Effects Of Lipid Oxidation Products Research Articles

Lipid Oxidation Products and the Risk of Cardiovascular Diseases: Role of Lipoprotein Transport.

Cholesterol has for decades ruled the history of atherosclerotic cardiovascular diseases (CVDs), and the present view of the etiology of the disease is based on the transport of cholesterol by plasma lipoproteins. The new knowledge of the lipoprotein-specific transport of lipid oxidation products (LOPs) has introduced another direction to the research of CVD, revealing strong associations between lipoprotein transport functions, atherogenic LOP, and CVD. The aim of this review is to present the evidence of the lipoprotein-specific transport of LOP and to evaluate the potential consequences of the proposed role of the LOP transport as a risk factor. The associations of cholesterol and lipoprotein LOP with the known risk factors of CVD are mostly parallel, and because of the common transport and cellular intake mechanisms it is difficult to ascertain the independent effects of either cholesterol or LOP. While cholesterol is known to have important physiological functions, LOPs are merely regarded as metabolic residues and able to initiate and boost atherogenic processes. It is therefore likely that with the increased knowledge of the lipoprotein-specific transport of LOP, the role of cholesterol as a risk factor of CVD will be challenged.

Molecular Mechanisms Underlying the Effect of Lipid Oxidation Products on Digestibility and Conformation of Myoglobin under Thermal Treatment.

Lipid oxidation can produce lipid oxidation products (LOPs), which further react with proteins and affect their structure and digestibility, although the underlying mechanism remains unclear. Herein, we investigated the conformation and digestibility of proteins induced by LOPs after thermal treatment. Digestibility of myoglobin (Mb) affected by trans,trans,-2,4-decadienal (2,4-Dec) was significantly reduced under high temperature (100-180 °C). The peptides digested from Mb modified with 2,4-Dec during thermal processing revealed that the quantity of peptides decreased with increasing 2,4-Dec concentrations. Proteomic analysis showed that 2,4-Dec covalently binds to Mb, and the extent of modification was in the following order: lysine > histidine > arginine. Moreover, the secondary structure, intrinsic fluorescence, and surface hydrophobicity results suggested that 2,4-Dec induced changes in Mb, leading to a tighter spatial structure and aggregation, and exposure of fewer recognition sites of the enzyme and thermal treatment assisted these changes in the structure. Meanwhile, molecular dynamics simulations elucidated the molecular mechanisms underlying the effect of 2,4-Dec and temperature on the digestion and structure of Mb.

Characterization of the action of the lipid oxidation product 4-hydroxyhexenal on Lactiplantibacillus plantarum, the dominant bacterium in dry-cured fish

(E)-4-Hydroxyhexenal (HHE) is receiving widespread attention as a representative reactive carbonyl species (RCS) derived from n-3 polyunsaturated fatty acids peroxidation due to its health hazard. However, few studies have addressed the potential effects of lipid oxidation products on microbial growth in food products. Therefore, this study attempted to elucidate the action of HHE on Lactiplantibacillus plantarum (L. plantarum), the dominant bacteria in dry-cured fish. The results showed that the minimum inhibitory concentration (MIC) of HHE was 250 μg/mL. Time-kill curves confirmed the strong inhibitory activity of HHE. Lactate dehydrogenase (LDH) assay indicated that HHE disrupted the integrity of cell wall. Scanning electron microscopy (SEM) and flow cytometry analysis revealed that HHE induced changes in cell morphology, compromised cell membrane permeability, and thus disrupted intracellular homeostasis. Based on untargeted metabolomic analysis, HHE disrupted multiple metabolic pathways in L. plantarum involving amino acid metabolism, lipid metabolism, protein biosynthesis, nucleic acid metabolism and energy metabolism. This research provide new insights into the HHE inhibition mechanism and theoretical support for studying the effects of lipid oxidation products on microorganisms in food matrix.

