FLAVONOIDS: BIOCHEMISTRY, BIOPHYSICS, MEDICINE - Tarakhovsky Yu. S. - 2013

PART 1. MAJOR FLAVONOIDS

1.2. Flavanans

Flavanans comprise four groups: flavanans proper, flavan-3-ols (also known as catechins), flavan-4-ols, and flavan-3,4-diols (Fig. 5). The latter two groups (flavan-4-ols and flavan-3,4-diols) are collectively referred to as leucoanthocyanidins, since their precursor in the biosynthetic pathway is an anthocyanin, whereas the prefix leuco- originates from the Greek word Xeukoq meaning "white" (in this context, uncolored or colorless).

Class="center">Fig. 5. Groups of flavanans.

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1.2.1. Flavan-3-ols (Catechins)

Catechins are one of the most thoroughly investigated groups of flavonoids, encompassing A wide variety of BIOLOGICALLY ACTIVE SUBSTANCES, namely catechins and catechin gallates. They also serve as precursors in the synthesis of proanthocyanidins. Catechin molecules (Fig. 6) differ from most other flavonoid molecules by the absence of a double bond between the second and third carbon atoms, which results in The formation of two chiral centers at these positions and yields four diastereomers. The two isomers with trans-configuration are called catechins, whereas the two isomers with cis-configuration are referred to as epicatechins.

Fig. 6. Diastereomers of catechin. Isomerization of these molecules is possible due to the absence of the double bond between the 2nd and 3rd carbon atoms, which is present in many other flavonoids.

Unlike enantiomers, diastereomers are not mirror-image molecules. Two diastereomers that differ at only one chiral center are termed epimers, and the prefix "epi-" is added to the name of one of the isomers. Unlike enantiomers, diastereomers can differ significantly in their Physical and Chemical properties. Among catechins, (+)-catechin is the most widespread, whereas (-)-epicatechin is the most prevalent among epicatechins.

Catechins derived their name from the extract of the cutch tree (Acacia catechu), used in Eastern countries (such as Japan and Malaysia) as a tanning agent for leather Processing and yellowish-brown dyeing. For centuries, this substance has been utilized in traditional Indian medicine as an astringent and in the Treatment of various conditions, including digestive disorders, wounds, and surgical Procedures. This material is rich in Tannins and catechins. Currently, researchers are exploring the potential use of extracts from Acacia catechu sap in the treatment of carcinogenic diseases, such as Skin epithelial carcinoma. It has been demonstrated that acacia catechins can mitigate the Toxic effects of carcinoma metabolic products owing to their potent antioxidant activity. There are also clinical reports on The Use of cutch wood preparations in the treatment of lepromatous leprosy. Furthermore, these substances exhibit hypotensive properties.

Tea (the leaves of Camellia sinensis) and cocoa beans (Theobroma cacao)—and consequently chocolate—are exceptionally rich in catechins. Green tea has the highest catechin content (Fig. 7). Green tea leaves contain 51–84 mg of catechins per gram of dry weight, which is several times higher than in black tea. Many fruits, vegetables, and wines are also rich in catechins, which can exert beneficial effects on human health. This is hardly surprising, given that plants utilize catechins to combat various pathogens and pests, including insects, Bacteria, Fungi, and Viruses. Catechins are well known as powerful antioxidants due to their ability to scavenge free radicals. Plants rich in catechins also contain oligomers of these substances known as polycyanidins. The most prevalent polycyanidins are formed by multiple epicatechin molecules, The Structure of which varies among different plant species. Cocoa beans and apples are particularly rich in procyanidins. They are also found in red wine and cranberry juice. For instance, cocoa beans are rich in A-type (-)-epicatechin dimers (4 > 8 linkage), whereas peanuts contain B-type (-)-epicatechin dimers (4 > 8; 2 > O7 linkages). Sometimes these molecules form long chains. In many plants, substances such as (-)-epicatechins form more complex, branched oligomers known as tannins (Fig. 7).

Fig. 7. Examples of catechins and their oligomers. The seven major green tea catechins are shown: catechin (C), epicatechin (EC), epigallocatechin (EGC), catechin gallate (CG), epicatechin gallate (ECG), gallocatechin gallate (GCG), and epigallocatechin gallate (EGCG). Examples of A2- and B2-form catechin oligomers formed by 4—> 8 and 2 —> O7 linkages, as well as a fragment of a branched tannin chain, are also illustrated.

