Effect of Oxidation and Protein Unfolding on Cross-Linking of β-Lactoglobulin and α-Lactalbumin

Protein oxidation and its effect on functional properties of livestock products during the processing and storage: A review

Protein oxidation is a complex chemical process that pervades the entirety of the food domain. It is governed by two primary mechanisms: the direct oxidation by active entities and the indirect oxidation by secondary oxidation byproducts like lipid oxidation, influenced by many factors. The oxidation of proteins in livestock products readily occurs post-processing and storage through techniques such as freezing, cooking, ultrasonication, among others, leading to protein carbonylation and subsequent alterations in structure. Consequently, the purpose of this manuscript is to scrutinize the impacts of conventional processing and storage methodologies on protein oxidation in livestock products, delineating potential mechanisms, action sites, and influential factors implicated in this progression. Additionally, we delve into the ramifications of protein oxidation on the processing attributes of livestock products, while venturing into forthcoming trends and obstacles to set a groundwork for ensuring and regulating the caliber of these commodities.

Goat Milk Exhibits a Higher Degree of Protein Oxidation and Aggregation than Cow Milk During Cold Storage

Due to their markedly distinct protein compositions and structures, goat milk and cow milk display substantially different characteristics. In this study, the quality and composition of goat milk and cow milk were studied after being refrigerated at 4 °C for 7 days, with a particular focus on protein oxidation and aggregation states. The results revealed that alongside increases in acidity, microbial colony count, and hydrolysis, there was a significant change in the protein aggregation state beginning on the second day. This change was characterized by increased turbidity, an elevated centrifugal sedimentation rate, and a right-shifted particle size distribution. After seven days of refrigeration, the centrifugal sedimentation rate of goat milk increased from 0.53% to 0.97%, whereas that of cow milk rose from 0.41% to 0.58%. The degree of aggregation was significantly greater in goat milk compared to cow milk. Additionally, both protein and lipids exhibited substantial oxidation, with the degree of oxidation more pronounced in goat milk than in cow milk. The malondialdehyde (MDA) content increased from 0.047 μg/mL to 0.241 μg/mL in goat milk and from 0.058 μg/mL to 0.178 μg/mL in cow milk. The results suggest that goat milk was more prone to oxidation, which further reduced its stability. Therefore, in the storage and transportation of dairy products before processing, it is essential not only to monitor sanitary conditions but also to effectively control protein oxidation to enhance the quality of milk processing.

Effect of chickpea protein modified with combined heating and high-pressure homogenization on enhancing the gelation of reduced phosphate myofibrillar protein

The effects of chickpea protein (CP) modified by heating and/or high-pressure homogenization (HPH) on the gelling properties of myofibrillar protein under reduced phosphate conditions (5 mM sodium triphosphate, STPP) were investigated. The results showed that heating and HPH dual-modified CP could decrease the cooking loss by 29.57 %, elevate the water holding capacity by 17.08 %, and increase the gel strength by 126.88 %, which conferred myofibrillar protein with gelation performance comparable with, or even surpassing, that of the high-phosphate (10 mM STPP) control. This gelation behavior improvement could be attributed to enhanced myosin tail-tail interactions, decreased myosin thermal stability, elevated trans-gauche-trans disulfide conformation, strengthened hydrophobic interactions and hydrogen bonding, the uncoiling of α-helical structures, the formation of well-networked myofibrillar protein gel, and the disulfide linkages between the myosin heavy chain, actin, and CP subunits. Therefore, the dual-modified CP could be a promising phosphate alternative to develop healthier meat products.

Insulin oxidation and oxidative modifications alter glucose uptake, cell metabolism, and inflammatory secretion profiles

Insulin participates in glucose homeostasis in the body and regulates glucose, protein, and lipid metabolism. Chronic hyperglycemia triggers oxidative stress and the generation of reactive oxygen species (ROS), leading to oxidized insulin variants. Oxidative protein modifications can cause functional changes or altered immunogenicity as known from the context of autoimmune disorders. However, studies on the biological function of native and oxidized insulin on glucose homeostasis and cellular function are lacking. Native insulin showed heterogenous effects on metabolic activity, proliferation, glucose carrier transporter (GLUT) 4, and insulin receptor (INSR) expression, as well as glucose uptake in cell lines of five different human tissues. Diverse ROS compositions produced by different gas plasma approaches enabled the investigations of variously modified insulin (oxIns) with individual oxidative post-translational modification (oxPTM) patterns as identified using high-resolution mass spectrometric analysis. Specific oxIns variants promoted cellular metabolism and proliferation in several cell lines investigated, and nitrogen plasma emission lines could be linked to insulin nitration and elevated glucose uptake. In addition, insulin oxidation modified blood glucose levels in the chicken embryos (in ovo), underlining the importance of assessing protein oxidation and function in health and disease.