Effects of Peroxyl Radicals on the Structural Characteristics and Fatty Acid Composition of High-Density Lipoprotein from Duck Egg Yolk

In this study, high-density lipoprotein (HDL) from duck egg yolk was subjected to oxidation with a system based on 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH)-derived peroxyl radicals. The effects of peroxyl radicals on the protein carbonyl, free sulfhydryl, secondary/tertiary structure, surface hydrophobicity, solubility, particle size distribution, zeta potential and fatty acid composition of HDL were investigated by using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), Fourier-transform infrared spectroscopy (FTIR), circular dichroism (CD), fluorescence spectroscopy, dynamic light scattering and ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). The results indicated that the content of protein carbonyl was significantly increased, that of free sulfhydryl was obviously reduced, and the ordered secondary structure was also decreased with increasing AAPH concentration. In addition, the surface hydrophobicity and solubility of HDL showed apparent increases due to the exposure of hydrophobic groups and aggregation of protein caused by oxidation. The fatty acid composition of HDL exhibited pronounced changes due to the disrupted protein–lipid interaction and lipid oxidation by AAPH-derived peroxyl radicals. These results may help to elucidate the molecular mechanism for the effect of lipid oxidation products on the oxidation of duck yolk proteins.

Oxidized Phospholipids in Control of Endothelial Barrier Function: Mechanisms and Implication in Lung Injury.

Earlier studies investigating the pathogenesis of chronic vascular inflammation associated with atherosclerosis described pro-inflammatory and vascular barrier disruptive effects of lipid oxidation products accumulated in the sites of vascular lesion and atherosclerotic plaque. However, accumulating evidence including studies from our group suggests potent barrier protective and anti-inflammatory properties of certain oxidized phospholipids (OxPLs) in the lung vascular endothelium. Among these OxPLs, oxidized 1-palmitoyl-2-arachdonyl-sn-glycero-3-phosphocholine (OxPAPC) causes sustained enhancement of lung endothelial cell (EC) basal barrier properties and protects against vascular permeability induced by a wide variety of agonists ranging from bacterial pathogens and their cell wall components, endotoxins, thrombin, mechanical insults, and inflammatory cytokines. On the other hand, truncated OxPLs cause acute endothelial barrier disruption and potentiate inflammation. It appears that multiple signaling mechanisms triggering cytoskeletal remodeling are involved in OxPLs-mediated regulation of EC barrier. The promising vascular barrier protective and anti-inflammatory properties exhibited by OxPAPC and its particular components that have been established in the cellular and animal models of sepsis and acute lung injury has prompted consideration of OxPAPC as a prototype therapeutic molecule. In this review, we will summarize signaling and cytoskeletal mechanisms involved in OxPLs-mediated damage, rescue, and restoration of endothelial barrier in various pathophysiological settings and discuss a future potential of OxPAPC in treating lung disorders associated with endothelial barrier dysfunction.

Oxidized Phospholipids in Healthy and Diseased Lung Endothelium.

Circulating and cell membrane phospholipids undergo oxidation caused by enzymatic and non-enzymatic mechanisms. As a result, a diverse group of bioactive oxidized phospholipids generated in these conditions have both beneficial and harmful effects on the human body. Increased production of oxidized phospholipid products with deleterious effects is linked to the pathogenesis of various cardiopulmonary disorders such as atherosclerosis, thrombosis, acute lung injury (ALI), and inflammation. It has been determined that the contrasting biological effects of lipid oxidation products are governed by their structural variations. For example, full-length products of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine oxidation (OxPAPC) have prominent endothelial barrier protective and anti-inflammatory activities while most of the truncated oxidized phospholipids induce vascular leak and exacerbate inflammation. The extensive studies from our group and other groups have demonstrated a strong potential of OxPAPC in mitigating a wide range of agonist-induced lung injuries and inflammation in pulmonary endothelial cell culture and rodent models of ALI. Concurrently, elevated levels of truncated oxidized phospholipids are present in aged mice lungs that potentiate the inflammatory agents-induced lung injury. On the other hand, increased levels of full length OxPAPC products accelerate ALI recovery by facilitating production of anti-inflammatory lipid mediator, lipoxin A4, and other molecules with anti-inflammatory properties. These findings suggest that OxPAPC-assisted lipid program switch may be a promising therapeutic strategy for treatment of acute inflammatory syndromes. In this review, we will summarize the vascular-protective and deleterious aspects of oxidized phospholipids and discuss their therapeutic potential including engineering of stable analogs of oxidized phospholipids with improved anti-inflammatory and barrier-protective properties.

