The relative lipophilicity of polyphenols depends on the number of contained hydroxyl groups [3]. Interactions of polyphenols with lipids such as lipid cell membranes are limited to the polar region of the lipid bilayer [3]. Their penetration through the lipid membrane depends on their structure, where planarity is preferred [3]. Polyphenols are generally more hydrophilic than lipophilic owing to their phenolic nature [25]. Therefore, free polyphenols along with aglycones, glycosides and oligomers can be readily extracted by solvents such as methanol, ethanol, acetonitrile and acetone, or by their mixtures with water [25].
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Widely established traditional spectrophotometric assays represent simple and fast screening methods for qualification of different classes of phenolic compounds in crude plant samples. Scavenging of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical has recently been applied to the phenolic compounds commonly present in natural tissues. This method represents the basis of a common antioxidant assay. The antiradical activities of various antioxidants are readily determined using the free radical DPPH, which shows a characteristic UV-Vis spectrum with an absorption band at 515 nm. The addition of an antioxidant is reflected in a decrease in absorbance proportional to the concentration and antioxidant activity of the added compound [52,53,54].
High-performance liquid chromatography (HPLC) and gas chromatography (GC) are the two most frequently applied technologies to quantify phenolic compounds. Other relevant methods include the determination of the disappearance of free radicals using UV-Vis spectrometry. Currently, HPLC coupled with ultraviolet detection, electrochemical detection, mass spectrometry (MS) or particle beam/electron ionization mass spectrometry; gas chromatography coupled with MS, high-speed counter-current chromatography; chiral capillary electrophoresis or Fourier transform near infrared reflectance spectroscopy are the most commonly used methods for the determination of phenolic compounds. Hyphenated methods based on the HPLC separation, like HPLC-MS and HPLC-MS/MS, provide information about the molecular mass and structural features of compounds. They are considered to be more useful than other techniques in the separation, identification and quantification of phenolic content. Gas chromatography represents yet another highly effective technique for the separation, identification and quantification of several phenolic species, such as phenolic acids and flavonoids. The major drawback of GC analysis is that phenolic compounds are of low volatility, therefore, their derivatization is necessary [63].
Flavonoids exert their antioxidative activity by effectively scavenging various free radicals (like superoxide anion and peroxynitrite), by regulating oxidative stress-mediated enzyme activity [12] and by chelation of transition metals involved in radical forming processes [79]. Other anticancerogenic properties include regulation of signaling pathways involved in carcinogenesis, interaction with proteins that control cell cycle progression and effective modulation of the wingless-related integration site (Wnt) signaling pathways in which most conventional therapeutics are ineffective [12]. Flavonoids can interfere with all three stages of cancer: the initiation, development and progression by modulating cellular proliferation, differentiation, apoptosis, angiogenesis as well as metastasis [70]. Moreover, chemopreventive effect of dietary flavonoids is quite specific as cancerous cells have shown to be more sensitive to polyphenol actions than normal cells [71]. Furthermore, flavonoids exhibit antibacterial, anti-inflammatory, anti-allergic and anti-thrombotic actions [79].
Measured total peak plasma concentrations of EGCG, EGC, and EC (free and conjugated) were around 2 to 3 μM or lower [24]. Therefore, when extrapolating the results of the animal in vitro studies to humans caution is needed because in a majority of the in vitro studies significantly higher concentrations of polyphenols are used than those attainable in vivo.
Tannins, like many other polyphenols, exert several biological effects including antioxidative, antimicrobial, anticarcinogenic, cardiovascular system protecting, and anti-inflammatory [118,121,123]. Their strong antioxidative action reflects in the free radical scavenging activity, chelation of transition metals, inhibition of pro-oxidative enzymes and lipid peroxidation [121,123]. On the other hand, they are considered as anti-nutrients due to the formation of complexes with proteins, starch, and digestive enzymes; and due to their detrimental influence on utilization of vitamins and minerals [120,121]. Moreover, they cause browning reactions in foods, damage to the mucosal lining of the gastrointestinal tract, alteration of excretion of certain cations and increased excretion of proteins and essential amino acids [120]. Like several other polyphenols they can act as pro-oxidants catalyzing DNA degradation in the presence of transition metal ions such as copper [124]. Even though tannin components have been implicated in the high levels of cheek and oesophageal cancer in certain regions of the world, probably due to their ability to cause irritation and cellular damage rather than due to their direct action on DNA mutation, they are not necessarily the main contributing factor [121]. Additionally, they can act as co-carcinogens or promoters in inducing skin carcinogenesis [121]. Discordantly, anticarcinogenic, antimutagenic, antiproliferative activity, suppression of malignant cell migration, invasion and metastasis by tannins have been reported in vitro and in vivo [8,71,121].