UVB pretreatment of β-lactoglobulin affects the temperature-induced formation of functional amyloid-like aggregates and promotes oxidative degradation

Unfolding in combination with or without acid hydrolysis is crucial for the formation of functional amyloid (fibrillar) or amyloid-like (worm-like) β-lactoglobulin (BLG) aggregates, which can be induced through temperature treatment for several hours at pH 2-4. A preceding conformational destabilization of BLG might affect its aggregation. We investigated ultraviolet (UV) B radiation as conformational perturbing treatment to facilitate temperature-induced protein aggregation. 2-h UVB pretreated BLG (UV-BLG) exhibited an accelerated worm-like aggregation at pH 3.5, while at pH 2 the formation of fibrils was decelerated. The UV-induced conformational destabilization lowered the thermal stability and thus facilitates unfolding during thermal treatment. Thereby, the formation of covalent and non-covalent intermolecular interactions was favored, which promoted assembly of intact proteins resulting in worm-like aggregates. The oxidative degradation of UV-BLG was suggested to alter fibrillation-prone protein regions and thereby impede peptide assembly.

Specific detection of protein carbonylation sites by 473 nm photodissociation mass spectrometry

The study of protein oxidation remains a challenge despite the biomedical interest in reliable biomarkers of oxidative stress. This is particularly true for carbonylations although, recently, liquid chromatography-mass spectrometry techniques (LC-MS) have been proposed to detect this non-enzymatic and poorly distributed oxidative modification of proteins using untargeted or carbonyl-reactive probe methods. These methods proved to be feasible but could not preserve the dynamic range of the protein sample, making it impossible to quantify oxidatively modified proteoforms compared with native proteoforms. Here, we propose an innovative method based on the implementation of a reactive carbonyl probe conjugated with a laser-sensitive chromophore, dabcyl-aminooxy, which confers optical specificity to the LC-MS approach. In addition, our protein carbonyl detection method allows us to localize individual carbonylation sites by observing fragments of derivatized oxidized peptides. Two model proteins, alpha-synuclein and beta-lactoglobulin, were oxidized and carbonylation sites were detected, resulting in the identification of respectively 34 and 77 different carbonylated amino acids. Thus, we demonstrated the application of a direct and sensitive method for studying protein carbonylation sites in complex protein extracts.

Multi-Oxidant Environment as a Suicidal Inhibitor of Myeloperoxidase

Tissue inflammation drives the infiltration of innate immune cells that generate reactive species to kill bacteria and recruit adaptive immune cells. Neutrophil activation fosters the release of myeloperoxidase (MPO) enzyme, a heme-containing protein generating hypochlorous acid (HOCl) from hydrogen peroxide (H2O2) and chloride ions. MPO-dependent oxidant formation initiates bioactive oxidation and chlorination products and induces oxidative post-translational modifications (oxPTMs) on proteins and lipid oxidation. Besides HOCl and H2O2, further reactive species such as singlet oxygen and nitric oxide are generated in inflammation, leading to modified proteins, potentially resulting in their altered bioactivity. So far, knowledge about multiple free radical-induced modifications of MPO and its effects on HOCl generation is lacking. To mimic this multi-oxidant microenvironment, human MPO was exposed to several reactive species produced simultaneously via argon plasma operated at body temperature. Several molecular gas admixes were used to modify the reactive species type profiles generated. MPO was investigated by studying its oxPTMs, changes in protein structure, and enzymatic activity. MPO activity was significantly reduced after treatment with all five tested plasma gas conditions. Dynamic light scattering and CD-spectroscopy revealed altered MPO protein morphology indicative of oligomerization. Using mass spectrometry, various oxPTMs, such as +1O, +2O, and +3O, were determined on methionine and cysteine (Cys), and -1H-1N+1O was detected in asparagine (Asp). The modification types identified differed between argon-oxygen and argon-nitrogen plasmas. However, all plasma gas conditions led to the deamidation of Asp and oxidation of Cys residues, suggesting an inactivation of MPO due to oxPTM-mediated conformational changes.