Longitudinal Effects of Dietary Oxidized Lipids on the Gut Microbiome and Mycobiome in Pigs

Gut microbial communities are modulated by host and environmental factors such as genetics, disease state, xenobiotics and diet. Western high fat diets are known to induce dysbiosis of the microbiome and are associated with chronic diseases – such as obesity and type 2 diabetes – but the effects of dietary lipid oxidation products (LOPs) on the gut microbial communities are unknown. Primary and secondary LOPs are abundant in western diets due to the extensive use of oxidation‐susceptible fats such as unsaturated fats and food processing methods such as deep fat frying that generate them. In this study, we utilized pigs as a translational model to elucidate whether a high fat diet containing high levels of LOPs would differentially alter the microbiome, mycobiome and host phenotype. We hypothesized that a high fat diet containing LOPs will cause selective alteration of gut microbiome and mycobiome compared to high fat diet alone (i.e., with low amounts of LOPs). We randomized twenty‐one day old pigs (N=48) to the following dietary regimens for five months: (1) Low‐fat diet (Control, 9% calories from fat); (2) High‐fat diet (HF, 44% calories from fat); (3) High‐fat with high amounts of LOPs (HF+LOP, ~44% calories from fat. Fecal samples for bacterial and fungal sequencing by Illumina were obtained from the pigs 2 and 5 months after they were placed on the above dietary regimen. The three groups of pigs initially exhibited similar microbiome alpha diversity (Shannon diversity index), but the groups receiving high‐fat diets (HF and HF+LOPs) showed reductions in diversity by the end point. Diet containing LOPs produced the lowest diversity by the endpoint. The HF diet elevated the phyla Firmicutes and decreased phyla Bacteroidetes compared to control diet. Compared to control diet, both HF diets increased Clostridiaceae and Turicibacteraceae, while the presence of LOPs induced a bloom in Enterobacteriaceae. The effect of LOPs was further explored using the binomial analysis DESeq2, revealing that Enterbacteriaceae, Epulopiscium, Elusimicrobiaceae, and Lactobacillus were significantly increased (when compared to HF alone, following FDR correction). The mycobiome of all the animals was dominated by the genus Kazachstania, a known porcine yeast. Pigs on the HF diets showed significantly lower levels of Aspergillus, Penicillium, Oidiodendron, and Wallemia when compared to those on control diet. The presence of LOPs in the diet led to significant elevation of fungi Schwanniomyces (compared to HF diet alone). Here, for the first time, we demonstrate that dietary LOPs can alter the bacterial and fungal communities in the gut, even when controlling for total dietary fat intake. These findings suggest that the oxidation state of dietary lipids is an important factor to consider when studying the effects of western diets on chronic disease risk.Support or Funding InformationThe research was supported by the Biomedical & Genomic Research Group Discretionary Fund, University of Wisconsin‐Madison.

Carbon Chain Length of Lipid Oxidation Products Influence Lactate Dehydrogenase and NADH-Dependent Metmyoglobin Reductase Activity.

The biochemical basis of lower metmyoglobin reducing activity (MRA) in high-oxygen modified atmospheric packaged (HiOx-MAP) beef than those in vacuum and polyvinyl chloride (PVC) packaging is not clear. To explore this, the effects of lipid oxidation products with varying carbon chain length on lactate dehydrogenase (LDH) and NADH-dependent metmyoglobin reductase activity were evaluated. Surface color, MRA, and lipid oxidation of beef longissimus lumborum steaks (n = 10) were measured during 6-day display. Further, two enzymes, LDH and NADH-dependent metmyoglobin reductase (n = 5), critical for MRA were incubated with or without (control) lipid oxidation products of varying carbon chain length: malondialdehyde (3-carbon), hexenal (6-carbon), and 4-hydroxynonenal (9-carbon). Steaks in HiOx-MAP had greater (P < 0.05) redness than vacuum and PVC, but had lower (P < 0.05) MRA and greater (P < 0.05) lipid oxidation on day 6. LDH and NADH-dependent metmyoglobin reductase activities were differentially influenced by lipid oxidation products (P < 0.05). The results indicate that the difference in reactivity of various lipid oxidation products on LDH (HNE > MDA = hexenal) and NADH-dependent metmyoglobin reductase (HNE = MDA > hexenal) activity could be responsible for lower MRA in HiOx-MAP.

