Green Tea Polyphenols: Novel Irreversible Inhibitors of Dopa Decarboxylase

The polyphenol epigallocatechin gallate lowers circulating catecholamine concentrations and alters lipid metabolism during graded exercise in man: a randomized cross-over study

PURPOSE

Physical exercise is shown to mitigate catecholamine metabolites; however, it is unknown if exercise-induced increases in sympatho-adrenal activity or catecholamine metabolites are influenced by ingestion of specific catechins found within green tea. This study explored the impact of epigallocatechin gallate (EGCG) ingestion on catecholamine metabolism during graded cycle exercise in humans.

METHODS

Eight males (22.4 ± 3.3 years, BMI:25.7 ± 2.4 kg.m²) performed a randomised, placebo-controlled, single-blind, cross-over trial after consumption (1450 mg) of either EGCG or placebo (PLAC) and performed graded cycling to volitional exhaustion. Venous bloods were taken at rest, 2 h post-ingestion and after every 3-min stage. Blood variables were analysed for catecholamines, catecholamine metanephrines and metabolic variables at rest, 2 h post-ingestion (POST-ING), peak rate of lipid oxidation (FATpeak), lactate threshold (LT) and peak rate of oxygen consumption (VO₂peak). Data were analysed using SPSS (Version 26).

RESULTS

Resting catecholamine and metanephrines were similar between trials. Plasma adrenaline (AD) was lower in ECGC treatment group between trials at FATpeak (P < 0.05), LT (P < 0.001) and VO₂peak (P < 0.01). Noradrenaline (NA) was lower under EGCG at POST (P < 0.05), FATpeak (P < 0.05), LT (P < 0.01) and VO₂peak (P < 0.05) compared to PLAC. Metanephrines, glucose and lactate increased similarly with exercise intensity in both trials. Lipid oxidation rate was 32% lower in EGCG at FATpeak (EGCG 0.33 ± 0.14 vs. PLAC 0.49 ± 0.11 g.min^-1, P < 0.05). Cycle time to exhaustion was similar (NS).

CONCLUSION

Acute EGCG supplementation reduced circulating catecholamines but not; metanephrine, glucose or lactates, response to graded exercise. Lower circulating catecholamines may explain a lower lipid oxidation rate.

Effects of bioactive molecules on the concentration of biogenic amines in foods and biological systems

Biogenic amines (BAs) are a group of molecules naturally present in foods that contain amino acids, peptides, and proteins as well as in biological systems. In foods, their concentrations typically increase during processing and storage because of exposure to microorganisms that catalyze their formation by releasing amino acid decarboxylases. The concentrations of BAs above certain values are indicative of unsafe foods due to associate neuronal toxicity, allergenic reactions, and increase risks of cardiovascular diseases. There are therefore various strategies that focus on the control of BAs in foods mostly through elimination, inactivation, or inhibition of the growth of microorganisms. Increasingly, there are works on bioactive compounds that can decrease the concentration of BAs through their antimicrobial activity as well as the inhibition of decarboxylating enzymes that control their formation in foods or amine oxidases and N-acetyltransferase that control the degradation in vivo. This review focusses on the role of food-derived bioactive compounds and the mechanism by which they regulate the concentration of BAs. The findings are that most active molecules belong to polyphenols, one of the largest groups of plant secondary metabolites, additionally other useful +compounds are present in extracts of different herbs and spices. Different mechanisms have been proposed for the effects of polyphenols depending on the model system. Studies on the effects in vivo are limited and there is a lack of bioavailability and transport data which are important to assess the importance of the bioactive molecules.

A Study for Therapeutic Treatment against Parkinson's Disease via Chou's 5-steps Rule

The enzyme L-DOPA decarboxylase (DDC), also called aromatic-L-amino-acid decarboxylase, catalyzes the biosynthesis of dopamine, serotonin, and trace amines. Its deficiency or perturbations in expression result in severe motor dysfunction or a range of neurodegenerative and psychiatric disorders. A DDC substrate, L-DOPA, combined with an inhibitor of the enzyme is still the most effective treatment for symptoms of Parkinson's disease. In this review, we provide an update regarding the structures, functions, and inhibitors of DDC, particularly with regards to the treatment of Parkinson's disease. This information will provide insight into the pharmacological treatment of Parkinson's disease.

