory regulatory role of the myofilaments as a critical physiological bridging mechanism to aid in preserving heart diastolic performance in failing hearts with severe Ca2+ handling deficits.

Structure of mavacamten-free human cardiac thick filaments within the sarcomere by cryoelectron tomography.

Heart muscle has the unique property that it can never rest; all cardiomyocytes contract with each heartbeat which requires a complex control mechanism to regulate cardiac output to physiological requirements. Changes in calcium concentration regulate the thin filament activation. A separate but linked mechanism regulates the thick filament activation, which frees sufficient myosin heads to bind the thin filament, thereby producing the required force. Thick filaments contain additional nonmyosin proteins, myosin-binding protein C and titin, the latter being the protein that transmits applied tension to the thick filament. How these three proteins interact to control thick filament activation is poorly understood. Here, we show using 3-D image reconstruction of frozen-hydrated human cardiac muscle myofibrils lacking exogenous drugs that the thick filament is structured to provide three levels of myosin activation corresponding to the three crowns of myosin heads in each 429Å repeat. In one crown, the myosin heads are almost completely activated and disordered. In another crown, many myosin heads are inactive, ordered into a structure called the interacting heads motif. At the third crown, the myosin heads are ordered into the interacting heads motif, but the stability of that motif is affected by myosin-binding protein C. We think that this hierarchy of control explains many of the effects of length-dependent activation as well as stretch activation in cardiac muscle control.

Myofibrillar myopathies due to a novel mutation in exon 8 of the LDB3 gene.

Myofibrillar myopathies (MFMs) are a group of genetically heterogeneous diseases affecting the skeletal and cardiac muscles. Myofibrillar myopathies are characterized by focal lysis of myogenic fibers and integration of degraded myogenic fiber products into inclusion bodies, which are typically rich in desmin and many other proteins. Herein, we report a case of a 54-year-old woman who experienced bilateral thigh weakness for over three years. She was diagnosed with MFMs based on muscle biopsy findings and the presence of a novel mutation in exon 8 of the LDB3 gene. Myofibrillar myopathies caused by a mutation in the LDB3 gene are extremely uncommon and often lack distinct clinical characteristics and typically exhibit a slow disease progression. When considering a diagnosis of MFMs, particularly in complex instances of autosomal dominant myopathies where muscle biopsies do not clearly indicate MFMs, it becomes crucial for clinicians to utilize genetic test as a diagnostic tool.

Lower basal and postprandial muscle protein synthesis after 2 weeks single-leg immobilization in older men: No protective effect of anti-inflammatory medication.

Muscle inactivity may reduce basal and postprandial muscle protein synthesis (MPS) rates in humans. Anti-inflammatory treatment alleviates the MPS impairments in younger individuals. The present study explored the influence of nonsteroidal anti-inflammatory drugs (NSAIDs) upon MPS during a period of inactivity in older humans. Eighteen men (age 60-80 years) were allocated to ibuprofen (1200 mg/day, Ibu) or control (Plc) groups. One lower limb was cast immobilized for 2 weeks. Postabsorptive and postprandial MPS was measured before and after the immobilization by L-[ring-13 C6 ]-phenylalanine infusion. The protein expression of select anabolic signaling molecules was investigated by western blot. Basal (0.038 ± 0.002%/h and 0.039 ± 0.005%/h, Plc and Ibu, respectively) and postprandial (0.064 ± 0.004%/h and 0.067 ± 0.010%/h, Plc and Ibu, respectively) MPS rate were higher pre-immobilization compared to basal (0.019 ± 0.005%/h and 0.020 ± 0.010%/h, Plc and Ibu, respectively) and postprandial (0.033 ± 0.005%/h and 0.037 ± 0.006%/h, Plc and Ibu, respectively) MPS rate post-immobilization (p < 0.001). NSAID treatment did not affect the suppression of MPS (p > 0.05). The anabolic signaling were in general reduced after immobilization (p < 0.05). These changes were unaffected by NSAID treatment (p > 0.05). Basal and postprandial MPS dropped markedly after 2 weeks of lower limb immobilization. NSAID treatment neither influenced the reduction in MPS nor the anabolic signaling after immobilization in healthy older individuals.

Solubilization of fish myofibrillar proteins in NaCl and KCl solutions: A DIA-based proteomics analysis.