Oxidant-modified amylin fibrils and aggregates alter the inflammatory profile of multiple myeloid cell types, but are non-toxic to islet β cells

Diabetes mellitus currently affects ∼10% of the population worldwide, with Type 2 predominating, and this incidence is increasing steadily. Both Type 1 and 2 are complex diseases, involving β-cell death and chronic inflammation, but the pathways involved are unresolved. Chronic inflammation is characterized by increased oxidant formation, with this inducing protein modification, altered function and immunogenicity. Amylin, a peptide hormone co-secreted with insulin by β-cells, has attracted considerable interest for its amyloidogenic properties, however, the effects that oxidants have on amylin aggregation and function are poorly understood. Amylin was exposed in vitro to hypochlorous acid, hydrogen peroxide and peroxynitrous acid/peroxynitrite to investigate the formation of post-translational oxidative modifications (oxPTMs, via mass spectrometry) and fibril formation (via transmission electron microscopy). Amylin free acid (AFA) was also examined to investigate the role of the C-terminal amide in amylin. Oxidant exposure led to changes in aggregate morphology and abundance of oxPTMs in a concentration-dependent manner. The toxicity and immunogenic potential of oxidant-modified amylin or AFA on pancreatic islet cells (INS-1E), human monocyte cell line (THP-1) and monocyte-derived dendritic cells (moDCs) were examined using metabolic activity and cytokine assays, and flow cytometry. No significant changes in vitality or viability were detected, but exposure to oxidant-modified amylin or AFA resulted in altered immunogenicity when compared to the native proteins. THP-1 and moDCs show altered expression of activation markers and changes in cytokine secretion. Furthermore, oxidant-treated amylin and AFA promoted maturation of THP-1 and pre-mature moDCs, as determined by changes in size, and maturation markers.

Unaided efficient transglutaminase cross-linking of whey proteins strongly impacts the formation and structure of protein alginate particles

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Microbial transglutaminase (MTG) cross-linked >70% β-lactoglobulin (β-Lg) at pH 8.5.

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Initial MTG catalyzed isopeptide bond formation caused partial unfolding of β-Lg.

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>75% of whey protein cross-linked, forming hetero-polymers containing β-Lg.

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50% less alginate is needed to form particles with cross-linked than with native β-Lg.

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Cross-linked β-Lg and alginate formed suspendable hydrophobically driven particles.

There is a dogma within whey protein modification, which dictates the necessity of pretreatment to enzymatic cross-linking of β-lactoglobulin (β-Lg). Here microbial transglutaminase (MTG) cross-linked whey proteins and β-Lg effectively in 50 mM NaHCO₃, pH 8.5, without pretreatment. Cross-linked β-Lg spanned 18 to >240 kDa, where 6 of 9 glutamines reacted with 8 of 15 lysines. The initial isopeptide bond formation caused loss of β-Lg native structure with t_1/2 = 3 h, while the polymerization occurred with t_1/2 = 10 h. Further, cross-linking effects on protein carbohydrate interaction have been overlooked, leaving a gap in understanding of these complex food matrices. Complexation with alginate showed that β-Lg cross-linking decreased onset of particle formation, hydrodynamic diameter, stoichiometry (β-Lg/alginate) and dissociation constant. The complexation was favored at higher temperatures (40 °C), suggesting that hydrophobic interactions were important. Thus, β-Lg was cross-linked without pretreatment and the resulting polymers gave rise to altered complexation with alginate.

Chemical Stability of Proteins in Foods: Oxidation and the Maillard Reaction

Protein is a major nutrient present in foods along with carbohydrates and lipids. Food proteins undergo a wide range of modifications during food production, processing, and storage. In this review, we discuss two major reactions, oxidation and the Maillard reaction, involved in chemical modifications of food proteins. Protein oxidation in foods is initiated by metal-, enzyme-, or light-induced processes. Food protein oxidation results in the loss of thiol groups and the formation of protein carbonyls and specific oxidation products of cysteine, tyrosine, tryptophan, phenylalanine, and methionine residues, such as disulfides, dityrosine, kynurenine, m-tyrosine, and methionine sulfoxide. The Maillard reaction involves the reaction of nucleophilic amino acid residues with reducing sugars, which yields numerous heterogeneous compounds such as α-dicarbonyls, furans, Strecker aldehydes, advanced glycation end-products, and melanoidins. Both protein oxidation and the Maillard reaction result in the loss of essential amino acids but may positively or negatively impact food structure and flavor. Expected final online publication date for the Annual Review of Food Science and Technology, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

SERS Investigation on Oligopeptides Used as Biomimetic Coatings for Medical Devices