It is hypothesized that Various Forms of catechins and their oligomers play a crucial role in the Prevention of various diseases. For instance, epigallocatechin gallate (EGCG) isolated from tea leaves may possess anticarcinogenic activity [40] by virtue of its ability to induce apoptosis in rapidly dividing tumor Cells. This effect may be related to METABOLISM/18.html">The Influence of catechins on cellular regulatory systems, particularly Tyrosine Kinases. The ability of EGCG to inhibit various stages of carcinogenesis—such as inflammatory processes, cellular transformation, proliferation, apoptosis, metastasis, and invasion—is attributed to this flavonoid's capacity to interact with various intracellular molecular targets, including the Transcription factor NF-kB (nuclear factor kappa B), which controls the expression of Immune Response and apoptotic genes, as well as The ability to induce cytokine production (e.g., monocyte chemoattractant protein).

It has been discovered that EGCG can counteract multidrug resistance in tumor cells mediated by P-glycoprotein by blocking the ATP-binding site of this protein. As a result, the intracellular accumulation of therapeutic drugs is significantly increased. Such a potent and versatile effect of EGCG on carcinogenic processes has drawn the attention of clinicians. The combined clinical use of green tea catechins and standard anticancer drugs is being explored, as it may exert a synergistic effect on tumor Cell apoptosis. In addition, these substances can lower Blood Cholesterol levels by reducing low-density lipoprotein (LDL) content, thereby decreasing the risk of atherosclerosis development.

The effects of tea catechins on The Nervous system are also under investigation, specifically regarding their potential as agents that prevent The Development of Parkinson's and Alzheimer's diseases. These substances help strengthen The Cardiovascular system. Furthermore, green tea catechins may help prevent obesity. This effect may be linked to their ability to inhibit catechol-O-methyltransferase and thereby prevent the degradation of norepinephrine (noradrenaline) in the Adrenal Glands.

1.2.2. Theaflavins of Black Tea

Unlike green tea, which is initially dried at temperatures well above 100°C—causing Denaturation of tea leaf Enzymes and preserving the catechin structure—black tea is subjected to rolling in specialized machines that disrupt the leaf Tissues and release enzymes capable of acting upon catechins. Following this maceration step, the leaves are left to ferment, during which significant structural changes occur in the catechins via enzymatic oxidation.

As a result of these enzymatic processes, catechins are converted into theaflavins and thearubigins, which impart the characteristic taste and color to black tea. Theaflavins are catechin oligomers (Fig. 8) that likely retain the preventive and medicinal properties inherent to catechins. Thearubigins, which are larger polymeric molecules constituting over 70% of black tea polyphenols, have been less thoroughly studied. In high-quality teas, the thearubigin-to-theaflavin ratio does not exceed 10, whereas in low-grade teas, this value may exceed 20.

Fig. 8. Examples of theaflavins and other polyphenols present in black tea. In addition to theaflavin gallate and theaflavin digallate shown here, black tea may also contain theaflavin trigallate and theaflavin tetragallate, which have been discovered recently.

It is widely believed that black tea consumption exerts a beneficial effect on numerous bodily processes, including protection against Cancer and cardiovascular diseases. This effect is attributed to theaflavins, which are the primary bioactive polyphenolic compounds in black tea. However, our knowledge regarding the bioavailability of these substances and their biotransformation in the body remains very limited, and most experimental evidence on the effects of theaflavins has been obtained under in vitro conditions, rendering researchers' Conclusions largely hypothetical. To the best of our knowledge, the first study on the biotransformation of theaflavins in rats was conducted only in 2011. Analysis of feces from animals administered theaflavin-3,3’-digallate revealed the presence of four metabolites: theaflavin, theaflavin-3-gallate, theaflavin-3’-gallate, and gallic acid. Glucuronidated and sulfated forms of theaflavin-3,3’-digallate were also detected, whereas methylated derivatives of theaflavins were present in trace amounts. This indicates that these substances undergo biotransformation in vivo. A comprehensive understanding of their Mechanisms of action also requires data on their presence in the bloodstream and their distribution across various Organs and tissues.

It has been demonstrated that in vitro theaflavins exhibit high antioxidant activity, comparable to that of EGCG. Theaflavin-3,3’-digallate displayed the highest activity in scavenging hydrogen peroxide and hydroxyl radicals, whereas theaflavin was the most active against superoxide radicals. Theaflavin-3’-gallate showed activity against singlet oxygen, hydrogen peroxide, and hydroxyl radicals, and protected DNA from hydroxyl radical-induced damage, which, According to the study's authors, may hold potential therapeutic significance.

It is known that, much like green tea catechins, black tea theaflavins can accumulate within the Cell Nucleus. It has been found that theaflavins interact with histone Proteins and all forms of DNA, including double-stranded and quadruplex DNA (G-quadruplexes). Notably, theaflavin-3,3'-digallate exhibited the highest affinity for DNA G-quadruplexes among all substances investigated to date. The Significance of this phenomenon can hardly be overstated, as G-quadruplexes are involved in cellular signaling processes and can also serve as targets for therapeutic agents used in the treatment of cancer and other diseases.