Understanding the basic solubilization of fish myofibrillar proteins (MPs) in common monovalent chloride solutions is crucial for muscle food processing. In this study, the differential proteomic profiles of MPs during extraction and solubilization in NaCl and KCl solutions were investigated by using advanced four-dimensional data-independent acquisition (4D DIA) quantitative proteomics for the first time. Compared to routine biochemical analysis, this could provide insights into the solubilization of muscle proteins. We ensure the consistency of the effective ionic strength of NaCl and KCl buffers by adjusting the conductivity. The results showed that NaCl extractor mainly facilitated the solubilization of cytoskeletal proteins, biochemical enzymes, and stromal proteins compared to KCl, such as tubulin, myosin-9, collagen, plectin, protein phosphatase, and cathepsin D. However, no significant difference was observed in the extraction of major sarcomeric proteins, including myosin, actin, troponin C, myosin-binding protein C, M-Protein, α-actinin-3, and tropomyosin.

Deficiency of Transcription Factor Sp1 Contributes to Hypertrophic Cardiomyopathy.

BACKGROUND: Hypertrophic cardiomyopathy (HCM) is the most prevalent monogenic heart disorder. However, the pathogenesis of HCM, especially its nongenetic mechanisms, remains largely unclear. Transcription factors are known to be involved in various biological processes including cell growth. We hypothesized that SP1 (specificity protein 1), the first purified TF in mammals, plays a role in the cardiomyocyte growth and cardiac hypertrophy of HCM. METHODS: Cardiac-specific conditional knockout of Sp1 mice were constructed to investigate the role of SP1 in the heart. The echocardiography, histochemical experiment, and transmission electron microscope were performed to analyze the cardiac phenotypes of cardiac-specific conditional knockout of Sp1 mice. RNA sequencing, chromatin immunoprecipitation sequencing, and adeno-associated virus experiments in vivo were performed to explore the downstream molecules of SP1. To examine the therapeutic effect of SP1 on HCM, an SP1 overexpression vector was constructed and injected into the mutant allele of Myh6 R404Q/+ (Myh6 c. 1211C>T) HCM mice. The human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from a patient with HCM were used to detect the potential therapeutic effects of SP1 in human HCM. RESULTS: The cardiac-specific conditional knockout of Sp1 mice developed a typical HCM phenotype, displaying overt myocardial hypertrophy, interstitial fibrosis, and disordered myofilament. In addition, Sp1 knockdown dramatically increased the cell area of hiPSC-CMs and caused intracellular myofibrillar disorganization, which was similar to the hypertrophic cardiomyocytes of HCM. Mechanistically, Tuft1 was identified as the key target gene of SP1. The hypertrophic phenotypes induced by Sp1 knockdown in both hiPSC-CMs and mice could be rescued by TUFT1 (tuftelin 1) overexpression. Furthermore, SP1 overexpression suppressed the development of HCM in the mutant allele of Myh6 R404Q/+ mice and also reversed the hypertrophic phenotype of HCM hiPSC-CMs. CONCLUSIONS: Our study demonstrates that SP1 deficiency leads to HCM. SP1 overexpression exhibits significant therapeutic effects on both HCM mice and HCM hiPSC-CMs, suggesting that SP1 could be a potential intervention target for HCM.

Damage to the orbicularis oculi muscle may impair the development of dermatochalasis.

The purpose of this article is to investigate the changes that occur in the orbicularis oculi muscle (OOM) in patients with dermatochalasis. The OOM specimens from 26 patients were collected during upper eyelid blepharoplasty. Each specimen was divided into three parts, which were then examined using different techniques: formalin embedding for light microscopy, free freezing for histochemical examination, and fixation in 3% glutaraldehyde for electron microscopy. The severity of dermatochalasis was classified according to the anatomical landmarks. 78 specimens from patients with dermatochalasis were evaluated. Under light microscopy, specimens showed an increase in muscle fiber size variation, rounding of muscle fibers, and lobulation of myocytes in a fibrotic background. Under electron microscopy, loss of myofilaments, vacuolar vesicles, and swollen mitochondria were observed, along with osmophilic aggregates resembling nemadine bodies and collagen fibrils. A statistically significant association between the progression of dermatochalasis and the presence of aggregates resembling nemaline bodies was found (p- value < 0.005). Significant changes occur in the OOM in patients with dermatochalasis and the presence of aggregates resembling nemaline bodies is correlated with the degree of eyelid drooping. Thus, OOM may contribute in dermatochalasis progression.