4-Hydroxy-7-oxo-5-heptenoic acid lactone is a potent inducer of brain cancer cell invasiveness that may contribute to the failure of anti-angiogenic therapies

Previously, we discovered that free radical-induced oxidative fragmentation of the docosahexaenoate ester of 2-lysophosphatidylcholine produces 4-hydroxy-7-oxo-5-heptenoic acid (HOHA) lactone that, in turn, promotes the migration and invasion of endothelial cells. This suggested that HOHA lactone might similarly promote migration and invasion of glioblastoma multiformae (GBM) brain cancer stem cells (CSCs). A bioinformatics analysis of clinical cancer genomic data revealed that matrix metalloproteinase (MMP)1 and three markers of oxidative stress – superoxide dismutase 2, NADPH oxidase 4, and carbonic anhydrase 9 – are upregulated in human mesenchymal GBM cancer tissue, and that MMP1 is positively correlated to all three of these oxidative stress markers. In addition, elevated levels of MMP1 are indicative of GBM invasion, while low levels of MMP1 indicate survival. We also explored the hypothesis that the transition from the proneural to the more aggressive mesenchymal phenotype, e.g., after treatment with an anti-angiogenic therapy, is promoted by the effects of lipid oxidation products on GBM CSCs. We found that low micromolar concentrations of HOHA lactone increase the cell migration velocity of cultured GBM CSCs, and induce the expression of MMP1 and two protein biomarkers of the proneural to mesenchymal transition (PMT): p65 NF-κβ and vimentin. Exposure of cultured GBM CSCs to HOHA lactone causes an increase in phosphorylation of mitogen-activated protein kinases and Akt kinases that are dependent on both protease-activated receptor 1 (PAR1) and MMP1 activity. We conclude that HOHA lactone promotes the PMT in GBM through the activation of PAR1 and MMP1. This contributes to a fatal flaw in antiangiogenic, chemo, and radiation therapies: they promote oxidative stress and the generation of HOHA lactone in the tumor that fosters a change from the proliferative proneural to the migratory mesenchymal GBM CSC phenotype that seeds new tumor growth. Inhibition of PAR1 and HOHA lactone are potential new therapeutic targets for impeding GBM tumor recurrence.

Influence of malondialdehyde-induced modifications on physicochemical and digestibility characteristics of whey protein isolate.

Impacts of lipid oxidation product malondialdehyde (MDA) on the properties of whey protein isolate (WPI) were investigated in this study. The incorporation of MDA into WPI promoted the formation of protein carbonyls, with the significant loss of protein sulfhydryls, impaired intrinsic fluorescence, and increased protein surface hydrophobicity. The visualized band profiles revealed by gel electrophoresis and immunoblotting suggested that WPI's main components β-lactoglobulin and α-lactalbumin were the targets of MDA, and the derivatives of MDA were involved in protein cross-linking and aggregation at higher molecular weights. Abnormal protein aggregation was further confirmed by scanning electron microscopy analysis of the surface microstructure of MDA-modified WPI. Finally, in vitro digestibility assay indicated that the modification of MDA reduced WPI's susceptibility to digestive enzymes. The present study demonstrated that the contribution of MDA to protein modification in dairy products can be substantial in complex foodstuffs composed of lipids and proteins. PRACTICAL APPLICATIONS: The present work enhanced our knowledge on the remarkable susceptibility of dairy product WPI to lipid oxidation product MDA. With the trend of application of highly unsaturated fatty acids such as fish oil or alga oils as functional ingredients in dairy products, it is obvious that apart from monitoring lipid oxidation products, the resultant changes in dietary proteins deserve more attention. The food industry must be aware of the importance of appropriate preventive measures in minimizing the negative effects of lipid oxidation products on dairy products.

Effects of Prepubertal Oxidized Dietary Fat Consumption on Body Weight, Adiposity and Adipose Distribution in A Swine Model.