Agricultural residues possess a great potential as a source of antioxidants, many of which belong to polyphenols. Extraction of polyphenols from the agricultural wastes seems a suitable technique for the isolation of these highly valuable, thermolabile compounds. However, extraction of polyphenols is hindered by two main factors: firstly, polyphenols may appear in plant tissues complexed with sugars, proteins or they may create polymerized derivatives with an increased resistance against an effective isolation which mainly depends on proper solvent selection in addition to terms and conditions of the extraction. Secondly, polyphenols are susceptible to oxidation. The recovery of phenolic compounds from plant materials is influenced by the extraction technique, the extraction time and temperature, the solvents used, the solvent to solid ratio and by the intensity of waves. High temperature, long extraction times and alkaline environment cause their degradation. Antioxidant power is usually related to the phenolic content and solvent extracting power represents the most important factor affecting antioxidant capacity. Compared to alternative methods, microwave-assisted and ultrasonication extraction techniques are considered as much more promising and have shown a greater potential and better efficiency for the extraction of polyphenols with a high antioxidant activity. The stability of proanthocyanidins can then be maintained for over two weeks by storing the extract in a freezer. A possible solution to avoid the degradation of polyphenols lies in encapsulation which also represents the most common solution in relation to their poor bioavailability that significantly reduces the polyphenol dose reaching the target cells. The encapsulation features additional advantages of masking the flavor, of control and targeted release. An alternative solution is the use of combinations of various polyphenols which brings synergistic effects resulting in lowering of the required therapeutic dose of a particular polyphenol. The combination of polyphenols with existing drugs and therapies has also shown promising results and has importantly reduced their toxicity. A healthy lifestyle remains crucial for disease prevention; the most important part represents a balanced diet abundant with various fruits and vegetables that contain vital mixtures of polyphenols.
Endothelial lipase (EL) is a potent modulator of the structural and functional properties of HDL. Impact of EL on cholesterol efflux capacity (CEC) of serum and isolated HDL is not well understood and apparently contradictory data were published. Here, we systematically examined the impact of EL on composition and CEC of serum and isolated HDL, in vitro and in vivo, using EL-overexpressing cells and EL-overexpressing mice. CEC was examined in a validated assay using 3H-cholesterol labelled J774 macrophages. In vitro EL-modification of serum resulted in complex alterations, including enrichment of serum with lipid-free/-poor apoA-I, decreased size of human (but not mouse) HDL and altered HDL lipid composition. EL-modification of serum increased CEC, in line with increased lipid-free/-poor apoA-I formation. In contrast, CEC of isolated HDL was decreased likely through altered lipid composition. In contrast to in vitro results, EL-overexpression in mice markedly decreased HDL-cholesterol and apolipoprotein A-I serum levels associated with a decreased CEC of serum. HDL lipid composition was altered, but HDL particle size and CEC were not affected. Our study highlights the multiple and complex effects of EL on HDL composition and function and may help to clarify the seemingly contradictory data found in published articles.
By acting on serum in vitro, EL promotes the formation of lipid-free/-poor apoA-I, associated with marked changes in serum and HDL functionality. We next assessed to what extent EL alters lipid and apolipoprotein composition of serum and HDL. EL markedly reduced the phosphatidylcholine (PC) (Fig. 4a), phosphatidylethanolamine (PE) (Supplementary Fig. S4a), phosphatidylinositol (PI) (Supplementary Fig. S4b) and triacylglycerol (TAG) content of serum, apoB-DS and HDL, respectively (Fig. 4b). As expected, EL significantly increased the LPC content of serum, apoB-DS and HDL (Fig. 4c). Lysophosphatidylethanolamine (LPE) (Supplementary Fig. S4c) and free cholesterol (FC) (Fig. 4d) were increased only in hEL-HDL, whereas the cholesterol ester (CE) content was unaltered (Supplementary Fig. S4d). Ceramide (Cer) and sphingomyelin (SM) content were only altered in hEL-HDL (Supplementary Fig. S4e,f). While EL did not alter the content of the major HDL apolipoproteins in serum (Supplementary Fig. S4g) and apoB-DS (Supplementary Fig. S4h), the apoA-II content was slightly, but significantly higher in hEL-HDL compared to hEV-HDL (Supplementary Fig. S4i). 2ff7e9595c
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