The cardiomyocyte origins of diastolic dysfunction: cellular components of myocardial "stiffness".

The impaired ability of the heart to relax and stretch to accommodate venous return is generally understood to represent a state of "diastolic dysfunction" and often described using the all-purpose noun "stiffness." Despite the now common qualitative usage of this term in fields of cardiac patho/physiology, the specific quantitative concept of stiffness as a molecular and biophysical entity with real practical interpretation in healthy and diseased hearts is sometimes obscure. The focus of this review is to characterize the concept of cardiomyocyte stiffness and to develop interpretation of "stiffness" attributes at the cellular and molecular levels. Here, we consider "stiffness"-related terminology interpretation and make links between cardiomyocyte stiffness and aspects of functional and structural cardiac performance. We discuss cross bridge-derived stiffness sources, considering the contributions of diastolic myofilament activation and impaired relaxation. This includes commentary relating to the role of cardiomyocyte Ca2+ flux and Ca2+ levels in diastole, the troponin-tropomyosin complex role as a Ca2+ effector in diastole, the myosin ADP dissociation rate as a modulator of cross bridge attachment and regulation of cross-bridge attachment by myosin binding protein C. We also discuss non-cross bridge-derived stiffness sources, including the titin sarcomeric spring protein, microtubule and intermediate filaments, and cytoskeletal extracellular matrix interactions. As the prevalence of conditions involving diastolic heart failure has escalated, a more sophisticated understanding of the molecular, cellular, and tissue determinants of cardiomyocyte stiffness offers potential to develop imaging and molecular intervention tools.

Effect of MDA-mediated oxidation on the protein structure and digestive properties of golden pomfret.

In this study, golden pomfret myofibrillar protein (MP) was used as the research object, and the oxidation system of malondialdehyde (MDA) as an inducer and the static digestion model in vitro was established for the analysis of the changes in protein structure and molecular morphology during oxidation and digestion. Subsequently, the effects of MDA-mediated oxidation on the structure and digestive properties of golden pomfret myofibrillar fibrillar protein were determined. The results showed that the hydrolysis degree and digestion rate of MP were inhibited with the increase in MDA concentration (0, 0.5, 1, 2, 5, 10 mmol/L), and the carbonyl group, surface hydrophobicity, irregular curling, and MDA content increased significantly (P < 0.05), whereas the total sulfhydryl groups, α-helices, free amino groups, hydrolysis degree, and MDA incorporation decreased significantly (P < 0.05), The molecular particle size was significantly reduced (P < 0.05), and the molecular morphology and molecular structure were analyzed (P >0.05). Finally, the molecular size and cross-linking degree gradually increased. In conclusion, MDA can alter the structure and morphology of proteins, resulting in a decrease in hydrolysis and digestion rate. This study can provide theoretical support and reference for the regulation of protein digestion.

Oxidation affects pH buffering capacity of myofibrillar proteins via modification of histidine residue and structure of myofibrillar proteins.

The pH buffering capacity is an important functionality of muscle proteins, and muscle foods are susceptible to being oxidized during storage and processing. In order to study the effect of oxidation on the pH buffering capacity of myofibrillar proteins, myofibrils extracted from snakehead fish (Channa argus) were oxidized with H2O2. Results showed that increased oxidation led to loss of free sulfhydryl groups, formation of carbonyl groups, increased surface hydrophobicity, and aggregation of myofibrillar proteins. In addition, there was a significant reduction in the content of histidine in oxidized myofibrillar proteins. The pH buffering capacity of myofibrillar proteins significantly decreased from 3.14 ± 0.03 mM H+/(mL × ΔpH) down to 2.55 ± 0.03 mM H+/(mL × ΔpH) after oxidation with 50 mM H2O2. Both oxidized myofibrillar proteins and histidine showed a high pH buffering capacity at pH near 5.8, which is the histidine pKa value. Here, we hypothesize that oxidation-induced changes in the pH buffering capacity of myofibrillar proteins were driven by oxidative modification of histidine and structural changes of myofibrillar proteins. The significance of this study to food industry may be the awareness that protein oxidation may affect pH through changes in buffering capacity. And the use of antioxidants, especially those targeting at histidine will be promising in addressing this issue.