The surface-enhanced Raman scattering (SERS) spectra of three amphiphilic oligopeptides derived from EAK16 (AEAEAKAK)2 were examined to study systematic amino acid substitution effects on the corresponding interaction with Ag colloidal nanoparticles. Such self-assembling molecular systems, known as "molecular Lego", are of particular interest for their uses in tissue engineering and as biomimetic coatings for medical devices because they can form insoluble macroscopic membranes under physiological conditions. Spectra were collected for both native and gamma-irradiated samples. Quantum mechanical data on two of the examined oligopeptides were also obtained to clarify the assignment of the prominent significative bands observed in the spectra. In general, the peptide-nanoparticles interaction occurs through the COO- groups, with the amide bond and the aliphatic chain close to the colloid surface. After gamma irradiation, mimicking a free oxidative radical attack, the SERS spectra of the biomaterials show that COO- groups still provide the main peptide-nanoparticle interactions. However, the spatial arrangement of the peptides is different, exhibiting a systematic decrease in the distance between aliphatic chains and colloid nanoparticles.

Efficient encapsulation of fish oil: Capitalizing on the unique inherent characteristics of whey cream and hydrolyzed whey protein

The effects of protein concentration and of blending a phospholipid-rich whey coproduct, Procream (Salibra 700 Procream, Glanbia Nutritionals), with intact or hydrolyzed whey protein concentrate, on fish oil microencapsulation efficiency and oxidative stability were assessed. Trypsin and protease M, from Aspergillus oryzae, were used to produce 2 unique hydrolysates. All microcapsules had excellent encapsulation efficiencies (>92%) and good physical properties, regardless of protein content and Procream inclusion. Intact α-lactalbumin and β-lactoglobulin and their peptides were involved in stabilizing oil droplets. Disulfide interchange resulted in formation of protein aggregates, which were more pronounced in samples containing Procream. Although all microcapsules had relatively good oxidative stability, most had better stability at 2 versus 0.5% protein. Protease M hydrolysate + Procream microcapsules had the highest stability, regardless of protein content. Results demonstrated that Procream, at a reduced protein inclusion level, can partially replace more expensive whey protein ingredients in microencapsulation, when blended with a select hydrolysate.

Modulation of amyloid fibrillation of bovine β-lactoglobulin by selective methionine oxidation

Deposition of oxidation-modified proteins during normal aging and oxidative stress are directly associated with systemic amyloidoses. Methionine (Met) is believed to be one of the most readily oxidisable amino acid residues of protein. Bovine beta-lactoglobulin (β-lg), a model globular whey protein, has been presented as a subsequent paradigm for studies on protein aggregation and amyloid formation. Herein, we investigated the effect of t-butyl hydroperoxide (tBHP)-induced oxidation on structure, compactness and fibrillation propensity of β-lg at physiological pH. Notably, whey protein modification, specifically Met residues, plays an important role in the dairy industry during milk processing and lowering nutritional value and ultimately affecting their technological properties. Several bio-physical studies revealed enhanced structural flexibility and aggregation propensity of oxidised β-lg in a temperature dependent manner. A molecular docking study is used to predict possible interactions with tBHP and infers selective oxidation of methionine residues at 7, 24 and 107 positions. From our studies, it can be corroborated that specific orientations of Met residues directs the formation of a partially unfolded state susceptible to fibrillation with possible different cytotoxic effects. Our studies have greater implications in deciphering the underlying mechanism of different whey proteins encountering oxidative stress. Our findings are also important to elucidate the understanding of oxidation induced amyloid fibrillation of protein which may constitute a new route to pave the way for a modulatory role of oxidatively stressed proteins in neurological disorders.

Bioactive proteins in bovine colostrum and effects of heating, drying and irradiation