A Comprehensive Insight on the Health Benefits and Phytoconstituents of Camellia sinensis and Recent Approaches for Its Quality Control

Tea, Camellia sinensis, which belongs to the family Theaceae, is a shrub or evergreen tree up to 16 m in height. Green tea is very popular because of its marked health benefits comprising its anticancer, anti-oxidant, and antimicrobial activities, as well as its effectiveness in reducing body weight. Additionally, it was recognized by Chinese people as an effective traditional drink required for the prophylaxis against many health ailments. This is due to the complex chemical composition of green tea, which comprises different classes of chemical compounds, such as polyphenols, alkaloids, proteins, minerals, vitamins, amino acids, and others. The beneficial health effects of green tea ultimately led to its great consumption and increase its liability to be adulterated by either low-quality or non-green tea products with concomitant decrease in activity. Thus, in this review, green tea was selected to highlight its health benefits and phytoconstituents, as well as recent approaches for its quality-control monitoring that guarantee its incorporation in many pharmaceutical industries. More research is needed to find out other more biological activities, active constituents, and other simple and cheap techniques for its quality assurance that ascertain the prevention of its adulteration.

Combined in silico approaches for the identification of novel inhibitors of human islet amyloid polypeptide (hIAPP) fibrillation

Human islet amyloid polypeptide (hIAPP) is a natively unfolded polypeptide hormone of glucose metabolism, which is co-secreted with insulin by the β-cells of the pancreas. In patients with type 2 diabetes, IAPP forms amyloid fibrils because of diabetes-associated β-cells dysfunction and increasing fibrillation, in turn, lead to failure of secretory function of β-cells. This provides a target for the discovery of small organic molecules against protein aggregation diseases. However, the binding mechanism of these molecules with monomers, oligomers and fibrils to inhibit fibrillation is still an open question. In this work, ligand and structure-based in silico approaches were used to identify novel fibrillation inhibitors and/or fibril binding compounds. The best pharmacophore model was used as a 3D search query for virtual screening of a compound database to identify novel molecules having the potential to be therapeutic agents against protein aggregation diseases. Docking and molecular dynamics simulation studies were used to explore the interaction pattern and mechanism of the identified novel small molecules with predicted hIAPP structure, its aggregation prone conformation and fibril forming segments. We show that catechins with galloyl group and molecules having two to three planar apolar rings bind to hIAPP structures and fibril forming segments with greater affinity. The differences in binding affinities of different compounds against several fibril forming segments of the peptide suggest that a mixture of active compounds may be required for treatment of aggregation diseases.

The Chemical Biology of Molecular Chaperones--Implications for Modulation of Proteostasis

Protein homeostasis (proteostasis) is inextricably tied to cellular health and organismal lifespan. Aging, exposure to physiological and environmental stress, and expression of mutant and metastable proteins can cause an imbalance in the protein-folding landscape, which results in the formation of non-native protein aggregates that challenge the capacity of the proteostasis network (PN), increasing the risk for diseases associated with misfolding, aggregation, and aberrant regulation of cell stress responses. Molecular chaperones have central roles in each of the arms of the PN (protein synthesis, folding, disaggregation, and degradation), leading to the proposal that modulation of chaperone function could have therapeutic benefits for the large and growing family of diseases of protein conformation including neurodegeneration, metabolic diseases, and cancer. In this review, we will discuss the current strategies used to tune the PN through targeting molecular chaperones and assess the potential of the chemical biology of proteostasis.