High‐fat diets, which are increasingly common in developed and developing countries, have been associated with an increased chronic disease risk. The effects of these diets take stage early in life with childhood consumption of fats increasing, and associated incidence of obesity, increasing in these countries. However, what is often neglected when studying the impact of fats in diets is that modern processing and western cooking (deep‐frying) practices generate lipid oxidation products (LOPs), and thus fats are not always consumed in their native forms. In this study, we examined whether consumption of high‐fat diets containing lipid oxidation products have a different effect on body weight, adiposity and adipose distribution using a swine model. The overwhelming genetic, anatomical, physiological, and pathophysiological similarities to humans make swine an ideal model for translational studies.Forty‐eight domestic swine (at twenty‐one days of age, immediately post weaning) were divided into three equal study groups and placed on the following diet regimens for five months: (1) Low‐Fat/Low‐Calorie Diet (LF/LC Diet, N=16, 9% of calories from fat). (2) High‐Fat/High‐ Calorie Diet containing LOPs (HF/HC+LOPs Diet, N=16, 44% of calories from fat). (3) High‐ Fat/High‐Calorie Diet without LOPs (HF/HC–LOPs Diet, N=16, 44% of calories from fat). Body weight (BW) measurements were made weekly. Total adiposity and distribution were assessed at five different time points on eight of the animals from each diet group using dual energy x‐ray absorptiometry (DXA). At the end of the five‐month study period, as expected, the animals from HF/HC Diet groups had a significantly higher body weight and adiposity than those from the LF/LC Diet group. However, the HF/HC–LOPs animals had 6.4% (12.5 lbs.) higher body weight than the HF/HC+LOPs animals. The adiposity of HF/HC–LOPs animals was also higher than their HF/HC+LOPs counterparts, although this did not reach statistical significance. Both HF/HC diet groups exhibited increased fat deposition in abdominal fat depots relative to the LF/LC diet group, however again the difference between the two HF/HC diet groups did not reach statistical significance. The results from the study suggests that fats containing LOPs do not have the same physiological effects as fats in the native form and the presence of LOPs should be considered in dietary studies. We are currently analysing differences in gene expression in tissues from the study animals to develop mechanistic hypothesis for the observed effects of LOPs.Support or Funding InformationThe research was supported by the Biomedical &amp; Genomic Research Group Discretionary Fund (University of Wisconsin‐Madison) and by USDA Formula Project Grant (WIS01972) National Institute of Food and Agriculture, USDA).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Covalent Binding of 4-Hydroxy-2-nonenal to Lactate Dehydrogenase Decreases NADH Formation and Metmyoglobin Reducing Activity

Lactate dehydrogenase (LDH) activity can regenerate NADH, which is a critical component in metmyoglobin reduction. However, limited research has determined the effects of lipid oxidation products on LDH activity. The overall objective of this study was to determine the effects of 4-hydroxy-2-nonenal (HNE) on LDH activity. LDH was reacted with HNE at pH 5.6 and 7.4, and LDH activity was measured as NADH formation following the addition of lactate and NAD. The effects of HNE on NADH-dependent metmyoglobin reduction also were analyzed. Mass spectrometric examination revealed that HNE adducts to LDH at both pH 5.6 and 7.4. More specifically, HNE binds with cysteine and histidine residues of LDH at pH 5.6 and 7.4. Covalent binding of HNE decreased NADH formation and metmyoglobin reduction (P < 0.05). These results indicate that secondary lipid oxidation products can inactivate enzymes involved in metmyoglobin reduction and have the potential to increase beef discoloration.

Effect of lipid oxidation products on the formation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in model systems

Ternary mixtures of creatinine, phenylalanine and lipids (or carbohydrates) were heated at 200°C for 1h to determine the potential contribution of lipids to the formation of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Although unoxidised lipids did not contribute to PhIP formation, their oxidation produced many compounds that significantly increased the formation of PhIP. Among the different lipid oxidation products (LOPs) studied, which included ω-6 and ω-3 derived lipid hydroperoxides, 2,4-alkadienals, 2-alkenals, 4,5-epoxy-2-alkenals, 4-oxo-2-alkenals, and 4-hydroxy-2-nonenal, 4-oxo-2-nonenal was the most reactive compound. It produced 32.48pmol of PhIP/μmol of creatinine in comparison with the 7.92pmol of PhIP/μmol of creatinine produced by the control phenylalanine/creatinine reaction mixture. 4-Oxo-2-nonenal reactivity was similar to that of most carbohydrates; although ribose and arabinose produced more PhIP (44–46pmol of PhIP/μmol of creatinine). In addition to single LOP, the addition of oxidised soybean oil for 24–144h at 60°C also increased PhIP formation. All these results pointed out to a potential contribution of LOP to the formation of PhIP in food products. This contribution will depend on the lipid content of the food product and its easiness to be oxidised.