Bovine colostrum (BC) contains bioactive proteins, such as immunoglobulin G (IgG), lactoferrin (LF) and lactoperoxidase (LP). BC was subjected to low-temperature, long-time pasteurization (LTLT, 63 °C, 30 min) or high-temperature, short-time pasteurization (HTST, 72 °C, 15 s) and spray-drying (SD), with or without γ-irradiation (GI, ∼14 kGy) to remove microbial contamination. Relative to unpasteurized liquid BC, SD plus GI increased protein denaturation by 6 and 11%, respectively, increasing to 19 and 27% after LTLT and to 48% after HTST, with no further effects after GI (all P < 0.05). LTLT, without or with GI, resulted in 15 or 29% denaturation of IgG, compared with non-pasteurized BC, and 34 or 58% for HTST treatment (all P < 0.05, except LTLT without GI). For IgG, only GI, not SD or LTLT, increased denaturation (30-38%, P < 0.05) but HTST increased denaturation to 40%, with further increases after GI (60%, P < 0.05). LTLT and HTST reduced LP levels (56 and 81% respectively) and LTLT reduced LF levels (21%), especially together with GI (47%, P < 0.05). Denaturation of BSA, β-LgA, β-LgB and α-La were similar to IgG. Methionine, a protective amino acid against free oxygen radicals, was oxidised by LTLT + GI (P < 0.05) while LTLT and HTST had no effect. Many anti-inflammatory proteins, including serpin anti-proteinases were highly sensitive to HTST and GI but preserved after LTLT pasteurization. LTLT, followed by SD is an optimal processing technique preserving bioactive proteins when powdered BC is used as a diet supplement for sensitive patients.

Advancement of food-derived mixed protein systems: Interactions, aggregations, and functional properties

Recently, interests in binary protein systems have been developed considerably ascribed to the sustainability, environment-friendly, rich in nutrition, low cost, and tunable mechanical properties of these systems. However, the molecular coalition is challenged by the complex mechanisms of interaction, aggregation, gelation, and emulsifying of the mixed system in which another protein is introduced. To overcome these fundamental difficulties and better modulate the structural and functional properties of binary systems, efforts have been steered to gain basic information regarding the underlying dynamics, theories, and physicochemical characteristics of mixed systems. Therefore, the present review provides an overview of the current studies on the behaviors of proteins in such systems and highlights shortcomings and future challenges when applied in scientific fields.

Modification of proteins by reactive lipid oxidation products and biochemical effects of lipoxidation

Lipid oxidation results in the formation of many reactive products, such as small aldehydes, substituted alkenals, and cyclopentenone prostaglandins, which are all able to form covalent adducts with nucleophilic residues of proteins. This process is called lipoxidation, and the resulting adducts are called advanced lipoxidation end products (ALEs), by analogy with the formation of advanced glycoxidation end products from oxidized sugars. Modification of proteins by reactive oxidized lipids leads to structural changes such as increased β-sheet conformation, which tends to result in amyloid-like structures and oligomerization, or unfolding and aggregation. Reaction with catalytic cysteines is often responsible for the loss of enzymatic activity in lipoxidized proteins, although inhibition may also occur through conformational changes at more distant sites affecting substrate binding or regulation. On the other hand, a few proteins are activated by lipoxidation-induced oligomerization or interactions, leading to increased downstream signalling. At the cellular level, it is clear that some proteins are much more susceptible to lipoxidation than others. ALEs affect cell metabolism, protein-protein interactions, protein turnover via the proteasome, and cell viability. Evidence is building that they play roles in both physiological and pathological situations, and inhibiting ALE formation can have beneficial effects.

Oxidant-induced glutathionylation at protein disulfide bonds

Disulfide bonds are a key determinant of protein structure and function, and highly conserved across proteomes. They are particularly abundant in extracellular proteins, including those with critical structural, ligand binding or receptor function. We demonstrate that oxidation of protein disulfides induces polymerization, and results in oxygen incorporation into the former disulfide via thiosulfinate generation. These intermediates, which have half-lives of several hours in vitro, undergo secondary reactions that cleave the disulfide bond, by irreversible hydrolysis to sulfinic and sulfonic acids, or reaction with thiols in a process that yields thiolated proteins (e.g. glutathionylated species in the case of reaction with glutathione). The adducts have been characterized by mass spectrometry (as ions corresponding to the addition of 306 and 712 Da for addition of one and two glutathione molecules, respectively) and immunoblotting. These modifications can be induced by multiple biologically-important oxidants, including HOCl, ONOOH, and H₂O₂, and on multiple proteins, demonstrating that this is a common disulfide modification pathway. Addition of glutathione to give glutathionylated proteins, can be reversed by reducing systems (e.g. tris(2-carboxyethyl)phosphine), but this does not repair the original disulfide bond. Exposure of human plasma to these modifying agents increases protein glutathionylation, demonstrating potential in vivo relevance. Overall these data provide evidence for a novel and facile route to glutathionylated proteins involving initial oxidation of a disulfide to a thiosulfinate followed by rapid reaction with GSH ('oxidant-mediated thiol-disulfide exchange'). These data elucidate a novel pathway for protein glutathionylation that may have significant implications for redox biology and cell signaling.