Studies on the isolated rat Heart following ischemia and reperfusion have demonstrated a cardioprotective effect at theaflavin concentrations of 10-40 nmol/L. According to the authors, this effect is associated with the opening of mitochondrial ATP-sensitive potassium channels and the inhibition of the mitochondrial permeability transition pore (mPTP) located on The inner mitochondrial membrane, which is responsible for cellular apoptosis.

Animal experiments have shown that tea theaflavins exert a protective effect on livers affected by non-alcoholic Fatty Liver disease under perfusion conditions, which is of great importance in liver transplantation. Theaflavins exhibit antioxidant, anti-inflammatory, and anti-apoptotic effects both on the liver as a whole and on hepatocyte cultures.

Theaflavins may hold therapeutic significance in the treatment of Parkinson's disease, as they are capable of suppressing inflammatory processes and apoptosis in dopaminergic Neurons of the substantia nigra.

The antioxidant, anti-inflammatory, anticancer, and antimutagenic effects of black tea theaflavins have been demonstrated in rat Bone Marrow macrophages. Upon exposure of the cells to lipopolysaccharide (LPS) pro-inflammatory agents, theaflavins blocked the signaling Pathways of the nuclear factor NF-kB and mitogen-activated protein kinases (MAPK), resulting in decreased expression of interleukin-6 (IL-6), monocyte chemoattractant protein-1 (MCP-1), and the intercellular adhesion molecule ICAM-1. A similar study was conducted on intestinal epithelial cells, where the protective effect of theaflavins against LPS action was also observed. The authors noted the suppression of Cell Adhesion protein expression (ICAM-1 and VCAM-1) resulting from the blockade of the nuclear factor NF-kB and the activation of c-Jun N-terminal kinase (JNK).

Topical application of theaflavins can effectively relieve edema, as demonstrated in animal experiments. When applied to the skin surface, theaflavins initiated an upregulation of the tumor suppressor protein p53 and the pro-apoptotic protein BAX, indicating an effect on Mitochondria. Cell-based models showed that theaflavins suppress the expression of the cyclooxygenase COX-2 Gene and reduce the expression of the inflammatory cytokines TNF-α, inducible nitric oxide synthase (iNOS), intercellular adhesion molecule 1 (ICAM-1), and the transcription factor NF-kB. A similar action involving the activation of BAX and p53 proteins promoted apoptosis in epithelial carcinoma cells.

Theaflavins are capable of activating various processes associated with their anticancer properties. For instance, theaflavins suppress the expression of matrix metalloproteinases (MMP-2), which degrade Extracellular matrix proteins and facilitate tumor invasion. Animal experiments have demonstrated that, owing to this mechanism, theaflavins can reduce melanoma size. Theaflavins also exhibit pro-oxidant activity toward certain cell lines; specifically, theaflavin-3,3'-digallate can inhibit the growth of carcinoma cells by inducing oxidative stress.

Theaflavins also possess bactericidal properties. For example, nosocomial infections are notoriously difficult to treat because their causative agents, such as Stenotrophomonas maltophilia and Acinetobacter baumannii, are resistant to most Antibiotics. However, it was found that theaflavins exhibit marked antibacterial activity against these bacteria in vitro. Notably, the antibacterial effect of theaflavins is enhanced in the presence of green tea catechins. Although the MECHANISM OF ACTION of these substances and the reason for the synergy between green and black tea polyphenols have not been fully elucidated, these findings may prove useful in the development of novel antibacterial agents.

Theaflavin derivatives, such as theaflavin digallates, as well as certain catechins, exhibit an affinity for the retroviral envelope protein gp41. Consequently, these polyphenols can suppress the virulence of such viruses, including the HUMAN IMMUNODEFICIENCY VIRUS (HIV). It has been shown that theaflavin-3,3'-digallate binds to the hydrophobic pocket located On the surface of the viral envelope protein gp41, thereby preventing Viral Particles from entering host cells by blocking their ability to fuse with membranes. Catechin derivatives obtained from green tea exhibit a similar action. Recently, a theaflavin-containing gel was developed that, upon intravaginal application, is capable of protecting against HIV infection. The authors note that the gel formulated using black tea components is characterized by high efficacy and low cost.

The black tea theaflavin fraction is also hypothesized to protect against Influenza virus infection by exerting an inhibitory effect on neuraminidase, an envelope protein of various virus strains (IC₅₀ = 9.27-36.55 ng/mL). This reduces the virulence of viral particles and the Replication of viral genes. Furthermore, cell culture studies have demonstrated that these substances suppress The production of the inflammatory cytokine IL-6, which has the potential to alleviate the course of the disease.