Oxidized lipid derived aldehyde, 4‐hydroxy‐trans‐2‐nonenal causes vascular inflammation by inducing endoplasmic reticulum stress

Oxidized lipoproteins‐derived aldehydes, such as 4‐hydroxy‐trans‐2‐nonenal (HNE) increase endothelial activation, however, mechanisms that mediate the effects of lipid oxidation products on the endothelium remain unclear. We observed HNE modified ER proteins in atherosclerotic lesions of apoE‐null mice. In vitro, treatment of endothelial cells (EC) and macrophages with HNE led to the modification of several proteins that colocalize with the endoplasmic reticulum (ER). HNE also caused the splicing of XBP‐1, an increase in the phosphorylation of PERK and eIF2á, and induction of ATF3 and ATF4. Phosphorylation of eIF2á by HNE was prevented by transfection with PERK siRNA. Treatment with HNE was also associated with a significant increase in the expression of ER chaperones, GRP78 and HERP. Pre‐treatment with a chemical chaperone, phenylbutyric acid (PBA) or adenoviral transfection with ATF6 attenuated HNE‐induced monocyte adhesion and IL‐8 induction. IL‐8 induction was also prevented by IRE1 siRNA. Moreover, PBA and taurine‐conjugated ursodeoxycholic acid (TUDCA) attenuated HNE‐induced leukocyte rolling and their firm adhesion to the endothelium in rat cremaster muscle. These data suggest that endothelial activation by HNE is mediated in part by ER stress. ER stress, induced by oxidized lipids may be a significant cause of vascular inflammation during atherogenesis.

The nuclear orphan receptors Nur77 and NOR‐1 are novel mediators of the inflammatory response induced by oxidized phospholipids

Nur77 and NOR-1, both members of the NR4A subgroup of nuclear receptors, are highly expressed in atherosclerotic lesions. However, factors responsible for NR4A expression are poorly characterized. LDL-derived phospholipid oxidation products of 1-palmitoyl-2-arachidonoyl-sn-3-glycero-phosphorylcholine (OxPAPC) have been shown to be present in the atherosclerotic plaque and induce a large set of pro- and anti-inflammatory genes, rendering them candidate triggers of the inflammatory process in atherosclerosis. Here, we show induction of NOR-1 and Nur77 mRNA and protein expression by OxPAPC in human umbilical venous endothelial cells (HUVEC) in a time and concentration-dependent manner, whereas native phospholipids have no effect. Using transient transfection assays and different NOR-1 promoter-driven luciferase constructs, we demonstrate the direct transcriptional regulation of NOR-1 by OxPAPC. Moreover, utilization of Nur77 and NOR-1 siRNA revealed a critical role of these nuclear receptors in the transcriptional regulation of a subset of OxPAPC-induced genes including vascular endothelial growth factor and the anti-inflammatory gene heme oxygenase-1. Thus, nuclear orphan receptors of the NR4A family mediate the effects of lipid oxidation products on the transcriptional level and might be involved in the initiation of chronic inflammatory disorders and tissue repair mechanisms.

The influence of oxidation on proteolysis in raw milk.

The link between oxidation and increased proteolysis in raw milk was studied. To accelerate oxidation, H2O2 (1 mM) was added to raw milk, resulting in enhanced proteolysis by up to 11.2% after 24 h incubation at 5 degrees C. Addition of Cu2+ (10 microM) to milk or exposure of milk to light (60 min) likewise increased proteolysis. To explain the mechanism responsible for increased proteolysis as a result of oxidation, the effect of lipid oxidation products on plasmin-induced proteolysis was tested. Addition of malondialdehyde to skim milk increased the formation of gamma-caseins, a proteolysis product from plasmin hydrolysis of beta-casein. The same observation was made in a model system containing 4.5 g beta-casein/l sodium tetraborate buffer at pH 8 and plasmin. Addition of a plasmin inhibitor blocked the formation of gamma-casein. The results indicate that aldehydes accumulated from lipid oxidation can modify beta-casein and thereby increase susceptibility of the proteins to proteolysis. Furthermore, the data suggest that proteolysis in raw milk may be connected to oxidative processes.

