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

PART 2. MECHANISMS OF ACTION OF FLAVONOIDS

2.1. Bioavailability and Pharmacokinetics of Flavonoids

Animal and human Cells are incapable of synthesizing Flavonoids; therefore, they enter the body exclusively through the consumption of plant-based foods. Daily dietary flavonoid intake can range from tens of milligrams to several grams, depending on dietary patterns. Flavonoids are absorbed via Transport Across the epithelial Cells of the gastrointestinal tract. The primary role in flavonoid adsorption is played by enterocytes (brush-border epithelial cells) of the Small Intestine, which line over 90% of its surface area. The bioavailability of flavonoids is remarkably low: less than 1% of the flavonoids ingested reach the bloodstream from the gut.

With the exception of flavan-3-ols, the molecules of most dietary flavonoids are Glycosides containing one or more pyranoside or furanoside carbohydrate residues. Within the intestinal lumen, these compounds are typically acted upon by Hydrolases exhibiting broad substrate Specificity toward flavonoid O-glycosides, thereby releasing flavonoid aglycones (Fig. 36).

Class="center">Fig. 36. Schematic diagram of the flavonoid metabolic pathway. In the intestinal lumen, flavonoid glycosides (FGlc) lose their glycosidic moiety through the action of lactase-phlorizin hydrolase (LPH) and are converted into flavonoid aglycones (FAgl), after which: (1) a fraction of these are absorbed by the intestinal epithelium and enter the bloodstream; (2) another fraction undergoes biotransformation mediated by uridine-5-phosphoglucuronosyltransferase (UGT). A glucuronic acid residue (GlcA) is subsequently attached, and the resulting conjugates enter the portal vein to be delivered to the Liver, where they undergo methylation (Met) and sulfation (Sulf) via catechol-O-methyltransferase (COMT) and phenolsulfotransferase (PST) Enzymes. (3) A significant portion of flavonoids is degraded by gut Bacteria, with the degradation products being excreted in the feces, or (4) delivered to the liver, oxidized by cytochrome P450, secreted via Bile back into the intestinal lumen, and eventually eliminated in the feces. Flavonoids and their modified derivatives present in the bloodstream are excreted in the urine.

The released flavonoid aglycone can be absorbed directly by the epithelial cells. Alternatively, Hydrolysis may occur after the glycosides penetrate the Cytoplasm of intestinal brush-border enterocytes (epithelial cells) through the action of β-glucosidase. Within the enterocyte cytoplasm, these molecules are deglycosylated, and a glucuronic acid residue is attached to them. Before entering the systemic Circulation, these metabolites are transported via the portal vein to the liver, where they are methylated and sulfated by respective transferases (Fig. 37). In the bloodstream, glucuronides represent the predominant form of circulating flavonoids. The enhanced Water solubility of these conjugates significantly prolongs their retention time in the Circulatory system.

Fig. 37. Selected Examples of quercetin metabolites present in the bloodstream. A - parent quercetin aglycone, B - 3’-O-methylquercetin, C - deprotonated form of quercetin-3’-O-sulfate, D - deprotonated form of quercetin-3-O-glucuronide.

The antioxidant activity and free radical-scavenging capacity of quercetin metabolites are generally somewhat lower than those of the parent molecules, although substantial differences only emerge when multiple hydroxyl groups are simultaneously modified. Under physiological conditions and neutral pH, flavonoid glucuronides and sulfates exist in a deprotonated, negatively charged state. This behavior is attributed to the low apparent acid dissociation constant (pK_a) of their functional groups; for instance, the pK_a of polyphenol glucuronides ranges from 2.9 to 3.1.

Flavonoid absorption can be hindered by their efflux Transport from the epithelium back into the intestinal lumen, a process driven by ATP-binding cassette (ABC) transporters associated with multidrug resistance. Furthermore, a fraction of flavonoids, along with their degradation and cytochrome P450-mediated oxidation products, is incorporated into bile and secreted back into the intestinal lumen via the bile duct.

A substantial proportion of flavonoids and their derivatives that escape Absorption in the small intestine reaches the colon, where the resident microflora cleaves the flavonoid molecules (Fig. 38). This microbial degradation yields phenolic acids and hydroxycinnamates, which are subsequently absorbed by the epithelium, transported to the liver for further metabolic Processing, entered into the bloodstream, and ultimately excreted in the urine.

Fig. 38. Degradation of quercetin by enterobacteria. The resulting acids are hydroxycinnamates (derivatives of cinnamic acid).

For instance, studies utilizing radioactively labeled quercetin-4’-glucoside have demonstrated that during gastrointestinal transit, it is converted into phenolic acids, with 69% of the radiolabel being cleared in the urine within 72 hours. As a result of these biotransformations, the plasma concentration of catabolic products derived from certain flavonoids can significantly exceed that of the parent compounds.