Investigate the binding of catechins to trypsin using docking and molecular dynamics simulation

To explore the inhibitory mechanism of catechins for digestive enzymes, we investigated the binding mode of catechins to a typical digestive enzyme-trypsin and analyzed the structure-activity relationship of catechins, using an integration of molecular docking, molecular dynamics simulation and binding free energy calculation. We found that catechins with different structures bound to a conservative pocket S1 of trypsin, which is comprised of residues 189–195, 214–220 and 225–228. In the trypsin-catechin complexes, Asp189 by forming strong hydrogen bonding, and Gln192, Trp215 and Gly216 through hydrophobic interactions, all significantly contribute to the binding of catechins. The number and the position of hydroxyl and aromatic groups, the structure of stereoisomers, and the orientation of catechins in the binding pocket S1 of trypsin all affect the binding affinity. The binding affinity is in the order of Epigallocatechin gallate (EGCG) > Epicatechin gallate (ECG) > Epicatechin (EC) > Epigallocatechin (EGC), and 2R-3R EGCG shows the strongest binding affinity out of other stereoisomers. Meanwhile, the synergic conformational changes of residues and catechins were also analyzed. These findings will be helpful in understanding the knowledge of interactions between catechins and trypsin and referable for the design of novel polyphenol based functional food and nutriceutical formulas.

Differential activities of fungi-derived tannases on biotransformation and substrate inhibition in green tea extract

Tannases are important enzymes in the antioxidant potential of tea leaves. In this study, we evaluated the effect of two tannases (T1 and T2) on biotransformation of tea polyphenols and antioxidative activities from catechins in green tea extract (GTE). The T1 tannase-catalyzed reaction was inhibited by the addition of >2.0% GTE substrate, whereas the T2-catalyzed reaction was not inhibited, even by addition of 5.0% GTE. Furthermore, the T1 tannase-catalyzed reaction was inhibited by addition of 10 mg mL(-1) EGCG, whereas the T2 tannase-catalyzed reaction did not display any inhibitory effect. These results indicate that T2 tannase was more tolerant than T1 tannase to substrate inhibition in degallation reactions. Specifically, the substrate EGCG (90,687.1 μg mL(-1)) was transformed into gallic acid (50,242.9 μg mL(-1)) and EGC (92,598.3 μg mL(-1)) after 1-h treatment with T2 tannase (500 U g(-1)). The tannase-mediated product displayed higher in vitro radical-scavenging activity than the control. IC50 value of GTE on ABTS and DPPH radicals (46.1 μg mL(-1) and 18.4 μg mL(-1), respectively) decreased markedly after T2 tannase treatment (to 35.8 μg mL(-1) and 15.1 μg mL(-1), respectively). These results indicate that T2 tannase treatment of GTE enhanced its radical-scavenging activity, an increase that was also observed in the reaction using EGCG substrate. Taken together, our results revealed that T2 tannase is more suitable for biotransformation of catechins in GTE than T1 tannase, and T2 treatment provides an enhanced radical-scavenging effect.

Novel inhibitors of human DOPA decarboxylase extracted from Euonymus glabra Roxb

Dopamine, a biogenic amine with important biological functions, is produced from l-DOPA by DOPA decarboxylase (DDC). DDC is a potential target to modulate the production of dopamine in several pathological states. Known inhibitors of DDC have been used for treatment of Parkinson's disease but suffered low specificity and diverse side effects. In the present study, we identified and characterized a novel class of natural-product-based selective inhibitors for DDC from the extract of Euonymus glabra Roxb. by a newly developed high-throughput enzyme assay. The structures of these inhibitors are dimeric diarylpropane, a unique chemical structure containing a divalent dopamine motif. The most effective inhibitors 5 and 6 have an IC50 of 11.5 ± 1.6 and 21.6 ± 2.7 μM in an in vitro purified enzyme assay, respectively, but did not inhibit other homologous enzymes. Compound 5 but not 6 dose-dependently suppressed the activity of hDDC and dopamine levels at low micromolar concentrations in cells. Furthermore, structure-activity relationship analyses revealed that p-benzoquinone might be a crucial moiety of these inhibitors for inhibiting hDDC. The natural-product-based selective inhibitors of hDDC could serve as a chemical lead for developing improved drugs for dopamine-related disease states.