Effect of milk serum proteins on aggregation, bacteriostatic activity and digestion of lactoferrin after heat treatment

To establish the effect of the presence of milk serum proteins on heat-induced changes to lactoferrin, lactoferrin alone, and lactoferrin mixed with either milk serum or β-lactoglobulin was heated at 65 °C, 70 °C and 75 °C for 30 min. After heating, the effect of milk serum proteins on aggregation of lactoferrin was characterized, after which the effect of such aggregation on digestion and bacteriostatic capacity of lactoferrin were determined. The presence of milk serum proteins accelerated the aggregation of lactoferrin during heating through thiol/disulphide interchange. Lactoferrin also formed disulphide-linked aggregates when it was heated with β-lactoglobulin. Protein aggregates formed at 75 °C were much more resistant to infant digestion, causing decreased peptide release from lactoferrin. Heating lactoferrin and milk serum proteins together accelerated the loss of bacteriostatic activity upon heating. In conclusion, heat-induced aggregation of lactoferrin with milk serum proteins affected both its digestion and its bacteriostatic activity.

Investigation of the effect of oxidation on the structure of β-lactoglobulin by high resolution mass spectrometry

In this work, high-resolution mass spectrometry was used to identify the oxidation sites and forms of β-lactoglobulin (β-Lg) induced by hydrogen peroxide with 1.5% concentration, and the influence of oxidation sites on the structure of β-Lg was discussed from the molecular level. Twelve kinds of oxidation products and 36 oxidation sites were identified, including sulfoxidation in sulfur-containing amino acid residue, hydroxylation in aromatic group residue, deamination in amino-containing amino acid etc. The destruction of hydrogen bonds and disulfide bonds in β-Lg caused by oxidation is the main factor causing its structural changes, which were manifested in the decrease of β-sheet component and increase of β-turns and random coil contents, intrinsic fluorescence intensity and surface hydrophobicity. In addition, several peptides as potential oxidative markers were found to be capable of monitoring the degree of oxidation of β-Lg. In short, this work provided insights into structural changes of β-Lg by oxidation.

Investigation and comparison of the protective activities of three functional proteins-lactoferrin, α-lactalbumin, and β-lactoglobulin-in cerebral ischemia reperfusion injury

The objective of this study was to evaluate the protection conferred by lactoferrin, α-lactalbumin, and β-lactoglobulin in cerebral ischemia reperfusion (I/R) injury. Rat pheochromocytoma (PC12) cells were used to construct an oxygen and glucose deprivation model in vitro, and ICR mice underwent carotid artery "ligation-relaxation" to construct a cerebral I/R injury model in vivo. The levels of toll-like receptor 4 (TLR4) and downstream factors including nuclear factor-κB, tumor necrosis factor-α, and IL-1β were measured. Metabonomics detection and data mining were conducted to identify the specific metabolic sponsor of the 3 proteins. The results showed that lactoferrin, α-lactalbumin, and β-lactoglobulin protected neurons from cerebral I/R injury by increasing the level of bopindolol and subsequently inhibiting the TLR4-related pathway to different degrees; β-lactoglobulin had the strongest activity of the 3 proteins. In summary, this study is the first to investigate and compare the protective effects of lactoferrin, α-lactalbumin, and β-lactoglobulin in a cerebral stroke model. The results implicate TLR4 as a novel target of the 3 bioactive proteins to prevent cerebral I/R injury.

Structural analysis and allergenicity assessment of an enzymatically cross-linked bovine α-lactalbumin polymer

Enzymatic cross-linking is frequently used in bio-processing of dairy products since it could change the physiochemical and functional characterization.

Formation and characterization of crosslinks, including Tyr–Trp species, on one electron oxidation of free Tyr and Trp residues by carbonate radical anion