Effect of lipid oxidation and frozen storage on muscle proteins of Atlantic mackerel (Scomber scombrus)

AbstractThe effect of storage on the lipids and proteins in Atlantic mackerel stored for up to 24 months at −20 and −30 °C was studied. Traditional methods including the peroxide value, thiobarbituric acid‐reactive substances (TBARS) and a reverse phase HPLC method were used to determine the primary and secondary lipid oxidation products. All tests showed an increase in lipid oxidation products with storage time and at a higher storage temperature of −20 °C compared with samples stored at −30 °C. Antioxidants had a significant effect (P &lt; 0.01) on the inhibition of lipid oxidation, as shown by the reduction in peroxide value and hydroxides, and malondialdehyde formation. Similarly, deterioration of protein structure and functionality in mackerel stored for 3, 6, 12 and 24 months was greater at −20 than −30 °C. ATPase activity in the myosin extract of Atlantic mackerel showed a significant decrease (P &lt; 0.01) with progressive frozen storage. Protein solubility in high salt concentration (0.6 M NaCl) decreased (P &lt; 0.01) during storage at both −20 and −30 °C but was greater at −20 °C. Interestingly, antioxidants BHT, vitamin C and vitamin E protected the proteins against complete loss of ATPase activity and protein solubility to a significant level (P &lt; 0.01) for up to 1 year at −20 °C compared with samples stored without antioxidants. This study confirms the deleterious effect of lipid oxidation products on protein structure and function in frozen fatty fish.© 2002 Society of Chemical Industry

Effect of lipid oxidation products on the transesterification activity of an immobilized lipase

The effect of varying amounts of linoleic acid hydroperoxidases on the activity of an immobilized lipase, Lipozyme, used for the transesterification of fats has been studied. The initial activity of the enzyme is not significantly reduced even by a peroxide value of 50 mequiv/kg, but the operational stability is reduced if the peroxide value of the oil is above 5 mequiv/kg, with the rate of enzyme inactivation increasing the peroxide value. A sample with initial peroxide value of 50 mequiv/kg caused a 50% loss of activity after eight batch treatments of 10 h per treatment. The loss of activity is due to generation of free radicals in the enzyme following hydroperoxide decomposition.

The antioxidant and nutritional effects of tocopherols, ascorbic acid and beta-carotene in relation to processing of edible oils.

Tocopherols belong to a class of phenolic antioxidants which can inhibit lipid autoxidation by scavenging free radicals and by reacting with singlet oxygen. In vegetable oils alpha-tocopherol inhibits the effects of singlet oxygen during sensitized photoxidation. Ascorbic acid has a complex multi-function, acting as a hydrogen donor, as a metal inactivator, and as a peroxide destroyer. beta-Carotene protects lipids by interfering with photosensitized oxidation, and behaves as a reducing agent by trapping radicals. After processing of vegetable oils, about 60-70% of the tocopherols remain in the oil. However, the tocopherol content in processed vegetable oils, is generally above the optimum range for antioxidant activity. On the other hand, beta-carotene is almost completely removed during processing of vegetable oils. For antioxidant purposes, both beta-carotene and ascorbic acid must be added to vegetable oils after deodorization. The vitamin E activity of alpha-tocopherol may be attributed to its very efficient inhibition of in vivo lipid oxidation. In addition to its singlet oxygen quenching properties, beta-carotene has good radical-trapping properties at low partial pressures of oxygen, which prevail in healthy tissues. In biological systems, alpha-tocopherol and beta-carotene exhibit synergism by reinforcing their mutual activities. Synergism also takes place in a cascade where ascorbic acid can be regenerated at the expense of more oxidizable substrates. Our results suggest that singlet oxygen and free radical species may significantly contribute to the fluorescence formed from the interaction of DNA with linolenate hydroperoxides in the presence of iron and ascorbic acid. A better understanding of the biological effects of lipid oxidation products is needed to conserve the nutritional value of foods containing unsaturated lipids.

Hydroperoxidation of unsaturated fatty esters.

Much work has been published on the hydroperoxides of unsaturated lipids. Only recently has significant progress been made, however, to clarify the mechanism and stereochemistry of lipid hydroperoxidation. Although there is considerable information available on the chemistry of lipid oxidation, the biological effects of lipid oxidation products are not well understood. Studies on the source of volatile oxidation products and on their physiological importance are still controversial and difficult to interpret.