Mammalian Dopa decarboxylase: structure, catalytic activity and inhibition

Mammalian Dopa decarboxylase catalyzes the conversion of L-Dopa and L-5-hydroxytryptophan to dopamine and serotonin, respectively. Both of them are biologically active neurotransmitters whose levels should be finely tuned. In fact, an altered concentration of dopamine is the cause of neurodegenerative diseases, such as Parkinson's disease. The chemistry of the enzyme is based on the features of its coenzyme pyridoxal 5'-phosphate (PLP). The cofactor is highly reactive and able to perform multiple reactions, besides decarboxylation, such as oxidative deamination, half-transamination and Pictet-Spengler cyclization. The structure resolution shows that the enzyme has a dimeric arrangement and provides a molecular basis to identify the residues involved in each catalytic activity. This information has been combined with kinetic studies under steady-state and pre-steady-state conditions as a function of pH to shed light on residues important for catalysis. A great effort in DDC research is devoted to design efficient and specific inhibitors in addition to those already used in therapy that are not highly specific and are responsible for the side effects exerted by clinical approach to either Parkinson's disease or aromatic amino acid decarboxylase deficiency.

Recent developments on polyphenol–protein interactions: effects on tea and coffee taste, antioxidant properties and the digestive system

Tea and coffee are widely consumed beverages across the world and they are rich sources of various polyphenols. Polyphenols are responsible for the bitterness and astringency of beverages and are also well known to impart antioxidant properties which is beneficial against several oxidative stress related diseases like cancer, cardiovascular diseases, and aging. On the other hand, proteins are also known to display many important roles in several physiological activities. Polyphenols can interact with proteins through hydrophobic or hydrophilic interactions, leading to the formation of soluble or insoluble complexes. According to recent studies, this complex formation can affect the bioavailability and beneficiary properties of both the individual components, in either way. For example, polyphenol-protein complex formation can reduce or enhance the antioxidant activity of polyphenols; similarly it can also affect the digestion ability of several digestive enzymes present in our body. Surprisingly, no review article has been published recently which has focused on the progress in this area, despite numerous articles having appeared in this field. This review summarizes the recent trends and patterns (2005 onwards) in polyphenol-protein interaction studies focusing on the characterization of the complex, the effect of this complex formation on tea and coffee taste, antioxidant properties and the digestive system.

Hydrolysis of green tea residue protein using proteolytic enzyme derived from Aspergillus oryzae

Free amino acids are important chemical components which impact the taste of green tea infusion. The hydrolysis of water-insoluble protein in the green tea residue helps to increase the contents of free amino acids components except theanine. Studies indicate that the hydrolysis of the tea protein could be restricted due to interaction of polyphenols with protein. The experiment indicates that the hydrolysis of tea protein by protease is the main trend when the polyphenols concentration is lower than 5 mg ml(-1), however, the proteins (including tea protein and protease) would interact with polyphenoles instead of hydrolysis when the concentration of polyphenols is higher than 5 mg ml(-1). The hydrolysis of tea protein is absolutely restrained when concentration comes to 10 mg ml(-1).

Structural features of mammalian histidine decarboxylase reveal the basis for specific inhibition

For a long time the structural and molecular features of mammalian histidine decarboxylase (EC 4.1.1.22), the enzyme that produces histamine, have evaded characterization. We overcome the experimental problems for the study of this enzyme by using a computer-based modelling and simulation approach, and have now the conditions to use histidine decarboxylase as a target in histamine pharmacology. In this review, we present the recent (last 5 years) advances in the structure-function relationship of histidine decarboxylase and the strategy for the discovery of new drugs.

Covalent modification of proteins by green tea polyphenol (-)-epigallocatechin-3-gallate through autoxidation

Green tea polyphenol (-)-epigallocatechin-3-gallate (EGCG) has various beneficial properties including chemopreventive, anticarcinogenic, and antioxidant actions. The interaction with proteins known as EGCG-binding targets may be related to the anticancer effects. However, the binding mechanisms for this activity remain poorly understood. Using mass spectrometry and chemical detection methods, we found that EGCG forms covalent adducts with cysteinyl thiol residues in proteins through autoxidation. To investigate the functional modulation caused by binding of EGCG, we examined the interaction between EGCG and a thiol enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Concentration-dependent covalent binding of EGCG to GAPDH was found to be coupled to the irreversible inhibition of GAPDH activity. Mutation experiments revealed that EGCG is primarily bound to the cysteinyl thiol group of the active center, indicating that the irreversible inhibition of GAPDH is due to the covalent attachment of EGCG to the active-center cysteine. Moreover, using EGCG-treated cancer cells, we identified GAPDH as a target of EGCG covalent binding through specific interactions between catechols and aminophenyl boronate agarose resin. Based on these findings, we propose that the covalent modification of proteins by EGCG may be a novel pathway related to the biological activity of EGCG.