Dityrosine and ditryptophan bonds have been implied in protein crosslinking. This is associated with oxidative stress conditions including those involved in neurodegenerative pathologies and age-related processes. Formation of dityrosine and ditryptophan derives from radical–radical reactions involving Tyr˙ and Trp˙ radicals. However, cross reactions of Tyr˙ and Trp˙ leading to Tyr–Trp crosslinks and their biological consequences have been less explored. In the present work we hypothesized that exposure of free Tyr and Trp to a high concentration of carbonate anion radicals (CO₃˙⁻), under anaerobic conditions, would result in the formation of Tyr–Trp species, as well as dityrosine and ditryptophan crosslinks. Here we report a simple experimental procedure, employing CO₃˙⁻ generated photochemically by illumination of a Co(iii) complex at 254 nm, that produces micromolar concentrations of Tyr–Trp crosslinks. Analysis by mass spectrometry of solutions containing only the individual amino acids, and the Co(iii) complex, provided evidence for the formation of o,o′-dityrosine and isodityrosine from Tyr, and three ditryptophan dimers from Trp. When mixtures of Tyr and Trp were illuminated in an identical manner, Tyr–Trp crosslinks were detected together with dityrosine and ditryptophan dimers. These results indicate that there is a balance between the formation of these three classes of crosslinks, which is dependent on the Tyr and Trp concentrations. The methods reported here allow the generation of significant yields of isolated Tyr–Trp adducts and their characterization. This technology should facilitate the detection, and examination of the biological consequences of Tyr–Trp crosslink formation in complex systems in future investigations.

Exposure of free Tyr and Trp to a high concentration of carbonate anion radicals (CO₃˙⁻), under anaerobic conditions, result in the formation of Tyr–Trp species, as well as dityrosine and ditryptophan crosslinks.

Structural Lesions of Proteins Connected to Lipid Membrane Damages Caused by Radical Stress: Assessment by Biomimetic Systems and Raman Spectroscopy

Model systems constituted by proteins and unsaturated lipid vesicles were used to gain more insight into the effects of the propagation of an initial radical damage on protein to the lipid compartment. The latter is based on liposome technology and allows measuring the trans unsaturated fatty acid content as a result of free radical stress on proteins. Two kinds of sulfur-containing proteins were chosen to connect their chemical reactivity with membrane lipid transformation, serum albumins and metallothioneins. Biomimetic systems based on radiation chemistry were used to mimic the protein exposure to different kinds of free radical stress and Raman spectroscopy to shed light on protein structural changes caused by the free radical attack. Among the amino acid residues, Cys is one of the most sensitive residues towards the attack of free radicals, thus suggesting that metal-Cys clusters are good interceptors of reactive species in metallothioneins, together with disulfides moieties in serum albumins. Met is another important site of the attack, in particular under reductive conditions. Tyr and Phe are sensitive to radical stress too, leading to electron transfer reactions or radical-induced modifications of their structures. Finally, modifications in protein folding take place depending on reactive species attacking the protein.

Whey proteins: targets of oxidation, or mediators of redox protection

Bovine whey proteins are highly valued dairy ingredients. This is primarily due to their amino acid content, digestibility, bioactivities and their processing characteristics. One of the reported bioactivities of whey proteins is antioxidant activity. Numerous dietary intervention trials with humans and animals indicate that consumption of whey products can modulate redox biomarkers to reduce oxidative stress. This bioactivity has in part been assigned to whey peptides using a range of biochemical or cellular assays in vitro. Superimposing whey peptide sequences from gastrointestinal samples, with whey peptides proven to be antioxidant in vitro, allows us to propose peptides from whey likely to exhibit antioxidant activity in the diet. However, whey proteins themselves are targets of oxidation during processing particularly when exposed to high thermal loads and/or extensive processing (e.g. infant formula manufacture). Oxidative damage of whey proteins can be selective with regard to the residues that are modified and are associated with the degree of protein unfolding, with α-Lactalbumin more susceptible than β-Lactoglobulin. Such oxidative damage may have adverse effects on human health. This review summarises how whey proteins can modulate cellular redox pathways and conversely how whey proteins can be oxidised during processing. Given the extensive processing steps that whey proteins are often subjected to, we conclude that oxidation during processing is likely to compromise the positive health attributes associated with whey proteins.

Tetramethylpyrazine Protects Oxidative Stability and Gelation Property of Rabbit Myofibrillar Proteins

Tetramethylpyrazine (TMP), an alkaloid rich in Ligusticum wallichii and fermented products, possesses multiple pharmacological activities in antioxidant, anti-inflammatory, and antibacterial. This study aimed to investigate the effect of TMP (15 mg/L) on the physicochemical and gelation properties of rabbit myofibrillar proteins (MPs) with/without oxidative stress. Results showed that compared to the control, oxidative stress to MPs decreased free thiol content, gel yield, whiteness, water-holding capacity, bounder water, immobilized water, and endogenous tryptophan fluorescence intensity, but increased surface hydrophobicity, dityrosine content, and free water content (p<0.01). Without oxidative stress, MPs treated with TMP increased free thiol content, whiteness, and bound water, but decreased dityrosine content and free water (p<0.05). Under oxidative conditions, all parameters were conversely affected by TMP (p<0.01). The results suggest that TMP can be an antioxidant to decrease the concern on oxidative deterioration during meat processing and storage by improving the oxidative stability, water retention, and gel forming property of rabbit MPs.