Flavonoids and their degradation products undergo extensive metabolic transformations within the intestinal epithelium and liver, including the methylation of hydroxyl groups, reduction of carboxyl groups, and conjugation with glucuronic acid. These conjugates are excreted in the urine as terminal metabolic end-products, while a portion may enter the bile and subsequently be eliminated in the feces. The overall pharmacokinetics of polyphenol compounds in The Human Body can be summarized as follows (Table 1).

Table 1. The Fate of dietary polyphenol compounds consumed over a 24-hour period.

Among daily food products, tea is particularly rich in flavonoids. Green tea deserves special mention, as its dried leaves contain up to 30% catechins by dry weight. Two hours after the consumption of a single cup of green or black tea (350-600 mL), plasma catechin levels reach 0.3-1.0 µM. With higher intake, plasma catechin concentrations can rise up to 10 µM. Although quercetin, naringenin, and hesperidin exhibit lower bioavailability than catechins, their plasma concentrations can also reach tens to hundreds of nanomoles per liter upon robust consumption of fruits and vegetables.

The question arises as to how flavonoids cross the hydrophobic barrier of the epithelial Cell Plasma Membrane. Flavonoid aglycones are known to be poorly water-soluble and sufficiently hydrophobic to spontaneously diffuse across the phospholipid bilayer of Introduction/36.html">Biological Membranes, as demonstrated using taxifolin. However, it has been discovered that water-soluble flavonoid glycosides—such as quercetin monoglycosides (though not diglycosides)—can enter epithelial cells via hexose transporters, including the sodium-dependent transporters GLUT1 or GLUT2. Carrier-mediated adsorption of glycosides proceeds even faster than that of quercetin aglycones. Furthermore, hydrophobic aglycones may also gain entry into epithelial cells with the assistance of the primary Insulin-sensitive glucose transporter GLUT4. Notably, the flavonoid genistein can inhibit the transport activity of this carrier, thereby interfering with insulin-mediated glucose uptake in adipocytes. Similarly, the flavonoids naringenin and phlorizidins (phlorizin) are capable of suppressing intestinal sugar absorption, a property that holds potential therapeutic value in diabetes management.

The bilirubin transporter bilitranslocase may also participate in intestinal flavonoid transport. Additionally, flavonoids can inhibit Proteins belonging to the monocarboxylate transporter family, such as MCT2 and SLC16, which are involved in the Transmembrane Transport of lactate, Pyruvate, Ketone Bodies, and various pharmaceutical drugs. Nevertheless, the physiological contribution of these transporters to flavonoid translocation remains debatable.

Bacterial degradation of flavonoids yields hydroxyphenylacetic acids. Acids generated during The breakdown of kaempferol and quercetin can exert tranquilizing effects. It is conceivable that in certain instances, the pharmacological activity is actually mediated by flavonoid metabolites, whereas the parent flavonoids serve merely as precursors to these bioactive agents.

Once in the bloodstream, flavonoids interact extensively with Plasma Proteins, most notably albumins. This interaction has been documented in studies involving kaempferol, galangin, diosmetin, luteolin, taxifolin, and catechins. Such binding occurs spontaneously, is accompanied by a release of energy, and is principally driven by hydrophobic forces. For example, apigenin spontaneously binds to bovine serum albumin (BSA) with a negative Gibbs Free energy change at site I within subdomain II. Fisetin binds to a homologous site on human serum albumin (HSA). Spontaneous interactions accompanied by free energy release have likewise been observed in studies with dihydrochalcone. Epicatechin binds to BSA with a binding constant of 1.0 × 10⁶ M^-1 at site II within subdomain IIIA, whereas epicatechin gallate exhibits a binding constant of 6.6 × 10⁷ M^-1 at site I within subdomain IIA. The authors attribute these observed affinity differences to the Influence of the galloyl group. Hesperetin binds to human serum albumin (HSA) with a binding constant of 1.941 × 10⁴ M^-1, while morin binds to BSA at site II of subdomain IIIA.

Through their binding to albumin, flavonoids can compete with certain toxins for shared binding sites. For instance, competition between ochratoxin A (a mycotoxin produced by Aspergillus ochraceus Molds and a common food contaminant) and specific flavonoids results in the Displacement of the toxin from the albumin molecule, thereby mitigating its toxic effects. Heavy Metal Ions (Cd²⁺, Hg²⁺, Pb²⁺) can also modulate flavonoid-albumin interactions, an effect likely linked to metal-induced Conformational Changes in the Protein Structure.

Flavonoids can also bind to blood Lipoproteins. For instance, quercetin binds to low-density lipoproteins (LDLs). When bound to the oxidized form of LDL, quercetin exhibits a protective effect on macrophages, for which oxidized LDLs are toxic and induce apoptosis. A similar protective effect against oxidative stress was observed when delphinidin-3-glucoside bound to oxidized LDLs.