Green tea catechin as a chemical chaperone in cancer prevention

Green tea catechins have recently gained significant acceptance as a cancer preventive, and one of the important features of catechins is their interactions with various target molecules. We recently found a functional and structural similarity between catechins and chaperones: Stochastic conformational analysis in silico revealed numerous conformations of (-)-epigallocatechin gallate, (-)-epicatechin gallate and (-)-epigallocatechin, showing a unique flexibility and mobility of the catechin molecules and suggesting the significance of a galloyl group in conformational variation. Since these conformations result in interaction with various types of molecules, we think that green tea catechin induces cancer preventive activity mediated through a chaperone-like property.

Food components inhibiting recombinant human histidine decarboxylase activity

Histidine decarboxylase (HDC) catalyzes histamine formation from histidine. Histamine is a bioactive amine acting as a neurotransmitter as well as a chemical mediator. Phenolic food components have been tested for their ability to inhibit recombinant human HDC. Epicatechin gallate (ECG) was found to be a potent inhibitor as it inhibited HDC activity in a competitive manner with Ki = 10 muM against l-histidine. Epigallocatechin gallate (EGCG) showed time-dependent inhibition which disappeared under anaerobic conditions. It is probable that time-dependent inhibition could be due to the result of autoxidation of EGCG. The initial burst observed for EGCG suggests that EGCG itself is involved in HDC inhibition as observed for ECG. Our present results have shown that the tested food components can inhibit HDC activity. This inhibition likely affects histamine biosynthesis and possibly leads to controlling the biological action induced by histamine. Therefore, those food components exhibiting HDC inhibitory activity might be potentially useful in controlling histamine-induced biological actions.

Mammalian histidine decarboxylase: from structure to function

Histamine is a multifunctional biogenic amine with relevant roles in intercellular communication, inflammatory processes and highly prevalent pathologies. Histamine biosynthesis depends on a single decarboxylation step, carried out by a PLP-dependent histidine decarboxylase activity (EC 4.1.1.22), an enzyme that still remains to be fully characterized. Nevertheless, during the last few years, important advances have been made in this field, including the generation and validation of the first three-dimensional model of the enzyme, which allows us to revisit previous results and conclusions. This essay provides a comprehensive review of the current knowledge of the structural and functional characteristics of mammalian histidine decarboxylase.

New Tyrosinase Inhibitors, (+)-Catechin−Aldehyde Polycondensates

In this study, new tyrosinase inhibitors, (+)-catechin-aldehyde polycondensates, have been developed. Tyrosinase is a copper-containing enzyme that catalyzes the hydroxylation of a monophenol (monophenolase activity) and the oxidation of an o-diphenol (diphenolase activity). In the measurement of tyrosinase inhibition activity, (+)-catechin acted as substrate and cofactor of tyrosinase. On the other hand, the polycondensates inhibited the tyrosine hydroxylation and L-DOPA oxidation by chelation to the active site of tyrosinase. The UV-visible spectrum of a mixture of tyrosinase and the polycondensate exhibited a characteristic shoulder peak ascribed to the chelation of the polycondensate to the active site of tyrosinase. Furthermore, circular dichroism measurement showed a small red shift of the band due to the interaction between tyrosinase and the polycondensate. These data support that the polycondensate acts as an inhibitor of tyrosinase.