Protein oxidation during temperature-induced amyloid aggregation of beta-lactoglobulin

Although the connection between protein oxidation, amyloid aggregation and diseases such as Alzheimer's is well known there is no information on such effects during preparation of beta-lactoglobulin fibrils. Different morphologies of amyloid aggregates of beta-lactoglobulin were prepared by incubation at pH 2 or pH 3.5 for up to 72 h. After 5 h, amyloid aggregates at pH 2 formed typical fibrils, which consisted of peptides. At pH 3.5, the amyloid aggregates were worm-like and consisted of intact protein. After 72 h, the building blocks at both pH values changed towards smaller peptides. The apparent tyrosine oxidation reached a maximum after 5 h at both pH values, whereas N-formylkynurenine and carbonyls increased continuously during 72 h. In case amyloid structures are used as edible material, the health related effects caused by protein oxidation needs to be considered.

Effect of Methylglyoxal-Induced Glycation on the Composition and Structure of β-Lactoglobulin and α-Lactalbumin

Glycation, and particularly reactions between aldehydes and nucleophiles (thiols, amines), can initiate changes in the structure, solubility, composition, hydrophobicity, conformation, function, and susceptibility to proteolysis of proteins. This can have adverse consequences for mammals, plants, foodstuffs, and pharmaceuticals. Low-molecular-mass dialdehydes such as methylglyoxal (MGO) are much more reactive than parent glucose and therefore potentially highly damaging. These are present at significant levels in some foods. This study investigated whether and how MGO exposure, with or without concurrent heat exposure, affected the major whey proteins β-lactoglobulin and α-lactalbumin. MGO diminished the formation of heat-induced, reducible, intermolecular disulfide cross-links for both proteins, with this being associated, at least in part, with alternative thiol consuming reactions of MGO. At long incubation times, nonreducible protein cross-links were formed in a dose-dependent manner, with LC-MS/MS and UPLC analysis showing the presence of methylglyoxal-lysine dimers (MOLD). UPLC analysis revealed MGO-dependent consumption of specific amino acids in the order Cys > Arg > Lys > Trp for both proteins, with α-lactalbumin affected to a greater extent than β-lactoglobulin. SDS-PAGE revealed altered protein mobility consistent with modification of charged residues. MGO exposure also resulted in increased binding of the hydrophobic dye, 8-anilino-1-naphthalene sulfonic acid, consistent with limited protein unfolding. Overall, these data are consistent with rapid reaction of MGO residues at Cys residues (when available) and surface accessible Arg and Lys residues, with formation of adducts and cross-linked materials. These alternative reactions of dialdehydes diminish direct heat-induced (disulfide) cross-link formation and result in limited protein unfolding.

Analysis of the soybean metallothionein system under free radical stress: protein modification connected to lipid membrane damage

Metallothioneins are small Cys-rich peptides capable of coordinating metal ions, and proposed to be involved in radical stress. The four Zn(ii)-GmMT complexes of soybean (Glycine max) were recombinantly synthesised and exposed to oxidative (HO˙) and reductive (H˙ atoms and eaq-) stress conditions. Gamma-irradiation was used to simulate the endogenous formation of the reactive species in both aqueous solutions and unsaturated lipid vesicle suspensions, a biomimetic model that showed that tandem protein/lipid damage occurs, in particular under reductive radical stress. This is due to the formation of diffusible sulphur-centred radicals, which migrate from the aqueous phase to the lipid bilayer and are thus able to transform the cis double bond of the oleate moiety into the trans isomer. Among the amino acid residues present in GmMTs, Cys is one of the most sensitive residues towards the attack of free radicals, thus suggesting metal-clusters to be good interceptors of free radicals. Also Met, Tyr and Phe residues are sensitive amino acid sites of attack under both oxidative and reductive conditions. The modification of the Zn(ii)-GmMT complexes, in particular isoform 2, was monitored by Raman spectroscopy and mass spectrometry. Free radical stress on the Zn(ii)-GmMT complexes is able to induce significant structural changes such as partial deconstruction and/or rearrangement of the metal clusters, but not the complete demetallation of the proteins nor breaking of the backbone, thus confirming their capability to act as protectors under free radical stress conditions.