Enzymology of methylation of tea catechins and inhibition of catechol-O-methyltransferase by (-)-epigallocatechin gallate

(-)-Epigallocatechin gallate (EGCG) and (-)-epigallocatechin (EGC) are the major polyphenolic constituents in green tea. In this study, we characterized the enzymology of cytosolic catechol-O-methyltransferase (COMT)-catalyzed methylation of EGCG and EGC in humans, mice, and rats. At 1 microM, EGCG was readily methylated by liver cytosolic COMT to 4"-O-methyl-EGCG and then to 4',4"-di-O-methyl-EGCG; EGC was methylated to 4'-O-methyl-EGC. The K(m) and V(max) values for EGC methylation were higher than EGCG; for example, with human liver cytosol, the K(m) were 4.0 versus 0.16 microM and V(max) were 1.28 versus 0.16 nmol/mg/min. Rat liver cytosol had higher COMT activity than that of humans or mice. The small intestine had lower specific activity than the liver in the methylation of EGCG and EGC. Glucuronidation on the B-ring or the D-ring of EGCG greatly inhibited the methylation on the same ring, but glucuronidation on the A-ring of EGCG or EGC did not affect their methylation. Using EGC and 3,4-dihydroxy-L-phenylalanine as substrates, EGCG, 4"-O-methyl-EGCG, and 4',4"-di-O-methyl-EGCG were all potent inhibitors (IC(50) approximately 0.2 microM) of COMT. The COMT-inhibiting activity of (-)-EGCG-3'-O-glucuronide was similar to EGCG, but (-)-EGCG-4"-O-glucuronide was less potent. The present work provides basic information on the methylation of EGCG and suggests that EGCG may inhibit COMT-catalyzed methylation of endogenous and exogenous compounds.

Tea catechin synergies in inhibition of cancer cell proliferation and of a cancer specific cell surface oxidase (ECTO-NOX)

The anticancer properties of tea catechins are most frequently attributed to the principal catechin (-)-epigallocatechin-3-gallate (EGCg). Efficacy was evaluated using growth of cultured HeLa cells and inhibition of the enzymatic activity of a putative cell surface tea target enzyme, a cancer-associated cell surface-located NADH oxidase (ECTO-NOX) designated tNOX. The amounts of EGCg required to inhibit by both criteria was reduced 10 times by combination with inactive catechins such as (-)-epicatechin (EC), (-)-epigallocatechin (EGC) or (-)-epicatechin-3-gallate (ECG). Various synthetic mixtures based on purified catechins and decaffeinated tea extracts treated enzymatically to reduce the ester bond-containing catechins varying in EGCg content from 0.065 to 40% were of comparable efficacy to decaffeinated green tea extracts as long as EGCg was present and the ratio of total catechins to EGCg + EGC was about 1.5. Such mixtures appear to offer potential cancer protection and therapeutic advantages over those of EGCg alone through lowered toxicity of the mixture to normal cells and for more efficient blood delivery of orally-administered catechins to a tumour site.

Inhibition of 15-lipoxygenases by flavonoids: structure-activity relations and mode of action

We have recently reported that flavonoids of cocoa inhibit the mammalian 15-lipoxygenase-1-a catalyst of enzymatic lipid peroxidation. To elucidate the structure-activity relationship of the inhibitory effect, we investigated the effects of 18 selected flavonoids of variable structure on pure rabbit reticulocyte and soybean 15-lipoxygenases using linoleic acid as substrate. Moreover, the inhibition by quercetin was studied in detail to gain insight into the mode of action. Quercetin was found to modulate the time-course of the reaction of both lipoxygenases by three distinct effects: (i) prolongation of the lag period, (ii) rapid decrease in the initial rate after the lag phase was overcome, (iii) time-dependent inactivation of the enzyme during reaction but not in the absence of substrate. A comparison of the IC(50) for the rapid inhibition of rabbit reticulocyte 15-lipoxygenase-1 revealed that (i) the presence of a hydroxyl group in the flavonoid molecule is not essential, (ii) a catechol arrangement reinforces the inhibitory effect, (iii) in the presence of a catechol arrangement the inhibitory potency inversely correlates with the number of hydroxyl groups, (iv) a 2,3-double bond in the C ring strengthens the inhibitory effect. The flavone luteolin turned out to be the most potent inhibitor of the mammalian enzyme with an IC(50) of 0.6 microM followed by baicalein (1 microM) and fisetin (1.5 microM).